In every motorcycle brochure and every engine specifications we see specified the 'Compression Ratio', but what is that number. The compression ratio is the ratio between the volume of the cylinder and volume of the combustion chamber. And on most road motorcycles there is a power increase to be had by increasing this ratio. There is always an optimum compression above which the increase in heat negates the proposed increase in power, and as most of you are not dealing with race engines it is wise to be conservative with increases in compression.
Typically, small increases in compression will be beneficial throughout the rev-range and larger increases in compression will benefit performance at lower revs but be detrimental to power at higher revs. For example a motocross engine would run a higher compression than a road race engine if they were both running on the same fuel.
On most older two-stroke secondhand motorcycles available in Thailand I would not recommend more than a 7:1 compression ratio and you would need to use the best unleaded fuel with a high octane rating.
To calculate the compression ratio you need to measure both the volume of the combustion chamber and the cylinder. I must have been very selective about the maths I remembered at school – how to measure volumes of cylinders was obviously deemed important enough not to forget! The formula is (Pi x r²) x height.
I always take Pi to be 3.142, r is the radius and the height is the stroke of the engine. In simple terms, for example on an engine with a bore of 64mm, the radius is half the bore, which is 32. The stroke is 54mm. So our little sum is: (3.142 x 32 x 32)x54 = 173.7cc.
This is the cylinder volume but if the cylinders have been rebored to say 1mm oversize that will alter the sum to (3.142x32.5x32.5)x54 = 179.2cc.
We now have to measure the other part of the ratio, the combustion chamber volume, this can't be calculated – it has to be measured. To do so you will need a burette or cc measuring tube.
Ideally, the engine will be on the bench not on the motorcycle. Remove the cylinder head, clean -up the head and barrel faces and removed the spark plug. Position the piston at the top dead center (TDC) and smear some grease around the edges of the piston to seal it and wipe off the excess. Use a new head gasket, fit the head and torque it down. Position the engine so that the cylinders are upright and check that the piston is still at TDC.
Fill you burette with paraffin to the zero mark and insert it into the plug hole, release the paraffin until it comes level with the top of the plug hole and read off the burette how much paraffin it has taken. If for example it shows 18cc deduct 2cc for the plug hole, which gives 16cc which is the volume of the combustion chamber.
To work out the compression ratio, take the volume of the cylinder, ADD the volume of the combustion chamber and then DIVIDE by the volume of the combustion chamber.
In my example this is (173.7+16)/16 = 11.85:1 compression ratio.
If we were talking about four-strokes that would be the compression ratio calculated, but with a two-stroke the compression does not start until the piston closes the exhaust port. The correct way to calculate the compression ratio on a two-stroke would be to measure from the top of the exhaust port to the top of the barrel, then when working out the volume of the cylinder instead of the stroke of 54mm in the equation, you would insert the port height, but for the purpose we are intending it is fair to generalize that the exhaust port is 50% of the way up the stroke i.e. 27mm from the top of the 54mm stroke. In this way the compression ratio we arrived at earlier would be halved i.e. instead of approximately 12:1 it would be 6:1. On a two-stroke this is called the corrected compression ratio.