I have a rebuilt 91 NA engine with less then 3k. compression reads with mazda factory tester front rotor 6.9 average and rear 8.7 average. Are these good results for a rebuild with less than 5k on the engine?

 

What do you mean "average"? The Mazda factory compression tester will give three results per rotor. What are the actual numbers? 6.9 is a bit low. 8.7 is a lot more reasonable. Still, you must factor in cranking RPM and whether one chamber has more or less lube (artificial compression effect)...also if new housings were used. By 5K, presuming proper break-in, one would expect higher and more consistent numbers, but there is a lot to factor in to get good results so don't jump to conclusions. Even with good equipment, the results are only as good as the operator.

 

the engine has to be warm to get proper readings... i know you know this cause you posted "on cold engine". i would not rely on cold readings...

 

strangest thing.

we had an rx8 in the shop, it came in flooded. we start it, its running on 1 rotor, no power, doesnt rev much past 5000, etc.

compression was good in the front, 9's in the rear it was 5.5 6 9.9

engines bad right? we call mazda, they say to put coils in it. we do, car starts right up runs fine

retest compression

f 9 9 9
r 8.8 9 9.2

somehow the coils fixed the engine.... wouldnt have believed it, if i didnt see it. id say it was the tester too, if the rear was evenly down, but it wasnt...

 

^ thats really weird! coils have nothing to do with compression... can you call them up and ask how on gods green earth it effects compression and how the hell they knew that?

 

if the rotor was evenly down, i could totally see compression being down, but it wasnt...