RX8Club.com - Compression Tested

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Compression Tested - Good or Bad?

Hey Guys,
I just had my engine compression tested and Mazda said it was ok...
Looking at the numbers I'm not so sure though. The numbers were:
Rotor 1- 243 rpm, 7.1, 7.4, 7 and Rotor 2- 235 rpm, 6.8, 6, 6.
The acceptable levels say 6.8 is the minimum. Doesn't my second rotor fail? How low do the levels have to be before Mazda will replace engine? Anyone have any idea?
Any help would be appreciated

My car is an '07 RX-8 GT with 157 000 kms. It had engine replaced at 92 000 kms, so really new engine has about 65 000 kms.

Quote:

Originally Posted by CRAZ8 (Post 4618737)

I believe those numbers are actually fine. Mine were down to 2.x, etc. before I got mine replaced.

I would also check to make sure your car still falls under the 8 year, 100,000 mile warranty.

I'm having a reman engine installed right now....my numbers on the front rotor were 6.5's and the back were 7.5's. I was told 6.9 was passing. Even after the decarb and retest, the numbers didn't change.

https://www.rx8club.com/new-member-f...4/#post4533734

Quote:

I got my compression test. What do these numbers mean? (re-write)

You should have gotten numbers in a format something like this:
Rotor 1: 7.5, 7.6, 7.5
Rotor 2: 7.2, 7.3, 7.3
250 RPM

- The 6 numbers, 3 for each rotor, are your compression scores, one for each face of each rotor in the engine.
- If you don't have all 6 rotor face scores and at least 1 RPM number, your compression scores are going to be vague and hard to interpret.
- You may also have your compression scores in a different scale, such as PSI.
- The RPM is the speed at which the test was done.
- The RPM is critical to interpreting the numbers, as rotary compression changes with engine speed, especially at low RPM.
- Since the engine is spun by your starter, this is ALSO a good indication of the health of your starter!

The general guideline for how good, or bad, the scores are, WHEN NORMALIZED TO 250 RPM AT SEA LEVEL!
- 8.5 and up: Congratulations, you have a stellar engine! Compression scores this high are rare. If you think that this may be too high, there may have been excessive oil in the housing.
- 8.0-8.4: This is a very good engine! You should have very good power and as long as you stay on top of the rest of the failure points possible, it should last a long time
- 7.5-7.9: This is an acceptable engine. Most engines from Mazda seem to be in this range after the break-in period. Stay on top of the other failure points possible, and you should get at least 40-60k more out of this engine, if not more.
- 7.0-7.4: The engine has some life left in it, but start looking for replacement options. Compression loss is going to start speeding up from blow-by combustion gasses eating away at the seals.
- 6.5-6.9: Officially failing. The engine doesn't have all that long to 'live'. Compression loss is accelerating due to blow-by.
- 6.0-6.4: Failing significantly. Very prone to flooding even with new starter, battery, and ignition. It will have trouble starting when hot, power loss especially down low, and noticeable difficulty idling.
- 5.5-5.9: Failing badly. Extremely prone to flooding. Will be nearly impossible to keep it idling when hot. Significant power loss.
- 5.0-5.4: This engine is probably only able to start with a pull start, daily use is nearly impossible.
- Under 5.0: How is this engine even running!?!

Caveats:
- If the test was done incorrectly, this can skew the numbers up or down.
- If the engine has excessive oil inside the housings, the compression numbers will be reported as higher than they actually are when the engine is running
- If the test was not done at 250rpm and at sea level (neither of which is likely), the numbers you have will need to be normalized to 250rpm at sea level. Use Mazda's calculator here: Foxed.ca - Rotary Compression Calculator
- This is largely my opinion, based on seeing hundreds of compression scores on the site over the years. I am not using anything scientific to back it up. As always, your mileage may vary.

The black line is the failing line. Any 1 rotor face at a 6.9 or lower is failing.
https://www.rx8club.com/attachment.p...1&d=1384536513

So... It looks like I have 1 rotor failing. I should be eligible for engine replacement. I will be contacting Mazda.
Thanks for the info guys

I just contacted dealership. They said my readings were low, but still passable. They are checking with Mazda head to see if we can get a reman while I'm still under warranty. (Fingers crossed)

I have a 2006 rx8 it keeps turning off when I press the brakes hard and when the heats up it takes longer to start. When idling it feels like it wants to turn off! Can someone please help me? I don't know what's going on with it. I put new spark plugs wires and coils on it n it's still turning off on me.

