What are acceptible compression numbers? '95 RX7.
 

Through searching, I was coming up with conflicting numbers for decent compression on the FD rotary engine. From what I've found on scuderiaciriani.com, it says the service limit is 690 kPa {7.0 kgf/cm2, 100 psi} minimum @ 250 RPM. So what is a proper compression number for these cars?

Reason I'm asking, is I'm looking at the 3rd gen 7 again, and am trying to get solid info. Thanks.

my last motor before i pulled it out was about 90psi's all across and it pulled strong still. from the top of my head i believe its like 65>psi. but im all about be happy if it idles.

Anyone have hard facts regarding this? I am looking for specific numbers (both in PSI, and kgf/cm2 if possible). I can't seem to locate hard data about it. I've gone through just about every thread that matched with the word "compression" in it. And I've visited all of the "major" guys' websites. Anyone with a definitive answer?

I've always heard:
60-70 replace
70-80 your pushing it if you don't replace
80-90 middle, your choice
90-100 good
100+ don't worry bout it

compression readings are very much rpm dependent. in other words if your motor cranks at 250 rpm versus 200 or 300 you will get significantly different numbers. there is a chart on it somewhere....
howard coleman

Anyone have a link to this chart? I'm considering a car, but am worried about it's compression. It seems a little low to me.

Mahjik? Where is he when we need him, I'd like to know for sure too.
art

Please someone inform me of what proper compression readings are!!

Go to www.iluvmyrx7.com, then look it up in the manual on page C-10. Acceptable is 7.0kgf/cm² (100psi) at 250rpm, with no more than 1.5kgf/cm² (21psi).

I believe that Mazda may have lowered the number to 6.0 after many engines failed (dumbasses who hammered the engine while cold).

Now go download that manual and read it before asking next time.

the problem is there is only one set standard and the tool costs alot to buy.

how the readings are obtained also play a big factor.

Rx7oneluv is about right for a ball park

too much difference between rotors is bad too.

How about creating a good link so I can go download that manual before coming across like you're better than me. I already posted that 7.0 is the "minimum" compression number, I'm trying to get NORMAL, DECENT compression numbers. You've given me information I already know. And I've searched and NOT been able to come up with the information I need. THAT is why I asked.

New cars are spec'd out to 8.5 kg/cm^2 or 121psi, and the minimum I go by (the updated minimum) is 6.0kg/cm^2 or 85psi. Anything above 7.0 kg/cm^2 or 100 psi I would consider "good" or "normal". Most properly functioning engines fall in the 7s range. Ported engines don't count. You won't start to notice power loss or idle/startup problems until below 6.0, although autos are more sensitive. The more consistant the numbers are between chambers, the better. I'd consider less than 0.5kg/cm^2 difference between the highest and lowest readings (out of 6 readings) good, most cars are under 1.0, and once the difference gets to 1.5 you start getting problems. Also, the test must be done HOT, it has been proven that doing it cold artificially inflates numbers. The standards also only apply to 250rpm at sea level. Higher elevations and/or lower rpms will lower your numbers and vice versa. Ok, I think that covers everything...

www.iluvmyrx7.com

without the comma

Thanks dude. Exactly what I was looking for.




Thanks dude. Didn't see the comma before. Thought the link was dead. (I've run into quite a few dead links trying to find these numbers). Anyway, I'm downloading that stuff now. Thanks for the link. :)

According to Racing Beat's catalog the lowest you should be looking at is 75psi, that's border line. My old FD was 75psi at and it ran fine and pulled strong too.

No problem. I came off sorta snappy, sorry bout that.

I strongly recommend buying the service manual in the flesh. It's very informative, and even if you only ever change your coolant using its directions it will pay for itself. I also just bought the two videos from www.rotaryaviation.com (Bruce Turrentine's 13B Rebuild, and 3rd Gen Engine Removal/Install). Excellent, excellent information. Just seeing the tools he uses and how he approaches the tasks is very good for a shade-tree mechanic like myself. It's also proof of why I need air tools.

Dave

ok, read through this thread and have a couple questions. My 95 has been sitting for a while so when I brought her back out I took it directly to the shop to have the new vac lines put in and replace worn hoses/etc. A couple problems have arrisen and I was wondering if they are interlinked or I can tackle them seperatly
1.) engine was compression tested at 6.5 front, and 8.0 rear when it was cold
2.) the car is consistantly generating only 4psi boost
3.) rough idle (found the cause of this- busted wiring harness on the idle motor- will fix soon)
4.) running extremely rich - I passed va emmissions by one point at 25mph on that alone, everything else on the emmissions test was perfect.

