Originally Posted by
drdsanders
I used to think pinch and compression were synonymous...they're not. I replaced 2 motors because I thought they had lost compression when in fact they had only lost pinch. I'm no longer concerned with either and just worry about performance. My V-spec just passed the 7 gallon mark and I'm still pleased with how she perfoms. She still holds a tune, doesn't flame out, and idles smooth.
Thats an engine has served you well in my book. You took care of it.
The engine makes best power, will rev into its RPM's, and will continue to combust a fresh fuel-air mixture best with minimal piston to cylinder clearance. Cylinder liner and piston surface finish conditions I find is equally important....."Fit + Finish". Half the work I receive daily "A Pinch Is Not Enough". We all know ON A NIB ENGINE, The "two" areas of physical resistance deminishes right above porting windows and at TDC and we then are in the "sweet spot"....usually with atleast 1/2 gallon , or more breakin process. I can share my opinions on alloy work hardening / stress relieving / piston to liner lapping relative a NIB engine breakin but thats out of context this tread about Engine Compression, so I won't. My point here is, I find the optimum running nitro engine cranking 40,000 RPM's 1) Gets the compression needed to make horse power well after 30% piston travel past porting windows (as is felt by hand) BUT still needs to seal well the engines total stroke for gas-air mixing 2) Will have great torque/gain rpm's quickly (engine inertia) with minimal physical resistance at point of combustion, TDC . In summary, Unless you are a racer with quite a bit experience and hundreds of engines under your belt its hard to feel that "sweet spot" testing a cold engine / piston+sleeve set in your hand.engine designs and their running charactoristis varry quite a bit, and half tenth one thousands in fit makes difference in some but not all. Watch the performance on the track / lap times / tunability.
RayAracing