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How to Do a Compression Test

By Anonymous

A compression test will give you a good idea of the health of your engine. It will indicate whether you have a broken apex seal or a blown side seal.
I just figured for $20 for my pressure tester, I got what I paid for. :) It really wasn’t meant to be an accurate compression tester. Remove the schrader valve from the compression gauge. That’s what holds the pressure reading. You can also just hold the relief valve down during the test so you can watch the needle bounce for each rotor face.
Procedure for a 3rd gen is (page C-11 RX-7 Workshop Manual:
1. Warm up engine to operating temperature, then allow it to cool for 10 minutes while you get the plugs out.
2. Remove the front and rear trailing (upper) spark plugs while the engine cools.
3. Disconnect the circuit opening relay, and the igniter connector. The CAS and EGI fuse are both unplugged so that no gas nor spark will occur during the test.
4. With both spark plugs out, fully depress the accelerator pedal to the floor, and motor the engine for 5-10 seconds. You should watch for 3 bounces of the needle (once for each rotor face). Specifications vary depending on the brand of shop manual. I’ve gone by 7.0 kgf/cm2 or 100 psi minimum @ 250 rpm as stated in my 93 RX-7 Workshop Manual. Other references say 6.0 kgf/cm2. Yes, you can compensate for higher cranking rpm during the test. There's a chart in the Workshop Manual.

More On Compression Tests

Several earlier posts have referred to a compression test for the rotary engine. Is this test conducted pretty much same as on a piston engine car? Which spark plug hole to use? or does it make any difference?
You can use the top spark plug hole (easiest to get to and there is no difference between top of bottom values). You need a special rotary compression tester allowing you to measure the difference between the 3 rotors. The test should be done on a warm engine and you should get above 200 rpm from the starter motor. (you should remove a spark plug from the second rotor to reach the 200 rpm (no counter-pressure). You have to floor the throttle while testing and don’t forget to disconnect the ignition (I had cars starting to run on one rotor while testing :-)) Cold compression tests mean NOTHING. I have seen differences of 5 Kg between hot and cold. If you don’t have a special rotary tester available, some hints:
You can use a normal tester but you will not see the difference between the chambers. I have never tried this and some guys are not very confident in this method [I've tried it and it works ok], Mazdatrix e.g. says you can as well hold a finger on the spark plug to measure compression with a piston tester.
I know of some others that have drilled a small hole through a normal spark plug. They turn this in and listen to the whistle. The higher sound you get the better. You can also hear the difference in the chambers. (peep peep - whoosh is no good)
What compression would generally indicate a healthy engine, or weak one needing rebuild? Any other diagnosis tips would be appreciated.
All compression above 7 kgf/cm2, 100 psi is good (no rebuild needed). Anything below 5.5 is likely to break down when driving the car home. You should not see differences of more than 1 kg between chambers and 1.5 between rotors for a good engine. Unless it has been partially rebuilt, where you can have strange values like 9 front and 6 rear.

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