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

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

Horsepower = (RPM x Torque) ÷ 5,252

# Torque

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

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.

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