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Computing The Compression Ratio of a Motor

By: Dennis Adams

Compression Ratio

To measure compression ratio of an engine there are two variables you need to be familiar with. One is swept volume and the other is total chamber volume. Swept volume can be thought of as the displacement for a single cylinder or the area a piston travels from bottom dead center (BDC) to top dead center (TDC). Total chamber volume is all the area above the piston at TDC. This area includes the piston dish volume, the piston valve relief pocket, ring land volume, compressed gasket volume, piston to deck volume, combustion chamber volume of cylinder head. Some other factors you'll need to know engine displacement, cylinder bore, and cylinder stroke. The compression ratio is the relationship between the swept volume to the total chamber volume.

Descriptions of Variables

Combustion Chamber Volume (Cylinder Head Volume)

This is the volume of the chamber in the cylinder head with the spark plug and valves installed. The volume is usually advertised for an aftermarket cylinder head by the manufacture or obtained in the repair manual for your vehicle. It can also be measured by placing the cylinder head face up (one chamber is sufficient) - and ensure the head is level. Then, with a suitably accurate burette or similar device place liquid (something like alcohol, although water with a small amount of detergent to reduce surface tension, can also be used) into the chamber until the liquid is level with the deck of the head. The amount of liquid used is therefore equivalent to the chamber volume.

Piston Volume

Pistons come in various forms, flat top, dish, or dome "pop-up". The flat top piston has an effective volume of zero. A dished piston (usually a stock or lower compression) adds volume to the total chamber volume. A dome "pop-up" piston (usually a higher compression) subtracts volume from the total chamber volume. The volume of either the dish or dome type pistons are usually advertised from the manufacture or listed in the repair manual for your vehicle or it can be measured by using the alcohol technique for dished pistons, with a suitably accurate burette or similar device place liquid in the dish. The amount of liquid used is therefore equivalent to the chamber volume.

Valve Relief Pocket

These are machined pockets in the top of the piston for clearance when the piston is at TDC and the intake valve is in the open position. Not all pistons have valve relief pockets. This volume is sometimes advertised with aftermarket pistons if there are reliefs. If not the alcohol method would work here as well.

Piston to Deck Volume

On a typical engine the piston is not level with the block at TDC. The area above or below the desk of the block must be accounted for.

Gasket Volume

This is the volume the compressed head gasket creates due to its thickness.

Ring Land Volume

This is the space between the piston and cylinder wall, and from the top of the piston to the top of the ring. This value is added to the total chamber volume. This is really only important if an engine has been bored and oversized rings are used. Otherwise this value is negligible.

Engine Displacement

The volume of all cylinders of an engine.

Cylinder Bore

The diameter of the cylinder that the piston fits in.

Cylinder Stroke

The distance the piston travels in the cylinder bore

Calculations

Compression Ratio = (swept volume total chamber volume) /total chamber volume

First we need to calculate Swept Volume
Swept Volume (cc) = cylinder bore diameter (mm) 2 x stroke (mm) x .0007854
Example
Cylinder Bore = 75mm
Stroke Length = 84.50mm
Swept Volume = [75mm] 2 x 84.50mm x .0007854
Swept Volume = 373.31 cc

Next we'll calculate Total Chamber Volume

Total Chamber Volume (cc) = cylinder chamber volume /- flat, dish, dome piston valve pocket volume head gasket volume /- deck clearance volume ring land volume.

Example
Cylinder Head cc = 38.5 cc
Piston = Flat-top w/ 2 valve relief pockets that measure a total of 2.5cc's
Head Gasket = 76mm round and .762mm thick
Deck Clearance Volume = Depth below block deck @ tdc = .254mm
Ring Land Volume = Piston to Wall Clearance = .05mm, Height from top of piston to ring = 16mm


Gasket cc = gasket bore diameter (mm) 2 x compressed thickness x .0007854
Gasket cc = [76mm] 2 x .762mm x .0007854
Gasket cc = 3.456 cc

Deck Clearance Volume = cylinder bore diameter (mm) 2 x deck clearance x .0007854
Deck Clearance Volume = [75mm] 2 x .254mm x .0007854
Deck Clearance Volume = 1.122 cc

Ring Land Volume = [bore diameter (mm) - [2 x piston to wall clearance (mm)]] 2 x height (mm) x .0007854
Ring Land Volume = [75mm-[2 x .05mm] 2 x 16mm x .0007854
Ring Land Volume = .00012 cc

Total Chamber Volume = cylinder chamber volume valve pocket volume head gasket volume deck clearance volume ring land volume.
Total Chamber Volume = 38.5cc 2.5cc 3.456cc 1.122cc .00012cc
Total Chamber Volume = 45.5781 cc

Now we can finally calculate the compression ratio
Compression Ratio = (swept volume total chamber volume) /total chamber volume.

Example
Swept Volume = 373.31 cc
Total Chamber Volume = 45.5781 cc

Compression Ratio = (373.31 cc 45.5781 cc)/ 45.5781 cc
Compression Ratio = 9.19:1

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