(ENGINE) Compression test using a piston engine tester
 

I wanted to post this in hopes of having a good reference for new members to learn how to check compression. I searched and found many telling members to listen this give you raw numbers to look at. Information is from a great builder Kevin of Rotary Resurrection. I found this information very helpful then I first started getting in to rotary's.


Compression test using a piston engine tester :

1) Note battery strength. A weak battery will yield low compression results.

1A). Cranking speed is measured at 250 RPM..

2) Remove both lower plugs and wires.

3) Remove EGI fuse from engine fusebox for injected models.


3A). On carbureted models remove the fuel pump fuse

3B). On both models remove the coil wires from the distributor.

4) Have a friend floor the accelerator pedal, opening the throttle for more airflow

5) Insert your tester into the leading hole

6) Hold the valve on the side of the tester open

7) Have your friend crank the car over for 5+ seconds.

7A). Alternative method is to use a remote starter switch

8) Observe the needle bounces. You should see 3 in succession without skips, even bounces, in roughly the 30-35psi range.

9) Let out on the valve now, and let the tester reach an overall compression value for all 3 faces(highest of 3 will be displayed).

9A). Compression numbers are available thru the FSM

10) Repeat for opposite rotor. Note difference in overall compression between rotors, which should be no more than 20psi max

Remember the compression numbers in the FSM are for stock engines.
Engines that have been ported will yield different numbers.

Many engines run well below 90 psi for a long time and aren't going to blow at any moment. Compression numbers are in the FSM, but they are listed in bars, not psi. Multiply them by 14.7 to get psi.

After re-reading the #'s i agree.. 90-100psi... I've had motors in this category that I ran for years then sold and are still running.... Had them at 90 and the same thing.. The bounce test is another thing, i've seen it both ways, I think it depends on your tester. I thought mine was broken when it did a low bounce test then checked it on a piston motor and was fine. Then let it hit regular compression on the rotary and it was fine?