Compression Test
 

hey i want to know the best way to do a compression test aslo i would like to know the acceptable numbers

mazda rotary engine compression tester.
depends on your engine.
also, some people will use typical piston engine compression testers, so those are completely different numbers.

im planing to do it with a piston tester
its a 1988 Na

if you use a conventional tester, remove the valve (looks like a tire valve) and watch it bounce. from what I remember, there shouldn't be more than 21 psi difference between rotors, but check the FSM

i just did mine.. its an s4 tII but i dont think the #'s change. mine were around 120 in both chambers... just make sure you hold the valve down for 3 bumps on each... its hard to see so you might have to do it a few times.
p.s. make sure you start the car and let it run for 10 mins first. if you test it cold the seals wont be sitting right and you will get a false reading.

p.s.s. wear gloves when taking out the plugs cause they are going to be pretty warm after she has been running.

I just did my S4 TII as well and also came up with 120, which is fantastic.

And yeah, I used the cheapo autozone piston tester.

120 is very good! BTW how much did you pay for the autozone tester (im thinkin about running down there and picking one up)?

the autozone tester is like $25 is you get the cheapie

i think you're supposed to remove both of the bottom spark plugs, both egi fuses, floor the gas and crank, get compression on front rotor....then the back

Factory limit is 3 equal bounces of 85psi... however it is 80psi is generally accepted.. you need to remove that one way check valve.. if you tested max compression w/o removing the check valve you will get a higher than normal reading because your letting the pressure build in the compression tester.

Oh thats not bad I think im gonna go pick one up. Thanks!

How many psi lost you should expect when the engine is cold ? Cause I will take compression on a jspec engine that is not in a car and as the transmission and the starter bolt on the engine.

Thanks!

Factory limit is 3 equal bounces of 85psi... however it is 80psi is generally accepted.. you need to remove that one way check valve.. if you tested max compression w/o removing the check valve you will get a higher than normal reading because your letting the pressure build in the compression tester.


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yeah i removed the valve but i held my thumb over the hole when doing the test...
maybe thats not the way you are supposed to do it.

i dunno.. i think ive tried it without holding the valve closed but it only jumps to around 60 on both.. with a new engine it shouldnt be that low....
so when i held it i let it jump until it hit its highest# (120) then i let go and did it again....

It's supposed to be the top plugs.
http://fc3spro.com/TECH/FAQ/compress.html



-Ted

- from fc3s.org

is that true? haha thats funny cuase i got 30-30-15 and 10-10-2 when my engine was cold (impossible to warm up, car is dead)

I was sure that seals expand when the engine was hot so the compression was better...

checking compression is great and all but i just dont see the point in going out and checking your comp., just for the sake of knowing unless you are ready for a rebuild. wow you know, but there is nothing you can do about it unless you rebuild.

Yah, but if you find your compression is a bit low you might hold off on some of the other upgrades until you get the engine rebuilt. Without knowing, you might blow your wad on some upgrades and then have nothing left when the engine dies. :: shrugs ::

compression also depends on how fast the battery/ starter can crank it, I think...

Also it gives you an idea of what shape your engine is in, I think thats important info. I mean for example wouldnt u wanna know if your heart was dying lol? There isnt much to do except a heart transplant (like an engine rebuild or swap). A bit drastic but it gets the point across :D

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indeed it is important to know that your engine is/isnt healthy, but the butt dyno is more than adequate for that purpose. if it still pulls strong, what do the comp #s matter. and if it runs bad and is obviously blown, again what good are comp #s.

heck a friend of mine's s4na only has like 85 psi and it will still keep up pretty well with my s5 that has a much newer engine (well, hes lighter and has more mods tho)

also, i dont think low comp. is as much as an issue on a rotary, espcialy if its from apex seal wear because the blow bye will just go into the next rotor face/chamber instead of wasted into the crankcase like in a piston engine.

Wouldn't this be a good test for a potential rotary vehicle you're looking to buy???



-Ted

i have a 87 tII me and my brother did a comp test and we got 99 psi in the front and 85psi in the back does that mean my engine is bad thanks nick

Mazda minimum spec is 60psi with front to rear difference of 15psi max (I think).
With that said, it's fine under Mazda guidelines.



-Ted