Various Automotive Formulas
There are probably many different ways to calculate for any of the desired results. These are one I use.
Cubic Inch Displacement: Bore x Bore x Stroke x 0.7854 x number
of cylinders
Example: 4 x 4 x 3.480 x 0.7854 x 8 = 349.849 cubic inches
Cubic Inches to Liters: Cubic Inches x 0.01639
Example: 350 x 0.01639 = 5.734 liters
Brake Horsepower: Torque x RPM / 5252
Example: 400 x 5000 = 380.8 horsepower
Carburetor CFM Requirement: (RPM x Cubic Inch Displacement
x .8x) / 3654
(Note .80 to .85 is considered good volumetric efficiency of engine.)
Example: (5000 x 350 x .85) / 3654 = 430.4 CFM
Miles Per Hour: Two methods depending on what variables are
known.
#1 (Tire Diameter x RPM) / (Final Gear Ratio x 336)
#2 (Tire Diameter x RPM x 0.002975) / Final Gear Ratio
Examples
#1 (27.0 x 3000) / (3.31 x 336) = 72.831 MPH
Final Gear Ratio: Two methods depending on what variables are
known.
#1 ((Tire Diameter x RPM) / MPH) / 336
#2 (Tire Diameter x RPM x 0.002975) / MPH
Example:
#1 ((27.0 x 3000) / 73) / 336 = 3.31
RPM: Two methods depending on what variables are known.
#1 (Final Gear Ratio x 336) / (Tire Diameter / MPH)
#2 (MPH x Final Gear Ratio) / (Tire Diameter x 0.002975)
Example:
(3.31 x 336) / (27.0 / 73) = 3000 RPM
Horsepower required: ((0.0426 x MPH) ^3) x Weight
Example: ((0.0426 x 70) ^3) x 3750lbs = 99.4 horsepower required
