Well my one of my friend in L.A. just bought a Single turbo FD. The car has a newly built engine with 2mm Rotary Aviation Seals. The motor has approx. around 3,000-5,000 miles on the engine. Anyways we did a compression check and the front rotor was like 100, 100, 100... And the rear rotor was like 80, 80, 80.... or 75, 75, 75... We were using a piston compression with that schrader valve out.

We did the compression twice to even confirm it. HOWever the motor runs PERFECT..? Vacuum is at 18hg to 20hg. Idle is at 800 to 700. Does not overheat. And the car boost SUPER STRONG at 14psi. And the car does not puff a single smoke... Even on cold start up. VERY CONFUSING... Has anybody experience this before?

Thank You very much.

 

So nobody have had experience with this before?

I'm guessing from reading and hearing a lot from Rotary Aviation seals.. It may just be because the seals sucks. Anyways we were wandering it wouldnt' hurt to just drive the car regardless of the compression, at least it's consistent correct?

 

I could only guess, but i'd say the motor was built with a rear rotor housing that was not in very good shape. But it could be lots of things especially sense that is not a accurate way to check the compression. It hard to see variations between rotor faces. Will it hurt the engine to drive it? Your only alternative is to rebuild, so if it runs good, drive it.

 

I know the Piston compression is not very accurate, but the engine is warmed all the way up. And using the Piston compression tester in this case is pretty close to accurate if you don't have a Mazda compression tester. I'm pretty sure the Mazda Compression tester results would be very similar.

 

i check compression w the same setup... a regular compression gauge w the valve removed. i video tape the gauge and replay it thru a vcr at very slow speed. i am able to note all power impulses so i can distinguish between an apex seal or a sideseal problems.

apex seal problem creates 2 sequential lower impulses

sideseal creates one

as to your situation... the rear rotor compression degrades first. i don't know how you were able to observe all the impulses but if you did you most likely have a weak rotor housing or rear rotor end or midplate. apex seals would be below the aforementioned on the probability list. (Gadd is most likely correct)

rotaries can make good power w lowish compression as the apex seal spins out and tends to seal at higher power levels.

in order to more accurately diagnose your situation i suggest you do the video camera compression test and repost your best accurate numbers for all impulses and we may be able to be of further help. (make sure your throttle plate is fully open for front and rear rotor tests and the motor spins at the same rpm for front and rear.)

good luck,

howard coleman

 

If it runs fine and starts fine then I wouldnt worry. Mines just rebuilt with 20miles on the engine but doesnt start off the key yet (nearly does) bump start it and it runs perfect.

But I have a aggresive half bridge ported engine and aggresive primary ports with solid corner seals and RA 3mm apex's and new housings, so I kind of expected it to be like this.
Once mine starts off the key I'll be happy.

 

I had somebody in the car crank the motor and full throttle while I hold onto the compression tester and watch the pulse. That's how I did it. The video won't do any good, as either way the rear rotor is still 20 psi lesser than the front rotor. But it does make sense that a worn rotor housing was reuse?

Anyways do you guys think theres a trick into probably bumping the rear rotor compression up.? I was thinking of the Marvel Mystery and ATF trick? Injecting it to the rear rotor. Crank it a couple of times and see if the compression comes up.?



Again thanks for the comment, but like I say, the video taping is not going to do any good. AS we had already seen the number and no matter what it is, the rear rotor is still 20 psi lower than the front rotor. BUT again regardless of the difference, both rotor has Complete consistent numbers.

80, 80, 80 is still a good number in my opinion. So I'm guessing that's why his motor sounds so nice and boost super strong... It's just the difference in the compression number that worries us. Anyways I'm leaning towards the blame on RA seals. I guess they are too hard, and prob. already sratched and worn the housing down by 2000 miles... We'll check the compression again when the motor has another 2,000 miles.

 

doesn't mazda give a 25psi tolerance between rotors??? not that I'd be satisfied, but I'm sure the motor will last a very long time.

 

Oh okay, that's something new.

Anyways I'm also pretty sure the motor will last a very long time. His car starts right up, no smoke at all. And boost super strong. The car was tune at 10:1. Idles very good. 800-700 rpm. And vacuum is always at 18hg to 20hg. And I think that is probably the best vacuum you will get.

 

howard coleman is right. you are testing for total compression without venting the guage while cranking. there is no other way to get up to 80/100 psi. venting the guage from it's valve while cranking can show differences between compression faces. you won't get a total compression number, but the needle sweeps can diagnose a problem more accurately.

smoking has little to do with compression. the seals responsible for compression are not responsible for keeping oil out of the combustion area. the oil control rings and thier respective o-rings do this. generally, dark smoke is a symptom of worn oil rings. white smoke is coolant seal failure.

snake oil and atf will not help you. if you want better compression, rebuild the engine with better housings. the only thing you can do is premix (with real 2 cycle oil) to help stem further wear of the rotor housings and seals.

 

Well whether it'll be accurate or not, it's still yield a 20 PSI difference between the two rotors. I had him call up Mazda to confirm whether it's good or not, and they say the difference in overall compression between the two rotors should be no more than 20psi MAX...

So I guess he be alright.