How do you calculate compression ratio? What’s the best way to figure out engine displacement?


In the automotive mechanics world, there are formulas and conversions for measuring just about everything—from horsepower and torque to carburetor cfm and gear ratio. The key is remembering them.

While cheat sheets might have frowned upon in your sixth-grade classroom, we strongly encourage them in the garage, shop, or pits. That’s why we’ve put together this list of 13 key performance formulas you should know when building or tuning your street or race vehicle.

Read it, print it out, and tape it to your beer fridge. (Quick—while your sixth grade teacher isn’t looking!)

Racing Carburetor CFM

Racing Carburetor CFM = RPM x Displacement ÷ 3456 x 1.1
Note: Summit Racing also offers this CFM Calculator to make the job easier.


Displacement = .7854 x Bore2 x Stroke x Number of Cylinders

Correct Compression Ratio (CCR)

CCR = FCR (Altitude/1,000) x .2
Note: You can also take this Compression Ratio Calculator tool for a spin.

Tire Diameter

Tire Diameter = (MPH x Gear Ratio x 336) ÷ RPM

Rocker Arm Ratio and Valve Lift

Gross Valve Lift = Camshaft Lobe Lift x Rocker Arm Ratio


Horsepower = (RPM x Torque) ÷ 5,252


Torque = (5,252 x HP) ÷ RPM

Rod Ratio

Rod Ratio = Rod Length ÷ Crank Stroke Length

Average Piston Speed

Average Piston Speed = Crank Stroke x RPM ÷ 6

Rear Gear Ratio

Rear Gear Ratio = (RPM at Finish Line x Tire Diameter) ÷ (MPH x 336)
Note: You can also save this link to a handy Gear Ratio calculator.

Volume (CCs) of Deck Clearance

CCs of Deck Clearance = Bore x Bore x 12.87 x Depth of Deck Clearance

Volume (CCs) of Head Gasket

CCs of Head Gasket = Bore x Bore x 12.87 x Thickness of Head Gasket

Compression Ratio

Combined Chamber CCs + Gasket CCs + Deck CI CCs + (Displacement x (N))


Combine Chamber CCs + Gasket CCs + Deck CI CCs

(N) = 2.0483 for  8-cylinder. (N) = 2.7311 for 6-cylinder. (N)=4.0967 for 4-cylinder.

Author: David Fuller

David Fuller is OnAllCylinders' managing editor. During his 20-year career in the auto industry, he has covered a variety of races, shows, and industry events and has authored articles for multiple magazines. He has also partnered with mainstream and trade publications on a wide range of editorial projects. In 2012, he helped establish OnAllCylinders, where he enjoys covering all facets of hot rodding and racing.