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MG MGB GT V8 Factory Originals Technical - Best Carb?
|What would be the best carberator to use on a stock Rover 3.5? I have installed a stock Buick 4 barrel manifold. Running an automatic trans. I would prefer something that bolted directly to the manifold without the use of any adapter plate. Thankyou, Clint|
|I'm using the Edelbrock 500 cfm carb on my 3.5 but with the Edelbrock Performer intake. Don't know if this is the same layout as your stock Buick one.|
Works great on the engine once it's set-up properly with new needles and jets. Got that info if you go this route.
The 500 is a bit big for this engine which explains the need to re-jet it. Nice thing about the Edelbrock is how easy it is to perform this.
Holley 390 is popular as well but apparently not quite as easy to work. Others who use this could confirm or deny this.
The Edelbrock 4-barrel is a newer version of the venerable Carter AFB... which used to be available in a 400cfm version. If your could find one used, the 400cfm AFB would probably be the best carb for the job. Its ports match the original Buick manifold. I believe it also sits a little lower than the Holley.
If you prefer to buy a brand new carb, the Edelbrock is clearly the most popular choice for your engine. Both the Edelbrock and Carter have progressive vacuum secondaries, which seems help.
Here's an article you might also enjoy: http://www.britishv8.org/Articles/Weber-Carburetor-Options.htm
I am a big fan of Holley carbs. I built my 215 with 1.25:1 cr, offy manifold and a isky 270 cam.
It took me about a year to gather all the parts, but over that year, I first bought a 600 double pumper (eBay, about 125.00), I then saw a 390 double pumper (the Holley NASCAR model). It retailed for around 700.00 and I paid about 350.00 for it.
Once I got my car finished, I put the Nascar carb on first. I could not get the idle below 2500 rpm. The reason is that in the primary blades on it, they have bleed holes to keep the engine idling. (remember, this is a full race carb, no vacuum takeoffs at all and 4 idle adjustment screws). Since I had it on the car I decided to drive it around. Other than no power brakes (remember, the brake hose usually connects to the vacuum takeoff on the carb base. Also, my intake manifold has no vacuum takeoffs either). The car ran like a bat out of hell, but the engine ran smoothly only about 3500 rpm and up. (Of course, it is race balanced with the 9 pound aluminum flywheel). My neighbors actually threatened to call the cops. (well, this initial blasting around was open headers. You can imagine what it sounded like. ::))) great fun!!).
I decided I could not do without the power brakes with this much power so I put the 600 double pumper on. It really has a little better low end than the 390 (bigger primaries is why), but I felt it didn't have the correct high RPM feel. (The level of insanity was reduced).
After about 400 miles on the 600, I decided to buy the recommended 390 vacuum secondary 4 bbl from Holley. I dropped another 350.00 on it (brand new) from Summit Racing. I have had it on for about 2 months at this point and till next summer I will probably leave it on. I do plan to put the 390 double pumper back on as it was simply breathtaking in response. (I used silver solder to plug the bleed holes in the primary blades after I talked to the Holley customer support team). So idle on it will be able to be lowered to normal levels.
I still have the stock MGB rear end under the car and cruising at 3400 RPM in 3rd gear, I can floor the throttle and the back tires break loose and smoke. Plenty of power. Glen Towery is bringing me a spare rear end which I have a set of 3.08 gears to put in. I think the 3.08 gears will make it a little less wild, but still thrilling.
Hope this answers some of your Holley questions.
|that was 10.25:1 compression ratio, not 1.25. |
|I have the standard Buick 4 barrel inlet manifold with Weber 500 and seems to perform pretty good. Can anybody recommend any improvements that can be done to this manifold ( gas flow smoothing etc).|
|I have a reasonably built 3.5 with Edelbrock 500 and Performer intake manifold, 10.25:1 cr, erson cam & springs, some head work & port matching and block hugger headers. I haven't installed the motor yet and would like to hear from others who have rejetted the Edelbrock.|
What are you using for jets and needles?
|> The Edelbrock 4-barrel is a newer version of the venerable Carter AFB... |
> which used to be available in a 400cfm version. If your could find one used,
> the 400cfm AFB would probably be the best carb for the job. Its ports match
> the original Buick manifold.
Yes. I've got both 400 and 500 CFM AFB's. The Carter 400 has smaller primary
and secondary venturis than the Edelbrock 500 which helps keep up the airspeed.
The 400 is likely better on unmodified 215's and applications where MPG is
more important than HP. The 500 CFM is likely better on higher HP applications.
> I believe it also sits a little lower than the Holley.
Yes and is a little lighter.
> Here's an article you might also enjoy:
For those who don't know, the Edelbrock AFB is marketed as a Weber in Europe.
Weber USA manufactures the current Carter and Edelbrock AFB and AVS carbs.
The Edelbrock AFB is the third generation of AFB carbs. The first version
of the Carter AFB was introduced in 1957 and used the smaller air filter
base, shared by the Carter WCFB and Rochester 4GC carbs and had a single
mounting flange bolt pattern of 4 1/4" by 5 5/8". Early AFB's were not flow
rated but, judging by venturi and throttle bore diameters, they flowed
approximately 450 to 625 CFM (probably a bit less), depending on the specific
model. Second generation AFB's use the standard 5" diameter air filter base
and have a dual mounting flange with both the 5 1/8" by 5 5/8" Holley 4150/4160
pattern and the 4 1/4" by 5 5/8" pattern used by spreadbore carbs (Quadrajet,
ThermoQuad, and Holley 4165/4175) and first generation AFB's. The Carter
400 is a 2nd generation AFB. For several years, the the Carter AFB was out
of production then Edelbrock reintroduced a revised version of the 2nd
generation AFB. It was produced on new dies by Weber USA and had a number
of changes (Torx head screws on the rod covers, a different top plate without
the center retaining screws, bottom feed primary boosters, rubber plunger
accelerator pump instead leather, some minor differences in the linkage and
fasteners, a shiney finish, etc.). Later, when Federal Mogul re-introduced
the Carter 9000 Series AFB's they simply put their badge on the 3rd generation
Edelbrock AFB's. The same carbs are marketed in Europe under the Weber name.
Edelbrock claims the bottom feed primary boosters, with exits on the lower
face of the delivery tube, provide more uniform atomization relative to
the original Carter side feed boosters. The boosters screw in place and
are supposedly interchangeable. Likewise, the rubber accelerator pumps
are supposed to interchange with the 2nd gen AFB's. I picked one up for
my Carter 400 so I'll find out if that's the case. The jets, rods, springs
and 2-step metering rods are fully interchangeable. Some 2nd generation
AFB's (and AVS's) used a dimpled rod cover and 3 step rods. It's possible
they can be interchaged but I've not verified that.
While I had the 2nd generation Carter AFB on the bench, I verified it's
step-up pistons are identical (diameter and depth of spring pocket) but
the cover screws are not. Edelbrock's are Torx head (Carter's are single
slot) with twice the thread length. I noticed the Carter 400 accelerator
pump arm has 4 positions (holes) while my Edelbrock only has 3. The Carter
also uses a skinnier primary booster in a smaller venturi hole while the
500 CFM uses the fatter booster in a larger venturi hole.
> Both the Edelbrock and Carter have progressive vacuum secondaries, which
> seems help.
