what are good numbers for a compression test?

[takirb]

i was awaiting a gsxr 750 motor today, and instead the seller accidentally mistagged the motor and it ended up being liquid-cooled so i was in the market for another motor, again...

found one with 6300 miles on it, and compression numbers are 160, 155, 158, 160. they're all consistent, so that's gotta be a plus. it's from a 92, the low mileage caught my eye...

[The CyberPoet]

Re: what are good numbers for a compression test?

i was awaiting a gsxr 750 motor today, and instead the seller accidentally mistagged the motor and it ended up being liquid-cooled so i was in the market for another motor, again...

found one with 6300 miles on it, and compression numbers are 160, 155, 158, 160. they're all consistent, so that's gotta be a plus. it's from a 92, the low mileage caught my eye...

Those numbers are superb for an older run-in engine

Cheers
=-= The CyberPoet

[The CyberPoet]

[ADDED]:
Checked the factory manual for the 98+ 600's and they say the acceptible range is 142 to 213 psi, with all cylinders being within 23 psi of each other. Service limit is 114 psi (i.e. - the point where repair is mandatory).

You may find that his valves are set to .0055" or so, and if you set them down to .0045, those values would go up.

Still a very healthy older engine...

You did check it warm, right?

Good article on this:

http://www.motorage.com/motorage/article/articleDetail.jsp?id=16170

Cheers
=-= The CyberPoet

[takirb]

oops, sorry, forgot to add that this is a 92 katana 950 engine, no gsxR

i haven't had my hands on it yet. this information was provided from seller. he said that he bought the bike from a guy on the side of the road 'cause he was selling it cheap and nothing was wrong with it. he drove it back, did the test, and tore it down. his words were "the engine was still warm when i pulled it". i wentt ahead and purchased the engine, and he said it ships within 2 days via fedX. i'm hoping to have it by the weekend, but it'll prolly be next week.

link bookmarked, very good info, thanks!

[The CyberPoet]

doing a little math (based on some curiosity) yeilded this:

If stock air pressure is 14.7 PSI (ambient);
and the compression on the engine by the engine design is 11.3:1 on the 600's;
Then max compression of a new engine should be 14.7 x 11.3 = 166.1 PSI...

With the 750, the compression by design should be 10.5:1 if I recall correctly, so max compression should be 14.7 x 10.5 = 154.35 PSI

Thus (at least if my logical progression is correct), if the numbers are higher than that, it must be because of either a high-pressure front coming through the area (more air pressure ambient) or carbon/ sulfated ash build-up in the cylinder that is causing higher displacement.

I can't imagine what combination of circumstances would have to exist to get the 213 PSI reading that is considered the top of the acceptible range for the 98+ Kats...

Cheers
=-= The CyberPoet

[takirb]

would there be enough significant carbon build-up to occur in just 6300 miles to cause a 160 reading? the seller is in MA, so there's no telling about the weather...

either way, i figure with the low mileage and close readings, it's going to run better than my old engine with 23K miles...

[The CyberPoet]

would there be enough significant carbon build-up to occur in just 6300 miles to cause a 160 reading? the seller is in MA, so there's no telling about the weather...

Could be, if it was poorly jetted... easy enough to clean out -- just run a the first dozen tankfuls of high detergent fuels through and throw some techron into the first few tanks -- that'll clean up just about all of it, if it's there.

Cheers
=-= The CyberPoet

[md86]

Huh , when I tested mine , all cylinders were at around 100psi . 94-101 . Runs great , though . Everything I've read pretty much says it's not so much the actual psi values , but more the variance between cyclinders .

[The CyberPoet]

Huh , when I tested mine , all cylinders were at around 100psi . 94-101 . Runs great , though . Everything I've read pretty much says it's not so much the actual psi values , but more the variance between cyclinders .

100 psi on a 771cc WiseCo engine kit (which should have 13:1 compression ratios)?
Curious: did you recam when you put in the 771 kit?

Cheers
=-= The CyberPoet

[ThAzKat]

213 PSI would mean alot of carbon build up to me on a stock engine but with low mileage that should not be a issue.

[md86]

Huh , when I tested mine , all cylinders were at around 100psi . 94-101 . Runs great , though . Everything I've read pretty much says it's not so much the actual psi values , but more the variance between cyclinders .

100 psi on a 771cc WiseCo engine kit (which should have 13:1 compression ratios)?
Curious: did you recam when you put in the 771 kit?

Cheers
=-= The CyberPoet

Yeah , I thought that didn't sounds right either , so I asked Wiseco about that while I had them on the phone . They said the same thing , should be higher . I didn't test exactly like that page you linked , either . Didn't hold the throttle WFO while cranking . Wonder if that made any diffence . I didn't do the engine work to it , the PO did . Or maybe the guy that had it before him did some stuff , too . I dunno , but it was all kinds of messed up when I got a hold of it . All I know is that right now the bike pulls hard enough to easily toss you back in the seat if you're not ready for it . I'll be hitting a semi-local shops dyno in the next couple weeks cause I wanna see how my jetting is (feels pretty close to me right now) . Hopefully the dyno will tell me that I'm not down on power .