 Here we have a graph of pressure measured within the very beginning of the expansion chamber while the engine was spinning at 10,000 rpm. The first pressure pulse seems strange and out of place but it is the result of the exhausts previous pressure wave having made two round trips from the exhaust port to the baffle and back again, twice. Fortunately it came back before the exhaust port opened. Unfortunately it makes another round after the second one and comes back to the cylinder again for a third time. Of course each time it comes back with lesser force (40%) and so the third wave will be 16% the strength of the original exhaust strength (.4x.4=.16). Usually pressure and or suction waves traveling from different directions just pass through each other like two ghosts. But when they occupy the same space after having come from the same direction they can subtract strength from each other if they have opposite pressure forces. That is the case here because the third generation pressure wave arrives at the cylinder (peaking at 205 degrees) right when the suction wave from the diffuser arrives (160-225 degrees), and the pressure wave diminishes the suction wave which is not good. Looking at the orange "return wave from baffle" you can see that the baffles return wave arrives too early which means the pipe is tuned for a higher peak rpm. The peak of that wave, if the pipe is designed right, should arrive so that its pressure peak coincides with EPC. The pipe in this example has a baffle distance correct for peak power at 11,400 rpm according to my calculations. If the pipe was correctly designed for 10,000 rpm then when would the 3rd generation pressure wave arrive? I figure at 271 degrees, just as the exhaust port is closing so that it adds to the supercharging effect. Next I show how I calculated it all. Assuming the exhaust pulse peaks at 17 degrees after EPO (which means a longer pulse at lower rpm but that is fine because lower ports for lower rpm means less pressure when the port opens which means a less sudden rise in pressure) here goes: EPO83+17=100, EPC277-100=177, 177+277=454 454-360=94 (2nd wave), 94+177=271 (3rd wave). So then I calculated for other port timings to see what range gave the most harmonious results. I calculated for the middle peak of the wave (see Excel file) and then guesstimated the strongest part of the wave occupied 50 crank degrees. Turns out that the 3rd generation baffle wave adds to the supercharging effect (extra cylinder pressure at port closing) when the exhaust port has timing from 80 to 86 degrees ATDC (durations of 200-188). From 87 to 88 degrees (184-186) the effect would be in a gray zone of neither good nor bad because it adds only slightly to the exhaust port blocking (of air/fuel escape) and begins to try to pressure back into the crankcase the intake charged that had been transferred into the cylinder (but due to charge flow inertia none would be pushed back down). From 89 to 98 degrees (164-182) the 3rd generation pressure wave returns during the opening of the transfers, between BDC and transfers closing which requires more diffuser angle to counterbalance. From 100 to 111 degrees (138-160) the 3rd generation wave comes back when the transfers are first opening (before BDC) and so requires more crankcase compression ratio to counterbalance for a normal entrance of the intake charge up from the crankcase. More than 111 degrees had the wave returning in harmony with the exiting pressure wave from the cylinder which hopefully will not cause any negative performance. Keep in mind that the timing of this wave depends only on the timing of the exhaust port and the tuning of the expansion chamber (meaning that the baffle has to be the right distance from the cylinder so that its return wave peaks at exhaust port closing). Here's a graph showing the return timing of the 3rd generation wave for different exhaust port timings of 79 (202 dur), 83 (194), 88 (184), 96 168), 102 degrees (156):  |