bobssuprsecrtsn

Hey guys I have a question.. How do you compression a rotary? (Since there's two spark plugs) and what range does the compressions have to be ? Iono I think it was 100 or around there..I'm not sure

NoDOHC

Either spark plug will work, leading is preferred (I don't know why.)

95+ is pretty good, 85+ is acceptable

An ordinary piston compression checker will work if you can hold the check valve open and watch the bounces (three equal bounces per rotor.)

Thread compression guage into plug hole, hold valve and watch while friend turns car over. Observe that there are three even pulses and that they are all greater than 85 psi, repeat for other rotor.

If compression is good (85 psi +), be happy. If compression is bad(< 85 psi), be sad.

rx7-kid

iTrader: (1)

So do you have to hold the valve where you let the air out while cranking.

RotaMan99

I find that when holding the pressure release valve open, or fully removed, you will get a lower PSI reading then what your engine actually has.

I did a compression check on my motor recently and held the valve open all the way and got 85-90psi. I then had it only opened about half way and got 105-110 PSI bounces.

If I had the valve closed the max psi it would read would be 115 psi without bounces of course. This doesn't mean its higher because its holding more pressure due to each bounnce, this is the highest pressure reading the meter was given from one of the chambers.

SpeedOfLife

iTrader: (16)

you're supposed to do it two times per rotor. once with the valve open, that's for checking that all seals are meeting the housing evenly. then with the valve closed, that's for checking the max compression for each face. however, you may want to do that second check a few times so you make sure to check each face, but it probably isn't completely necessary if you got nice even bounces on the first check.

RotaMan99

This will give you the highest compression on ONE face. If one face is 115 and the other 2 were 110, the gauge will read 115 and you won't know what the other faces were. There is no way you can tell what the compression is on each face doing it this way.

RX7Tuner.

When doing the compression test it would be wise to remove the main fuse to keep fuel from flowing into the housings.

Brian

SpeedOfLife

iTrader: (16)

that's exactly why I said it can be useful to do that particular check several times.

SpeedOfLife

iTrader: (16)

um, I'm not 100% sure, but I don't think the car could turn over with the main fuse removed.

OP, just search 'compression test' and find a writeup. it's easy to do as long as you do it right. otherwise you could hurt yourself and or your car.

btw you do have to remove fuses to eliminate the risk of blowing stuff up, but just search like I said. this thread has been done so many times...

RX7Tuner.

Uhm, are you serious? Do you think doing an overall compression test multiple times will give you each rotor face?

RX7Tuner.

The starter gets 12v directly from the battery. Your car will turn over with the main EGI fuse removed. What you won't have is fuel and spark. You should remove the EGI fuse.

Brian

SpeedOfLife

iTrader: (16)

your odds go up. I really hoped I wouldn't have to explain that because it should be obvious. and like I ALSO said, if you get even bounces with the valve OPEN then you shouldn't have very different individual face results ANYWAY, so having the exact number for each face wouldn't be as important.

if you want the best compression test, shell out the $450 or whatever ungodly amount it is for a Mazda rotary compression gauge and have at it and stop bitching about minor discrepancies that rise from using a cheap gauge that isn't intended to check multiple combustion chambers at the same time.

RotaMan99

You have a good point, but doing the compression test with the valve closed multiple times will not give you each rotor face. Thats the point we are trying make.

RETed

Actually, WRONG.
Mazda guidelines are to use the trailing holes.

Don't be too preoccupied with the numbers unless you're using a genuine Mazda SST.
As long as ALL the rotor faces are about the same, you're good.

http://fc3spro.com/TECH/FAQ/compress.html


-Ted

pfsantos

iTrader: (1)

Haven't seen mentioned a couple of points overlooked:

1. Make sure the throttle plates are held open when doing a compression test.
2. Make sure battery is charged.

Also, download the FSM, especially chapter 1 (links in my sig.). If you want all the numbers, pay an hour's worth of labour to the local Mazda dealer, but make sure you read the FSM first and watch the procedure (some are good, some are not).

