Throttle Body Size
To calculate throttle body size with bore, stroke and number of cylinders:
To calculate throttle body size by engine displacement in cubic inches:
To calculate throttle body size by engine displacement in liters:
The equations above give the best throttle body size for overall performance on the street. For race applications, add 15%.
How accurate are these equations?
Click here for a throttle body size calculator
Pictured above is a throttle body. It's diameter influences air velocity into the
What you do:
Choose a throttle body size which is appropriate for your engine.
Factor to consider:
The size of the throttle body controls the velocity of the air. Smaller
throttle bodies make the air go faster, larger throttle bodies make the air move slower.
Since the velocity of the air is limited to the speed of sound, which is 767.58 mph
(miles per hour) or 67547.4 fpm (feet per minute), a throttle body which is too small
could limit air flow in the engine. If throttle body size is too
large for a particular engine, the air will move too slowly and reduce power.
The HiPerMath equation is:
throttle body air velocity in fpm = (4.317)(2.616)(number of cylinders)(2)(stroke)(rpm)(3.14159)(bore / 2)2
(((throttle body size / 25.4)2)/4)(3.14159)(12)(10)
which reduces to:
throttle body air velocity in fpm = (121.43)(number of cylinders)(stroke)(rpm)(bore)2
throttle body size2
So the throttle body size that would not restrict air at the rpm where your engine makes the most power would be:
For HiPerMath, if an engine has a throttle body and no carburetor, it is assumed the engine is fuel injected.
If an engine has a carburetor, no throttle body is needed. If an engine has a throttle body
and a carburetor, the smaller of the two will dictate air velocity.
fpm = feet per minute