Effective Compression Ratio Calculator

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This Calculator will figure the effective compression ratio using your inputs.

The effective compression ratio is what the engine sees while running. While the static CR is defined simply by the geometry of the engine, the effictive CR is influenced by multiple factors such as the engine geometry, cam timing, intake pressure, connecting rod length, and volumetric efficiency.

It is the effective CR that determines how much the fluid is actually compressed during engine operating and therefore the minimum octane rating necessary to avoid pre-detonation. This is why some engines require 100+ octane with an 11:1 compression ratio while others are perfectly fine on 91 octane with a 13:1 compression ratio. E-85 has an equivalent octane rating of 105, and with an expected operating temperature of around 180°F, the maximum dynamic CR is slightly above 10.5

To get higher VE% from high performance engines, the camshaft intake valve is usually closed significantly after BDC compared to a 'street' type of engine. This later closing intake point helps contribute to additional cylinder filling using 'ramming' properties of the intake flow.

This later closing point reduces the engine's 'effective compression ratio' to below the static compression ratio.

Basically the piston is traveling up from BDC on the compression stroke when the intake valve is still open. This is effectively bleeding off the compression pressure, while still filling the chamber with fresher air/fuel mixture. (better VE%)

It is basically using a percentage of the compression stroke before it starts compressing the fuel/air mixture!

This 'effective compression ratio' can be lower than the popular 'Dynamic Compression Ratio' however. (or even higher depending on cam data)

It calculates all the effects of the cylinder intake pumping and ramming processes with the trapped air/fuel at the intake closing point.

(Use seat to seat specs for Intake Close spec for best results)

Effective Compression Ratio

Rod Length:  inches
IVC (ABDC):  Degrees
Stroke:  inches
Boost Pressure in PSI :
Target Altitude : (Feet)
VE:  %

Input All Parameters, Please

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