sbest
Active Member
Sweet Jeff.
I started as a machinist, rebuilt engines as a hobby, then switched over as an industrial mechanic, electrician, then electronics and programming, troubleshooting in a factory. Not as specific experience as you so nice to have you here working on this stuff and sharing.
The beauty of these engines is the head comes off and on in 5 min so experimenting is easy. Previous experience show just about any engine can run at at 0.010" with no immediate damage but typically lose power at that small a gap. I did not progressively try different gaps on the China Girl engine, but why not you give it a try? According to Gordon (and others) the sweet spot should be somewhere between 0.010" and 0.040". Easier to take metal off than to add it so maybe try shaving the head 0.010"or less at a time from 0.040"? When you feel it loses power throw a paper gasket under the base to bring the squish gap back up to where the power was. The ports will hardly notice a 0.006" rise. Like the RC engines Jeff, these are wonderful to play with.
As for damage, I was amazed that even with the noise made at 0.010" there was no metal contact on the 200cc engine. The engine heated up noticeably and produced ( I'd guess 10%-20%) less hp. I suspect there was sudden piston deceleration at tdc that would eventually result in the bearing or piston failure. I'd also guess that the rings are taking a pounding too. So after that I always erred on the top end of the tolerance. In other words if I felt no power difference between 30-40 thou, I went to 40.
I do the same with compression too. All these things give diminished returns. For racing you want everything you can get so you may run it to max, but for longevity I run it to the safe side where I cannot feel any difference. As the cc's decrease closer to detonation, less and less power gain is felt. So if I have detonation at 16cc, I'd probably be safe at 17cc, but I really didn't feel any power gain from 18.5cc so that is where I left it. The problem with heat is that it causes a rapidly cascading failure. Run forever while everything is right but when something fails it rapidly cascades into meltdown. I love these engines, I'm only out $30 for another top end!
So Jeff, if you don't experiment with squish gap, when the snow melts, I probably will.
Steve
I started as a machinist, rebuilt engines as a hobby, then switched over as an industrial mechanic, electrician, then electronics and programming, troubleshooting in a factory. Not as specific experience as you so nice to have you here working on this stuff and sharing.
The beauty of these engines is the head comes off and on in 5 min so experimenting is easy. Previous experience show just about any engine can run at at 0.010" with no immediate damage but typically lose power at that small a gap. I did not progressively try different gaps on the China Girl engine, but why not you give it a try? According to Gordon (and others) the sweet spot should be somewhere between 0.010" and 0.040". Easier to take metal off than to add it so maybe try shaving the head 0.010"or less at a time from 0.040"? When you feel it loses power throw a paper gasket under the base to bring the squish gap back up to where the power was. The ports will hardly notice a 0.006" rise. Like the RC engines Jeff, these are wonderful to play with.
As for damage, I was amazed that even with the noise made at 0.010" there was no metal contact on the 200cc engine. The engine heated up noticeably and produced ( I'd guess 10%-20%) less hp. I suspect there was sudden piston deceleration at tdc that would eventually result in the bearing or piston failure. I'd also guess that the rings are taking a pounding too. So after that I always erred on the top end of the tolerance. In other words if I felt no power difference between 30-40 thou, I went to 40.
I do the same with compression too. All these things give diminished returns. For racing you want everything you can get so you may run it to max, but for longevity I run it to the safe side where I cannot feel any difference. As the cc's decrease closer to detonation, less and less power gain is felt. So if I have detonation at 16cc, I'd probably be safe at 17cc, but I really didn't feel any power gain from 18.5cc so that is where I left it. The problem with heat is that it causes a rapidly cascading failure. Run forever while everything is right but when something fails it rapidly cascades into meltdown. I love these engines, I'm only out $30 for another top end!
So Jeff, if you don't experiment with squish gap, when the snow melts, I probably will.
Steve
