If the opening point of the KACR has no effect, then . . . is piston velocity and swept volume vs. crank angle constant?
Didn't think the volume swept by the piston is CONTINUOUS and UNIFORM through the 180 degrees of the compression stroke.
Rather, thought the volume varied with the SINE of the crank angle; i.e., considering BDC as zero degrees, a maximum volume vs. rotation (and maximum piston speed at any given rpm) would be found at 90 degrees, decreasing to a minimum volume (as in zero) at zero and 180 degrees . . . volume swept vs. rotation would be a sinusoidal function . . .
Since I thought the piston velocity varied with crank angle, I thought the volume swept by the piston varied with crank angle also. Thus, the crank angle where the KACR cracks a valve would affect the compression released, and thus the PSI registered in a compression test. The closer to 90 degrees the cracking of the valve, the more compression released.
If the piston velocity is constant throughout the compression stroke, my assumptions appear invalid and I stand corrected.
This is a long thread and the last post was over a year ago, but I want to add some more info because it appears people find and refer back to it frequently.
First, in the quoted post above from Damocles, he states that the speed of the piston as it slides up and down is sinusoidal. Actually, it is not. If you graph out the piston speed, or the piston position, compared to crank angle, you will find that the sinusoid actually is more peaked at the top of the graph, and more broad at the bottom of the graph. If the connecting rod was very long relative to the crank stroke then the graph would approach a true sinusoid. But in real world engine designs, we need to keep the engine compact, so the con rod length is generally between 1.6 and 1.8 times the stroke. Given this physical reality, near the bottom of the stroke, the big end of the con rod swings with the crankpin a large number of degrees without pushing the piston up or down much. Whereas, at the top of the stroke as the crank and con rod swing through TDC, the piston moves up and down more for the same degrees of rotation. Thus the piston has less time near TDC than near BDC. With a longer rod ratio, the piston spends more time near TDC. This result affects cam designs and setting engine timing. This ratio is an important factor in engine design, so it has a name: the “rod ratio”. For more info, see this link:
How Rod Lengths and Ratios Affect Performance
Second, people keep talkin’ bout the MC mod and lamenting that no one, or very few, have done before and after dyno tests. I understand that dynos are expensive, and paying for the use of someone else’s is also expensive, however, you don’t need a gold-standard dyno to figure out whether your mod had an effect. You can compare before and after roll-on acceleration measurements. I also suggest doing a roll-on from 30-50 MPH in 3rd gear, so that you cover the low and midrange RPM, and 50-70MPH in 4th to cover the midrange to high RPM. And I suggest doing it at least 3 times, averaging the results. Obviously, pick a flat section of highway too.
ideally, you can do several sets of roll-on acceleration measurements around noon, come back to your garage and havea quick lunch while the engine cools off enough to work on, do the MC mod in less than an hour, and go out and do the same tests, on the same day, on the same stretch of road. And do it on a day that is overcast when the air temperature doesn’t change much.
Anyone up for it?