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Compression with rotating assembly

Sometimes you know the rotating-assembly dimensions but not the piston's position relative to the deck. This calculator works out piston-to-deck from the stack-up of rod, crank radius and compression height against the block deck height, and then gives the static compression ratio.

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Units
The cylinder's diameter.
The piston's stroke.
Centre to centre.
Pin centre to piston crown.
Measured from the crankshaft centreline to the block deck surface. This is NOT the piston-to-deck clearance.
The volume in the cylinder head's combustion chamber.
Leave blank to use the cylinder bore.
Compressed thickness.
A dome is entered as plus, a dish as minus.

Compression ratio

11,77:1

Piston to deck: 0,500 mm
Cylinder volume: 716,7 cc
Total clearance volume: 66,5 cc

Static compression ratio worked out from the rotating assembly. The deck height is measured from the crank centreline to the block deck. Combustion chamber and piston dome are entered in cc (dome +, dish -).

How the calculation works

At top dead centre the piston crown sits at the height rod plus crank radius (half the stroke) plus compression height above the crankshaft centreline. Subtracting that height from the block deck height, which is also measured from the crank centreline, gives piston-to-deck. A positive value means the piston sits below the deck and adds extra volume, a negative one that it pokes up. That volume is added to the rest of the clearance volume.

Then the compression is worked out as usual: the whole volume above the piston at bottom dead centre divided by the volume left at top dead centre. The clearance volume is the chamber volume plus the deck volume plus the gasket volume, minus any dome on the piston (a dish is added instead). Be careful with the deck height, it is measured from the crank centreline to the deck and must not be confused with the piston-to-deck clearance itself.

piston to deck = deck height - (rod + stroke/2 + compression height) clearance volume = chamber + deck + gasket - piston dome CR = (cylinder volume + clearance volume) / clearance volume

Example

101,6 mm bore, 88,4 mm stroke, 144,78 mm rod, 31,0 mm compression height and 220,48 mm deck height give a piston-to-deck of 0,50 mm. With a 58 cc chamber, 104 mm gasket at 1,0 mm and a 4 cc dome it works out to about 11,77:1.

Guide values for compression and fuel

Fuel or setupGuide value
95 octane (E10)9,5-10,5:1
98 octane10,5-11,5:1
E85up to 13:1
Race fuel12:1 and up
Turbo or superchargerlower 1-2 steps compared with naturally aspirated

Guide values for a naturally aspirated engine on pump petrol. An aluminium head often tolerates half a step to a full step higher than cast iron.

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Compression ratio

Static compression ratio from all the volumes in the head.

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Displacement

Cylinder volume, displacement and total capacity from bore, stroke and cylinders.

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Common questions about compression with rotating assembly

The block deck height is the distance from the crankshaft centreline up to the block deck surface, often around 220 mm. Piston-to-deck is the small clearance between the piston crown and the deck at top dead centre, often a few tenths of a millimetre. This calculator works out piston-to-deck for you from the deck height and the rotating-assembly dimensions.

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