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MG MGB Technical - HIF SU- Port or manifold vacuum?

I just installed a rebuilt set of HIF 1 1/2 on the 1977 MGB Roadster, replacing the drafty HS which had been in place since the smog conversion rebuild 10 years ago. I attained 27.8 MPG up to Reno, over the summit (7272'), and 33.3 mpg on the way back down. That's the best to date I've done over 40 years with these cars.
I'm using the manifold vac for the dizzy.
Since the carbs had no port fitting ( and I did not convert the lower plug as I am aware can now be done), I used the intake manifold with a "tee" coming off the "anti-over run" hose to the Schlemmer rebuilt/recurved D-25, set at his suggestion to 16* BTDC with the light at 900 RPMS, vac plugged.
FWIW, it is always reassuring to see a dead steady timing mark line on the crank under the light, no wobble, with the Pertronix, 6/12 ballasted B+ voltage to the ultra low ohm "Flamethrower" coil, NGK-BPS-7 at 38k" gap.

Seems to be a mixed bag with the manifold vacuum though. This car features a Burgess FRBV head, Peco header/exhaust, 60 over 9.00:1 CR, 3* advance key on crank, with the much misunderstood Piper 285 camshaft and hardened lifters. Previously, with a barely detectable "loping idle" at 850 RPMS and the old HS carbs, the acceleration response was strong, steady and continuous to 6500 RPMS- went like the wind, and in OD I could surprise myself, slipping past 100 mph late at night.
Now it's different. The idle, with manifold vac, can be set to 250 RPM and the car will idle.... seemingly oblivious to the "fact" that it should not with that cam. When set to 500 it is dead steady, sounds like bacon frying, no lope at all. But the take off response is sluggish, and at freeway speed in OD, it has about 80% of the pedal power as before, but seems to poop out around 4800 RPM. And when I tried, on the flat at sea level, I couldn't get past 90 MPH: Like a Buddy Holly song, it just faded away to my grave concern. However, it cruises like a champ up to 70-75 and that is where I got that impressive gas mileage noted above.
Timing light, with vac connected, showed 18* at 1000RPM, and 48* at 3500 RPM, and topped out at 52* at 4200 RPM and above. Sounds like I need Jeff's services again, or port vacuum.
What is the thinking out there from you knowledgeable guys?
Cheers, VEM
vem myers

Manifold and carb vacuum only vary at idle. As you start to open the throttle they very quickly converge, then both reduce gradually as the throttle is opened further, and after a certain point significantly short of full throttle there is no advance at all (although that does depend on the vacuum capsule in use), see

With everything that has been done to the car the vacuum source is the least likely cause of any problems.
PaulH Solihull

Where did you get the bright idea to set/check timing with vac connected? Not done.

Assuming you have a 77-on distributor, there are two possibilities.
Fed gives 24 +/- 2 crank deg vac start @ 3inHg, end at 11
Cal gives 14 +/- 2 start @ 5inHg, end at 11
Both give 13-17 crank mech @2000, 28-32 @ 3500, 33-37 @ 4500
That gives a possible timing of 63 at 4500 with vac connected, plus any static, and I've seen that on stock smoggers.

At idle with manifold vac, you have all the vac in, and even at high RPM have most of it, since you are running no load/small throttle opening, in the shop.
Result is you are running way retarded. If you have the Fed dist, with all the vac in at 3500 possibly contributing 26deg, then your real timing at WOT is 22, or 10+ retarded. Performance will suck,and so will mileage, so you have a lot to gain. (I expect considerably better mileage and performance than you state, even stock.)

The hot cam with high compression more or less duplicates the poor compression and valve timing of the wretched smog engine at idle, since you have poor cylinder filling, so it idles great at 18.

But, that dist is probably not a reasonable choice for the engine as it is. I'd be worried about the possible 37 mech + static at high speed when the cam should be doing its thing and filling the cylinders nicely. The smog engines needed that to even keep running at 5000.
Call Jeff!