Quote:

Originally Posted by God_of_mula (Post 4618916)

Get a compression test. The engine is failing.

No, I'm not joking.

Yea its the compression. I had some of the same symptoms.

Not to hijack the thread but if your clutch is trashed can it give a false compression test reading?

I took my car in and the stealership because it wouldn't get any power till about 7,000 rpm. Said I needed a new clutch and a new engine because one of the rotors has failed.

Well I took the car to a trusted local shop (doesn't work on rotarys) and they just replaced the clutch and basically didn't want to take the engine out because the car is running fine.

Got the car back and it's running absolutely amazing right now, and it's only at 48,000 miles I've been the only owner.

No, a bad clutch might possibly in rare circumstances reduce the cranking speed of the test, but otherwise won't affect the test. It also won't affect 'getting any power till 7000 rpm' either though.

It's more likely that you have two different problems, and replacing the clutch only solved one of them.

The local shop's judgement that "the engine is fine", is probably just opinion, and is not actually based on a proper compression test,.

RIWWP is correct here. A RX8 even with a sub-par engine can still run and "seem" fine for a long time. The proper compression numbers will tell the real story of the engine's health.

Thanks for the replies, definitely not what I wanted to hear though even though I had been expecting that answer sadly.

It may cost more in the long run but with the way the cars running I'm inclined to drive it until I really notice a problem.

Oh and I guess saying lack of power till 7,000 was slightly misleading. It started at like 4500 rpm before the car would even start to move, then by the time I took it to the shop I had to rev it to almost 8500 rpm before the car would even move. Which the new clutch did fix :). (The clutch was in terrible shape)

Thanks again for your replies!

You're inside the 8 year/160,000km warranty. Get it replaced?

[edit] sorry, there are too many people starting their own threads inside this thread, got confused. You do have the new members section where you can start whatever you want ya know.

Quote:

Originally Posted by Dave Davidson (Post 4620513)

Oh and I guess saying lack of power till 7,000 was slightly misleading. It started at like 4500 rpm before the car would even start to move, then by the time I took it to the shop I had to rev it to almost 8500 rpm before the car would even move. Which the new clutch did fix :). (The clutch was in terrible shape)

From stopped? Yes, that's a failing clutch, and has nothing to do with the engine.

I took my car to a local mechanic today and he plugged car in, he said that my apex (sp) seals are bad. The car wouldn't start after a quick run into the store after the engine was warm. I've called Mazda and they say that I need to replace the engine. I have already had one engine change - why is this happening? I have a 2004 and they replaced the engine in 2006. My husband did some looking on line and found some chatter about after the engine had been replaced that there was another 8 yrs and 100,000 on new engine. Need advice please!!

Quote:

Originally Posted by Loki (Post 4620551)

Yeah my bad I didn't want to confuse or hijack I just had a quick question, which you guys answered thanks!

I've been on here for 8 years haven't needed to start a thread luckily. If I do I'll keep in mind the new members section, thanks and sorry to confuse ya.

Mazda has stepped up and my new engine is on order. I will have my new engine (3rd one) installed next week. I had all my records of servicing and no questions were asked!

Quote:

Originally Posted by cmahoney24 (Post 4620601)

The 8yr 100k doesn't reset with an engine replacement, it is always referring to the original purchase date of the car and the car's odometer mileage.

The RX-8's engine isn't a strong engine, and it wears out easily, even more so if it isn't cared for properly. There are many different reasons for engine failure though, so since I don't know anything about your driving habits, maintenance, or the car's history to make a guess at why the 2nd engine might be failing.

"A mechanic says the apex seals are bad" is a terrible diagnosis method though. If he doesn't have a rotary specific compression tester (he doesn't, not even all Mazda dealers have them), then he is just making wild guesses based on what he knows, and when it comes to a rotary, that is more likely to be faulty knowledge than good knowledge. You will need a proper compression test at a dealer to confirm a failing engine.

That being said, harder to start when it hot than when it's cold is typically a low compression problem.