rx8-rus

Warranty will soon expire for my car (56000 miles already), so I decided to do a compression check, to make sure that I do not need a new engine soon. Results are:

Rotor 1 : 7.7, 7.7, 7.6 at 314 RPM
Rotor 2 : 7.6, 7.5, 7.6 at 302 RPM

Comparing with expected numbers from service manual, compression seems to be low. It specified to be 6.8 - 8.3 at 250 RPM. But at more than 300 RPM it should be well over 9. I haven't talk with service adviser about results yet. Can somebody share their measurements? I need to decide if I should press Mazda to replace engine under warranty.
Right know car runs well, but I do not want to pay out of pocket for engine overhaul in the near future.

rx8-rus

I hope it was done at proper temperature. It was done by a dealer, I asked them to do it in addition to regular maintenance. I picked car late in the evening, so I didn't have a chance to talk about numbers. Technician wrote that numbers are within specs. I would agree if they measure it at 250 RPM. But at 300 RPM results should be higher.

nycgps

iTrader: (1)

do not accept the results.

Take it back, say do it at 250 rpm.

Cuz your numbers seems low (if its really at over 300rpm)

nycgps

iTrader: (1)

I know, but right now its hard to tell that if his motor is still within the specs.

nycgps

iTrader: (1)

I didnt look at the service manual :P

maybe he should file a report with MNAO first ? Just to be safe ?

Brettus

iTrader: (2)

I could be very wrong here but just throwing this in as a thought :
The fact that all your numbers are very close may suggest a healthy engine that is perhaps a bit carboned up .
Have you tried any of the decarbonising products available ?
Some hard driving at the track could also help

nycgps

iTrader: (1)

I think some Rx7 people put water into the vacuum port to clean it up.

I forgot what was it call tho, it claims to be able to clean most of the carbon up, and its almost free.

Brettus

iTrader: (2)

don't know - that guy Jax is always recommending BK44 (or something) but I've never tried it myself .

I'm just thinking that in my (limited) experience - when you have a worn out engine you normally don't get even readings. Thats's what I remember for piston engines anyway.

RoXanneBlack8

some aftermarket rotary compression checks will run the test at whatev rpm the starter spins and u can push a button and ask it to recalculate it for 250 rpms.

04-05 cars didnt have a mop problem, it was 04-06s that had mop problems! hahaha they injected too little oil at idle with the AC on and the oil on the autos would vaporize b4 even entering the rotor housings bc the owner was forced to idle in traffic for hours on end in the 100 degree weather and they had only 1 oil cooler to rely on


mid 7s are fine.....anything at 6.9 or lower and ur in trouble.....

change the oil, put in some 93 octane and go redline the little ***** and ull be set

nycgps

iTrader: (1)

I will find some topic about it . wait

Or you can ask RG, he should know better.

nycgps

iTrader: (1)

http://www.rx7club.com/showthread.ph...steam+cleaning

Something like this. I will try it one day.

swoope

iTrader: (5)

charles is correct. that reading is low.. very low at that rpm...

documentation is you friend..

as to the 44 k idea.. the stuff works great the problem is the over 7200 k ports need to be open to clean them..

means run the 44k at a track day. or i dont know drive a tank of gas in 4th gear on a trip...

or we have to find a fogger that goes into the upper ports..

beers

carbroke

Hey I had compression test at neptunespeed this week saturday. Looks like I'm having same reading as you. My car had only 27k on odometer. The average compression reading on my car was 7.4 on 2rotor. shop owner Yoshiya checked my car and he said carbon build up inside of the rotor housing. Also when he took out spark plug. The life left on spark plug was expact to be 5000mile So I just decide to replace with brand new NGK. After the inspection and repair is done He gave me suggestion that when I'm running on the highway put the gear at 4th and keep the RPM above 5~6 about 2~3minute will be make carborn burn out. Is there Anyone has other result? or solution? to bring compression back?

Oh!~ one more thing. when yoshiya taking out the spark plug he was struggle with the losen the spark pulg out. That moment I realized the reason. It was when I took my car to Long Beach mazda dealer last summer for emssion recall.. and probably one of newbby mechanic tighten the spark plug too much. Result become grind the 2nd rotor housing spark plug hole.. ****..

DO NOT TRUST DEALER

Galen Darkmoon

Looks very good for a car with 55k on it, ya want new numbers? buy a new car. Seems like ya have taken very good care of this one.

rx8-rus

I understand that number has to be lower than for a new one. But I expect engine to last at least 100K miles. And though now I can not demonstrate power loss and dying at idle, it all can change in July-August when the next heat wave hits here. And warranty will likely be over at that time.

8is>enuff

I went through the sme thing a couple years ago. Here were my numbers - https://www.rx8club.com/showpost.php...4&postcount=36

I wrote detailed letters to MNAO and the dealership. A year and a half later there has been no resolution on my end. I hope you fare better.

