Airflow to Cubic Inches
This formula seems to indicate the heads need to be better than what I’ve seen in the real world.
For some of the combos I’ve tried, it’s almost 100cfm off.
Most of the formulas and or rules of thumb got to take with huge grain of salt, just curious what cid and rpm was off by 100 cfm's ?
Just in case your curious, you can modify the formula for any VE%, below
100% VE would be cid x rpm x 0.0007895 / 8 = cfm demand
"Cubic capacity * peak rpm * (variable) / number of cylinders = CFM target at 28” test pressure
The variable is a number that is derived from 1.01055 / fps / 2 * VE
The common variable is 0.0009785, represented by:
1.01055 / 640 / 2 * 1.2394 = 0.0009785
1.01055psi equivalent to 28” water column test pressure
640fps is in between our 2 max airspeed ranges
2 is needed because the intake cycle is every second crank rotation
1.2394 is the VE as a decimal, which is 123.94%
These numbers are used as a base because 640fps is around the average of our target maximum port speeds
and 123.94% VE is used because that is approaching the limit for the majority of naturally aspirated (NA)
engines. Therefore 0.0009785 is seen as a “best case scenario” target for us to work from. Now because the
(variable) is variable we can use it to cross reference future results from dyno testing or even give us an idea
of what the VE or port speeds may be in certain situations, This is going a bit beyond the scope for now but I
will revisit this another time."