Originally Posted by
wingracer
The problem with the compression testers is that I have yet to see one that will work in a turbo plug motor.
Plus, it's not really needed. When these things have good compression, they run great. The moment something happens, they become a nightmare to tune. They will flame out, not want to idle and the tune will change throughout a run. When that happens, time for a new motor or a rebuild.
+100%.
Plus, those testers really do not test compression, they test blow by which is a totally different thing, compression is the rate between the total BDC and TDC chamber volumes. You can really have an engine with all the blow by in the world, super soft with 20 gallons and not have idle, flameou and tune problems if the piston and sleeve fit(more like "shape" this one) is still a good one.
Right know I have an engine with about 8 gallons and it has absolutely no pinch at all, when hot you can turn it with your pincky if you want, but it still makes a perfect tune from top to bottom and has all the power it ever had.
Remember that for blow by to affect compression it needs to be very bad and by that I mean VERY, like having a piston with a different shape than the sleeve (which technically speaking IS what happens) but if there are no abnormal things like that just think of this:
If you turned your fly wheel when the motor is hot and it is soft but you can still feel the cycle, you can still feel the stroke with your flywheel spin at what 15 RPMS? how fast can you really go and at that rpm you CAN actually feel the pulse of the cycle.
well if at 15 rpm, and hot the motor gets to the point where blow by is not enough for you to NOT feel the compression stroke, then divide 15/idle speed lets say: 15/3500 = 0.43%, thats what you loose, AT IDLE!!! if you do it 30,000 rpm its virtually nothing.
xe