Cylinder bore size is also a factor, and the bigger the bore, and subsequently the combustion chamber, the more time it takes for the flame front to travel from the spark plug to the farthest areas of the combustion chamber, thus giving the mixture more opportunity to detonate. This is where engine “knock” comes from, and it is very damaging.Ī mixture can ignite prematurely due to the type of fuel (higher-octane fuels resist detonation), ignition timing (advanced timing gives the mixture more time to experience detonation as the piston moves upward) and how much the mixture is compressed (compressing a gas creates heat). So, instead of a well-timed expansion of gasses that begins burning when the spark plug fires as the piston nears TDC, and continue expanding as the piston begins to move back down, there’s a blast that hits the piston like a hammer blow while it is still on the upstroke. The biggest detrimental factor is “detonation.” This is when a fuel mixture ignites elsewhere in the combustion chamber other than at the spark plug when it fires. However, it gets a bit more complicated than that in practice, and there are more factors working against a high compression ratio than there are benefits. This is because the more a fuel mixture is compressed, the more complete it will burn and expand. Put simply: the higher the compression ratio, the more power will be produced for a given displacement, and for a given amount of fuel. Therefore, if the total volume of the cylinder is 100 cc, and the volume with the piston at top dead centre (TDC) is 10 cc, you have a mechanical compression ratio of 10:1. The compression ratio is the ratio between the total volume of a cylinder when the piston is at the bottom of its stroke and the volume of the combustion chamber when the piston is at the top of its stroke, including the head gasket thickness and the shape of the top of the piston. The more efficiently the fuel mixture burns, the harder the push on the piston will be, and the more power that will be produced.Īside from factors like cubic displacement, and intake and exhaust design, an engine’s compression ratio has a great effect on output and fuel efficiency. Power transfers to the rear wheel of a motorcycle by burning an air/fuel mixture in the combustion chamber of a cylinder, and the expansion of the burning gasses pushes on the piston, which turns the crankshaft that eventually propels you forward. It can be a complicated process to determine a compression ratio.