hey guys, could anyone tell me whats the ideal compression on a 3rd gen rx7 and if matters a lot if the front and rear compression is different. thanks a lot guys.

 

anything over 7kg/cm^2 or 100 psi is good (under standard conditions of 0 elevation and 250 cranking rpm)

More important is that all six rotor faces have consistent compression. Check the service manual for the exact specs. Also, most motors fail by coolant seals, not apex seals. A compression check is good info but does not mean the engine is 100% ok.

 

Perfect would be around 9.0 and I believe Mazda says less than 6.0 is out of spec. No more than .3 variance between faces on the same rotor and no more than .5 between the 2 rotors.

 

Is there any way to check the coolant seals?

Mike

 

Perfect is 8.5kg/cm^2 or 120psi, as per the shop manual. Everyone thinks "9.0" (I assume 9.0kg/cm^2) is perfect for some reason but this is not correct. The car has a 9.0:1 compression ratio but that is not the same thing. I agree on the importance of the consistancy between faces as a better indicator, as the overall numbers have been proven to vary by quite a bit.

 

I always heard 90psi or above is pretty much perfect... I checked compression after getting a street ported engine from pettit with the 3mm apex seals and it ran 90-90-90 and 90-89-90... I dunno...

 

The number's I'm quoting are out of the shop manual so they are correct, but don't get too hung up on them. Your numbers sound plenty good, imho it is more important for the numbers to be consistant across the 6 faces than to be high, because a lot of factors can affect the absolute numbers. Also, 3mm seals don't seal as well at low rpms because they are heavier and wider so that will yield lower numbers. Lastly, you may see compression go up a bit as the engine breaks in, I'd get another check in 5000 miles and see if it goes up. A common misconception is that a street port will lower compression, this is not true. It will lower your vacuum at idle but during a compression test, peak compression occurs when both ports are closed so it shouldn't matter. I'll have to check up on this but I'm pretty sure about it.