Hey guys, I've been trying to find some info on compression tests. I've been looking into buying an RX-7 and after getting the compression test done, the guy gave me 4 #'s. Isn't there suppose to be 6? He supposedly had it done at the dealer.

From what I've read this is what I should be getting in terms of results:
He said:

4 Spark plugs? Does this have anything to do with the number of results I'm suppose to get? Isn't it one number for each face? I'm not quite sure what I'm suppose to make of this.

Thanks for your help,
Josh

 

you need 6 numbers, 3 for each rotor....John

 

http://www.scuderiaciriani.com/rx7/c...sion_test.html

http://robrobinette.com/compression.htm

-Alex

 

I've read that, but I should still be getting 6 readings right? The dealership should be following this standard procedure.

I'm guessing the first two 7.7 and 7.7 are for the first rotor. But Is that leading and trailing readings? Shouldn't I be getting 3 results one for each face?

The second one is 6.5 and 5.5. Those numbers are pretty low right?

Thanks,
Josh

 

If you read through the links I gave you, you'd be able to answer all your questions.

Yes, you should have 6 readings. The dealership should follow the same procedure, if they know what they're doing. In this case, they obviously do not. In a piston engine, you take the reading from each spark plug hole. In a rotary, you only use the trailing spark plug hole to get your readings. On the front rotor, you get 3 readings, then on the 2nd rotor, you get 3 readings. The 3 peak readings from each rotor housing will tell you the compression of each face. Period.

It's hard to tell anything from the numbers, because we have no clue what the hell they did. He needs to get a second compression test done. If he refuses, and says you have no clue what your talking about, find another car. It's already quite clear that he has no idea what he's talking about.

-Alex

 

I think thats bar not psi what we are used too.... so lets pretend those are the numbers (we know they aren't cause we need 6 not 4) but hell lets pretend.

1. 7.7 = 111.68
2. 7.7 = 111.68

3. 6.5 = 94.27
4. 5.5 = 79.77

Those are WAY low and as stated before WAY WRONG.

Seth at Pure sports just got in a fancy new compression tester that I'm sure he's dying to try out. Give him a call so he can do the test for you. 760.787.5759

 

renkenkyo,

Compression results are based on the number of faces on the rotor. So, the rotor is a triangle shape which means it has 3 faces. So, per rotor, you have 3 numbers for compression (one number per rotor face). Now, you have 2 rotors, so that means you'll have 3 readings per rotor for a total of 6 numbers.

Sparkplugs have nothing to do with it except for where you plug in the compression tester.

Also, that means you NEED all 3 numbers, per rotor, to have an accurate compression reading. Just one number per rotor doesn't mean anything.

 

Saying what the others said differently:

If he's giving you four numbers, then he's probably running the compression test once at each spark plug like a piston engine, since each piston has its own compression value and one plug. However, that's not how it works on the rotary. You only use two plug holes (one for each rotor), and measure 3 values from each one. It's like that since each rotor forms 3 chambers, and the chambers pass by the plug in quick succession, 1-2-3-1-2-3-1...etc.

The Mazda special tester is set up to record every value and average every third one into one number. It should create a printout that shows the compression spiking with each chamber, and every 3rd number should closely match each other. If you use the other plug you should see essentially the same thing (I think both plugs are exposed when the chamber reaches peak pressure). See Here for an example printout.

So it sounds like he doesn't exactly know what he's talking about, especially if he thinks its only 4 values.

In any case, no matter what the units are (I'm guessing kg/cm², and assuming each number was a plug hole) the 5.5 and 6.5 suggests one rotor is pretty low. The 7.7 and 5.5 suggests the difference between rotors is also out of spec.

Dave

 

The car is a 93' touring with 39k miles on it so I thought it would turn out ok.
Thanks for all the help, I guess the final word is I demand he take it to someone who knows what they're doing or look for another car.
-Josh