I heard from mazda that the minimum compression spec of 6 kg/cm^2, which equals 85 psi is that true? So to say that 85 psi on all three faces is bad?

 

It's not horrible, but not great. The main thing is that the compression is consistant across the board.

 

Just for a reference, my brother just bought a '93 FD , with 26K miles and all the compression numbers were like 130psi. ( he's a lucky bastard for gettting that car for only $16,300.)

 

Not to hijack the mans thread, but what's a healthy or a great compression of a rotary motor ?

 

130 is near perfect...90psi is 6:1 compression, for reference

 

130 psi = 8.84:1

6:1 compression = 88.20 psi

stock FD rotors are 8.5:1 right? So that would be 124.95 psi.

Of course most compression testers aren't accurate to the .01.....but just to keep things accurate

 

mine was 90lbs. on both side on both rotors

as long as it is consistant than you should be okay

 

The reason I asked is b/c I'm gonna buy on of those compression testers.

 

interesting

 

Stock FD rotors are 9.0:1. Nominal (new) compression is 120psi, minimum is 85psi. Although compression ratio is related to your absolute compression readings, they are not equivalent. That is to say, if atmospheric is 14.7psi and you have a 9.0:1 compression ratio, you will not achieve 9x14.7psi = 132psi on your compression meter. That is simply the maximum possible static compression. Since the test is dynamic compression, plus in the real world no engine seals perfectly, you are always going to see something lower than that. Another interesting thing to note is testing the car cold will result in higher numbers than testing it hot (this has been demonstrated by a list member and was also what Mazdatrix told me). Testing hot is the proper procedure.

 

Cool, I didn't think of including real world limiting factors. My numbers were just rough theory, perfect atmospheric conditions, etc etc. I thought and posted too quickly. Thanks for correcting me.

So fully warmed up 85-120 psi is within spec. What is acceptable as far as difference in compression from face to face? 10psi? 20psi?

 

The '93 FSM allows a 21psi differential.