Wet compression test

12031 Views 22 Replies 9 Participants Last post by clevermetal, Jun 27, 2010

G

Discussion Starter · Jun 16, 2010

Just want to clarify the meaning please guys.

From what I have learned, I should just put a syringe of motor oil down the spark plug hole and attach the same pressure test guage and perform the test giving the bike another 4 cranks?

I know what ratio the bike should be (i think 11.5:1), can someone please explain the algorithm to determine the compresion ratio given a wet and dry test result? or is there more to it than that?

Thanks

1 - 20 of 23 Posts

CBR_RR

· #2 · Jun 16, 2010

cant help with the test but can confirm the compression ratio is 11.5:1

King Kenny

· #3 · Jun 16, 2010

Nake sure you do the dry test first before comparing the results. The should be within 10% of each other - any more indicates ring issues.

A aerosol can of spray oil (3 in 1, etc.) with a nozzle stray through the spark plug hole for a sec does the trick nicely too.

juxt

· #4 · Jun 16, 2010

Make sure you don't put too mcuh oil in there...

When I was testing compression on the postie, someone told me that the 11.5:1 and the 140-160 that you will find on the compression tester are 'different' scales and cannot really be compared.

I may be wrong, and I'm sure someone will correct me if I am...

Humbug

· #5 · Jun 16, 2010

11.5:1 * 14.5 psi (atmos) = 166.75psi abs no?

G

Discussion Starter · #6 · Jun 16, 2010

Humbug said:
11.5:1 * 14.5 psi (atmos) = 166.75psi abs no?

what is atmos? atmospheric pressure? and is abs absolute? small words for the non-talented please

smiley_666

· #7 · Jun 16, 2010

compression ratio is the mechanical compression - the reduction in chamber volume and is fixed. ie, with a set of given pistons, rods, crank, head, block the compression ratio will be the same everywhere under all conditions. in a perfect world the measured compression will match this, but the world aint perfect. there will always be a (very) small amount of leaking around the rings, through valves etc so it will be a bit lower.
and dont use the full syringe of oil in it - 3 or 4 drops is plenty

Drew

· #8 · Jun 16, 2010

If you use too much oil in the wet test you'll get too high a result. Most of the things I've read say to put in a teaspoon, but a few drops or some spray lube (as Kenny said) would work perfectly.

Humbug

· #9 · Jun 17, 2010

roobars said:
Humbug said:

11.5:1 * 14.5 psi (atmos) = 166.75psi abs no?

Click to expand...

what is atmos? atmospheric pressure? and is abs absolute? small words for the non-talented please

correct. don't forget you would need to subtract one atmos to obtain guage pressure...

G

Discussion Starter · #10 · Jun 17, 2010

Humbug said:
roobars said:

Humbug said:

11.5:1 * 14.5 psi (atmos) = 166.75psi abs no?

Click to expand...

what is atmos? atmospheric pressure? and is abs absolute? small words for the non-talented please

Click to expand...

correct. don't forget you would need to subtract one atmos to obtain guage pressure...

Cool man, cheers.

Now where did the 14.5psi come from

a hurricaine has 12.5psi atmospheric pressure in its center, so I would be really supprised if 14.5 was normal. Or did I miss something? I need an engineering handbook or something I think!

edit: ok my bad, 14.5psi = 1bar

wiki says:-

for a compression ration of 10:1 - So I assume that

pressure TDC = 14.5 * 11.51.4 (power of 1.4 for air heat ratio)

correct my errors please!

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Drew

· #11 · Jun 17, 2010

14.7 PSI = 101.3 kPa = 1 Atm = 1.03 Bar = 29.92 inHg = 415.2inH2O = 760 mmHg

I prefer to stick to metric units because they're not totally insane and bullshit, but there are some conversion factors for you.

smiley_666

· #12 · Jun 17, 2010

i dont understand what all this is about. (i know about pressures etc)
you do a compression test. you do a wet compression test. you compare. if wet is better than dry by a bit more, rings are stuffed. if both are lower than everyone says they will be, youve got other problems (valves, head, gasket etc)
you dont really need to work it all out perfectly if you have a base to start with ~150ish psi

G

Discussion Starter · #13 · Jun 17, 2010

smiley_666 said:
i dont understand what all this is about. (i know about pressures etc)
you do a compression test. you do a wet compression test. you compare. if wet is better than dry by a bit more, rings are stuffed. if both are lower than everyone says they will be, youve got other problems (valves, head, gasket etc)
you dont really need to work it all out perfectly if you have a base to start with ~150ish psi

I like to know details mate. Anyone can take measurements and compare them to what's in the manual, I like to be the fool that knows why you do these measurements, and how they are calculated.

I find by learning anything with this amount of detail helps me identify faults, potential problems, dangers etc. without being explicitly shown by someone in the "do this but never do that" fashion. This also creates a better base for intuitive and resourceful fixes/solutions to problems.

Just have a thirsty mind...

smiley_666

· #14 · Jun 17, 2010

yes, but by figuring it out this way means not a whole lot. like i wrote above, it will always be out by a fair bit to the 'theoretical' pressure from the compression ratio

G

Discussion Starter · #15 · Jun 17, 2010

smiley_666 said:
yes, but by figuring it out this way means not a whole lot. like i wrote above, it will always be out by a fair bit to the 'theoretical' pressure from the compression ratio

I agree, couple that with my $30 compression tester made in china and its going to be way off. But like I said, It's just for better understanding of the process.

Humbug

· #16 · Jun 17, 2010

smiley_666 said:
yes, but by figuring it out this way means not a whole lot. like i wrote above, it will always be out by a fair bit to the 'theoretical' pressure from the compression ratio

hardly scratched the surface man

smiley_666

· #17 · Jun 18, 2010

^in what way? (not sure if your agreeing with me or arguing :lol: )

K916

· #18 · Jun 18, 2010

So what were the test results like?

G

Discussion Starter · #19 · Jun 19, 2010

K916 said:
So what were the test results like?

Well, I have a sneaking suspision that this chinese compression tester isn't worth a pinch of s..t but this is what i got:

cyl 1 2 3 4
dry 110 105 102 100
wet 140 120 130 110

This engine ran smooth as, even started properly when I crashed, ran smooth no problems all the way home, then got parked... then tested then. So, I thin the guage is rooted. However, there was some excessive amount of black crap/out on the cyl 3 plug (has been doing that for a while) so I think new pistons and rings are in order... seeing as this is educational project and all.

are ebay pistons + 0.5mm rings worth it (i think i remember studricho having some problems) or just go oem? (ebay is 100/cyl)

edit: ignore the ebay thing... eom are only $20 more for a cyl (according to cmsnl) so I will just go with them.

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smiley_666

· #20 · Jun 20, 2010

id say the compression tester is fine. seems like all the rings have seen better days

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