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Old 04-29-2004, 07:22 AM
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Theory: Intake volume vs cylinder volume / engine vacuum

having had and still having problems with my engine combo, I got interested in this subject.

My theory is the following:

A large intake volume Iv compared to a small cylinder volume Cv leads to low idle vacuum and related problems.
If carb is on the large side problem worsens... requiring you to run a smaller carb than would be normally expected....

Consider this:
A torker single plane intake has and RPM range of ~2500 to 6000 for a 350cui engine. For simplicity we set the relation Iv/Cv =1.

Putting that intake on a 283cui engine with a lot smaller cylinder volume we now have that Iv/Cv = 1.24.

Now the intake volume is suddenly 24% larger compared to the cylinder volume on this small motor

This means that when ever we need to fill the cylinder we can now draw from a 24% larger volume, meaning that the "pressure drop" in that volume will be smaller (less vacuum) thereby drawing less air through the carb lowering the velocity which make the proper carb function harder to obtain.
This also makes it necessary to have a smaller carb compared what is normally expected in order to create velocity through it to atomise the fuel properly.... Carb acts a a restrictor to get vacuum in the manifold.

This resembles the theory about small cylinder head intake runner volume for smaller motors.
Compares to install a set of 220CC heads on a 283 cui engine, which is much too large....

The larger volume will also move the useable RPM band up, in order to compensate for the larger volume we must add more rpm to ontain the optimum air speed as the cylinder volume is not variable.
In turn this 2500-6000rpm manifold might now be a
3000-7000 rpm manifold....

This really means that installing a single plane manifold on a 283 for street is nuts....
In order for it to work reasonably it must be a dual plane on which the runners will be better sized for the cylinders, and a 350cui idle-5500 rpm manifold =idle-6800 for a 283cui, would probably be optimum for such a small motor

Does this make sense or does anyone think different...?

/Bjarne
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