How to tell if I have good compression?

[7fire3]

I don't have a comp tester nor do I know anyone with a rotary comp tester close. Is there a way to tell if the car has good compression?

[vex]

There's a poor mans compression test, it's not accurate, but it might give you a digital reading of either the engine is blown or not, but not if it's got low compression.

With the car warm: Remove EGI fuse, have friend hold accelerator down, pull leading plug on one rotor, listen for 3 even large pulses. If the engine is not significantly warm (as in you're not shooting out hot air or fluids from the spark plug) you can put your hand close to the spark plug hole and feel the pulses to judge the strength of the seals. Repeat for the other rotor.

With the car cold (will not be as accurate for when it is warm): do the same as above but put your finger over the spark plug hole and make a seal. have your friend crank it. If your finger comes off the hole with the pressure, you have okay compression. If it does not, you do not have good compression on that rotor. (your finger should come off 3 times per orbit of the rotor).

Disclaimer: These tests are significantly inaccurate and will only allow you to know if your engine is blown/not blown. See Teds post for more information

[To_Slow]

Originally Posted by 7fire3

I don't have a comp tester nor do I know anyone with a rotary comp tester close. Is there a way to tell if the car has good compression?

Get a piston compression tester from your local auto parts store.....

That will give you an idea on your compression #...

[Cp1]

you can use a regular piston compression gauge but you have to remove the valve that Holds the reading. then crank it over with the egi fuse removed and watch for the height of the needle to get your readings.

[RETed]

No, you need a gauge.
You can only discern between motor-has-even-compression versus motor-has-a-dead-seal by your ears.


-Ted

[7fire3]

Originally Posted by vex

There's a poor mans compression test, it's not accurate, but it might give you a digital reading of either the engine is blown or not, but not if it's got low compression.

With the car warm: Remove EGI fuse, have friend hold accelerator down, pull leading plug on one rotor, listen for 3 even large pulses. If the engine is not significantly warm (as in you're not shooting out hot air or fluids from the spark plug) you can put your hand close to the spark plug hole and feel the pulses to judge the strength of the seals. Repeat for the other rotor.

With the car cold (will not be as accurate for when it is warm): do the same as above but put your finger over the spark plug hole and make a seal. have your friend crank it. If your finger comes off the hole with the pressure, you have okay compression. If it does not, you do not have good compression on that rotor. (your finger should come off 3 times per orbit of the rotor).

Disclaimer: These tests are significantly inaccurate and will only allow you to know if your engine is blown/not blown. See Teds post for more information

Thx for the detailed info, I'm going to head out and take a look at it.

Originally Posted by To_Slow

Get a piston compression tester from your local auto parts store.....

That will give you an idea on your compression #...

I actually have one I just wasn't sure if that would work correctly?

Originally Posted by Cp1

you can use a regular piston compression gauge but you have to remove the valve that Holds the reading. then crank it over with the egi fuse removed and watch for the height of the needle to get your readings.

I'll have to try that also, I have one sitting in the garage. thx for the info

[Fidelity101]

you dont have to remove the valve but you can hold the relief valve button down.

[TitaniumTT]

Banzai-Racing has a very detailed write-up on thier site.

FWIW - I was talking to one of the tech guys @ Motec who is also a Rotary guy. He was telling me that he uses his M800 to check his compression. So simple it's genius actually.

He goes into Dig 4 and sets it up as the ignition switch, basically turning off the ign/fuel pump. Then he gets a 0-5v 0-250PSI sender, adapts it to a piston compression tester and plugs it into his Oil Pressure input. Rolls the engine over a few times. Shuts the ignition off, moves to the other chamber and repeats. Pulls his datalogs and BAM! Digital compression test. Logged with RPMs, temp, Throttle position etc etc. I'll be rocking this method when I get the young girl rebuilt.............. again. 3 engines in 3 months

[JustJeff]

I'm looking into renting a digital compression tester. I was recently using a piston compression tester and found that my rear rotor had nice even compression all around so it was easy to read the analog gauge bouncing. The front one was a different story. I have uneven bounces and each compression pulse comes too quickly for me to tell the exact numbers.

Rear is about 110-110-110
Front is about 80-50-80

I'm told that a bad apex seal usually shows two low readings and one higher. Or that a bad side seal can cause one low reading.

I won't get enough use out of buying a digital compression tester or rotary compression tester, but I'm looking into renting a digital tester.

[TitaniumTT]

Go to a Mazda dealer. They should have one.

You are right though. A bad Apex seal, or corner seal, will cause two low bounces. A bad side seal will cause one. "Bad" can mean a bunch of different things though.

Your Front Rotor reading 80-50-80 could be a number of things but it definately sounds like your housing is scored.

[TehMonkay]

The dealer wanted WAY too much money for a compression test. Forget that, I think $600?

[TitaniumTT]

That was probably the tester. It should take them all of 30 minutes. Tell them to charge you normal labor rates as opposed to assraping you with a moose, less they want you to report them to the BBB for gouging