Every time (and I mean EVERY time) I see a post where a member is working on their car and trying to diagnose a compression issue, someone will pedantically reiterate that piston compression testers cannot be used to check a rotary engine.

I say poppycock! But only because I love old-time cockamamie curse words.

Granted, it's not 100% accurate, but it CAN save you hours of labor reinstalling your engine after a rebuild only to find your issue has not been corrected. Here's how it's done:

At the bottom end of the tester hose there is a Schrader valve, just like in a tire stem. Use a valve stem tool to remove it.

Now that the stem is out, you will need a camera capable of slow-motion video. Most current cell phones have this capability. Find a way, either with an assistant or with a phone holder, to set your phone to record the dial of the tester while you crank the engine. What I did was I attached the bell-housing from my transmission to the engine while it was on the engine stand. I bolted the starter to the engine on the stand, and used jumper cables to the battery to operate the starter. I clipped the negative to the engine lift bracket and touched the positive to both the battery attachment post and the solenoid lead. Using a cell phone, I recorded slow motion video.
Here is my setup:

That's my cell phone holder wrapped around the oil fill neck.

I tried to upload my results, but I was informed my file was of the wrong file type? HOGWASH!

I will attempt to upload the results to my PC first, then to the forum. But it DOES work. This helped me diagnose a bad rotor after multiple engine install-and-removal procedures. I just HAD to come up with a way to check it before re-installing it AGAIN.

 

This method has been posted and discussed before.
If you're going through all that effort isn't at least the worth of buying a proper rotary compression tester?

I give you A+ for the write up but this method is not news

 

Ok, I had to run these through a converter. Some of the quality is gone.

Front rotor is attached. By the time I figured out how to use the online converter, I had exceeded my 10-file limit.

The front rotor was the one I was having problems with, anyway.

 

Lol! Speaking of pedantic replies, it looks like I've gotten one already!

No, it's not worth buying a proper rotary compression tester.

One more thing, in case anyone is unclear. Connect the compression tester to the leading spark plug hole, and leave the trailing spark plug inserted. If you're doing this on the car, make sure the trailing spark plug is disconnected. Failure to do so will result in a highly inaccurate result - like 250 psi - when the gas ignites.

 

This is how I used to do it before I got my rotary tester . It's ok for a once off situation and isn't as accurate but ...it works fine . One thing you didn't mention is that with the check valve removed you get a very low reading due to the fact that the rotor has to compress the air in the tester line each time . I got around this by doing one test like you did plus another using a check valve ... which gave me a peak value. This was typically 15psi or so above the no check valve values .
So then you have the peak value plus the variance between the rotors ...which is everything you need.
BTW ... you can also work out rpm by counting the pulses in slow mo.

 

^^+1, the adapter+tester hose volume is not trivial

"Helped me after multiple engine reinstalls and removal"
"No, its not worth buying the proper tool"

I mean... isn't it though?
Wouldn't you like to know your compression accurately, rather than having a tool that gives some result? How do you know its right?
Also you're supposed to test the engine warm..


You can also build a proper rotary tool for about $80, there's a DIY here on this forum.
I can see how knowing compression is valuable in your situation, and would have probably been more valuable earlier, but that's not really an argument for choosing a tester.