Actually both the Edelbrock and Carter AFB's have mechanical secondaries.
However, instead of using a secondary side accelerator pump, they use a
weighted air door (flap) over mechanical secondary throttle blades. This
keeps it from bogging when the secondaries are opened too soon or abruptly.
The downside to this is the opening rate is not easily adjusted. Carter
made a version of the AFB (aluminum four barrel) called the AVS (air valve
secondaries) that uses an adjustable spring loaded air door in conjuntion
with fuel spray bars (instead of the secondary side boosters). Carter
never sold them as aftermarket carbs like they did with the AFB and
ThermoQuad but they did install them on Chrysler products. Edelbrock
recently re-introduced the AVS and show one in a 500 CFM size. Be aware
that as delivered, the 500 AVS comes with rods, jets and springs picked
for dual four barrel carb installation on a larger V8. You'll need to
re-jet for installation on a 215 or 3.5L.
> The 500 is a bit big for this engine which explains the need to re-jet it
When sizing a carb for your application, the usual carb sizing formulas
are simple volumetric relationships and consrvative tot he point of being
useless. An alternative is to realize that 100 HP requires 140 CFM based
upon a reasonable assumption for Brake Specific Fuel Consumption (BSFC).
The BFSC assumption keeps us from having to guess at volumetric efficiency.
A 250 hp engine uses an actual 350 CFM but that doesn't mean a 350 CFM
rated 4 barrel carb will provide the required flow. You have to convert
the pressure drops. 4 barrel carbs are rated at 1.5" but that is too
restricitive. 0.7" is more reasonable for a tuned engine to keep the carb
from being overly restrictive. It takes slightly over 500 CFM (rated at
1.5" Hg pressure drop) to do that:
Flow @ 0.7 In Hg = (CFM Rating @ 1.5 In Hg)/SQRT(1.5/0.7)
350 = X / 1.46385
X = 512.3475 CFM flow rating required
> Nice thing about the Edelbrock is how easy it is to perform this.
> Holley 390 is popular as well but apparently not quite as easy to
> work. Others who use this could confirm or deny this.
Holley makes at least 3 different 390 CFM carbs. List number 8007 is meant
for small displacement, street driven V8's and has an electric choke. The
8007 is a 4160 Holley which means it has a plate with fixed size orifices
for metering. These plates are replaceable but cost much more than jets. If
you plan on doing any tuning (and you should), you may want to convert to the
4150 secondary metering block which has removable jets. However, you'll still
have to pull the fuel bowls off to change the jets, unless you install the
Holley quick change fuel bowls. They have a threaded plug that allows access
to the jets without disassembly of the carb. Highly recommended. One
potential drawback is the quick change bowls are only compatible with center
hung floats and the 8007 has side hung floats. If you plan on hard cornering,
you'll want the center hung floats anyway since they are much less susceptible
to cornering loads and fuel slosh. Side hung floats have a pivot at one side
of the bowl (the pivot pin runs front-to-back in the carb, hence "side pivot")
and a long float that extends to the other side. With that arrangement,
mechanical leverage is significant and a slight slosh at the far tip can cause
the entire float to move. With the center pivot floats, the pivot pin runs
side-to-side and the mechanical leverage is much lower, so slosh in the
corners doesn't lift or drop the floats as easily. You can change from side
hung to center hung floats by simply by changing bowl and float assemblies.
Holley will be glad to sell you the parts or you can pick up a core carb at
a swap meet. There also aftermarket kits (Percys and Willys) that achieve
the same thing as the Holley kit via different means. They may not require
a float change.
The 8007 is a vacuum secondary carb and has a vacuum diaphragm spring which
controls the opening point/rate of the secondaries. To make spring changes
easy, Holley makes an inexpensive quick change kit (P/N 20-59). It consists
of a two-piece secondary diaphragm housing cover which allows spring
replacement without housing disassembly. Also, make sure you install power
valve blow out protection. Holley makes a power valve protection kit that
consists of a tiny ball that forms a check valve. During normal operation
the check valve is open but when a backfire occurs, the valve quickly closes
blocking the pressure wave from damaging the power valve diaphragm. The kit
is inexpensive and comes with check balls to do two power valves, a drill bit,
and instructions but no gaskets. Installation requires carb disassembly.
I did the quick change fuel bowls, the quick change secondary kit and added
power valve protection to the Holley 735 I ran in my Pantera and it mades the
Holley much more user friendly. Still not as nice as a Carter AFB but
From Holley, the 8007 would have come with a 6.5 power valve, #51 main jets,
and a 134-34 metering plate. What works for a given engine is dependent upon
the engine modifications (cam, headers, compression, etc.). As a starting
point you might want to try #51 or #52 primary jets, #54 secondary jets, 7.5
power valve and a 0.28 squirter. However, there's a lot more to tuning a
Holley than just jetting changes.
To tune a Holley for best fuel economy, you usually have to alter the power
valve channel restrictors (PVCR's). You have to first get the power valve
opening point right but the power valves only determine when the additional
fuel is added. How much fuel is added is determined by diameter of the
PVCR's. Unfortunately, standard Holley carbs provide no easy way to do this
so you have to modify the metering block, either drilling the passages larger
or epoxing them shut and re-drilling smaller.
List number 6299 is meant for 4 and 6 cylinder applications. I checked my
listings and the jetting doesn't look much different but the 6299 doesn't
show a power valve. That may mean it has no power enrichment which is
surprising. I also checked the Holley site and it also lists no power
LIST # CFM STOCK JETTING POWER VALVE TYPE
6299-1 390 (F)50, (R)plate 134-34 N/A 4160
8007 390 (F)51, (R)plate 134-34 6.5 4160
80507 390 (F)65,(R)65 3.5,3.5 4150HP
If that's really the case, I'd bet the metering is quite different in the 6299
than in the 8007.
List number 80507 is meant for carb-restricted racing applications, typically
on larger displacement engines. 80507 is an HP Series carb and has no choke
tower, mechanical secondaries, a streamlined venturi inlet, high flow metering
blocks, non-stick gaskets, a four-corner idle system, stainless steel throttle
plates, button head throttle plate screws, center hung dominator style fuel
bowls with notched floats and jet extensions, spun-in (conventional) boosters,
power valve blow-out protection, screw-in air bleeds and double 30cc
Some also use the larger Holley 450 CFM 4 barrel. There are several, being
a popular size for dual quads installations on larger V8's:
P/N CFM Jetting PV Model
4548 450 (F)57 (R)plate 134-30 6.5 4150
6407 450 (F)58 (R)plate 134-5 8.5 4160
8159 450 (F)59 (R)plate 134-32 8.5 4160
9776 450 (F)58 (R)plate 134-6 8.5 4160 no choke
> I have a reasonably built 3.5 with Edelbrock 500 and Performer intake
> manifold, 10.25:1 cr, erson cam & springs, some head work & port matching
> and block hugger headers. I haven't installed the motor yet and would like
> to hear from others who have rejetted the Edelbrock. What are you using
> for jets and needles?
