Do any of you guys out there compression test to determine piston/ring wear? I’d like a way to measure wear instead of the old “20 hour” replacement program. I know the top guys just say replace often before important races, but that seems wasteful.
If you do, what kind of compression do you normally see? When do you replace?
Ray, I have been doing some compression testing . My rotax was at 150 pounds cold in Grand junction. It tests less with less air density like at my shop. I tested Norberts Pavesi after a fresh piston and ring at SBR and got a weird low reading , even tried a different gauge and still got a low reading while it ran great on track.
I am finishing up my shifter motor rebuild and will be doing a low pressure 10 pound leak down test on the bottom end just to see how the case gasket and seals perform.
I just did a couple of tests that you might be interested in.
I took a measurement at cold with an old piston/ring (about 5 races) and it measured 95 psi. I warmed it up and measured 110.
Looking at the piston is was past its prime. So I changed it out (new top end) and now I’m getting 110 cold and 120 at hot. Seems like I should be getting more pressure out of it. Do you know what this could mean? Worn out jug? The cylinder is pretty darn good… If I had a new cylinder it would be nice to try out.
That is interesting Ray, It sounds like the odd readings we got with the Pavesi.
A few things that may be influencing the readings. My rotax turned over at starter rpms with a full charged battery, should make note of that rpm. Be sure full throttle is applied during tests. Squish measurement should be recorded with the test along with air density.
I haven’t tested my new build yet. I have a starter bolt mounted inside my rear axle so that I can us an electric starter on it. I will get that reading then compare it to a reading with a few pulls of the strap around the tire. But what gear should be used when starting and testing? I use 2nd gear usually yet right now I have 14 X 28 gearing on it for ice racing which is regulating the cranking speed.