I have an A12. I pulled both top plugs and measured the compression. They both measured at 90 PSI. That seems low to me. What do you guys think?

 

In the Factory Service Manual for 85RX7, the specified compression ratio for a 12A (&12B) is 94:1.

You can follow the link in the 2nd post at at the top of the forum to find your year's FSM.

 

A strong number for a stock engine is in the 120 PSI range. I think below 80 PSI is when you might want to tear it down.

 

Tim's missing a decimal point in there...

Stock COMPRESSION ratio is 9point4 to 1 (9.4:1), but the OP is asking about single face peak pressure. Below 80psi, you'd be feeling a loss of power and hard hot-startup because the engine can't get enough compression when hot to start the fire. Went through this with my original 13b at 222k miles, but was suffering from the symptoms for about 5k miles.

You really need the multi-chamber tester to know how each chamber face is doing, however.

 

Are you testing the engine cold or hot?

 

Compression is at its best when the engine is warm. You may want to try a little Seafoam with the engine running or perhaps try soaking the internals in Marvel Mystery Oil for a week or so to help dissolve some carbon. It may not be carbon, but could be worn seals but its worth a shot. The good news is that you have even compression on both rotors.

Mazda states that below 85psi the compression is out of spec. The engine will still run but as stated, you may experience hot starting issues.

 

How did you measure? With the special mazda compression tester or one like it? You can use
a standard compression checker by holding in or removing the valve but that will just get you
the highest compression of the 3 rotor faces. Watching the dial bounce can give you a clue to
whether one face is weak but thats about it.

Like everyone said, do it hot and make sure theres some lube getting in there so it seals to get
an accurate reading.

 

Thank you for the info. This is exactly what I was needing. I did the check with the motor cold and used a standard, not Mazda, compression checker. I'll take your advice and check it with the motor at running temp, with it well lubed, and I will measure at all 4 plug holes. I think this motor is probably in need of rebuild but it would be great to get another 10,000 or more miles out of it if possible without damaging the motor.

 

You only need to check compression on the leading (lower) plug holes - top and bottom will always read the same. Obviously, only remove the ONE spark plug you're inserting the test sensor into. Let us know what you find,