Compression results
 

So I just picked up a beautiful S4 TII with about 109k on it. Everything is bone stock and in really great shape.

The engine runs fine, pulls hard, boosts well, and idles smooth and where it should be.

I am beginning to really assess her condition and ran a compression test as per Banzai racing instructions. Banzai Racing Compression Test

Results were grim 70psi in the front and close to 70 in the rear.

It seems to me that many people have slight differences in the way they do a test and I have heard that anything this low would probably not even fire up. But she runs great.

I'm concerned that I have done the test incorrectly. I used a rental tester so I couldn't remove the schrader valve. If I let the tester run it would build after cranking for a bit to the final result of ~70,
which I would understand as being an overall compression reading. If I held the valve it would not bounce at all.

Please let me know what I am doing wrong. Would it be worth trying again with a different tester? Also tried the test with a brand new battery, just to remove a variable.

[QUOTE=Originally Posted by jackhild59 View Post
rotary compression tester

Here is jackhild59's methodology for uniform compression test results on a rotary engines


Fully charge the battery. Slow rotation will yield deceptively low results.

Use the bottom spark plug hole; take both bottom spark plugs out for the duration of the test.

Remove the EGI fuse: you don't want the thing trying to try to run.

The throttle should be held to the floor or the throttle plate tied fully open. Failure to do this will yield very low results.

You will need an assistant to operate the ignition (and throttle) or a remote start switch with the throttle tied open.

Ideally the test should be done while the engine is warm. Cold is ok, but warm numbers are what should count. If you test a cold engine, be aware the what looks like an unacceptably low number may actually turn out to be quite all right. There is no hard fast number that will be the difference from cold to hot. Warm will be consistent, cold may not be. If an engine fails the test cold, it may well pass the test when warm.

Crank the engine for about 10-15 seconds max for each test.

There are two different tests to perform on a rotary engine with a piston compression tester! Both tests should be performed separately. Most people blend together the methods and smear the results, leaving a muddy view of the compression state of the engine.


Bump Test:
This verifies that the apex seals are operating properly, ie. not broken or stuck.


Remove the Schroeder valve in the BOTTOM of the compression tester (NOT the side release valve) for the 'look for three uniform bump test'.

Leave the side release valve alone for this test.

Test each chamber front then rear, make notes! Those bumps should be even and somewhere around 60-70 psi minimum.


Compression Test:
This is to test the actual compression in the engine.


Replace the Schroeder valve in the BOTTOM of the compression tester.

Retest each chamber, this time allowing the pressure to pump up to the maximum psi. Write this number down.

PSI should be above 85 psi per the FSM. Lower than this and the engine is in need of a rebuild. This engine will be prone to flood. It may however run for quite sometime, especially if it is NA.

90 psi+ should be enough to run reliably;

95 psi + is great,

100psi+ is excellent.

NOTE: All pressures will tend to be lower with longer hoses and higher with shorter hoses. Experts agree that ideally, the hose should be as short as possible; Best would be if the hose were eliminated and the unit screwed directly into the sparkplug hole essentially with the gauge as closely coupled to the compression chamber as possible.

NOTE 2: In general, Turbo II's will be lower numbers than NA, S4's will be lower than S5's.

NOTE 3: This write up is a compilation from several authors, Reted, Kevin Landers, Aaroncake and others combined, clarified and hammered into one place. The intent is to create consistent documented methodology in one place so that the newbs and accomplished alike can agree on the results.[/QUOTE]

My rule of thumb is If its not broke don't fix it. But that doesn't mean you can't start buying the parts to rebuild before it fails....

Thanks for the input. I will try again as I performed the test where the trailing plugs go.
Any idea as to how much lower a TII would read, as well as s4 compared to an s5? How would one eliminate so many variables?

I'm just trying to see whether it would be best to pull it now and rebuild or chance catastrophic failure with an occasional drive to the grocery store.

You could call a mazda dealer and see if they still have an old rotary compression tester. A coworker here has one and it was awesome to use. Worth a shot even if they charged a little for the test.

If the car is on its original engine, then for a turbo you're well into the mileage range where a rebuild is normal. But with the car running well and 70 psi, I would definitely retest. Those 2 facts seem out of sync with each other.

In my tester I can unscrew the schrader valve in the bottom. It should come out with a tire core removal tool. If you can't do that on the rental tester then try another for a retest.

As stated above, make sure the cranking speed is not low and that the accel pedal is pressed all the way to the floor.

Ran the test again, using two different testers with the schrader valve removed. Also used the leading plug holes. Looks more like it should with an average of the two giving me 95 psi in the front and approximately 80 in the rear.

Strangely, just doing a bump test gave me interesting results, tester one gave me three bounces of roughly 40psi in front and 35 in the rear, whereas tester two gave me 95 front and 80 rear.

Time to save up for a build. Question is, should I risk driving it here and there? Everything was looking so great on this FC its a sad day..

I still don't think the results are accurate, if it starts and idles well, pulls strong, and doesn't smoke then I'd say roll with it for now while you save for a build.

sounds like something is wrong with the tester.

keep in mind that precision (all bullets in a tight group but in the 9-ring) is local to the gauge and that accuracy (all bullets in a tight group in the bulls eye) is more universal.

id see if i could borrow a known, good third gauge and test it again. pedal floored, EGI fuse pulled and warm engine.

maybe if you can test on a piston engine, run both gagues on the same cylinder and see if you still get wacky results. you'll know to disregard that tester's numbers.

Thanks for the input. I'll see if I can find a fourth tester :lol:

I bought a new one at NAPA for $30

You've already bought the car. It starts easily, presumably hot starts too? Idles smooth and runs fine.

Not trying to be rude, but what's the point of a compression test at this point? The time to "really assess her condition" and run a compression test was before you bought the car. The thing to do now, IMHO, is to do regular maintenance and most importantly, enjoy the car.

Exactly!

The thing that you are doing wrong is overthinking and Over medicating yourself with the "horror stories" of a Rotary vehicle!

Sometimes I wish that someone would show the alternative for a Compression tester...on a Human Body!)

If the car runs Fine,then just maintain it and for God's sake..Go out and DRIVE THE CRAP OUT OF IT.
Enjoy the fact that your car is mobile and working!

IF it develops a problem then Post up!

Thanks guys, I wholeheartedly agree with what you're saying, just scared of having to have to replace a rotor or housing when it decided to go. Going to try water injection now to see if that changes anything.

You can speculate that at 110K your compression isn't all that great. But if it's hot starting without difficulty and has a smooth idle, it's apparently not all that bad either. WI is a great idea. Cooling benefits as well as carbon control. If you keep it at stock boost levels a simple boost-activated system is appropriate, inexpensive and reliable. With WI you could even consider pre-mixing a little 2 cycle with the gas to help with hard seal lubrication and sealing. But WI or pre-mix isn't likely to increase compression noticeably at this point. Housing and hard seal wear can't be reversed by it. Neither will worrying.