what are good numbers for a motor i want to run well if i just drop it in? this is a TII motor by the way. the guy i talked to said anything above 40psi on a cold motor is good. and this motor COLD had 80 or 85 psi. keep in mind im not going to be rebuilding it, and it cost's $750.

 

Anything over 90 is good.
70 to 80 is average.
Below 70, I would budget a rebuild soon.

In our experience, doing a compression test when cold is actually higher than hot.
I'm not talking freezing cold...down to about 60F.


-Ted

 

I think the numbers are actually similar to the "warm" numbers, because I've started to track mine with a cold engine for future comparisons, & got 110+ cold (5,000 miles on the rebuild).

If you think about it, what's the difference between a cold & warm engine anyway, as far as the apex & side seals are concerned, which provide the sealing properties that create compression in the first place. Sure, the rotor housing will expand with heat, but so will the rotors, which means the spring pressures under the seals are comparable.

And, if using OMP OR premix, a lubricating film should remain on the rotor housing walls for quite some time after shutdown, so the relative amounts would be the same whether cold or warm...

 

IN my experience with many engines and the mazda digital tester, you'll get 5-15psi HIGHER numbers when COLD, which is why you're supposed to do the test when warm, to get the "worst case scenario" as a true representation of the engine's health...after all, you spend 99% of your time with the engine warm.

This is (in my theory) why many rotaries have problems starting when warm, but not when cold. You have less compression when warm.