Ok most compression testing is done when the motor is in the car, but is there any other way when the motor off the car?

 

no, not really. unless you have the tranny attached and can crank the starter with 12V.

 

^He's correct. You won't get perfectly accurate results out of the car though. Variables that will skew your data would be the engine being STONE cold and cranking speed (RPM's). Those two are very important for interpretation but you can reasonably get a good idea about the condition of the motor either way.

 

thank for the input guys, i guess it really dont matter since i going to tear in to the block and do some measurement and porting.

 

You can use a differential compression tester. It tells what % of leakage you have. Summit or Jegs sells them.

 

Interesting, but I have never heard of using a leakdown tester on a rotary. What would it mean?

I use it on my racecar (piston engine), but I always need a baseline value to compare the results to.

 

They are primarily used on piston engines(in the aircraft industry) to let you know how much leakage you have past the rings and valves. We are looking for the same thing with the normal compression testers we use. Some guys here say that 110# on each rotor face with the rotary compression tester is good. We know that every combustion chamber leaks some amount. What would the rotary tester read if there was no leakage whatsoever? I don't know. The DCT is a 2 guage set-up with regulator and orifice set-up. Standard input pressure is 80 psi. Insert the threaded end of the tester in 1 spark plug hole while the other plug is left in. The 2nd guage will read lower. Say you read 80 input and 60 output. Your leakage is 25%. In piston engines, this indicates close to the allowable tolerance before new rings or valve seating is required. Rotor needs to be at TDC. I would think that similar readings and conclusions could be similar between rotary and piston engines with regards to seal condition.

 

Good explanation (especially for those not very familiar with leakdown tests), but I wonder how relevant it would be for the rotary, given the much larger seal area, and the large effect of lubrication on the amount of leakage. Leakdown %, IMO, would be MUCH greater in the rotary, and subject to much more variation, depending on engine temperature, amount and type of lubrication present, etc.

However, having said all that, it should still be a better # than what you could obtain in a compression test while cranking the engine over by hand out of the car. You would, for sure, need a baseline to compare to in order to determine the degree of health of the engine. It might, though, be quite good to find out if one rotor or one face was much worse than the others.

 

Maybe someone here has access to both types of testers. Do a test with the rotary tester on a motor with known good compression and compare results to establish baseline acceptable readings for the DCT.