The following equations can be useful when designing or building an engine and the following quotation can help keep everything in perspective.

*"Today’s scientists have substituted mathematics for experiments, and they wander off through equation after equation, and eventually build a structure which has no relation to reality."*Nikola TeslaBasic Airflow Rule: The airflow (cfm) to the engine (through the air cleaner, throttle body, intake manifold, and cylinder head) must support the horsepower trying to be achieved at the specific rpm and the port velocity must also be at the correct speed.

If the engine power is low, perform a compression and leak down test to confirm sealing. Analyze the camshaft, dynamic compression ratio, and the exhaust system for problems. Assure the tune-up is correct for the air/fuel ratio and the ignition timing. Test the battery and the charging system.

If the engine power is low, perform a compression and leak down test to confirm sealing. Analyze the camshaft, dynamic compression ratio, and the exhaust system for problems. Assure the tune-up is correct for the air/fuel ratio and the ignition timing. Test the battery and the charging system.

**If you are not using math to design and build your engines you are probably missing out on potential power.**

circumference of a circle = diameter x 3.14159

area of a circle = (radius x radius) x 3.14159

actual valve lift = (cam lift x rocker arm ratio) - valve lash

degrees of duration = opening + closing + 180

use positive numbers if opening is BTDC and closing is ABDC for the intake and opening is BBDC and closing

is ATDC for the exhaust, use negative numbers for other positions

intake centerline = (intake duration / 2) - intake opening

this assumes cam profile is symmetrical

exhaust centerline = (exhaust duration / 2) - exhaust closing

this assumes cam profile is symmetrical

lobe separation angle = (intake centerline + exhaust centerline) / 2

this assumes cam profile is symmetrical

the lobe centerlines are also the maximum lift points for symmetrical profiles

maximum flat tappet velocity = circumference of tappet face / 360

I have a valve timing spreadsheet program that will do the opening and closing calculations for both symmetrical and asymmetrical cam profiles. Email me for a free copy.

cubic inches = liters x 61.02398

cubic inches = (bore x bore) x stroke x (0.7854 x number of cylinders)

average piston velocity = (stroke x rpm) / 6

rod to stroke ratio = rod length (center to center) / stroke

total theoretical airflow (cfm) = (rpm x cubic inch) / 3456

theoretical airflow (cfm) is used to determine the volumetric efficiency (ve)

ve = (actual cfm / theoretical cfm) x 100

actual cfm is measured on a dynamometer

rpm peak horsepower = (1925 / cubic inch per cylinder) x cfm airflow @ 10"

cfm airflow is through the complete intake system

cfm airflow @ 10" = (rpm x cubic inch per cylinder) / 1925

cfm airflow is through the complete intake system

horsepower @ crank = (cfm airflow @ 10" x 0.4) x number of cylinders

cfm airflow is through the complete intake system

flow bench pressure conversion factor = square root of (new pressure / current pressure)

The formulas above using 10" are older formulas when many shops used a 10" standard.

port velocity = (rpm x (bore x bore) x stroke) / (pcsa x 260.9)

pcsa = minimum port cross sectional area in square inches, or the

**(intake valve diameter x 0.82)**

__area of__690 ft/sec is ideal, 660 - 720 ft/sec is acceptable, Mach 0.55 - 0.60 based on 1200 ft/sec.

pcsa = (rpm x (bore x bore) x stroke) / 180,000

rpm peak horsepower = (pcsa x 180,000) / ((bore x bore) x stroke)

horsepower = (torque x rpm) / 5252

torque = (horsepower x 5252) / rpm

brake mean effective pressure (bmep) = (horsepower x 792,000) / (cubic inch displacement x rpm )

stock 125-145

modified street 150-185

race 185-230

bmep = (torque x 150.8) / cubic inch displacement

horsepower = (cubic inch displacement x rpm x bmep) / 792,000

good video showing relationship between drag and mph https://www.youtube.com/watch?v=F76-npz0CeI