Quick clarification on compression check (not quite the usual).
 

Hey All,

So I've been researching tons of information about FDs and rotary engines over the past few months and there is one thing I can't seem to get a definitive consensus on, the exact compression number to compare against Mazda's specs. From my research, I know the following information should be accurate for the procedure:

1. Test should be performed on a warm engine otherwise the cold numbers will be higher than when the engine is warm.

2. Either remove the schrader valve from the compression gauge or hold the pressure release valve during the test.

3. Ensure that the throttle is kept wide open during the entire test.

4. Remove both the leading (bottom) spark plugs (although I have heard a decent number of people say it doesn't matter if you use leading or trailing).

5. Insert the compression gauge, hold the release valve, and turn the motor over for a good 5 to 7 seconds. What you are looking for is a reading off each face of the rotor. As the engine turns, the needle should bounce and the readings should look something like:

0-90-0-90-0 etc.

I'm clear on all of that, here's the part where I'm not clear on and I have read conflicting things both ways from a few different builders and random members..

Question: The test you perform with the release valve open where it bounces from 0-90-0-90-0 (or whatever your number is), is this the number you compare against Mazda's factory specs or is this simply the number you use to check the health of each Apex seal?

I have heard people say you use this to check the compression on each face of the rotor but that this isn't the total compression number you use to determine the overall compression on the engine. The people I've seen state this usually say you need to see at least around 60 psi bounces on each face and then you stop holding the release valve on the gauge and let the rotor spin completely around all 3 faces to build a final compression number.They state THAT is the number you reference against Mazda's specs.

So is this accurate? From what I initially read, I thought the number you use to compare your engine's total compression to Mazda's specs is the number shown during the 0-90-0-90-0 sweeps when the release valve is held. Can we get a complete consensus on this? Obviously, Mazda's tool would perform both the sweep test and the overall test in one so as to graph the motors overall compression, but we have to improvise with a standard piston engine compression gauge.

PS: I'm aware this test isn't 100% perfect and Mazda's tool is best, that varying RPM comes into play with the numbers, and this won't tell you anything about coolant seal condition :)

Thanks in advance for the clarification on this everyone.

PPS: Here's two videos that show someone doing the 0-90 sweeps for 5 seconds and then letting the motor build compression for the final reading..

http://www.nightwalkermods.com/night...nt%20rotor.mp4

http://www.nightwalkermods.com/night...ar%20rotor.mp4

you did it right and it looks good.

Hey thanks for the reply! Yeah, I believe that motor is decent compression wise but I'm still confused as to which number you compare against Mazda's spec. Is it the 0-90-0 sweeping number or the final number with the release valve no longer held down?

Basically, if you look at those videos, that motor has about 90 PSI for each rotor face (which means each apex seal passes) and around 105 to 110 for total compression with the valve no longer held down. Which value is the one to compare against spec for total compression?

Thanks again!

I believe mazda does cumulative compression, but actually watching the sweeps is whats realy important so you can actually see the health of EACH rotor face, cause sometimes you can have a dead one, and the cumulative will not show it

Mazda has a tester that reads BAR I believe..and if it is something like 8.1 I just multiply that by 14.7.
Each face gives you a reading..example 90..then again 90..then again 90..PSI.
overall compression is the average of all three.

If you had a bad apex seal you would get a reading of 90.....BLIP....90...

TR-01 Rotary Engine Compression Tester makes a decent aftermarket tester if you are willing to shell out some money.It is user friendly.

Thanks for the responses guys!

Good to know this is how it's done