This is our theoretical based Throttle body calculator that takes into consideration not only an engines VE curve, but air speed at the throttle body and the pressure drop that is created. 

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Throttle body sizing can be an area of confusion. The first problem comes as many manufactures flow test throttle bodies at 28"H2O (2in/Hg). This is the same depression/pressure drop as used to test cylinder heads. But here is the problem. Not even a Carburettor is tested at such high pressure drops. Most 4 barrels are CFM rated at 1.5in/Hg or 20"/H2O and we know that a running Carburettor will only see 0.8-1.2in/Hg of vacuum if setup right. 

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To help us understand why even a carburettor's test pressure is too high to use for a TB. We must first understand the fundamental differences between a Carburettor's function and a Throttle body. The main difference is a carburettor needs a set amount of pressure drop to allow the carburettor to lift fuel up into the main venturi. This isn't needed with a Throttle body so to maximise Not only Horsepower, But torque as well. We need lower Air speeds which directly rates to a lower pressure drop. 

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Now it goes without saying that not all Throttle bodies 

are equal.  Some use flat shaft designs to minimize the 

turbulence which increases mass flow and reduces PD

in the same size diameter TB. For this reason Holley, 

Accufab and others will give a CFM rating to help you 

understand their efficiency.