XanderCage

iTrader: (1)

Hi y’all,

Maybe my search skills aren’t what they used to be, but I’m having a tough time finding an answer to my specific question-

What is a healthy PSI number (or at least the minimum limit) for a compression test on a stock port REW engine with 3mm seals?

From what I’ve read on the forums, I understand that the 3mm’s have somewhat lower compression than your typical 2mm, but I haven’t exactly found how that should translate to numbers on a basic compression test, if it affects that at all.

For reference, just tested my engine last night.
-Hot engine
-Front and rear leading plugs removed
-Throttle floored
-Healthy battery

All faces of each rotor tested around 80~83 PSI using the piston tester with schrader valve removed. Unfortunately, RPM is sort of a question mark although normal cranking speed as indicated with PFC is around 230.

ItalynStylion

iTrader: (1)

I'm interested to know the answer to this. I'd have thought the compression would be slightly higher due to the seal being thicker and taking up slightly more combustion chamber space. Hopefully some Sage chimes in here with the actual answers.

j9fd3s

iTrader: (3)

see pic, basically the minimum limit is 6 kg/cm at 250rpm, adjusted for altitude, from the 1985 shop manual

XanderCage

iTrader: (1)

Interesting. Hadn't thought of looking at the '85 manual, 'course that's also referring to the 12A engine, correct? Any reason to believe that the same size seals in the 13B would yield different ideal tolerances? Maybe I'm overthinking this?

XanderCage

iTrader: (1)

Whoops, should have read more closely! "The standard values are the same for the 13B and 12A engines"

DaleClark

iTrader: (55)

The 84 and 85 GSL-SE was a fuel injected 13b with 3mm apex seals.

The rule of thumb is 3mm apex seals don't seal as well at lower RPM. How much is hard to say, of course a well-built motor with good rotor housings and well-fit side seals will have better compression than used housings and poorly fit side seals, for example.

3mm apex seals were a band-aid for years against blowing motors. Back when "tuning" a car meant turning up fuel pressure, adding injectors in a throttle body elbow, and getting a chipped ECU and hoping it did OK. Now with modern apex seals and modern EFI systems (even the PowerFC) you aren't seeing the tons of blown motors from back in the day.

All that said, the process to mill a rotor for 3mm apex seals is very tricky. It's easy to mess up. I've even heard of a motor put together by a reputable engine builder who milled the rotors properly for 3mm seals and it burned oil. Reason was the milled slots just BARELY got into the oil gallery in the rotor due to the nature of that particular casting.

Anyhow, to go back to OP's question, you may want to look into getting one of the new rotary compression testers or finding someone who has one. They are super easy to use and give good readings. The piston style testers are really only good to tell you that there isn't a compromised seal (even pulses) and the general health of the engine. If you are wanting to really know the exact health and compression a rotary tester is the only way - it will correct for cranking speed, give exact numbers for each rotor face, etc.

Also, the compression could be affected by other things in how the motor was built and the health of the motor. The compression numbers could possibly have nothing to do with the 3mm seals.

Dale

j9fd3s

iTrader: (3)

i recently went back and re-read some old threads, and 3mm seals were popular for a while, but results were very mixed at best.

BLUE TII

iTrader: (9)

Piston compression testers with the Schraeder valve removed just give you an idea of whether the engine is blown up and you cannot judge an engines health with it.

This is because they do not have engine cranking rpm compensation, peak pressure recall function and you have to watch the needle bounce and judge the psi.

80s psi was normal for me, 60s was blown side seal, 30 was blown apex seal 0 was missing apex seal and stuck side seals.

I never noticed a difference on my 3mm seal engines vs 2mm seal engines using my piston engine ompression gauge, but the 2mm engines did feel a tad better off idle when actually driving.

If the motor hot restarts no problem then its probably in good shape.

I actually prefer Mazda OEM 3mm seals for my built engine as they were strong enough on my S5 FC that it would break front or rear irons before the apex seals with detonation, but they didnt destroy the rotor housings like the 3 kinds of aftermarket 2mm seals I tried.