Quote:
Originally Posted by Jakobi
Yeah as per the link.
"The Black line is the stock engine, in this case 2.5mm squish, 11.9:1 comp, 1.3mm base gasket stack, piston level with port floors at bdc"
From here they speak of dropping the cylinder (less gaskets).
0.7mm in this scenario boosts overall with a slight drop at top end, however going to 1.0mm results in a major sign off up top.
The figures will change depending on your bike, your compression ratios, squish etc, but basically regarding the port timing having the piston sit higher above the exhaust port results in exh duration drop, transfer duration drop. This increases bottom end at the expense of a good flowing top end. More back pressure, more grunt.
Increasing compression has the same effect on an engine. Adds to bottom end, but will be less willing to rev as high.
So by running less gaskets you end up increasing the compression and changing the ports to all focus on improvements in the bottom to mid. The top end is sacrificed.
If you time ports flush with piston at bdc you get a softer bottom end more inclined to rev out. Add some compression by changing your squish and you pick some up all over.
Make sense?
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That helps immensely. Keeping the piston above the floor effectively lowers the top of the exhaust port (reduces exhaust port duration, right?), thereby "choking" top end. Reducing squish "recovers" some top end. My numbers equate (roughly) to the .7mm reduction in gasket stack, so I just need to measure squish. If my squish is ~1.8mm, I'm in the sweet spot. Sounds like I'm pretty close, now that I understand the basic concept. I'll measure the piston/floor difference more accurately tomorrow and go from there. 0.7mm of piston showing (roughly) and ~1.8mm of squish. I want bottom end right now, not worried about revving it out. When I get more experienced I'll chase the RPM's, but most of my riding is single track with limited transfer areas. Sounds like a plan. Thanks for your help.