Quote: I believe that lower crankcase compression would resault in lower cylinder compression pressure, Is that right?
With a good modern pipe design and porting, I'd say the reverse is true; a larger crankcase (i.e.; lower primary compression) = more fuel/air mixture available for the ports and pipe to pull up into the cylinder = possibly better scavenging / cylinder filling = more cylinder compression, and potentially more power. I think nowadays the primary (crankcase) compression mainly affects the initial puff of mixture when the transfers open, then the pipe and porting do the 'heavy lifting' and pull the remainder into the cylinder as best they can.
With vintage and early EVO porting and pipe designs, I think the crankcase pressure is more important, as scavenging and filling are more dependant on the pumping-in effect of the crankcase.
In the course of several hundred simulations on my MR175 motor using the engmod2t program, the one variable I really couldn't change - crankcase volume / primary compression - seemed to be the most effective at increasing power potential. The MR175 is basically a 250 piston on a 125 bottom end, so the crankcase is tiny, the primary compression way high (1.6:1), and power potential is limited because of it, regardless of how good the porting and pipe may be.
I always thought reed block spacers were intended to tune the intake-tract resonance lower by increasing its length, but the spacer would also increase crankcase volume which would surely have an effect as well.
Intake tuning can be very effective at increasing power in specific rpm ranges. For what it's worth, the MR175's unusual intake tract has a very noticeable effect on the very low end (from idle to maybe 3,000 rpm) especially the air boot from the carb to the airbox (!). After seeing the simulator results I said "that can't possibly be right!" and pulled off the air boot; goodbye bottom end.
Ray
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