FR Millmore

FR- Thanx for the kind nod on my bright idea. Yes, it was to demonstrate that at idle, as noted above with vac plugged the advance was 16*. With vac connected at the same idle, the advance was 18*. Ya folla?
Why assume my friend, I noted the distributor was a D-25 recurved by Jeff Schlemmer at Advanced Distributors, last year
Thank you for the candid input. It does idle great, best ever. I'm thinking based on the vac enhanced advance numbers above 3500 RPM, Jeff needs to stretch some springs for me.
You out there Senor Jeff???? Cheers, Vem
vem myers

Sorry, totally missed the whole Jeff dizzy bit. In that case, there is nothing I can say about it, without knowing the advance curve currently installed, and the characteristics of the vac capsule.

I would be interested in knowing these curves, as I have issues with setting timing in the range around 900-1500, since that is where most curves have the greatest change. A small rpm error results in a lot of timing difference. Early engines set timing at 600, a pretty stable range in the stock curve. Later engines give horribly sloppy ranges for advance curves as stock. Of course, with a good distributor, the curves should not be sloppy, but it is still the wrong range for setting. I'm old school - if the distributor is good, you can set timing static far better than using a strobe in a high slope advance area.

Based on 16 @900 without vac, and 18 @1000 with, it seems the vac unit is totally dysfunctional; the +2deg could easily be in the mech advance over 100rpm. Vac advance would be at full possible with manifold vac, connected at idle.

PS and PLEASE: Quit using "k" as an abbreviation for "thousandths" as in "38k" gap". "k" is thousands, in correct metric terminology. It is not "thousandths" and it is not correct to use it with Imperial units.

Thinking about that plug gap, it is too much for a 25D distributor, designed for .025 plus some safety. The 45D was made bigger, specifically to allow larger plug gaps without danger of the spark jumping across the cap internally. The plug gap is then .035" plus safety. When you go over this gap, you are eating the safety provided. It is conceivable that your more efficient engine is pushing the spark demand to the point that you are losing sparks inside the distributor.

FR Millmore

Vic as far as performance is concerned the advance readings with vacuum disconnected are the only ones that matter, as vacuum advance only comes in when in a light cruise or at idle. As FMR said you are getting a high reading at fairly high revs because of the no load situation. Although vacuum advance can to a small extant effect throttle response its main reasons for being there are to give a more complete burn of the fuel and keep the combustion chambers and spark plugs clean. When on light cruise the compression pressure in the cylinders drops way down because of the throttle restriction and this makes the air/fuel take longer to burn and needs to be ignited earlier, this in turn improves mpg. All this changes when load is placed on the engine and then only mechanical advance counts. Denis

More I think about this, the screwier it gets!
I'll stand by my evaluation of the vac at idle, but your advance figures at high rpm indicate that it must be working - backwards. Seems impossible - call ze Jeff!
And please publish the old and new numbers!

Might be that the internal spark jumping I mentioned is triggering the #1 plug off the #2 spark at high advance, giving false readings, or true readings of bad sparkles. This all having to do with the phasing of the Pertronix to the rotor, which I don't remember the details of. To the effect of a late spark on 2 is an early spark to 1, if the rotor is midway between the cap contacts.

FR Millmore

The purpose of vacuum advance is to give better cruising mpg, although it also aids part-throttle acceleration. The fuel-air mixture burns at a constant rate regardless of throttle opening and rpm, and this is why we need centrifugal advance and why vacuum advance is beneficial for touring as opposed to competition. If the spark come soon too soon before TDC the additional compression when getting to TDC will convert the burn to an explosion which is harmful. But as the revs increase more and more of the burn, with a fixed timing point, occurs on the expansion stroke and is lost as heat. Because the engine gets to TDC quicker at higher revs, but the fuel burn remains the same, the timing can be advanced but still not result in an explosion, and the more burn that occurs near the top of the stroke the more forward progress you get.

