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Certainly, increases in compression ratio, which may be accomplished simply by trimming a few thousandths of an inch from the cylinder head's lower surface, can work minor miracles with an engine's performance.īut higher compression ratios can also bring about a mechanical disaster: improvements in power gained in this manner are purchased at a disproportionate cost in peak cylinder pressure, leading to reduced bearing life and sometimes to an outright failure of a connecting rod or crankpin. That is, of course, a very great gain, and the consequences - measured at an engine's output shaft-are the reason for many experimenters' fixation on “raising the compression”.
For example, at a compression ratio of 5:1, air standard efficiency is 47.5-percent, while at 10:1, it is 60.2-percent. Even so, this theoretical level of efficiency, calculated against compression ratio, provides a useful yardstick against which actual efficiency can be measured - and it tells us a lot about the effects, on power output, of compression ratio. In this, it is assumed that the cylinder is filled only with dry air, and heat then added, which ignores the fact that in practice the air contains some moisture and a considerable percentage of hydrocarbon fuel.
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For the Otto-cycle engine, of which the two-stroke is an example, there is a theoretical level of efficiency, in terms of converting heat into work, referred to in basic engineering texts as “air standard efficiency”.