Mazda Rotary RX-7 FC3S/FD3s Compression Tester
A rotary engine seems elegant until you try to perform a compression test on it. Each rotor has three faces, and each face has its own apex and side seals. You need to check all three for each rotor. You also need to check them at cranking speed, not by turning the engine over by hand. Compression varies with rpm, and Mazda specifies that the check be performed at 250rpm, the normal cranking speed of the engine.
You could do what many others have done: hook up a traditional dial-style pressure gauge up to each one of the cylinders and watch the needles “bounce” with each revolution. This gives a crude approximation that may be good for most purposes. You could also buy Mazda's “Special Service Tool” (SST), a nice (if dated) digital unit capable of providing a more accurate indication. However, this tool is extremely expensive, and probably beyond the budget of somebody building only a single engine.
My engine will be for aircraft use, and I plan to check compression regularly. This can be a helpful early warning sign of seal failure
Or, if you have more time than sense, you could build your own compression tester as I did. Although it's an added expense of time and money, I'm glad I did. It allowed me to get a very accurate measurement for each rotor face, eliminating one pre-start worry.
Ignore the tape - this is my prototype, so I taped the case shut instead of using screws, to make it faster to open it up and tweak it.
This compact unit uses two Honeywell stainless-steel, sealed sensors with threaded adapters that fit into the spark plug holes. You can use just one sensor to save money (the sensors are a large portion of the device's cost), although then you have to move it from one rotor to the other to test the second rotor.
The tool is USB-powered, and an upcoming software update will allow it to record and graph compression data on a Windows laptop.