I recently went through a tuning exercise on my Triumph TR8 using a wide
band O2 sensor. I highly recommend using a wide band O2 sensor to check
your mixture. Besides making the process of tuning a carb go much faster,
it's found problems I didn't know I had. When I started tuning the Rover
3.5L in the TR8, it was rich in 1st, 2nd and 3rd but would go lean at the
top of 4th gear and in 5th. I figured there were 3 possible causes:
1. Fuel pump capacity insufficient
2. Carb inlet needle and seat too small
3. Fuel filter too restrictive
Turns out the fuel filter I had (one of those metal mesh filters in a glass
tube) was too restrictive. Holding it up to the light, I could see through
the mesh so it didn't appear clogged but the wideband knew better. By the
time I hit fourth, the fuel bowls had drained enough to cause the mixture
to go dangerously lean. A larger diameter clear plastic fuel filter
(Purolator F21111) fixed the problem. Since the Purolator filter has a
plastic body, I moved it to the firewall to keep it away from direct engine
heat. The fuel filter originally on the car was maybe an inch diameter with
a glass body, a metal mesh filter element and 3/8" inlet and outlet. I held
it up to the light and could see through the mesh so it didn't appear clogged.
I kept a detailed log of my tuning. Here's a summary:
The engine is a stock low compression (8.1:1) Rover 3.5L V8 engine with
the following modifications:
Edelbrock triangular foam air cleaner
Edelbrock 1404 500 CFM four barrel carb (Carter AFB clone)
Offenhauser/JWR Dual Port intake manifold
Crane hydraulic flat tappet cam (unknown specs)
New but stock rate TR8 valve springs
Rhoads fast bleed rate lifters
Rimmers tri-y headers and dual exhaust (H-pipe, no catalytic converters)
Mallory Unilite distributor with vacuum advance
MSD 6AL spark box
MSD Blaster 3 coil
NGK BPR5EY11 spark plugs gapped at 0.040"
I installed the Mallory, MSD and plugs. Everything else was installed by
the previous owner. The Edelbrock 1404 is a clone of the Carter AFB. The
easy stuff to change or adjust includes:
idle mixture screws
primary side jets
primary side rods (controls primary cruise and primary power mode mixtures)
step-up spring (controls when the transition occurs between cruise and power
accelerator pump lever hole
Instead of a power valve for enrichment like a Holley, the Carter/Edelbrock
AFB's use a rod and jet arrangement. A 2 step tapered rod, controlled by a
vacuum-referenced, spring-loaded piston, moves up and down in the jet to
provide a two different area main metering orifices. This allows the carb
to adjust the air-to-fuel ratio for differing loads, as sensed by the vacuum
level. When the vehicle is cruising, the rod is on the lean step. Under
high load, low vacuum, conditions the rod moves to the rich step. The
metering rods and step-up springs can be changed without removing the top
cover but the cover has to come off to R&R the jets. There's a series of
screws that hold the top cover on but you must first remove two small clips
that hold the accelerator pump and throttle lever linkages in place. These
are very tiny and easy to drop, so keep a spare pair on hand and cover the
carb when removing them. I used a very small pair of needle nose to remove
the accelerator pump clip but found a dental pick with curved hook worked
better on the throttle linkage. You should also remove the rod and pistons
(uses torx headed screws to hold the covers on). Unlike a Holley, there's
no need to drain the float bowls when changing jets. Use a screwdriver with
a tip that is the same width as the jet, otherwise it's easy to scrape a
little aluminum of the carb body.
The goal of my tuning was to get it lean of stoichiometric at cruise for
best fuel economy while in the range for maximum power at wide open
throttle. 14.7:1 is the stoichiometric ratio. It's the "chemically ideal"
where there is no excess fuel or oxygen left after combustion. Leaner means
there's excess oxygen left after combustion. Richer means there's excess fuel
left. Generally, you want to run rich of stoichiometric at wide open throttle
(WOT) and a bit lean at cruise. There's no single ideal ratio that applies
to all engines. Some engines make best power at 13:1, others closer to
12.5:1. Note that the air fuel ratio is by weight. 13:1 means 13 pounds
of air are mixed with 1 pound of fuel. The usual target values for normally
aspirated 4 stroke engines are about 12.5 to 13 for WOT, 14.0-15.5 at
part-throttle cruise and 13.5-14.0 for part throttle acceleration (or
climbing a long hill, pulling a load, etc.). If you want to lean out the
mixture at cruise for best fuel economy, be aware that you'll also need to
adjust timing. Combustion gets much slower under lean conditions and if
you don't adjust spark timing, the combustion occurs much later and exhaust
temperature climbs. That's bad for the exhaust seats and valves. However,
if you adjust for MBT spark at each A/F ratio, exhaust temperature will
actually decrease relative to stoichimetric (rich will still be somewhat
cooler). For typical gasoline engines, the range or ratios is:
5 Rich burn limit. Combustion is weak and/or erratic.
6-9 Extremely rich. Black smoke and low power.
10-11 Very rich. Some supercharged engines run in this range at full power as
a means of controlling detonation.
12-13 Rich. Best power A/F for normally aspirated WOT.
14-15 Chemically ideal. At 14.6 the A/F is at the theoretical ideal ratio
with no excess fuel or oxygen after combustion. Good A/F target for part
throttle cruise and light to moderate acceleration.
16-17 Lean. Best fuel economy A/F ratio. Borderline for part throttle
drivability (worse than borderline if EGR is used).
18-19 Very lean. Usual lean limit (Driveability).
20-25 Lean burn limit. Varies with engine.
If your engine has a cam with a lot of overlap, your wideband will likely
read rich at idle, even though it isn't. In cases like that, I usually
set the idle mixture use the RPM drop method. Adjust mixture to yield
maximum RPM, then lean it so the idle drops 20 to 40 RPM. Even with milder
cams on engines with carbs, I usually only get around 13:1 at hot idle
if the idle speed is low enough. Raise the RPM a bit and the air fuel
ratio will quickly go to the cruise value. Also placement of the sensor
(in collector versus near tailpipe) can yield different results. The best
thing to do is to correlate air fuel ratio on a dyno with maximum power
for your set-up. You can accomplish the same thing with an accelerometer
based G-meter or at the drag strip.
I started the tuning session by drinving the car around to warm it up before
setting the initial idle speed and mixture. Timing had been set previously.
