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vacuum advance

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I don't always do explanations that everyone understands.

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But mostly, Dave, and that's what I appreciate on your explanations. I've now understood the principle of ported vacuum ignition advance and it seems that it works in a nearly identical manner as manifold vacuum ignition advance.

Thanks for all your efforts. /ubbthreads/images/graemlins/cheers.gif
 
I read this thread yesterday and I thought about the Benoulli's law explanation and read the other posts and I have some questions. The key to the vacuum system and being able to set the advance properly is in Rick's initial post (Leaking line and correcting it). Is not the distributor and the vacuum line to the intake(wherever connected) considered a closed system? This closed system is just connected to the vacuum pump, that being the pistons during their intake stroke. The pressure at the inlet to the air cleaners is local atmospheric pressure (29.9 in. Hg.at sea level), so if you are to get air into the combustion chamber you must either suck it in or force it in (turbo charge) so the SU carburetor before and after the throttle disc and the intake manifold are all less than atmospheric during engine operation. The typical way is to suck it. Now my second question, does the vacuum advance pressure specifications change , say if you are in Colorado at elevation?
 
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The pressure at the inlet to the air cleaners is local atmospheric pressure (29.9 in. Hg.at sea level), so if you are to get air into the combustion chamber you must either suck it in or force it in (turbo charge) so the SU carburetor before and after the throttle disc and the intake manifold are all less than atmospheric during engine operation. The typical way is to suck it. Now my second question, does the vacuum advance pressure specifications change , say if you are in Colorado at elevation?

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Not sure what you are getting at. Short answer, yes. The pressure on the engine side of the throttle disc/SU piston will be below atmospheric pressure, the pressure after the throttle disc/piston will be at near atmospheric. The pressure differences take place across the restriction. At wide open throttle, the pressures will be nearly the same & of course, no vacuum advance.

Because atmospheric pressure at 5,000 ft is about 14% lower than it is at sea level, there will be 14% less vacuum at 5,000 ft. The mixture in the cylinders will be less dense & combustion pressures lower. The amount of vacuum advance at altitude will be less under these conditions, when it should actually be more, due to lower combustion pressure.

Since vacuum advance only aids part throttle economy & driveability, you would not be likely to notice much difference due to changed vacuum advance operation at altitude. There would however, be a noticeable reduction of power & fuel mixture would be richer than optimum due to lower air density. SU type carbs are somewhat more self compensating for altitude changes than other designs.

If you happen to be driving with a vacuum gage & go from 5,000 ft to sea level, you would notice a considerable increase in vacuum reading, engine power, & fuel economy.
D
 
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