For any of you interested in builing an engine some useful formulae for you
Engine Capacity/Displacement
Info Required:
1: Cylinder bore
2: Crankshaft Stroke
3:Number of cylinders
Calculation: 0.7854 x bore x bore x stroke x number of cylinders
Example: Bore = 81mm (8.1cm), Stroke = 77.6mm (7.76cm), Cylinders = 4
Solution: 0.7854 x 8.1 x 8.1 x 7.76 x 4 = 1599.5cc/97.6cu.ins
To convert from cubic centimetres (cc) to cubic inches (cu.ins)
divide by 16.387
to convert from cu.ins to cc miltiply by 16.387
Crankshaft Stroke
Info Required:
1: Bore size
2: Engine Capacity
3: Number of cylinders
Calculation: Divide engine capacity by: (0.7854 x bore x bore x cylinders)
Example: 1600 cc 4 cylinder engine with an 81mm bore
Solution: 1600/(0.7854 x 8.1 x 8.1 x 4)
Compression Ratio
Info Required:
1: capacity of ONE cylinder
2: Compressed volume
Cylinder capacity it the total engine capacity divided by number of cylinders: 1600/4 = 400cc per cylinder
Compressed volume is the area above the piston crown when the piston is at its highest point or Top Dead Centre (TDC)
This area comprises:
1: the combustion chamber, usually in the cylinder head but can be formed partly by the area of the piston crown if the pistons are dished
2: the head gasket thickness
3: the area between the cylinder block face and the piston crown, commonly refered to as "deck hieght"
Procedure: Measure the volume of the cylinder and/or piston chambers using a suitable burette filled with parafin. Calculate the volume of the gasket and deck hieght and add these to the chamber volume, this will give you the total compression volume.
Calculation: (cylinder volume + compressed volume) / compressed volume
Example: 2000cc 4 cylinder engine with a compressed volume of 54ccs
Solution: One cylinder = 2000 / 4 = 500
(500 + 54) / 54 = 10.26
Therefore giving a 10.26:1 compression ratio
Valve size
The most effiecient airflow through any valve occurs when the valve is lifter 25% of its diameter
For example a 38mm(1.5") valve will require a lift of no more than 9.5mm(0.375")
a 45mm(1.770" valve will require a lift of 11.25mm(0.433") to reach optimum flow capabilities
Further opening of the valve will not increase flow
HTH Tony
