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MG MGB GT V8 Factory Originals Technical - Edelbrock 600cfm carb on a 4.6

I think my 4.6 needs to breath better at high rpm.
Spec is Big valve ported heads, 10.8:1 cr, piper 285 cam, Rv8 exhaust. 240rwhp.
Anyone using the 600 edelbrock?

Would I gain anything.
Car used mainly for track days.

Mark
Mark

Mark,

This enlightening thread should give you the answers.

Kevin

http://www2.mgcars.org.uk/cgi-bin/gen5?runprog=mgbbs&access=&mode=archiveth&subject=36&subjectar=36&thread=200610151836193581
Kevin Jackson

Thanks Kevin
So much info.
As my car never needs any choke I have removed the complete choke assembly as this must cause some restriction and dispuption to the air flow entering the carb. Seems to make a difference.

Still tempted to try the 600

Mark
Mark

Mark,

Have you considered converting to injection, I have a source who will sell you a guarateed working Hot wire EFI set up.

It includes manifold and plenum, air flow meter,ECU, loom and the high pressure pump for £200.00

All you would need is a large filter to act as a swirl pot at the rear to be fed by you existing pump and a return fuel line and pressure regulator.

You could do some flow work on the manifold and fit a larger air flow meter, Jaguar? and shorten the trumpets, and that should give you all the fuelling potential you would ever need. There are also lots of goodies available to upgrade the EFI if you want to spend the money, an upgrade to an Emerald or Megasquirt ECU would be worthwhile. RPI do upgrade chips for the standard ECU but I consider them expensive for what they are.

Kevin
Kevin Jackson

Kevin
I cant be doing with all that Voodoo fuel injection stuff.
You know where you are with a carb.
Next upgrade will be 96mm bore to 4.8L
Simple rod change 10mins
With fuel injection I would have to pay big bucks to get it sorted!

Bought Dave Walkers Engine management book and hell, it scared me to death. Got it locked away where it cant do any harm.

Mark
Mark

Mark,

Just call in your friendly local witchdoctor!

Having re-read Dan Jones extremely comprehensive explanations on how to set up and fine tune a carb, a EFI system looks pretty simple in comparison.

Re the carb size Dan suggests 140CFM of flow for each 100BHP, can't say I understand the equations but his rule of thumb would suggest that a 500CFM would be suffcient for up to 347BHP and you would struggle to get that with original Rover castings, even in Stage 3 spec, the only way to achieve those sort of outputs would to use the Wildcat Heads, very very expensive, or possibly the new Real Steel re-cast heads.

From what I've read and understood, the exhaust port on the Rover casting is the most limiting factor, if you look down the ports it's almost a right angle just above the valve and obviously not very efficient.

As the experts state it's always the last few extra BHP that cost you dearly, so I suppose it's down to what your happy to spend, taking the bores out to give 4.8L will really only increase Torque and do nothing at the top end with your existing heads.

Kevin.

Kevin Jackson

> Having re-read Dan Jones extremely comprehensive explanations on how to
> set up and fine tune a carb, a EFI system looks pretty simple in comparison.

It can be that way sometimes, particularly when the circuit you need to
tweek is a fixed orifice in the carb. Luckily some of the newer Holley
replacements have replaceable orifices for things like the PVCR's and
air bleeds.

> Re the carb size Dan suggests 140CFM of flow for each 100BHP, can't say I
> understand the equations but his rule of thumb would suggest that a 500CFM
> would be suffcient for up to 347BHP

You missed the crucial part of converting from actual carb flow to the
rated pressure drop. 500 CFM of actual carb flow (at a non-restrictive
0.7" Hg) will be sufficient for approximatedly 357 HP but to get that
500 CFM of flow, you'll need a carb rated at 732 CFM.

Flow @ 0.7 In Hg = (CFM Rating @ 1.5 In Hg)/SQRT(1.5/0.7)
500 = X / 1.46385
X = 731.925 CFM flow rating required

> you would struggle to get that with original Rover castings, even in
> Stage 3 spec, the only way to achieve those sort of outputs would to
> use the Wildcat Heads, very very expensive, or possibly the new Real
> Steel re-cast heads.

Or ported Buick 300 heads. My Buick 300 heads will support well over
350 HP.