Some points not mentioned, see FSM!

RX7Tuner.

If you leave the valve in the compression tester and turn the engine over, you are only reading compression for the highest rotor face. If you have three rotor faces and they read 30, 60, and 90, every time you did a compression test with the valve in it would read 90. You could do the test 1,000 times and it would always read 90.

The valve will only show the highest compression for the entire test. If you watched it during the test you would see the needle hit 30, then hit 60 and then hit 90 (all within about .5 seconds) and then it would stay at 90 even though you keep turning the engine over. If you repeat the test, you wil hit 90 again.

Brian

RotaMan99

If the compression is the same on each rotor face, the way Speed Of Life does it will work fine, but if your like me and want to know oowhat each face has for compression, then I would only hold the valve open half way to reach the highest psi possible with a place fopr the pressure to vent....

SpeedOfLife

iTrader: (16)

you're right if you just turn it over then go check the gauge.

instead you can watch the gauge as you're turning the motor, so if you have a decent eye you'll be able to catch the lower pulses fairly accurately. of course it's easier with two people.

RotaMan99

Your confusing me. I stick the gauge where I can see it and use electrical tape to hold the valve open to where I want it.

Thats what I do with the valve half way open, not closed. When the valve is closed, you won't get accurate bounces.

SpeedOfLife

iTrader: (16)

this is taking way too much time. I'm done. find a way that works and do it.

bobssuprsecrtsn

Ok. I'm gonna hold the valve half way open and examine the gauge while a friend cranks over the engine 3times with the gauge in the trailing hole. Correct?

Suppose the first crank is 30 and second is 60 and third totals to 90 on the gauge. That means first face is 30, second is 30, and third is 30. Correct?

Do the test one more time and if its still above 90 then its good?

pfsantos

iTrader: (1)

No. Hold it all the way open. You'll see the needle bounce and should be able to tell where the bounces are maxing out as each chamber shows its compression. Yes, the pressure shown will be slightly lower because you're holding the release, but it will give you a good representation of each chamber relative to the others. When you let go of the release, you'll get the max pressure of the three chambers.

Time to see what we mean, and get your hands dirty.

1. Put the battery charging if it's not fully charged.
2. Download FSM (see my link, once again), read the procedure there.
3. Remove top (trailing) spark plugs.
4. Screw in compression tester.
5. Remove EGI fuses under the hood.
6. Have buddy hold throttle wide open, car in neutral, parking brake set.
7. Press in release valve.
8. Crank engine.
9. Watch.

Record the gauge with a camera if you want to review slowly later. Guys, did I miss anything?

SpeedOfLife

iTrader: (16)

you don't add those numbers. if one face yields 30 psi, and the next face yields 40 psi, and the next yields 30psi, you will see a max of 40 psi on the gauge. however with the valve open at all you might not get the full pressure. that's why you do two tests, one for bounces and one for peak compression. it's a safe bet you need to replace or unstick side seals if you don't have even compression for all 3 faces per rotor (within around 5psi of each other).

I don't think holding your valve halfway open will get you dependable results. do it open, then do it closed.

RotaMan99

He could get a more accurate reading holding it only half way open.

Max pressure of ONE chamber. hopfully all 3 chambers are the same so the number you get can represent all 3 chambers. Other then that, you are getting the highest psi in ONE chamber.

It sure does because by the time the next chamber comes around, the pressure in the gauge from the previous chambers is about 80% gone. This will yeild a high pressure reading on the gauge.

Getting the max pressure of the ONE chamber when all three faces are off wont tell you much of anything.

RX7Tuner.

It's funny that this is taking so long to explain. Anyway, I wouldn't use eletrical tape to hold the valve open. Just unscrew the schreider valve from the bottom on the tester.

If anyone still has a question on how to do this, just read the FSM. Don't listen to any of us, we have no idea what we're talking about.

Brian