It drove well with no bogs or flat spots but the old plugs and tail pipes
suggested the mixture was rich. The carb was installed out of the box
without any jetting changes by the previous owner. The baseline specs of
the manual choke 1404 carb are:
1423 jets (0.086" diameter) in primaries
1460 rod (0.065" x 0.052", stamped 6552)
orange step up springs (5" Hg)
1426 jets (0.095" diameter) in secondaries
center position accelerator pump
0.0935" needle and seat
11/32" float height (+/- 1/4" float drop)
middle accelerator pump link hole
0.028" accelerator pump nozzle diameter
500 CFM rating
I set the initial hot idle speed to 750 RPM. BTW, with the MSD, it starts
just fine without the choke and will idle down to 400 RPM. I set the initial
idle mixture using the method suggested in the Edelbrock carb owners manual
(richen to maximum idle speed then lean to a 20 RPM drop). I then installed
a wide band O2 sensor in a bung I had welded in previously. It's located aft
of the passenger side header collector with the sensor cable routed through
the engine compartment (zip tied away from anything hot) out the rear of the
hood into the passenger side seat.
The wideband showed the the idle mixture at 13:1. I was initially going to
try for stoichiometric (14.7:1) but settled on 13.5:1. Then I noticed the
representative charts in the Edelbrock catalog show a 13:1 idle mixture. Be
aware that cams with a bunch of overlap can fool the wideband at idle due to
the unsteady nature of the idle (misfires result in unburnt fuel passing over
the sensor). This will go away quickly as the revs rise. Since it's
dangerous to drive and read the gauge at the same time, I had a neighbor do
the driving while I read the meter. It was immediately obvious that the base
calibration was way too rich (2 points across the board) for my engine.
Edelbrock provides nice little charts that correlates the rod and jet
combinations to changes in mixture:
Each step is approximately a 4% change. With the mixture readings from
the meter, it's possible to calculate how many steps are required to get
a desired mixture. Be aware that not all possible combinations appear on
that chart and only the highlighted ones are possible using the tuning kit.
You'll need to purchase additional tuning parts for the other combinations.
I wrote a little program to calculate all the possible rod/jet combinations
and it helped me find a couple of rod/jet combos that worked better in my
application. I'm now cruising at 15.5:1 (lean of stoich for better fuel
economy) with transition mixtures (climbing a hill, pulling a load, part
throttle acceleration) in the 13.5 to 14:1 range and WOT in the 12.5 to
13:1 range. Pretty much ideal. When I started, it was rich across the
board. I've picked up 3 MPG and power to boot. Using the combos on the
chart, I couldn't get the primary power mode lean enough so I had to
compensate by leaning down the secondaries a bunch. Getting the right
primary rod/jet combo fixed that. Depending upon where your cruise mode
mixture is, you may want your primary power mode to go between 15 and 25%
The initial reading was so rich, I didn't even bother to write the air-fuel
ratios down. Returned home and went three steps (12%) lean on both the
primary cruise and primary power modes:
#10 on chart, #1422 jets 0.083", #1463 rods (0.067" x 0.055")
Effective Cruise Jet Area (sq. in.) = 1.8849555921538760E-03
Effective Power Jet Area (sq. in.) = 3.0347785033677404E-03
3.0348 / 1.8850 = 1.61 or 61% more area
Flow is proportional to area squared
0.61**2 = 0.372 or 37.2% power enrichment flow
That was too lean at cruise (had a mild surge) so I went to #11 on the chart
(2 stages lean on both cruise and power modes):
#11 on chart: #1422 jets 0.083", #1460 rods (0.065" x 0.052")
Effective Cruise Jet Area (sq. in.) = 2.0923007072908024E-03
Effective Power Jet Area (sq. in.) = 3.2868913138183213E-03
3.287 / 2.0923 = 1.571
0.571**2 = 0.326 0r 32.6% power enrichment
That put me near stoichiometric (14.7:1) at cruise but was rich in power
mode which also made WOT rich, even after leaning the secondaries from
the baseline 0.095" jets down to 0.080" (five 4% lean steps). The power
mode was too rich, causing me to try compensate by leaning out the
secondaries. The tuning chart doesn't show any other combinations that
would allow me to fine tune this set-up but using the program I wrote, I
came up with some additional combinations not on the chart that worked
not on chart: #1422 jets 0.083", #1461 rods (0.065" x 0.057")
Effective Cruise Jet Area (sq. in.) = 2.0923007072908024E-03
Effective Power Jet Area (sq. in.) = 2.8588493147667119E-03
2.860 / 2.092 = 1.367
0.367**2 = 1.135 or 13.5% power enrichment
not on chart: #1421 jets 0.080", #1441 rods (0.062" x 0.052")
Effective Cruise Jet Area (sq. in.) = 2.0074777056438778E-03
Effective Power Jet Area (sq. in.) = 2.9028316119169689E-03
2.9028 - 2.0075 = 0.8953
0.8953 / 2.0075 = 0.446
0.446**2 = 0.200 or 20% power enrichment
I purchased the additional rods and jets for these two combinations.
When I went to install them, I noticed the #1461 rods were machined
differently. Both carry the same part number, though it's obvious
they are from different production batches as the stamped lettering
is different. Also, the length of the machined step is different
and the tip of one of the rods was blackened which solvent failed to
remove. I put the micrometer on the rods and they varied more than
I cared for. I also have a Carter 400 AFB here and, as luck would
have it, it was equipped with rods with the power mode of the #1461
rods but a slightly leaner cruise mode. Since the rods are so easy
to change out, I swapped the rods over and took it for a test drive.
not on chart: #1422 jets 0.083", Carter 400 rods (0.066" x 0.057")
Effective Cruise Jet Area (sq. in.) = 1.9894135478857367E-03
Effective Power Jet Area (sq. in.) = 2.8588493147667119E-03
2.860 / 1.989 = 1.437
0.437**2 = 0.191 or 19.1% power enrichment
Cold, without the choke, there was some mild lean hesitation but that
quickly went away as the engine warmed. The leaner rods did the trick.
By leaning out the primary mode, WOT also leaned out without a further
seconday jet change. With the Carter rods, I'm now cruising at 15.5:1
(lean of stoich for better fuel economy) with transition mixtures
(climbing a hill, pulling a load, part throttle acceleration) in the
13.5 to 14:1 range and WOT in the 12.5 to 13:1 range. Pretty much ideal.
I got 19 MPG on the last tankful when I was cruising at 14.7:1, up a
couple of MPG from where I started even though I was mainly WOT tuning
the carb or cruising at 75+ MPH. With the latest carb changes, I think
it'll do 20+ MPG around town. Edelbrock doesn't have a 0.066" x 0.057"
rod listed. The next size listed is #1436 rods (0.068" x 0.057"). That
would be too lean at:
not on chart: #1422 jets 0.083", #1436 rods (0.068" x 0.057")
Effective Cruise Jet Area (sq. in.) = 1.7789268400952206E-03
Effective Power Jet Area (sq. in.) = 2.8588493147667119E-03
With all Edelbrock parts, the best bet would likely be the other
combination listed above that's not on the charts (#1421 jets 0.080",
#1441 rods 0.062" x 0.052")
There are some other rod/jet combos that give similar but slightly different
effective jet areas. If I was really trying to fine tune this combo, I'd
test each on the same day with an accelerometer to see which gives the best
performance. Vizard has gone so far as to put O2 sensors in each header
primary so he could stagger jet to better equalize the mixture distribution.
