all else being equal how much hp can be gained by going up to 12 to1 and how much to go to 13 to 1 thanks
Phil
Increasing compression alone is just as viable a means of increasing HP, as when installing cams with otherwise stock pistons & compression. As with anything however, there are always advantages and disadvantages. If you are going to increase compression, make it worth your while and make as large an increase for the money as possible meaning, don't stop at 12:1, go with 13:1.
One advantage of going with higher compression pistons is, so long as you installed them properly, which is a fairly easy task technically speaking, they produce their yield. In comparison, when installing cams, degreeing them can be a real pain in the ass, as well as time consuming.
Another advantage of installing higher compression pistons alone is; they increase HP throughout the engine's entire RPM range. This is because higher compression pistons (assuming they actually are increasing compression as advertised) will occupy a greater portion of the remaining space in the cylinder. In so doing, at the end of the exhaust-stroke the pistons WILL displace (exhaust) more of the spent charge and when they begin the intake down-stroke, they begin drawing from a lesser space, thereby more quickly producing a greater differential pressure between cylinder pressure and the outside 14.7 PSI atmospheric pressure. This results in greater air density per intake down-stroke and with more Oxygen available, more fuel may be added to increase HP. End result, it adds pep to the engine from idle through redline.
However, combustion engines suffer from intake ineffiencies as per increases in their RPM. This is due to the short time durations associated with the piston's down-stroke time on its intake stroke. For instance, an engine that's idling at 1,000 RPM (a slow idle for a Hayabusa) has only 30 milliseconds (.030 seconds) to draw in a cylinder's worth of fresh air.
At 1,000 RPM:
1,000 / 60 seconds = 16.666666 RPS - crankshaft Rrevolutions Per Second
1 / 16.666666 RPS = .060 seconds - the time for a single crankshaft revolution
Since half of the crankshaft's revolution time is up-stroke, the other half is down-stroke:
.060 seconds / 2 = .030 seconds - this represents the pistons' down-stroke time during intake
Try to envision the true nature of how very short this intake time duration actually is at idle RPM. This 30ms intake down-stroke time is only slightly more than a mere 1/4 of 1/10 of a second! So this time is fairly short even at idle speed. Air is thin, so this works well enough at these low engine RPM, but this intake down-stroke time only grows shorter with increases in engine RPM.
By the time 10,000 RPM has been achieved, the intake down-stroke time is only .003 seconds (3/1000 of a second)! Air is thin, but it moves like molasses in comparison to low engine RPM operation. These time durations have become too short to accomplish the task of completely filling the cylinder with 14.7 PSI of fresh air, therefore the engine begins to suffer torque loss as it can no longer draw in as much air density per down-stroke time at these higher RPM, so less fuel is being injected per cylinder. HP can still increase however, because HP is merely a measure of how much work can be performed in a given time. Since the cylinders are receiving lesser powerful A/F mixtures as the RPM increases BUT is producing MORE of them per second due to increases in RPM, the computation for HP allows it to increase even though the cylinders are slightly less powerful per ignited A/F mixture. However, once the point in RPM is reached at which the ratio of A/F mixture power release has decreased to the point where having more of these lesser ignited A/F mixtures per second can no longer make up for the loss in power release per A/F mixture, then torque begins to fall off as demonstrated on the dyno.
If the dyno were able to measure the actual torque produced as per the power-stroke of each cylinder, it would demonstrate that the cylinder begins losing torque from the time it begins to increase in RPM above idle. Dynos however, measure torque over time, not as per the power-stroke of each cylinder, so what the dyno sees in its method of torque detection is an engine that begins to increase in torque somewhat as engine RPM begins to increase above idle, then it sees a peak, then an early decline in torque in comparison to the engine's redline. Dynos simply cannot reveal the true workings of the engine with their method of torque detection over time verses actual power-stroke measurement of individual cylinders.
