my car has been hard to start for the last 4-6 months but I've never bothered with a compression test as it is out of warranty and there is no way mazda will give me a new motor.

however after an extended de-flooding session over the last two days I finally got the car started and decided a compression test was a good idea.

numbers are very low. but in true dealer fashion they don't give you all the info.
their verdict was: even but low compression.

I got a graph of the pressure pulses but no rpm.

my front rotor is about 80psi even between the faces.
the rear rotor is 90psi on one and 79 on the other two.

from the graph and time between pulses it took around 0.95-0.99 seconds for 3 compression peaks according to the scale.

how do I convert the compression phase to rpm?

one diagram I found seemed to indicate 1 revolution of the e-shaft for every face. this ends up being about 180-188rpm which seems low for the test.

however one e-shaft rev per face seems to high because I thought the rotor was 1/3rd e-shaft speed.

can anyone set me straight?

 

Your calculations are correct. For a more precise rpm calculation, take the elapsed time for 9 compression peaks and solve for "x" in the simple ratio below where et=elapsed time and x=engine rpm at the time of the test:

9/et=x/60
et*x=9*60
x=540/et

That will refine the number a bit, but your numbers are in the ballpark.

Yes, one full rotation of a rotor equals 3 rotations of the eccentric shaft. And you are correct, 188 rpm is very low. The recorded numbers need to be normalized to 250 rpm. Use the graph in the factory service manual to do that. Seems like your battery was very weak at the time of the test. Had it been fully charged, the rpm would have been closer to the 250 rpm "Mazda standard".

Of some concern is the differential compression on the rear rotor faces, regardless of the absolute compression numbers, suggesting a possible minor problem with one apex seal. Not fatal at this point, but certainly of some concern.

FYI: If the dealer is a "Metro" dealership, they are probably using a tester system that I am very familiar with which requires them to calculate the engine cranking rpm at the time of the test just as you have done. Some techs apparently don't want to bother doing that or they are not aware of how to do it even though the procedure is spelled out in the user manual. Basically you did it for them.

 

thanks Go48,

I used your calculation and improved the calculation slightly but they ended up lower at 182 front and 185 rear.

these are off the normalisation chart in the pdf manual I have. I found a spread sheet on the forums that seemed to handle the normalisation to 250rpm.

front rotor was 693kpa and the rear rotor was 730-770. the imbalance in the rear rotor wasn't as bad as I first guesstimated.