IIRC, he said it was worth 20 HP on a 300 cube engine.
Some miscellaneous observations. While it starts just fine with no choke,
you don't get the fast idle cam so it idles at 500 RPM cold versus 750 hot.
The Offenhauser Dual Port/AFB combo is very smooth and I could not tell when
the secondaries opened. It will pull 3rd gear idling through the subdivision
and is, in fact, smoother than the fuel injected 5.0L in my 1987 Mustang.
In case you aren't familair with it, the Dual Port design has the runners
split into top and bottom sections with the plenum split fore and aft (rather
than the usual side-to-side). It's essentially two single plane intakes
stacked on top of each other with the 4 barrel carb primaries feeding the
longer path lower runners and the secondaries feeding the shorter path upper
runners. It's an interesting concept but the packaging required to fit within
a carb intake envelope does compromise the design. The Offy Dual Port has the
reputation of being a good low to mid range intake with excellent throttle
response and fuel economy (probably the best cruise fuel economy based upon
BSFC tests performed by David Vizard... I've got the dyno results here some
place). It's not a high rpm intake but Offenhauser says you can trim the
dividers back at the head flange to pick up some top end power. An Edelbrock
Performer Rover intake will make better power but is taller so hood clearance
may be an issue. The Edelbrock foam air filter has got to go. The air fuel
readings were not quite as stable as the readings I got on my buddy's 428CJ
powered Cobra replica. That might be due to the size of the motor relative to
the carb (750 CFM on a 428 versus 500 on a 215). Also, the accelerator pump
shot and/or clogged fuel filter may have had something to do with it. Make
sure to tighten the fuel line after you've re-jetted or you'll spray fuel on
the hot manifold. Doh! It's tough to read the numbers on the rods and jets
once they get stained with fuel. I resorted to the calipers (for the rods)
and a tapered rod (for the jets) to verify the sizing. I then bagged and
tagged the rods in individual zip-loc baggies (the really small snack size).
I noticed a little sediment in the bowls and cleaned it out. I'll check
again later. I was shifting at 6000 RPM and managed to bump the 6400 RPM rev
limiter once while watching the meter. I bought a dual outlet for cigarette
lighter so I could use the radar detector during tuning. The wideband O2
uses the lighter socket to provide power for the sensor heater. The primary
side venturis of the 600 CFM AFB's are the same size as the 500 CFM AFB's
though the boosters are different. Need to get a G-meter to correlate
acceleration (power) with mixture.
> I have the standard Buick 4 barrel inlet manifold with Weber 500 and seems
> to perform pretty good. Can anybody recommend any improvements that can be
> done to this manifold ( gas flow smoothing etc).
The Buick 215 has 4 holes at the carb base. You can open the space between
the front and rear holes so it's two oval holes or even open the plenum up
to one big hole. A friend did that on an autocross car and got it to work
about as well as an unported Edelbrock Performer. If I were doing it, I'd
go the dual oval opening route and experiment with an open spacer if you
haveenough hood room.
> What would be the best carberator to use on a stock Rover 3.5? I have
> installed a stock Buick 4 barrel manifold. Running an automatic trans.
> I would prefer something that bolted directly to the manifold without
> the use of any adapter plate.
Best of the easy to obtain carbs would be the 500 CFM Edelbrock AVS.
The AVS version with the adjustable air door might come in handy with
the automatic transmission. Does the transmission require a kickdown
linkage. The Carter 400 AFB is pretty rare but would likely be better
for MPG. Another carb that would work well would be 1.08" venturi version
of the Autolite 4100. The Autolite was used by Ford and features annular
boosters which are superior to standard boosters. For higher HP engines,
something like the Demon 525 would likely work well.
|Has anyone used the AED carbs? I know they may be a little large for the 3.5 aplication.|
I have purchased a 650 for my 302 with 350+ at the flywheel. Just wanted to know if any of you have history with this carb
|Dan Jones, please contact me off-line. My e-mail address, phone number, etc. appear here: http://www.britishv8.org/British-V8-Contact-Info.htm |
|Thanks for the tremendous job, Dan! |
There ya go, Curtis. Article?! That's practically a book!
I have eyeballed the differences between the stock Buick manifold & the Edlebrock Performer. The mods you mention (open up carb inlet holes plus spacer) will essentially turn the stock manifold into an Edelbrock Performer. Be interesting to compare the flow numbers.
"The Edelbrock foam air filter has got to go."
That's the first thing I noticed. I used to have one of those on top of a Holley 390 in my ols S10 pickup. I have read that they are quite restrictive.
|On the air cleaner, I have a corvette part on my Holley. Works great and sits down tight over the carb. |
Here is a link to one like I bought.
Even with my Offy intake and holley carb, I still have close to 1/2" space to the hood with the air cleaner on.
|> Has anyone used the AED carbs? I know they may be a little large for the 3.5|
> aplication. I have purchased a 650 for my 302 with 350+ at the flywheel. Just
> wanted to know if any of you have history with this carb
I've not used an AED carb but it's probably the right size for your engine:
350/100 * 140 CFM = 490 CFM at 0.7" pressure drop need for 350 HP
Flow @ 0.7 In Hg = (CFM Rating @ 1.5 In Hg)/SQRT(1.5/0.7)
490 = X / 1.46385
X = 717.2865 CFM flow rating (at 1.5" pressure drop) required
AED likely starts with a Holley 650 body and increases flow via streamlining
the carb. Contact them for the actual flow rating but I bet it's over 700
CFM which should be just what you want air flow-wise.
> Dan Jones, please contact me off-line. My e-mail address, phone number,
> etc. appear here: http://www.britishv8.org/British-V8-Contact-Info.htm
I just sent you an email message with my contact info.
> I have eyeballed the differences between the stock Buick manifold & the
> Edlebrock Performer. The mods you mention (open up carb inlet holes plus
> spacer) will essentially turn the stock manifold into an Edelbrock Performer.
> Be interesting to compare the flow numbers.
If you decide to do this, get a copy of "How to Build Horsepower, Volume 2:
Carbs and Intake Manifolds" by David Vizard. He shows how to do this in
detail. Vizard's earlier book "Performance with Economy" also has some
good info on modifying factory dual plane intake manifolds.
> "The Edelbrock foam air filter has got to go."
> That's the first thing I noticed. I used to have one of those on top of a
> Holley 390 in my ols S10 pickup. I have read that they are quite restrictive.
I've got a better filter set up for it but the Edelbrock filter was easier
to R&R so I used that while I was tuning.
> On the air cleaner, I have a corvette part on my Holley. Works great and
> sits down tight over the carb.
I have a TR8 which doesn't permit a large diameter round filter without
removing the fresh air plenum. I ended up using an oval filter turned
90 degrees. My next air cleaner set up will duct cold air to the filter.
St. Louis, Missouri USA
|Dan, I didn't realize you were such a carb guru. Very impressive indeed. I believe your tuning tecniques would apply very well to efi systems also. Do you have any interest in exploring that area?|
|> Dan, I didn't realize you were such a carb guru. Very impressive indeed. |
I'm not really a guru but I try to do my homework.
