To Vacuum Or Not [ Distributors, Vacuum Advance ]

[Michael Oritt]

I'm considering replacing the distributor on my wife's 3000 with a Mallory Unilite, which is available with or without vacuuam advance. I've been told that the need for vacuuam advance has been pretty much eliminated by the more efficient combustion characteristics of modern fuels. Any input, even if just conjecture, will be appreciated.

 

[CDK]

Funny you should mention this. After reading an artical on vacume advance, I was left with the impression that it was more designed with emmisions in mind then performance. Disclaimer I do have a Mod Healey with a 302. Having a small bent toward R&D I saw an experiment coming. This one is pretty easy. Dissconnect the vacume advance and plug it. Then drive around for awhile and see what you think. Mine has been off for a month and a half and is not going back on. The next thing you can try is advancing your timing a degree or two at a time, with the V.A. gone you can increase your base timing. I am very happy with the way the motor is running and am planning to upgrade to an MSD unit soon sans no advance.
CDK

 

[Dave Russell]

Re: To Vacuum Or Not [ Distributors, Vacuum Advanc

It is true that many more modern engines require less spark advance for optimum combustion. The more compact combustion chamber designs require less time for the mixture to completely burn so less advance is needed. But a fair amount of advance is still needed to have the burning fuel at maximum pressure at the required 15 to 20 degrees after TDC piston position.

The density of the compressed fuel-air mixture varies with the amount of fuel that is let into the cylinder by the throttle. At smaller throttle openings, very little mixture is let into the cylinder to be compressed by the piston as it rises. The relatively large distance between fuel/air molecules in the low density mixture results in slow burning of the mixture & the need for a considerable amount of spark advance which is provided by the vacuum advance. Vacuum is high at small throttle openings & decreases as the throttle is opened more until it is near zero at wide open throttle.

As the throttle is progressively opened, more mixture is taken into the cylinder & the molecules are packed closer together when compressed to the same combustion chamber volume & so burn faster. At the same time vacuum advance is decreasing which compensates for the faster burn of the fuel.

The fuel mixture always requires a minimum time to completely burn & since the time to get burned efficiently decreases with engine speed increase, the burn must be started earlier in the cycle. This is the purpose of centrifugal advance. To have combustion pressure at the optimum when the piston is 15 to 20 degrees after TDC.

Since the mechanical advance (centrifugal) cannot sense the amount of throttle opening, only rpm, it cannot compensate for the different optimum burning rates required by a thin vs dense fuel mixture. (Closed vs open throttle). Not to be confused with rich vs lean mixture.

There is an advantage to vacuum advance in that it usually gives better part throttle performance & fuel economy. Especially on the old technology slower burn combustion chambers. It has no advantage at full throttle which is why racers usually dispense with vacuum advance.
D

 

[Johnny]

Re: To Vacuum Or Not [ Distributors, Vacuum Advanc

Great explanation on vacumn advance Dave. You always astound me with your knowledge. I've always found the vacumn advance on Healeys to not be operational. Usually the small diameter line is either kinked, clogged or the compression missing and the line can just be pulled away from the distributor. One of the first thing I do is fix this by replacing the line and unclogging the fitting. Usually results in a smoother running engine at idle and slower speeds; as you so well explained.

 

[Roger]

Re: To Vacuum Or Not [ Distributors, Vacuum Advanc

Well said, Dave. If I may be so bold as to add another benefit, it can help prevent pinking or detonation on sudden throttle openings as the reduction of vacuum tends to retard the ignition.
As a young man back in the UK, it was my understanding that was why and how it first came about in the standard family underpowered overweight four-seater. It's less relevant with better power-to-weight ratios, and it won't hurt performance to retain it since it's pretty much ineffective at wide throttle openings and high inlet gas speed.