Want to make a little more sense of your timeslip? Looking for bragging rights after a trip to the drags? Don't know what overboring your engine will do to your compression ratio?

Do some quick compression ratio calculations or just have some fun with our power/speed calculators - they are based on maths algorithms to calculate precise compression ratios and engine displacements. The power/speed calculators are the real thing too; calculating realistic estimates of horsepower figures at the flywheel as well as at the wheels based on performances at the track.

The rear wheel power figures have an arbitrary 20% drivetrain loss factor included - this is an approximation, as parasitic losses vary from vehicle to vehicle and don't change when the engine is modified. Manual transmissions absorb less power than autos, torque converters have different slippage rates, diffs have different hypoid angles, etc which all contribute to varying the total losses from 17% to over 30%.

There are usually discrepancies between power calculated from trap speed versus power calculated from mile elapsed time - this occurs if the car does not leave the start line efficiently, hence wasting precious time. Horsepower calculations based on trap speed usually provide more accurate figures when applied to street cars.

The compression ratio/displacement calculator is very handy for evaluating changes in compression ratio/engine size and can convert from metric to imperial displacement, and visa versa.

A few handy conversions:

To convert horsepower to kilowatts, multiply horsepower x .746
To convert kilowatts to horsepower, multiply kilowatts x 1.3404
To convert cubic inches to cubic centimetres, multiply cubic inches x 16.39
To convert kilograms to pounds, multiply kilograms x 2.2
To convert MPH to KPH, multiply MPH x 1.6
To convert KPH to MPH, multiply KPH x .624
 

Calculate HP from Trap Speed
   

Vehicle weight in pounds (including driver)
lbs

Trap speed at end of 1/4 mi in m.p.h.
m.p.h.


You are making : HP at the rear wheels and HP at the flywheel*.

JavaScript must be enabled in your browser for the calculation to work.
 

 
Calculate HP from ET

Vehicle weight in pounds (including driver)
lbs

ET in seconds
sec


You are making : HP at the rear wheels and HP at the flywheel*.
JavaScript must be enabled in your browser for the calculation to work.

*Note: Assuming a 20% drive train loss.
 

Calculate Tyre Diameter
 

The tyre diameter calculator is a quick and easy way to work out the rolling circumference of a tyre- although many racers swear the only way to accurately measure it is to mark a spot on the centre of the tyre with white paint/chalk, then roll the vehicle to imprint the mark on the ground. Continue on until you get a second mark on the ground and measure the distance between them. A bit more work than sitting at the computer!
 

Tire width - Series - Wheel Diameter
Example 275 - 60 R 15
Tire Width
Tire Series
Wheel Diameter


Tire diameter in inches

JavaScript must be enabled in your browser for the calculation to work.
 

Calculate MPH for RPM range
   
Tire diameter
Gear Ratio



This calculation assumes the car's transmission is in drive or 1:1 ratio, no overdrive. For Overdrive cars multiply the overdrive ratio times the rear and gear ratio. Example a 700R4 transmission has an overdrive ratio 0.7:1. For a 4.11 rear end multiply 4.11 x 0.7 = 2.877. Enter 2.877 as the number for the gear ratio to calculate speeds in overdrive.

JavaScript must be enabled in your browser for the calculation to work.

 

1000 rpm mph 4500 rpm mph
1500 rpm mph 5000 rpm mph
2000 rpm mph 5500 rpm mph
2500 rpm mph 6000 rpm mph
3000 rpm mph 6500 rpm mph
3500 rpm mph 7000 rpm mph
4000 rpm mph 7500 rpm mph
       


Calculate Rear End Gear Ratio
 

Enter the tire diameter in inches, rpm's of the motor at a certain speed as indicated by your speedometer in mph. This calculation assumes the car's transmission is in drive or 1:1 ratio, no overdrive.
 

Tire Diameter (in)
RPM
Speed at RPM (mph)


Rear End Gear Ratio

JavaScript must be enabled in your browser for the calculation to work.
 

Calculate compression ratio and engine displacement


The Engine Parameters calculator is designed to assist engine builders ascertain what components and/or machining is required to achieve a desired compression ratio. It is also handy to evaluate the effect of modifications, eg what thickness head gasket is required with a specific combustion chamber volume to achieve your target compression ratio, or evaluating the effects on compression ratio by changing the crankshaft stroke, etc.

The calculator will use whatever data is entered, so if there is insufficient data entered some of the output objects will be blank, undefined, or infinity. It will also convert displacement into Metric or Imperial units and visa-versa.

The calculator is designed to recalculate the values each time an input object is changed, so therefore no [Calculate] button is required. Use the Tab key to move to the next data entry point.


Enter the number of cylinders, the bore, and the stroke of the engine. This information will be used to calculate the displacement and the compression ratio.
 

Number of Cylinders

Bore

Stroke

Inches
Centimeters
Millimeters

Enter the volume of the pistons and combustion chambers (in the heads). The head chamber volume is always positive. The piston volume is positive if it is dished, zero if it is flat, and negative if it is a dome piston. This data is used to calculate the compression ratio.
 

Head Chamber Vol

Piston Volume

Cubic Inches
CC

Enter the Piston Deck Height and the Compressed Head Gasket Thickness. The Piston Deck Height (also known as the "Installed Height") is the distance that the piston protrudes above the top of the block. It is negative if the piston top is below the top of the block. The total piston to head clearance is the Compressed Head Gasket Thickness - the Piston Deck Height. This data is used to calculate the compression ratio.
 

Piston Deck Height

Head Gasket Thick

Inches
Centimeters
Millimeters

This section contains the results of the calculations. The displacement only can be entered if you want it translated to other units.
 

Displacement

Cu In Litres CC

Compression Ratio

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