One of the benefits of the upgraded starter (from Mazda's standpoint at least) is higher cranking RPMs and the accompanying higher compression readings. It skews the readings of questionable engines, bumping them up into the "acceptable" range. The fact that Mazda fails to acknowledge the upgraded starter's influence on compression readings is in keeping in line with Mazda's customer service policy - Deny any and every problem as long as possible in the hopes that the customer tires of fighting.

savedsol

Anyone tried a flushing oil to clear the gunk? I read about it here...
http://www.carbibles.com/engineoil_bible.html#flushing
I've heard a lot of great reviews about Seafoam in piston engines, just curious if we'd get equall results...
http://www.seafoamsales.com/motorTuneUpTechGas.htm

rx8-rus

So did your engine eventually fail? I do not mind to have "low" measurements as long as it does not affects engine performance to the point when it needs an overhaul.

8is>enuff

No, my engine hasn't failed yet. On the upside I don't have to use premium gas anymore.

Good luck pressing the issue with Mazda if you're so inclined. They didn't seem to give a **** about my numbers or accompanying complaint and explanation. Their response was simply "The numbers are within spec."

MadDog

Just wondering how the effects of the new starter and the accompanying higher cranking RPM could be ignored by Mazda if the specs for compression are given as a function of cranking RPM... Help me understand.

rx8-rus

If engine didn't fail completely, then Mazda may be right and measurements are acceptable. I will probably rise concern to the dealer, just to have a paper trail of complaints.

Can anybody else share their story?

r0tor

iTrader: (1)

Firstly, I would not say the *nearly* linear relationship between compression and rpm should be extrapolated off of the graph. The graph at 300 rpm shows a perfect engine is at 9.5 kgf/sqcm. Perfect compression at idle speed of 900 rpm (where the seals are acted on with more centrifugal force and the MOP is in operation) is 10.0 kgf/sqcm (otherwise known as a 10:1 compression ratio). Clearly the linear relationship has to come to an end between 300 and 900 rpms.

Secondly, using the graph and converting the minimum pressure of 6.8 kgf/sqcm at 250 rpms to 300rpms would give you around 7.8 kgf/sqcm which is in the ballpark of the original poster.

Lower compression at starter motor speed is not necessarily an indication of the compression at idle speed and above where you have the centrifugal force and MOP oil creating the pressure seal (along with intense heat shrinking tolerances) instead of basically using the spring tension of the rotor seals. If you are experiencing no problems while driving, the engine is probably fine.

The problem you run into with low compression at the starter speed is the engine not cranking over to begin with. If you are not having that problem then you are probably fine.

8is>enuff

MadDog, I wondered that too.

rOter - I haven't checked the charts out in a bit, but I was unaware a perfect engine would be at 10:1 at 900 RPMs.

Compression at starter motor speed is the only reading we are able to measure (to the best of my knowledge). Also, All engines, including the hypothetical perfect engine, would experience the mentioned centrifugal forces and MOP oil seal at increased RPMs, so that should already be accounted for as it pertains to the targeted compression numbers. Therefore the question remains - why does Mazda have a sliding scale of acceptable readings if a customer's compression results are not read as such?

rx8 rus - I would definietly document it with MNAO. It can't hurt.

rx8-rus

Talking about compression specs we always refer to the original 2004 service manual. Have somebody seen updated manual for 2007 model? They come with the new starter from factory, and thus all have higher cranking speed. Manual is probably updated with the new data to reflect this.

r0tor

iTrader: (1)

I'm looking at the graph now... at 250rpms its at 8.7 and 300 rpms its at 9.7 kgf/sq cm. Therfore, the slop of the theoretical perfect line is 1 kg-f/sq cm per 50 rpm... other wise known as .02 KGf/sq cm per rpm

The line ends at 9.7 kgf/sq cm and 300 rpm. The line with the above slope would hit 10.0 kgf/sq cm at 315 rpm. It could never reach more then 10.0 beacause the engines compression ratio is 10:1. Atmospheric pressure is 1 kgf/sq cm and working with the ideal gas law you have P2 = (v1/v2)*P1 or the compression ratio*P1 or 10.

For me to buy that the engine should hit full compression at 315roms, I have to ignore that the effect of the MOP on compression and cetrifugal forces acting on the seals (heat decreasing tolerances of the sealing surfaces can be ignored since the engine should be up to temp when doing this - however this is a huge variable as to what the technician considers a warmed up engine!!!). I'm not exactly willing to buy that. Nor am I exactly willing to buy the relationship is perfectly linear up until it hits the 10 mark and then the line goes perfectly horizontal.


Then when thats all said and done, none of that matters because mazda give you no clue on what the minimum compression curve looks like at all - only a single point. The could say that all you need is 6.9 kgf/sq cm at any rpm because that may very well be the needed compression to light the engine off and thats leaves the MOP and higher speeds take care of the rest of the pressure.... and they'd probably have a point that could stand up to alot of scrutiny. The orginal posters compression is well above this number.


This is not as cut and dry as some people are seemingly convinced it is.. Its only 1 data point and needs to becombined with other things to make any sense of.