Similarly at light throttle openings there is a relatively small charge in the combustion chamber and a significant depression, which resists explosive combustion, so the timing can be advanced by vacuum. As the throttle is opened the charge gets bigger, the depression reduces, and the cylinder pressures increase, the more heat is generated on compression, and the more likelihood there is of explosive combustion, so the amount of advance need to be reduced.

16 degrees with no vac, only changing to 18 degrees with vac, both at idle, shows either a problem with the vacuum or the advance mechanism. However some MGB vacuum capsules gave as little as 4 degrees, but others went as high as 24 degrees, which was usually the case for low-compression engines. You need to look at the number stamped on the capsule, you should see something like 3-11-12, which shows vacuum advance starts at 3 in.Hg, ends at 11 in.Hg, and gives a maximum of 12 *distributor* degrees at 11 in.Hg or higher, which translates to 24 crankshaft degrees.
PaulH Solihull

Paul has a point about the different vacuum advances on the B engine over the years. Some being suitable for manifold connection and others carby port. However I disagree with his statement on mixture burning rates. There are many thing that affect the speed of the flame front. Mixture, charge density or compression pressure, temperature and turbulence as well as a few lesser things and also you get a compounding effect of some such as compression and heat. If the mixture always burnt at the same rate there would be little need for a vac advance. Back in the 50s we learnt that the fuel mixture burning rate was constant but we know a lot more now. Both the mechanical and vacuum advance are designed to have the mixture burn develop maximum cylinder pressure between 15-20* ATDC, and if fuel always burnt at the same rate and the vacuum + mech adv gave 50*+ as some do we would have max pressure BTDC creating wast-full pumping losses. Its just that at a light cruise the low cylinder pressures take longer for more complete combustion. Denis

"you get a compounding effect of some such as compression and heat."

That's what I said, and when the compression/heat gets high enough the steady burn will change to an explosion which is when you get the tinkling pinking/pinging. Both centrifugal and vacuum advance take account of that change from burn to explosion, and hopefully avoid it. Modern unleaded fuels are more prone to it in high compression engines, as is lower octane fuel, which is why the timing has to be *manually* retarded to prevent it. My 73 won't run now without pinking at the book figures, even on 99 octane. However in the 70s with a different engine on 4-star leaded I could run more advance without pinking and so get better performance and economy. The 73 was OK at the book figures on 4-star leaded, but couldn't be advanced more than that.
PaulH Solihull

Paul I have no problem with your explanation of the advance mechanism and their reason for being there, including detonation control. Its just that their scope is much greater than that, coping with variations in speed, load and density of charge. There are a few good articles around about the rate of combustion in a petrol engine and a good easy to read one is by JCM MACHINE and called 3.0 Engine Combustion Dynamics Normal Combustion
Lets know what you think. I think if you type in jcm machine and look around you will get there.
All the best, Denis.

"coping with variations in speed, load and density of charge."

Again, that is exactly what I have said:

"Because the engine gets to TDC quicker at higher revs, but the fuel burn remains the same, the timing can be advanced but still not result in an explosion, and the more burn that occurs near the top of the stroke the more forward progress you get."

"As the throttle is opened the charge gets bigger, the depression reduces, and the cylinder pressures increase, the more heat is generated on compression, and the more likelihood there is of explosive combustion, so the amount of advance need to be reduced."


"Because the gas combustion is designed to burn at a constant rate ..."

"As the engine RPM increases, the ignition spark must be advanced tens of crankshaft degrees ..."

There's not that much more on timing that I can see, and none at all on using vacuum to lessen the amount of advance as the throttle is opened.

The bulk of the piece is on tuning and modification aspects, fuel composition and combustion chamber design having changing burn rate, but they are not factors in a given engine and a given tank of fuel.

Enough already.
PaulH Solihull

This thread was discussed between 10/04/2012 and 16/04/2012

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