> From what I've read and understood, the exhaust port on the Rover casting
> is the most limiting factor, if you look down the ports it's almost a
> right angle just above the valve and obviously not very efficient.

and the valve is quite small. Larger valves tend to help flow throughout
the lift range as long as the short side radius is not compromised.
I run a 1.5" exhaust valve in my Buick 300 heads (and 1.71 on the intake).
These are cosidered large in the Rover world but still on the small side.

> taking the bores out to give 4.8L will really only increase Torque and
> do nothing at the top end with your existing heads.

True. If you don't change the limiting factor(s) in a combination,
increasing displacement will usually bump up torque but do little
for peak power. One area that's often overlooked is the intake manifold
porting. You can learn a lot by flowing the heads and intake together on
a flow bench. Often, particularly on dual plane intake manifolds, you
will see a large variation in CFM from one port to another. We recently
ported a dual plane intake to bring the poorest flowing cylinders up to
match the best and found a surprising amount of HP on the dyno (40+ on a
485 HP engine). I've found even the most highly regarded intakes to
have problems when put on the flow bench. We dyno'd the same engine
with mufflers and found a 50 HP difference between the stock mufflers
and open pipes. Magnaflow mufflers got nearly all that back (within
2 HP of open pipes)

Dan Jones
Dan Jones

I am now standing in the corner with a pointy hat on.

Dan thanks for correcting my understanding of stated and actual flow, I did say that I didn't understand the equations.

I'm sure the additional information you provided will be of interest and assistance to Mark.

Kevin.
Kevin Jackson.

Dan,

Just a thought, do you know what the choke/venturi sizes are for the primaries and secondaries on the Edelbrock 500? and are they changeable ie. can you upgrade a 500 to a 600.

Kevin.
Kevin Jackson

Just had the car on the Rolling road.
314lb ft Torque and 282hp. Engine was running very lean.
Without me mentioning anything about carb size, the guy suggested that I go for a bigger carb.
Just ordered a 600edelbrock with calibration kit.
When its all fitted and set up Ill go back to the same rolling road for a direct comparison.
Will post results for those that are interested.

Mark


Mark

> Just a thought, do you know what the choke/venturi sizes are for the
> primaries and secondaries on the Edelbrock 500?

> and are they changeable ie. can you upgrade a 500 to a 600.

The difference in flow rating is primarily on the secondary side,
with the 500 having a smaller venturi than the 600. The clusters
can be swapped but I'm not sure if their are other changes like
air bleed diameters.

> 314lb ft Torque and 282hp. Engine was running very lean.
> Without me mentioning anything about carb size, the guy suggested
> that I go for a bigger carb.

I would have tried richening it with a rod/jet change. That said,
a 600 should work fine on a 4.6L.

> Just ordered a 600 edelbrock with calibration kit.

Did you get the AVS or AFB version? The AVS version is more flexible
since you can tailor the secondary opening rate. Don't be surprised
if you find you need rods and/or jets that are not in the kit to
dial the carb in for both WOT and part throttle cruise.

> Will post results for those that are interested.

Thanks.

Dan Jones
Dan Jones

The 600cfm Edelbrock is now fitted and Ive been playing with the jets/rods.
Decided to start off on the lean side from stock so went with #24 on the calibration chart for the 1405 carb. It started on the button and driving revealed a low speed hesitation on small throttle opening but mid range and top end seemed OK.
Then moved to #21 on the chart which improved things but the hesitation was still there so I new I was moving in the right direction. Then #5 which is base calibration on cruise mode but 8% lean on the power mode.
It drives fantastic with no hesitations at all.
In fact I seem to have more performanc for less throttle opening if that makes sense.
Gonna book an hour on the rolling road this week if I can get in as I need to know what the AFR is doing.
Back to back figuers should be interesting.
Mark
Mark

Mark,

Sounds good, look forward to hearing the results.

Kevin.
Kevin Jackson

Rolling road revealed its running a but rich.
Slightly down on power but peaking higher @ 6400rpm
Torque was down a lot.

Think im going to invest in a wideband lambda setup maybe the
LM-1 by innovate motorsport.
Now I need metering rods that are not in the calibration kits and as no one seems to stock them it looks like they have got to come from the states.

Mark
Mark

This thread was discussed between 01/05/2007 and 27/05/2007

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