I had a compression reading done on an FD on the weekend and the mechanic had one of those official Mazda compression testers. We did front and rear rotors and it measured something around 8.3 on both. I've never seen a compression test done before but I observed that he disconnected the ignition module thing on the side of the engine bay and connected the tester to the positive and negative on the battery before taking out the plugs and inserting the tester.

I cranked the car over for about 5 seconds and you could hear a loud pshhh pshhh pshhh sound like one of those old steam trains.

Does anyone know what units the 8.3 reading is in?

The guy who did it just said thats a very healthy engine, equivalent to about 105psi so does this sound right?

 

It's in kg/cm^2. Those are very healthy readings.

 

thats 118 psi. sounds good to me.

 

you have a good engine.

the min Compression of the Stock apex seal is 6KG or 1.0 kg different for front/rear rotor. or in one blank

J

 

Some tests are done in kg/cm^2, and some are done in bar. The conversion is almost the same to psi, however. You multiply the reading by about 14.2 to arrive at psi.

90-100psi = weak

100-110psi = healthy

110-120psi = strong

120-125psi = perfect

Without the assistance of higher than standard (250-260rpm) cranking rpm, or oil in the chambers to artificially raise compression, 125 is as high as you will ever see.