> I believe your tuning tecniques would apply very well to efi systems
> Do you have any interest in exploring that area?
Yes. Once I finish up this:
it's slated to get this injection system:
A friend in the club is a former pattern maker and cast the intakes
up from scratch (those patterns are his handiwork, not mine) to match
my high port heads and Fontana aluminum race block.
Once that's finished up, the next engine project is a Rover/Buick stroker
(4.0L Rover block, Buick 300 crank, ported Buick 300 heads, etc.). I'm
seriously considering injection for it. I had a Rover 3.9L EFI system:
but it's ports were just too small to support the ported Buick 300 heads:
I sold it to a local guy who plans to use it on his 3.5L, probably with
a Megasquirt ECU. Should work well in that application. Tim Lanocha
makes this EFI intake:
which fits under a TR8 hoodline. Even though it's listed on his
website (along with other parts like 4-into-1 headers), you can't
buy it. I tried for a couple of years to get him to make me one
but he was always too busy. Willpower (Bill Laney in Australia) makes
a version of this single plane intake that has drilled injector bosses:
I'm supposed to pick up mine over the Thanksgiving weekend. If the
ports are a reasonable match (they are supposed to match a ported Rover
head like a Harcourt intake), I'll likely use it as the manifold for the
TR8 injection project. Failing that, I've got the Huffaker single plane
intake with ports that match the ported Buick 300 heads perfectly:
Given that it's such a rare intake, I'd hate to drill it for EFI
bungs. I guess I could always go TBI. The cheapest and easiest
approach would be a GM TBI on a single plane carb intake. You could
continue to run a carb while installing the system, so there'd be
little downtime and it would be easy to convert to port injection
at a later date. A couple of small TBI's on one of the Edelbrock
or Offy 2 x 2 intakes would be neat:
I'd love to get my hands on one of those Mickey Thompson cross ram IR
and mount a set of TWM side draft EFI throttle bodies.
As far as engine computers go, there are several low cost approaches
including hacked GM or Ford, Megasquirt, VEMS, etc.
|Dale - I'm running .080" primaries, .086" secondaries and .062x.052" needles in my Edelbrock.|
After much trial and error, checking emissions on a gas anaylyzer and running on a dyno, I've found this is the best combination for my car. We have emissions testing here so this had to be considered as well.
Air filter is a K&N 1 7/8" filter with an "Extreme" top on a 1.5" drop-base assembly. It all seems to work.
If you'd like any more info or pics, let me know.
That's a fantastic load of information on the Edelbrock Carb, and helps to explain how the carb functions, many thanks for the input.
I have the 500 on my Rover 3.5 (215ci)in my MGB V8 with the Performer manifold,Heads with the throats opened to seat size, bulleted guides and port matching, 9.75-1 compression and a Viper Hurricane Cam from real steel in the UK, which is the cam they recommend for fast road use (where?). I have changed to the 1441 rods and 1421 jets,same as Simon and as recommended by Real Steel and suggested by yourself above. It runs pretty well considering the timing is yet to be optimised and the vacuum canister is no longer working, the only critisism would be a slight hunting on a light throttle a low speed, I have adjusted the slow running jets by the RPM drop method and this has improved the situation but it is still noticeable, perhaps sorting out the timing will improve matters.
I was very interested in your comments re the fuel filter, I am using a clear plastic filter similar to the one you describe, although probably a little smaller. It is mounted horizontally in line with the carb but never seems to completely fill, only half way up most of the time and I am wondering if this is important and might promote fuel starvation at large throttle openings. I have not had it at WOT yet as it only has 500 miles on the engine since a full rebuild to the above spec.
I would be interested to hear you comment re the filter,
|> It runs pretty well considering the timing is yet to be optimised and the |
> vacuum canister is no longer working, the only critisism would be a slight
> hunting on a light throttle a low speed
Get the timing dialed in before doing the final carb tuning. You'll
want to get the vacuum advance working again. It'll run cooler and
use less fuel. The small chamber Rover heads shouldn't need much total
advance 30 to 32 degrees (with vacuum disconnected).
> I have adjusted the slow running jets by the RPM drop method and this has
> improved the situation but it is still noticeable, perhaps sorting out the
> timing will improve matters.
The idle mixture has a minor effect on cruise. You may be a tad lean
or it may be over advanced.
> I was very interested in your comments re the fuel filter, I am using a
> clear plastic filter similar to the one you describe, although probably
> a little smaller. It is mounted horizontally in line with the carb but
> never seems to completely fill, only half way up most of the time and I
> am wondering if this is important and might promote fuel starvation at
> large throttle openings. I have not had it at WOT yet as it only has 500
> miles on the engine since a full rebuild to the above spec.
Possibly. The carb draws from the float bowls. When they are full,
WOT will get all the fuel it needs (assuming the needle and seat are
large enough which they should be). If either the fuel pump or filter
can't keep up you'll eventually drain the fuel bowls, leading to a lean
condition. Be aware that AFB's are sensitive to fuel pressure. No more
than 6 psi at idle. If you have a regulator, set it at 5.5 PSI. At
wide open throttle, you need to maintain at least 2 PSI.
Thanks for your comprehensive reply.
|Some good links Dan, of course I'm partial to blowers, especially on these little motors and I like the megasquirt controller pretty well. I figure if a fella is going to go to the trouble of making an intake manifold it might as well be one for a blower.|
Otoh it isn't very hard to add injector bungs and there are any number of intakes that will do for starters. Still, with your friend the moldmaker and his connections with foundries I could show you how to make a great blower intake that you could close the stock MG hood over. There's not much doubt that there would be some guys who would like to have such a piece. Sound interesting?
I would try and find a used Carter 400 AFB, and overhaul it. D+D had some listed on they're web site, however I'm not sure if they have any left.It's a direct bolt on to the Buick manifold. I found it's calibration for the 215/3.5 to be very good right out of the box.
I finished my intake manifold for the S/C. Almost done with the blower case mods, just have to drill/tap for the throttle body mounting studs. Working on the throttle body adapter plate now.
Pulled the 4bbl intake, and temporarily bolted on the S/C intake with the blower. Good news, I was able to close the hood with room to spare!
Next comes making fuel rails, and a belt drive system.
|Has anyone used the original Rochester 4 bbl carb designed for these engines? I've always had great luck with Rochesters, not that the Edelbrock aren't as good or better!|
I haven't personally... but I know it's been done. This article may be a little helpful, although it doesn't provide an actual performance comparison:
|> Thanks for your comprehensive reply.|
> I figure if a fella is going to go to the trouble of making an intake
> manifold it might as well be one for a blower.
Neat car. One of these days when I'm in Cincy I need to stop by and
check it out.
> Otoh it isn't very hard to add injector bungs and there are any number
> of intakes that will do for starters.
I think a lot of the Roots type blower manifolds start out as an
Offy 360. It's easy to modify for that purpose. For centrifugal
blowers, a plenum intake like the Rover 3.9 EFI would work better.
