Thread: More about Vacuum Sources and Timing

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More about vacuum sources and timing


What we’re dealing with here is in effect a variable venturi. At least it is as far as ported vacuum goes. The variable venturi bit due to throttle blade position.

I got curious about a comment I heard about manifold and ported vacuum going to zero at WOT (Wide Open Throttle) and ran a little experiment.

The car - 32 roadster - weighs 2400#, engine is an overbored 455 with 462 cid, 9/1 compression ratio, Edelbrock Performer intake, Carter 750 cfm competition carb with electric choke added later and a Crower Compu-Pro #1 cam which has about 262 & 266 degrees advertised duration intake and exhaust with 112 degree lobe centers. It’s a smooth cam and the car when warm idles @ 19" vacuum.
The dash carries a large (2 5/8") S-W vacuum gauge which compares favorably with the vacuum/pressure test gauge I have.

Advance is 8 degrees initial and all in at about 2600 rpm with a total of 32 degrees. Vacuum advance is about 16 degrees and sourced from Manifold Vacuum (MV).
The car runs very well on 87 octane in summer and winter and does not overheat in traffic.

Firing the car from dead cold and on the elec choke, MV reads 18-19" and idle is around 900-1000 rpm.
Ported Vacuum (PV) read 12" on startup.

Once the engine warmed up, MV reads 19" and PV reads zero at about 500-600 rpm.

Cruise at 40 mph with a light throttle setting on a flat road gives you 18.5 - 19" MV and just about the same on PV.
Rolling the throttle in about half way shows 8 - 10" of vacuum on both MV and PV during light acceleration.

Once at 60 mph MV read 18 - 18.5" vacuum (keep in mind this is a very light car) and PV read
10".

Flooring the throttle at 40 mph or 60 mph brought the MV down to 1" or so and PV to zero.

The key thing is, at idle with a fully warm engine, MV reads 18.5 - 19" and PV reads zero.

The lack of additional timing at idle is what creates an overheating problem in the GM engines.
It takes time to burn the lean idle mixture and additional advance is required to get the process underway early and avoid overheating.
Exactly the same thing (overheating) would happen with the timing severely retarded in an engine under load at a higher rpm level.

There’s a lot of confusion out there about timing, both centrifugal (mechanical) and vacuum as well as the vacuum sources to use.

The key thing is to realize they are two different systems that work together to give optimum spark advance for a particular condition and key on rpm as well as load.

To my way of thinking perhaps there would be less confusion if the vacuum advance cannister was called the vacuum retard cannister.

I’ve been amazed at the lengths some go to, to cure an overheating problem that can be solved in most cases simply by selecting the correct vacuum source.
Granted, most of my experience has been in cars with small engine bays and many times not the biggest radiator in the world, but I note, the bigger cars have the same amount of timing and overheating problems as the small car guys do and for some reason many car owners avoid doing something as simple as swapping vacuum sources to cure overheating and prefer to throw money at the problem.

As far as spinning up a little experiment, I’m not trying to prove anyone wrong here, just got curious, had some free time and those are the results I came up with.


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An additional note; for those of you who live at a higher altitude than where these tests took place, you’ll find that your vacuum levels at no-load (idle) rpms will read lower.
To the tune of a 1" vacuum loss for every 1000' of altitude.

The tests took place at 350' altitude and manifold vacuum at idle read 18.5".
After moving to Sunny Arizona and ending up at 3300' altitude the manifold vacuum now reads 15.5".
Highway figures and under load vacuum levels remain the same.

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This combined article was written from research on timing figures and real world experience.

You can learn a lot by taking the time to set up some inexpensive instrumentation and taking the time to run a few simple tests.
What you’ll gain is a better understanding as to what’s going on with your engine and gain a small bit of education about the particular thing you’re researching.

Learning from books is one way to do it and there’s nothing quite like taking advantage of what smart, experienced and educated people have done and written down for your educational pleasure.
I’m not including myself in this group.

What I’m talking about is the stubbornness and unwillingness to learn from those who’ve been down the road before us.
It’s amazing sometimes to talk to an individual who thinks factory engineers don’t know much.
Thing to recognize here is the factory engineers know a helluva lot more than we do and get into sophisticated areas that the great majority of us know nothing about.

Keep in mind too, factory engineers are constrained by the bean counters, the necessity to build a vehicle that is useful to the majority and seldom are let loose to pursue a dream or even an interesting idea.
When they do get the freedom to investigate particularly interesting areas, the results can be astounding.

The name, “Zora-Arkus Duntov” should ring a bell....

i agree with every thing said c9x. i dont remember fixing a lot of heating problems with timing, but i always made sure the timing was close, and used mv most of the time. good post