RR's compression test information states that if you use a standard piston compression tester you should get between 30-35psi per face. I'm seeing about 60psi per face, so either I'm still doing something wrong (and i followed his instructions to the letter), my tester doesn't work, or he has a typo.

Any suggestions?

 

lol, I did 90 front even and somewhere btwn 85 and 90 even on the rear on my TII motor, and 90 even rear and 70 even front ( I think it was because the 6PI actuator was stuck open...) on the s5 n/a motor.

this is using the piston compression tester.

 

Yeah, i hear it's suppose to be closer to 90, bt i'm seeing 60-70psi. Would the 6PI actuator make that much difference? Maybe mine is stuck open as well...

 

Just make sure the bounces are even. After that, do the test where you let compression build up and see what your max compression number is. If the bounces are even, and your built up comp number is good, chances are you're fine.

 

I got around 90 psi every "face" before the rebuild with a "regular" compression tester, and 110+ now with the same tester, which sounds eerily like the ranges of pressures given in the FSM. Therefore I have to conclude that the "regular" piston testers do a pretty damn good job of checking our rotaries...As long as your eyeballs are good enough to pick up the needle movements and "high" points, lol...

 

Every piston tester I've used on a 2 different rotaries hasn't been able to build up that high of compression between faces before it "bounces" for the next face. I never understood how everyone could get numbers like that. That's why I always just looked for even bounces on the faces and then what the max compression that built up was.

 

The bumps are even, no doubt about that. I have not done a max compression test yet, but i will and get back to you one it.

 

Yah that sounds good, all the test I've done on both rotaries was about what Kevin says on his site. Obviously other people seem to get varying results. I'm sure it has to do with the different types/quality of testers that everyone uses, but oh well.

 

Even bounces are key, 30 per face is great, if you missing some seals. Mazda says a min. of 85psi at cranking speed.

 

I've always used just craftsman level compression guages with the check valve removed and have never had probelms distinguishing between a good running rotary and bad one. Which plugs do you remove?

-Marc

 

The actuator would make no difference.

-marc

 

Well the FSM says to remove the trailing (top) ones I believe, but everyone else says to remove the leading (lower) ones. I've done tests on both and gotten the same exact compression, so I really don't think it matters.

 

I did the same with the same results.

 

Make sure the battery is fullycharged, and your at Wide Open Throttle while cranking away.

 

Yep, always do that, doesnt change the bounces.

 

if there is no change between throttle closed vs WOT during the compression test, something is probably not right with the test.

throttle closed == less than atmospheric before any compression occurs
throttle open == ~atmospheric before any compression occurs

your numbers should be noticably lower with the throttle closed than with it open.

also, you really have to KNOW the RPM for the compression test to be accurate, I've had cars range from 200RPM to 300RPM @ cranking, battery and starter condition play a huge role. If you look in the FSM there is a correction table for you to plot the RPM and also a correction for the barometric pressure, both of these affect the test enough to care.

I usually have a guy crank the engine for 15 seconds, and i'll have a watch w/second hand and tell him when to stop cranking. I'll count the spikes during the cranking over the 15 second duration, multiply spikes by 4 and you have your test RPM.

beware though, extended cranking periods can heat up the starter, make sure the rotor you're not testing has it's plug out too, will help get the RPM up.

 

I also live well above see level (little over a mile) in Colorado so i know that has something to do with it. According to the FSM at my elevation 60psi should be normal...i think....

 

I said on the bounces.... On every test I've ever done with every piston tester on TWO different rotary engines the bounces have never gotten over 30-40ish because the bounces occur so quickly it cant build up higher compression before the next face comes around. On full compression when I let compression build up with the relief valve closed (not venting) yes I do it at WOT and it is noticeably higher than with the throttle closed.


Also, I don't thinking cranking for that long is safe, lots of people have said cranking for that long can fry the starter from being overheated. And I don't see why having the plug out of the other housing would make a difference in compression for the housing I'm testing, but maybe I'm wrong.

 

I think the plug in the other rotor is left out so the engine will crank at a higher rpm and give a better reading. Without having to crank against two compressing rotors, the starter should be able to crank a little faster. So indirectly it should give a higher pressure by cranking a little faster.

Scott