> Still, with your friend the moldmaker and his connections with foundries
> I could show you how to make a great blower intake that you could close
> the stock MG hood over. There's not much doubt that there would be some
> guys who would like to have such a piece. Sound interesting?
I think Kelly is set for projects, as am I. The IR EFI intake project
started at as a winter hobby deal and was never meant to be a money-maker.
Just hoping to be able to recover the costs. Kelly has a couple of other
casting projects already lined up so it'll be a couple of years before
he's interested in anything new.
> Has anyone used the original Rochester 4 bbl carb designed for these
I rebuilt a 215 version for a guy one time but never ran one myself.
The Rochester 4G/4GC (also known as the 4 Jet but not to be confused with
the later and much more sophisticated Quadrajets which is an entirely
different animal) was Rochester's first four barrel carb and was used on
a variety of GM automobiles from 1952 to 1967, including the 1961 to 1963
Buick/Oldsmobile/Pontiac aluminum 215 V8's. The 4G/4GC carbs use a square
bore layout with a conventional fixed jet, power valve and accelerator pump
metering scheme. Secondaries are mechanically actuated but use an adjustable
spring loaded air valve. Adjustment is achieved by loosening an allen head
set screw and inserting a screwdriver. Rotating the screwdriver counter-
clockwise increases tension. The accelerator pump is also adjustable, using
a link with 5 adjustment holes. Four center pivot floats are used. The
4G's used the smaller diameter air cleaner, shared by the first generation
Carter AFB's, so the usual aftermarket air filter cases will not fit.
However there are adapter rings and street rod sources usually carry
compatible air filter cases. Three main versions were produced with 486,
553, and 692 CFM ratings though looking at small version, I think those
numbers are optimistic.
> I've always had great luck with Rochesters, not that the Edelbrock aren't
> as good or better!
It should work acceptably well on something with a stock or near stock
camshaft but I'm not sure what sort of tuning parts (jets, power valves,
etc.) are available. I've robbed jets from 2GC's before but only some
use the same jets. IIRC, there some that had aluminum bases and others
that had cast iron bases. The latter is noticeably heavier. Doug Roe's
Rochester book (Rochester Carburetors By Doug Roe, ISBN 0-89586-301-4,
HP Books, Copyright 1981) covers the 4G and 4GC carbs but your best bet
for information is probably a GM shop manual from the early 1960's.
St. Louis, MO
Hey I saw that car on Laughatrice.com!!
|This has been a great thread and I'm happy to see it bumped back up to the top of the screen, even if the troll that "bumped" it there is a real idiot!|
What a GREAT post. I'm overloaded my carb intake for today! This one is worth of a cut and paste/print and read/save!
|No fair Bill, Memphis. You didn't link to the post. No way to find it.|
Bill Jacobson: That's great news! You *will* send me photos I hope. What are you using for the throttle body and induction?
Dan J: I'm only 2 miles or less off I-75 on a pretty direct route, love to have you visit if you are up this way. Drop me an email or call if you're in the neighborhood, my number is listed.
|Wow, information overload! Great thread.|
Of course the main question is what do you want to do with your car. For the stock 3.5, I have to go with the Carter 400 for general all round use. (If you can find one.) The Holley 390 carb is prone to leak and that's a bad thing. I ran the Carter 400 on my 3.5 and it's now on my 4.0 and still is what I need. Somewhere in the archives is a chart about the CFM requirments for the 3.5 and other engines.
The added bonus is that with the Buick manifold, it's about the shortest thing out there.
| Carter is a very good carb and it can hide mismatch tuning. The Holley is more prone to tuning. As for leaks, if the correct gaskets are used and the plates are not ever tightening they can provide great performance.|
There are several manufacturers that can provide different plates to tune your Holley. Weber makes a plate that uses Weber emulsion tubes, air jets and fuel jets to fine tune the carb. There is also a adjust- a jet which can be use by screwing the jet in or out to change the jet size.
Then there is the Demon, same principal as the Holley but with CNC machine plates and they flow more than advertise.
I am using a 525 cfm Demon which it took a bit of tuning for the V6 3.4 liter engine. I get 26 mpg on the hwy and 20 on town.
Kelly Combs said it best, what is the purpose of the car. A Holley with vacuum secondary is best for the street.
A Carter/Edelbrock is best when matched with the cfms for the engine requirements. The trick is getting the power piston spring correct for the application, not an easy job. The Edelbrock/Carter is very nice due to the available butterflies that work on engine demand The Holley works almost the same with the vacuum secondary.
The forgotten carb. The mighty Rochester Quadra jet is almost a carb that fits any engine and can be tune to run. This carb only comes in two sizes and it can be run on a small engine with the right tuning.
I ran one on my drag car as per rules. This carb has small primaries and large secondary that are control by the air butterflies and only feed as much as the engine needs. This carb is very easy to tune and there plenty of books on how to tune your carb.
Pic's and info sent.
|Thanks Bill, VERY impressive work. Guys, you are going to love getting to see this car.|
I still have the orginal Rochester that came on my 215. Didn't use it because it is taller than the Edelbrock/Carter.
Can't wait to see Bill's newest creation!
|> Dan J: I'm only 2 miles or less off I-75 on a pretty direct route, love |
> to have you visit if you are up this way. Drop me an email or call if
> you're in the neighborhood, my number is listed.
Jim, I'll be in Ohio the week of Thanksgiving. My dad's place is
a few miles of I-275 at SR-125.
> Somewhere in the archives is a chart about the CFM requirments for the 3.5
> and other engines.
The ones that use the straight volumetric relationship aren't useful.
Vizard has done a couple that use modifiers that can be decent but
the relationship between CFM and HP is a better general rule.
> The added bonus is that with the Buick manifold, it's about the shortest
> thing out there.
> Weber makes a plate that uses Weber emulsion tubes, air jets and fuel jets
> to fine tune the carb.
Actually it was Edelbrock that made the plates that used Weber parts
and they discontinued them many years ago. There is a specialty shop
that will make them (at least they did a few years back) but they
are quite expensive. The plates were nice but an assortment of Weber
tuning parts can also be expensive.
> There is also a adjust-a-jet which can be use by screwing the
> jet in or out to change the jet size.
Percy's adjust-a-jet. These replace the jets only (unlike the Weber
plates which replace several of the carb circuits including the emulsion
tubes). One down side is they lengthen the carb which may cause an air
cleaner interference problem. If you have problems you can install them
temporarily to find the right jets then replace them with the standard
Holley metering plates with the correct jets. There are several similar
systems, including one by Holley. Be aware some of these are aimed
at drag racers for weather condition tuning and have a finite range
equivalent to around 5 jet changes. Others have a tapered needle with
a larger range of equivalent jet sizes.
> Then there is the Demon, same principal as the Holley but with CNC machine
> plates and they flow more than advertise. I am using a 525 cfm Demon which
> it took a bit of tuning for the V6 3.4 liter engine. I get 26 mpg on the
> hwy and 20 on town.