The only thing I have done to the car is get a DP to help with heat.
Thanks!

I had my engine checked yesterday at Rick's Rotary with the Mazda factory tool.

Cranking speed was ~280 rpm.

Front Rotor: 9.0, 8.9, 8.9
Rear Rotor: 8.8, 8.8, 8.5

Engine is a Mazda reman with about 20k miles on it. Pulls real good. :)

Sonny

To approximate psi from the mazda tester, multiply the results by 14.7.

90psi and up is a decent engine, but turbo rotaries are very good at turning 90psi into zero. 100 and up is healthy. 110+ is very strong...120+ is like new. Below 90, you're on borrowed time.

Oh, I rebuild engines for a living, so I know what Im talking about :)

Also, you dont need the mazda tester for a good idea of whether or not the engine needs a replacement. A piston tester, though not accurate for comparing measurements one face to another, is pretty good for measuring overall health of the engine.

At what RMP are you checking those at?

Spec is 250, you'll usually see 235-275 from a car with a decent battery.

kevin- so if I got 6.5 on the front, then that would be around 95-96 psi?

could my boost issue and this be linked together?

Right at it, yeah. It tells you that it's on the decline and you'll want to look at replacing it within 1-2 years or sooner. The only time this is not accurate is with a rebuild which is still on its way up.

Not really. It takes really low compression to lose boost...say 80 and below.

does it matter that I got 6.5 when the engine was cold?

Yes. Hot engines tend to seal better than cold, especially if it's an o-ring. I don't recall offhand which it's supposed to be. Also, the cranking speed needs to be factored in. Hopefully the shop handled those things correctly.

You know, actualy, with a rotary anyway, you get better compression when COLD. IN my experience, anyway. You tend to lose 5-15psi when the engine warms up to op. temp. This is why a lot of worn out rotaries crank right up when cold (more compression) and flood/hardstart when warm (less compression).

That is the exact opposite from what I have heard and seen.

The mazda compression tester doesnt lie.

Ok, I believe you. I just have seen people start dead engines with ATF. Before they put ATF in the engine had hardly any compression. They were using an modified normal one, so it was tough to see the exact compression. After ATF the compression numbers increased. Once the engine started and warmed up they were able to let it die or shut it down and restart it. Yes it was hard to restart but it had more compression then when it was cold and was able to fire on its own. Although it didn't last long.

I am going to have to have them do the test again. When I took it in they did the test before they fixed anything. They replaced the o-rings, a crack in the intake tube, and a fuel leak.....would those make a difference?
btw....still trying to find the boost prob.

a cold engine has higher compression.....NO

your mazda tool must not be working too well.

Yes they do. This has been tested, I still have the post from the "big list" saved from the guy who did it. This is also what Mazdatrix told me and they've done quite a few compression tests in their day. In any case, the proper procedure is to test the engine at operating temperature, meaning you should warm up the engine first, then test it.

*Edit: dug out the old post, in this example we see about a 20psi increase when doing the test cold. Admittedly, there is more mileage during the hot test, but it was done at the same altitude and barely a month later:

quote:
I had them do the compression test twice cause the dealer screwed it
up-go figure! Anyhow the first test (2/14/01, 61,678 miles, engine cold)
was-
Rotor 1; 9.1, 8.2, 9.1
Rotor 2; 9.0, 9.1, 8.4
Second test, (3/21/01, 66,312 miles) after I complained about the test
procedure (showed them their own Mazda manual on how to do it) and made
them do the test hot-
Rotor 1; 7.9, 8.1, 7.2
Rotor 2; 8.3, 8.0, 7.1

Now what.

:owned:

I have NEVER seen a higher compresison number when hot. To check this again, I tested a car I just rebuilt, an 88 nonturbo. IT has 7 miles, and is still a bit weak on compression.

Test performed using mazda digital tester, trailing plugs out, using trailing hole for test.

When cold (sitting overnight) @ 240rpm
7.5, 7.3, 7.5, 7.4, 7.2, 7.4

Ran 15 minutes out on the road, immediately pulled plugs and rechecked warm @ 275rpm
7.1, 6.9, 7.1, 7.0, 6.9, 7.0

Had the cold test cranked @ 275rpm, it probably would have bumped to 7.7's. So, we can say that it lost about 10psi compression when warm.

This backs up my theory about rotaries and hard warm starting. When warm, they lose compression, which can sometimes cause cranking and starting problems. Anyone who's ever dealt with an old FC or a really tired FD can tell you that they crank right up when cold, but have trouble (or won't at all) when warm. BEcause the lose some compression, enough to cause a problem.