Nominal compression ratios, as I have said before, have little meaning in high-output two-stroke engines. However, you can work with trapped compression ratios almost as effectively as by measuring cranking pressures. An engine's trapped compression ratio is the ratio between the cylinder volume at the moment of the exhaust port's closing and the volume with the piston at the top of its stroke. To find this, you must first measure the combustion chamber volume, with the piston in position at top center. The job can be done with the engine assembled, using n graduated cylinder and pouring in oil until the level comes up to the spark plug hole. Or you can calculate the volume. When the combustion chamber has a simple shape (part-spherical, conical or cylindrical) I prefer to do the job by calculation, but more complex shapes send me scurrying for a can of oil and a graduated cylinder. In fact, the process of actual measurement may appeal to you as a regular thing, because you will need a graduated cylinder for more than this single task, and a slide-rule may not be a part of your basic equipment. In any case, remember when figuring the compression ratio, that it is not the ratio between piston displacement and combustion chamber volume, but between cylinder volumes from the point of exhaust port closing to top center, as in the following formula :
Where CR is compression ratio
V1 is cylinder volume at exhaust closing
V2 is combustion chamber volume
Traditionally, compression ratios have been measured "full stroke". That is to say, V, would represent the combustion chamber volume plus piston displacement from bottom center to tog center. Thus, a combustion chamber volume of 28cc and a piston displacement of 250cc, calculated full-stroke, would be
CR= 9.93:1
But a far more realistic figure is obtained when V1 represents the cylinder volume above the upper edge of the exhaust port, and if we assume that our hypothetical engine has an exhaust port height equal to 45-percent of stroke, then V1 becomes 55-percent of piston displacement plus V2, and calculation goes like this:
CR= 5.91:1
Coincidentally, that compression ratio (5.91:l) is very nearly all a non-squish combustion chamber will permit in an otherwise fully-developed two-stroke engine. With small-bore engines you may push the compression ratio up to perhaps 6.5:l without serious consequences, using a non-squish cylinder head, but that is very near the limit. Good squish-band cylinder heads, on the other hand, permit compression ratios up to as much ns 9.5:l in motocross engines with exhaust systems that provide a wide boost without any substantial peaks, but for road racing engines I cannot recommend anything above 8.5:l even when unit cylinder size is only 125cc. You will find that higher compression ratios than those suggested can produce marvelously impressive flash readings on a dynamometer; as soon as the engine has a chance to get up to full temperature, the output will drop well below that sustained by an otherwise identical engine with a lower compression ratio. Sustained, and not flash horsepower, is what wins races.
