|
Volumetric Efficiency
| Actual Air/Fuel Mixture Intake |
= Volumetric Efficiency |
| Theoretical Air/Fuel Mixture |
|
Engine/Carburetor Sizing Relationship
The larger the engine, the more air it's capable of "gulping". In math terms, it looks like this:
|
Cubic In. Displacement |
X Maximum RPM |
= Maximum CFM |
|
2 |
1728 |
|
An Example:
|
289 Cubic Inches |
X 8000 RPM |
= 669 CFM |
|
2 |
1728 |
|
This is still not the final answer for sizing a carburetor. The volumetric efficiency must be taken ito consideration.
In our example a 289 @ 8000 RPM can use 669 CFM
however
660 CFM x .90 volumetric efficiency = 602 CFM
669 CFM x .85 volumetric efficiency = 568 CFM
Our example is based on a Hi-Po 289. Ford installed a 600 CFM Autolite 4100 on this engine. Did Ford engineers know what they were doing? You Bet!
A standard "A" code or "D" code 289 4 barrel with the factory valve train will not run much above 6000 RPM.
6000 RPM requires 501 CFM x .85 volumetric efficiency = 426 CFM. Ford installed a 480 CFM carburetor!
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