I have one of those in a box that I was going to test. Should make
more power than an equivalent AFB.
> A Carter/Edelbrock is best when matched with the cfms for the engine
That goes for just about any carb.
> The trick is getting the power piston spring correct for the application,
> not an easy job.
What's difficult about it? The step-up springs simply determine when
(based on vacuum) the rods transition from the lean cruise step to the
rich power step. The operate like a Holley power valve but are easier
to change. The springs can be changed very quickly via the top covers
and are color coded to manifold vacuum level. Edelbrock's colors are:
3" Hg (blue)
4" Hg (yellow)
5" Hg (orange)
7" Hg (pink)
8" Hg (plain)
BTW, Carter has some springs like 10" Hg that Edelbrock doesn't list.
> The Edelbrock/Carter is very nice due to the available butterflies that
> work on engine demand. The Holley works almost the same with the vacuum
> The forgotten carb. The mighty Rochester Quadra jet is almost a carb that
> fits any engine and can be tune to run. This carb only comes in two sizes
> and it can be run on a small engine with the right tuning.
The Quadrajet is an excellent carb. It used to be hard to get tuning
parts for them but when Edelbrock started reproducing them, they also
reproduced the rods and jets so you can buy them by size now. The
ThermoQuad is an even neater carb but tuning parts haven't been made
for many years. You can sometimes find Strip Kits but they will be very
> I ran one on my drag car as per rules. This carb has small primaries and
> large secondary that are control by the air butterflies and only feed as
> much as the engine needs. This carb is very easy to tune and there plenty
> of books on how to tune your carb.
Yup. GM used them on everything from 230 cubic inch Pontiac inline OHC
sixes and Buick 252 cubic inch V6's to 500 cubic inch Cadillac V8's. Ford
used some on 429's and 460's. Mercedes even used them on some of their
sixes. There are two basic flow ratings. The ones with 1 3/32" primary
venturis were rated at 750 CFM and the ones with 1 7/32" primary venturis
were rated at 800 CFM. However, some models were fitted with stops that
limit the secondary air valve opening, reducing maximum flow capacity.
For instance, most of the computer controlled versions were limited to
between 590 and 655 CFM. The Buick V6 version was likewise limited.
There were models with side inlet and front inlet fuel fittings, integral
and divorced chokes and a number of other variations over the years.
The problem with running a Rochester Quadrajet carb on a Buick/Rover V8
is none of the intakes will mount one directly because of the spreadbore
pattern. You have to use a spacer/adapter plate which is a bit of a
kludge because of the difference in venturi spacing and also raises the
stack height which could lead to hood clearance problems.
> I still have the orginal Rochester that came on my 215. Didn't use it
> because it is taller than the Edelbrock/Carter.
That's the 4GC "4 Jet" and is completely different (and much less
sophisticated carb) than the later Quadrajet.
|Dan, you've just about covered every carb except the two I use on my 3.5 Rover powered MGs Viz;- Holley 350 2 barrel (very popular in Oz) & Holley 465 4 barrel. Any further comments ? Barrie E|
|Barrie, I have one on my 2.8 ford using the Weber plate and 50 cc pump that came from his big brother a 500 cfm 2v. YES Weber which later Edelbrock bought the rights|
Mine is one of the first made and have another one that was given to me back in 1983 before they were sold to the public.
If interested Barrie e-mail me. It's new in the Weber box.
|> Dan, you've just about covered every carb except the two I use on my 3.5 |
> Rover powered MGs Viz;- Holley 350 2 barrel (very popular in Oz)
Understand that 350 CFM from a two barrel carb is not the same as 350 CFM
from a 4 barrel carb. Two barrel carbs are usually rated at a different
pressure differential (3.0 In Hg). The reason for this is primarily
historical. When 4 barrel carbs first came into popular use, the vacuum
pumps used to rate 2 barrel carbs were unable to pull the same pressure
differential across a 4 barrel carb, so 4 barrels were rated at a lower
pressure drop (1.5 In Hg). Flow ratings from one standard can be related
to flow ratings from another standard. For 2 and 4 barrel carbs:
Flow @ 1.5 In Hg = (CFM Rating @ 3.0 In Hg)/SQRT(3.0/1.5)
Which is approximately:
Flow @ 1.5 In Hg = (CFM Rating @ 3.0 In Hg)/1.414
This relationship is derived from the fact that, for incompressible flow
(assuming subsonic flow... flow will choke at Mach one), the volumetric
flow rate through a venturi is proportional to the square root of the
Q = K1*A2*SQRT(2*Gc/Rho)*SQRT(P1-P2)
or more simply:
Q = K2*SQRT(P1-P2)
Q = volumetric flow rate
K1 = flow coefficient
A2 = downstream area of the venturi
Gc = gravitational constant
Rho = density
P1 = inlet pressure
P2 = pressure at venturi minimum area
K2 = K1*A2*SQRT(2*Gc/Rho)
Computing the relationship for volumetric flow rate at the two flow
differentials and equating yields the conversion formula. Applying
the formula to your 350 CFM 2 barrel carb:
Flow @ 1.5 In Hg = (CFM Rating @ 3.0 In Hg)/1.414
= 247.5 CFM
So your 2 barrel carb flows about half what a 500 CFM 4 barrel would.
Using the relationship that 100 HP requires 140 CFM based upon a reasonable
assumption for Brake Specific Fuel Consumption (BSFC). A carb starts to
become restrictive at 0.7 In Hg pressure drop:
Flow @ 0.7 In Hg = (CFM Rating @ 1.5 In Hg)/SQRT(1.5/0.7)
= 247.5 / SQRT(1.5/0.7)
= 169 CFM
Ratio 169 CFM with 140 CFM and multiply by 100 HP:
= 121 HP
If I did my math correctly, your 350 CFM carb starts hurting HP at around
120 HP. This doesn't mean your engine will be limited to 120 HP. It means
that above 120 HP a larger capacity carb will make more power. 2 barrel
carbs (on V8's) are restrictive and used primarily for cost reasons. Four
barrel carbs will make better power and provide as good or better throttle
response and fuel economy at cruise.
> & Holley 465 4 barrel. Any further comments ? Barrie E
I've not used the 465 myself but it looks like a reasonable upgrade over
the Holley 390 if te engine needs the extra CFM. If I were buying new,
I'd probably opt for the Demon 525 with annular boosters, though. Booster
design can make a larger carb atomize as well as a smaller. It's the
primary reason the simple Autolite 4100's work so well (and similar
discontinued Holley 4010 models).
> YES Weber which later Edelbrock bought the rights.
Interesting. I wasn't aware of that but iut makes sense since Weber USA
manufactures Edelbrock's AFB and AVS carbs.
| Thanks Dan,I haven't really noticed much difference between the carbs. One car, the GT, always seems faster than the Roadster no matter which carb is fitted. I put this down to cam & diff & gear ratios & less drag. Barrie E|
Bill, will contact you.
This thread was discussed between 15/10/2006 and 31/10/2006
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