Formula to Calculate Vacuum??

PeterC

PeterC

Well-known member
My 59A is 284 CID - twin Stromberg 48's and a cam with 365 lift, 270 duration and 112 centerline - compression 7:1. curious how this would compare to a stock 59A in terms of inches of mercury it pulls for vacuum. Are there other parts of the equation that would need to be factored in?
 
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Now that is one of the most-interesting questions I've heard asked here in a long time, and I surely don't have the answer right off hand, either. DD
 
Assuming the engine is sound with good valves and rings, etc. and in good tune for the components in it I would think the biggest factors are camshaft duration and overlap. Increasing duration will have a tendency to lower the idle vacuum reading but lobe center angle will have an effect too. Keep greater duration the same but increase the lobe centers and you can help the idle vacuum because of the decrease in overlap. Put a big cam in and even though you might increase the static compression you will see a decrease in idle vacuum and a decrease in cranking compression compared to a stock engine as tested, but try widening the lobe centers with that same big duration and now you are not bleeding off so much cylinder pressure- I think you will see an increase in cranking compression but the effect on idle vacuum will be negligible. Late ignition timing for a given combination can cause low vacuum readings too, that's why I mentioned an engine in good tune.
I'm not a tech writer per se, just a hobbiest giving my observations from things I picked up over the years. And I'm not saying I'm right either, I've been wrong plenty of times, just ask my wife. As to there being a "formula" to compute an expected vacuum an engine might pull? Sounds unlikely to me, but that's JMO.
 
Peter, all I can do is offer a comparison for you.
A Stock engine in good condition will produce about 20-22 inches of vacuum.
With a cam of 280 advertised (Potvin 425) duration and an engine of similar displacement, I get 11 inches of vacuum @ an idle speed of 900 rpm
 
Fordors said:
Assuming the engine is sound with good valves and rings, etc. and in good tune for the components in it I would think the biggest factors are camshaft duration and overlap. Increasing duration will have a tendency to lower the idle vacuum reading but lobe center angle will have an effect too. Keep greater duration the same but increase the lobe centers and you can help the idle vacuum because of the decrease in overlap. Put a big cam in and even though you might increase the static compression you will see a decrease in idle vacuum and a decrease in cranking compression compared to a stock engine as tested, but try widening the lobe centers with that same big duration and now you are not bleeding off so much cylinder pressure- I think you will see an increase in cranking compression but the effect on idle vacuum will be negligible. Late ignition timing for a given combination can cause low vacuum readings too, that's why I mentioned an engine in good tune.
I'm not a tech writer per se, just a hobbiest giving my observations from things I picked up over the years. And I'm not saying I'm right either, I've been wrong plenty of times, just ask my wife. As to there being a "formula" to compute an expected vacuum an engine might pull? Sounds unlikely to me, but that's JMO.
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Very well said, and I totally agree.

Sal
 
PeterC said:
My 59A is 284 CID - twin Stromberg 48's and a cam with 365 lift, 270 overlap and 112 centerline - compression 7:1. curious how this would compare to a stock 59A in terms if inches of mercury it pulls for vacuum. Are there other parts of the equation that would need to be factored in?
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I'm sure you meant 270 duration, not overlap. An engine with 270 overlap would not run. There is no formula for vacuum. The stock 59 cam has 6 deg overlap and your cam probably has at least 3 - 4 times that. So your vacuum will be considerably lower.
 
flatjack9 said:
I'm sure you meant 270 duration, not overlap. An engine with 270 overlap would not run. There is no formula for vacuum. The stock 59 cam has 6 deg overlap and your cam probably has at least 3 - 4 times that. So your vacuum will be considerably lower.
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sorry - yes meant 270 duration
 
There are all sorts of things to affect the manifold air pressure. I run several different displacement engines and the normal idle MAP is relatively consistent for each individual type but varies a bit from one type to another. I don't think you would ever be able to calculate what it would be due to so many changes in barometric pressure each day and operation at different altitudes. All the factors I base my readings on are 29.92" Hg at sea level. A 1000 foot altitude change will change it a bit. 10,000 feet will change it a lot.

The work an engine is doing also has an affect. The weight of a flywheel can affect the MAP for a given engine. Light weight flywheels will pull lower pressure than heavy ones will at idle since the engine is doing less work.
 
That sounds very nice, what kind of boost do you see? I have a 294 in progress with an Eaton blower, we hope to keep the boost below 5 lbs, CR is around 7.8
 
This looks like a project that two guys that I worked with tried to figure out how energy it took to blow soap bubbles.
 
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