Just had a compression test done, and was wondering what the numbers mean, and are they good.



Rotor 1 FT. 10.1 Rotor 2 FT. 10.4 Rotor 3 10.6
Rotor 1 RR. 11.0 Rotor 2 RR. 10.6 Rotor 3 10.6


Don't know anything about compression tests, but a potential buyer wanted to see one done.

Thanks for any help.

 

those seem abnormally high, assuming they are measured in kgf/cm^2. Those numbers should be 9.0 as a practical upper limit.

Could those actually be psi, like: 101,104,106?

Dave

 

Hmm, i don't know exactly...i literally posted the numbers the advisor gave me in the same order. No indication of kgf/cm^2 or psi.

 

Cranking rpm? Mazda tester? What shop?

 

I would go back and ask this advisor about the readings. Those are off the scale. As dave alluded to, perhaps they are in psi and you just have to add a zero and lose the decimal point, but that seems kinda odd.

 

Don't know, i assume so, dealer

Guess i'm calling on Monday since he won't be there tomorrow

 

Quick question for my own knowledge...

What is the average compression *supposed* to be for the 13B-REW engine??

Assuming that 1kgf/cm^2 = 14.223 psi than this engine would be pushing:

#1 FT: 143.65psi #2 FT: 147.91psi #3 FT: 150.76psi
#1 RR: 156.45psi #2 RR: 150.76psi #3 RR: 150.76psi

I know that a stock configured car shouldnt be pushing less than 100psi (preferably). I think I remember reading somewhere that from the factory they were supposed to be around 128(ish)psi.

If the rebuild was done with 3mm apex seals would that effect the compression??

I did a search of the forums and didnt find much in regards to this subject.

 

with a streetport and 3mm seals it should be slightly lower.

 

Slightly lower than what is listed, or slightly lower than stock compression?


Hrm... that eliminates one possibility...

Has anybody run compression tests on a FD with high carbon deposits?? I've heard that this could be a possible result/indication of carbon build-up.

(Everything I say is pure speculation resulting from research in this forum... only engine I have ever *personally* worked on is the H22A1 in my 'lude)

 

Correction to myself.. I think carbon deposits lower compression... Anybody know for sure?

 

Here's another question to throw at you guys, the engine has about 12k miles on it, so compression shouldn't be an issue.

 

Check the FAQ.

Dave

 

Are you sure that new apex seals and rotor housings were used? based on the readings, it seems to me that the mazduh dealership you went to doesnt know what the hell they are doing. bring in the shop manual to them and show them the proper ranges.

 

Ok, so higher numbers are better(obviously). But what about *too high*? Is there such thing? There is a clearly defined bottom acceptable number, would the same rule apply on the opposite end of the spectrum.

Is this an uber strong engine, or is it likely a case of some mechanic taking a pipe break before doin a compression test???

 

It's clearly a case of the numbers not being right, or in some esoteric units of measure.

Dave

 

I'll take choice (b) for a gazillion, Alex

 

Oh boy, i just saw this.

 

Okay, just spoke with my service advisor and the numbers are in kgf/cm^2 which means that yes...it's an "uber strong engine" as fendamonkey put it. There's nothing wrong with the numbers for a new engine.

 

no, the dealer fucked up

those compression numbers are physically impossible without boost

 

I agree. They just aren't physically possible.

Could the engine be built with higher compression rotors?

Dave

 

When I first bought my MB car, the compression test came out to something in the 10's or 11's but the rear's were in the 6's cranking around the 250 RPM's.

They tested it out like 3 times.

You can ask Atkins Rotary since I took it to them.

FYI, they used a Mazda Rotary Digital Compression Tester.

 

if i'm not istaken, atmospheric pressure * compression ratio = compression

14.7 * 9.0 = 132.3

this is the max compression that you can make with 9.0 c/r rotors and no boost assuming the surfaces seal perfectly

11 kg/cm^2 in psi equals 156.42 PSI

 

Yeah, I *was* the potential buyer that asked for compression tests (owner switched buyer last minute... It's a business deal, whatev... I have cash and time, another car will come along so I'm not worried or upset)

HOWEVER, I was quite curious as to how this anomoly would occur. I called RP Performance (of Manassas, VA) yesterday (they are the guys that did the rebuild) and explained the situation, along with the compression numbers.

Their baseline assessment was that compression in these numbers can occur (though very rarely) when there is a LARGE amount of carbon deposits in the engine. These numbers occur more often in the 2nd gen n/a engines when the drivers treat the car like they are granny driving to church all day, everyday... RP P went on to say that this can be bad (pretty sure I remember this right, I made the call after a 14 hour work day and didn't write anything down) in that it can screw up the springs on your Apex seals. Best way to check and see if the springs/seals are busted or not is to take the car out for a drive. It's a sports car, drive it like one.. "flogg" the engine for a good 20-30 min of spirited driving to break up and spit out the carbon (the second gen forum confirmed this tactic for cleaning out carbon build-up). After that turn the engine off and let it cool (after a cool down period while driving, back to the whole turbo timer vs. being responsible debate) down, but not completely cold. Re-start the engine and feel the idle. If it's rough and *obviously* not happy than you have fucked up apex seals/springs.. if not, lucky you.

Would any of the more experienced forum members like to comment on this? I would love to hear the general consensus on this subject.. if nothing more than as a clear reference point for the future in case somebody else runs into this problem..

Thanks,
Levi

 

Guess i'm lucky then, cause that's exactly what i did yesturday, and there's no problems at all with the car. Boost is normal, no rough idle or anything.

Edit: Should add that a potential buyer told me about the carbon build up thing, and suggested that i run the car hard like it should be and do the whole cool down thing and check the idle.

 

it is not physically possible to make that many psi with a 9.0:1 rotary.
this means physics wont allow it.

i dont think you're lucky. i think you're unlucky -that you dont know how to do a compression test- or find someone who knows how to do one correctly.

and as for the carbon buildup, that would have to be a SHITLOAD of carbon. enough to **** something else up before the compression numbers got 25psi higher than brand new (~125psi).