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CFM Calculations & Volumetric Efficiency

The larger the engine, and the faster it spins, the more air it's capable of gulping.

In math terms it looks like this:

  Cubic In. Displacement

Maximum RPM

=  Maximum CFM



An example using a 250ci inline six.

 250 Cubic Inches

6000 RPM

=  434 CFM



However, this is still not the final answer.

When calculating the CFM requirements, volumetric efficiency (VE) must be taken into consideration. Volumetric efficiency in internal combustion engine design refers to how efficiency the engine moves the air/fuel charge into and out of the cylinders. More correctly, volumetric efficiency is a ratio (or percentage) of the volume of fuel and air that actually enters the cylinder during the induction cycle, to the actual capacity of that cylinder under static conditions. Engines with higher volumetric efficiency will generally be able to run at higher speeds (commonly measured in RPM) and produce more overall power due to less parasitic power loss moving air in and out of the engine. Naturally aspirated engines normally operate between 70-90%, rarely reaching 100%. On the other hand, boosted engines (with induction manifold pressures above ambient pressure) commonly have volumetric efficiencies ranging from 100-120%, depending on the amount of boost.

There are several ways to improve volumetric efficiency of an engine. A common approach is to use a camshaft profile that has more duration and/or lift to increase air flow. Another method is to use larger valves, however larger valves increase the weight of the valve train, which in turn increases the engine parasitic losses. Streamlining and/or polishing the ports also increases flow capability. This is referred to as Porting & Polishing, and is usually done with the aid of a flow bench. Improving the air flow or induction, by swapping to a high performance intake manifold can also help. And finally, increasing the exhaust flow with headers or improving the exhaust system. While naturally aspirated performance motors generally reach 85-90% VE, they can hit 100% if properly built and tuned. However using forced induction, such as supercharging or turbo charging, can easily push the volumetric efficiencies up to 120% or more.

Actual Air/Fuel Mixture
= Volumetric Efficiency
Theoretical Air/Fuel Mixture

In our example a 250ci @ 6000 RPM's can use 434 CFM

Calculations using volumetric efficiency look like this:

434 CFM x 80% (.80) volumetric efficiency = 347 CFM
434 CFM x 90% (.90) volumetric efficiency = 391 CFM
434 CFM x 100% (1.00) volumetric efficiency = 434 CFM
434 CFM x 120% (1.20) volumetric efficiency = 521 CFM

Click here for an online calculator that will do the math for you.

Remember calculated results are just that, calculated.
Consider the following "Rules of Thumb"
before you make the final decision.

Rule of Thumb
Bigger Carb
Smaller Carb
Results in More Horsepower
Results in More Torque
Results in a Higher RPM Range
Results in a Lower RPM Range
Prefers Higher Compression Ratios
OK with Lower Compression Ratios
Best with Higher Rear Gear Ratios
Best with Lower Rear Gear Ratios
Likes More Cam Duration
Good with Less Cam Duration
Better with Manual Transmissions
Better with Automatic Transmissions
Better with High Stall Converter
Better with Low Stall Converter
Needs More Mechanical Adv.
Needs Less Mechanical Adv.

Click here to purchase Carburetors, Adaptors, and Related Products
Related Tech Articles - Induction
How to Calculate CFM
Future Article
Related Tech Articles - Cylinder Heads

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