Originally Posted by
fyrstormer
I did increase exhaust timing a little bit by raising the sleeve 0.1mm, though that means I also increased the transfer-port timing as well. I also dropped the total height of the shim stack 0.1mm, down to 0.3mm from 0.4mm. I could drop it further, down to 0.2mm, but that would leave only a single 0.1mm shim between the sleeve and the head since one of the shims is under the sleeve instead of above it.
I agree that increasing compression increases power, especially in the mid RPMs, I've seen that myself on other engines. Not on this one though, for some bizarre reason. I have no doubt that cleaning out the transfer-port channels must've helped somehow, because they were really badly obstructed before. But the effect on the engine's power output appears to have been academic.
Doesn't increasing compression have a negative effect on the top speed of the engine? I read that somewhere but I confess I have no firsthand knowledge to back it up.
The whole concept of higher compression hurting top end is only a half truth and very misunderstood, there is a ideal sweet spot for compression where the engine will make its best power and RPM but once you go further then and increase compression even more the engine then will start losing power and some cases the symptoms become worse on the top end. Now if your engine has too many shims ( which 95% of all engines do, especially RTR ) then removing shims will actually increase top end, and can be substantially too, even 0.1 mm too many shims can cost you thousands of RPM and a huge % of torque.. Out of all the mods you can do getting the head at the sweet spot is by far the most critical... I am lucky and own a good functioning dyno system so I am able to test all of this stuff and separate the fact from the fiction !