ok so i'm going to get my rx-7 to a compression test. what is the range in numbers of my compression? whats good, ok, bad, very bad?

 

Are you doing it with a piston tester, or getting it done by mazda with the rotary tester? With the piston tester, 110+ is excellent, 95-110 is good, 85-95 is decent, and below 85 is getting ready need a rebuild. Keep in mind those are general numbers, just a general guide. There are also several variables in checking with the piston tester, such as battery charge, or if its been sitting a while.
As for the rotary tester, I'm not sure off the top of my head.

 

Won't it also be dependant on the compression ratio of the rotors? An S5 NA should get higher compression numbers than an equal condition S4 TII due to the compression differences of the 9.7:1, vs the 8.5:1.

 

I don't think it really makes much difference. I'm sure it effects the numbers, but not enough to really alter the outcome, epecially since the numbers are only a general guide anyway.

 

yep just as its above 85 then your good

 

compression ratio of the rotors has nothing to do with the cars actuall comression numbers.

 

Really. Could'a fooled me when I put 10:1 pistons in a stock 9:1 engine the compression pressure shot up QUITE a bit.

 

^I would imagine so! Just make sure it's done right and you get THREE compression numbers for each rotor.

 

90 psi and above is good...

 

i just had the compression checked on the '89 vert and got 110 on one rotor and 100 on the other ............ 210,000 km ( 185,000 miles) origonal NA

 

don't listen to this guy.....

 

compression is going to be the same no matter what rotors. If they where going to be differant the fsm would specify differant compression numbers for the differant rotors but it does not. I've taken compression readings from s4 to s5's, high and low compression rotors and they are all the same unless they where blown or something. you can also do searches and it's been said time and time again. You sir are the one not to be listened to.

edit: if high and low compression rotors really did make a differance, perfect compression wouldn't be 120 for all rotary enginges it would be more like 120 for low compression rotors and 130 or something for high but as we all know that is simply not true.

 

some people've driven on 80 fine

 

No. Higher compression equals a higher figure.

Attached at the end of this post is a thread from a fellow who was trying to make a digital compression tester. He dealt with both series four and series five motors/rotors. The figures speak for themselves.

There's no way a stock series four turbo motor with low compression rotors are going to read the same as a series five n/a with higher compression. It falls under common sense imho.

The site: http://www.marx7.org/~dsmith/comp.htm

It takes a bit of time to download because of the graphs, if using dial up.

 

with the rotary compression tester 800+ is damn near brand new

anything under 500 and you're having some problems

i just had mine done cause i'm having some issues and i ran higest was 834 on #1 rotor and 797 on #2

my old engine had around 480 on each rotor and it didn't run

 

Higher compression = higher PSI reading. It's as simple as that, I wouldnt try to argue it.

Now the condition of the engine is another story...

 

problem with that site is they didn't know that the 85psi that the fsm quotes is the last tollerable compression numbers. It's not like on thier "theoretical" graph that shows "well if the s4 is supposed to have 85 and since we got 110 then that must be what high compression rotors should be as opposed to low compression s4 rotors". No they have a healthy engine and the factory service manual says 85 as in that is the number that is still "within spec".

 

Well, then ... it does make sense that compression ratio will have an effect on your readings.

Because I was getting rather depressed. I rebuilt my first engine and used brand new housings , RA seals, OEM springs, and S4 turbo rotors. My compression check came out at 90psi. I guess that isn't so sucky.

James

 

I guess for aguments sake I'd like to see kevin landers in here just because he rebuilds a dumb amount of these engines and can tell us real quickly what he sees when doing compression tests. I don't mind saying it when I'm wrong but I had to do lots of research on this because I had the same question and from what I found it doesn't make a differance.