Bought an Autozone compression tester.. pulled out the valve on the dial so it wouldnt hold a reading.. warmed the motor to normal Op temp and tested my compression. Pulled the EGI fuse, held the throttle wide open and cranked the motor.

I saw about 115 psi rear rotor and about 118-120 front! The motor is a j-spec used motor with about 60k miles on it... now i hear most people saying they have about in the 90's to low 100's.

What gives? Is this normal compression?

 

That is wild...you pour some oil in there or something?

James

 

Sounds good to me. I wouldnt believe what I was told about the mileage on the engine. Jspec resellers quoting mileage on their engines are like boyfriends wondering about their new girlfriend's sexual history...they have no clue what all was done by the previous owner before they got their hands on it.

 

well, you never know what shape the engine is in when you get it. maybe someone did some internal work.. i just got a jspec engine and did compression check, the rear has only one good face on the rotor. the others dont read any compression..did you start it yet?

 

The thing that bothers me is that you said you took out the valve, so the tester wouldn't hold a reading. Usually, that would mean you just get the needle bouncing to 30 psi or so as each sealing face makes compression.

You leave the valve closed for a total reading and open it to get individual bounces...

BTW, I recently did a test and got even higher than those numbers.

 

n/a engines are generally about 10-20PSI higher than TII engines, because of the differences in compression. TIIs usually are in the 90-100 range where n/a's are in the 110-120 range.

alex, he means he took out the valve in the hose, not the gauge.

 

I have never noted more than 5psi difference in them. I have seen 125psi t2/FD engines more times than I have seen 125psi NA's.

 

depends on conditions mainly, i do notice the difference quite often and it isn't because the internals are in bad shape. most i have seen on n/a is 115 really but 120 isn't impossible. i don't compression test FDs much because theyre a bitch to get to the plug holes so i just do the poor mans compression test on them often because they have blown seals and is quite obvious.. S4 TIIs are the lowest of all generally ranging around 90-100 often because the housings are semi worn already but the compression is still average.

i am kind of skeptical about this jspec 125psi TII motor though, that is rather high, i thought it was an n/a at first until i re-read the post. an S4 would have to be pristine surfaces to get that reading, at least on my gauge.

 

I have seen 2 125psi s4 t2's, and one 120psi s4 t2. One of them had 75k one owner miles on a US car.

 

Ive bought 2 jspec 13b-re's, both came with compression readings, and where in the 90-100 range. The one I am using now the compression has gone up he first time I tested it it was around 90 a few months later under the same conditions it was around 105. I did the water treatment on it, but thats all I can think of.

 

Well i just checked mine the other day but now Im not so sure I did it right. My gauge only has a valve with push button on the side that you cant remove. I checked it without venting the valve so I guess i just got an overall compression. How many times do you turn the engine over? I let mine do about 5 full rotations or 15 puffs. About right? If I didnt hold open the throttle how would that have effected my #'s? Higher or lower? I only read around 75psi

 

Usually piston compression testers have TWO Schraeder valves. One is for releasing pressure after the test is done to possibly do the test again without unscrewing the tester. The other valve is located at the insertion end (snicker snicker) of the compression tester. THAT'S the one that you should unscrew.

If I'm wrong, someone please correct me.

 

By the way, the FSM clearly states how to do a compression test. You crank it between 5-10 seconds. Sorry if that sounded snarky, I'm just trying to help.

 

if you don't believe it, do it again.

or go to Mazda and get a real one done for $200 (that's what they quoted me.. TWO HOURS OF LABOR. I didn't do it though)

 

is it possible there is alot of carbon build up? piston engines read higher with alot of buildup. just a thought!

 

Carbon buildup will generall lower the compression on a rotary since it keeps the seals from sealing as well.
I'd say you just got damn lucky on the engine.
Also, excellent analogy, Kevin. I'll have to remember it for future discussions.

 

The reason that may seem high on the net is because most engines are either worn out or have a half assed rebuild.

I've seen 125psi on an s4 n/a engine, I don't remember the ALT, but I believe it was around 1100ft, can't remember cranking RPM either, it was 5 years ago.

2psi difference between rotors to boot.

 

That's so wrong...

 

The valve was unscrewed so it wouldnt hold a reading. It was not FULLY removed so the air would vent. Upon cranking (@ about 2000ft elevation) i saw the needle bounce between 80-120 psi. (meaning the guage pressure would only be able to drop to 80psi before it 'puffed' again). I saw a solid 118-120 psi on all rotor faces for the front rotor.

I dunno, it just seemed really high. I was surprised cause first i tested it when cold and got REALLY low numbers and i thought my motor was bad.

 

meh.........

My DD 86 N/A with approx. 50,000kms on a re-man has 113 front (all faces) and 116-113 range on all faces on the rear rotor.

Thats nothing out of this world.

edit: no idea what the cranking RPM was and these were on a warm motor.

 

Yeah, how would you find out? I was watching my tach when I did my compression test and I didn't get anything. Would I have to have an external dwell/tachometer thing hooked up to see that?

 

Count I guess you can do math? 250RPM is what it says in the FSM, so 250 revolutions equates 250 RPM at the ROTORS right? If so, there should be 750 puffs. Divide that by 60 (seconds in a minute) and 750/60=12.5. So you should see 12.5 puffs a second if the motor is rotating @ 250RPM. Hmm, that seems like alot.... this is of course, theory... HAH

 

Haha. I wonder what a half puff sounds like. ff? pu?

 

ah, well i'll shut up then...

 

Ha.