# Various Automotive Formulas

There are probably many different ways to calculate for any of the desired results. These are one I use. You might also check my Tools page HERE for interactive calculators.

Cubic Inch Displacement

How to calculate your engine's cubic inches displacement and how to convert that to liters.

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

Some of the more popular Chevrolet engine sizes.

Brake Horsepower

What is my theoretical brake horsepower?

Brake Horsepower: Torque x RPM / 5252

Example: 400 x 5000 = 380.8 horsepower

Carburetor CFM Requirements

How big of a carburetor do you really need in normal circumstances? Everyone assumes 'bigger is better' but it's very easy to go too big with a carburetor. Naturally your altitude above sea level, driving conditions, engine mods, and much more come into play when choosing a carburetor; this formula is just a starting point.

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

Calculate Miles Per Hour

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

Example #1 - (27.0 x 3000) / (3.36 x 336) = 71.747 MPH

Calculate Final Gear Ratio

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

Calculate RPMs

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 Requirements

Horsepower required: ((0.0426 x MPH) ^3) x Weight of vehicle

Example: ((0.0426 x 70) ^3) x 3750lbs = 99.4 horsepower required