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Thinking these are good numbers,


jon haney

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Did a compression check on the old BB drag bike last night, and wondered if these sound like good numbers:

Cyl. #1,2 & 4 190 psi. Cyl. #3 175 psi

This was done with throttle wide open, the engine stone cold, but cranked it enough that I'm sure the oil jets were squirting the cylinder walls. Figured that out when doing #4. The pressure shot up, so I went back and did the other 3 again. All 3 went up 20-25 more psi.

Motor is not stock. 81mm Wiseco pistons with valve pockets machined about .030" (didn't need to though). Machined .020" off the block, .004" off the head, and use a .018" head gasket. Stock head with stock cams and stock valves that the seats were re-ground. I'm sure I have at least 500 passes on it since this work was done. Spark plugs were all a nice light grey color. I have not done a leak-down test yet, but I'm not sure I need to.

I'm looking at building something different, so don't want to put any money into it, if I don't have to.

Should I just put some new plugs in it and race her another year?

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Did you check the valves to make see if anything is going on with that #3 from that end?

That's what the leak down test would do, but since it's within 10% of the others, I don't think I need to bother.

My question is whether the PSI pressure is good for a motor in that state of tune. I don't believe a completely stock Bird motor will be much higher than 160, but want to hear from someone who has performed the test, on a Bird.

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Looks acceptable to me, run a leakdown. Did you notice a decrease in mph during your latest runs? A lack of power would show there, and I think possibly a better indicator of issues.

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squirt some oil in and retest, will determine ring or valve for the difference. Also what was the baro pressure at the time. convert to psi and divide for static compression ratio. Calculate dynamic and compare.

10% differance in any is reason to renew.

But you knew that...

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squirt some oil in and retest, will determine ring or valve for the difference. Also what was the baro pressure at the time. convert to psi and divide for static compression ratio. Calculate dynamic and compare.

10% differance in any is reason to renew.

But you knew that...

Never thought about figuring compression ratio that way. The local airport weather site still had info for Sunday night, so I was able to get baro pressure (30.33"Hg). I assume I have to subtract the baro pressure from my readings before I divide for the ratio? If so, I got 11.75 to 1, which sounds right.

I'll try the oil thing. Just hope my dad has an oil can with a lonnggg, flexible tube on it.

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Also be aware that different testers will give different numbers. The gauge itself can read off, the valves in the testers will vary, and probably most important on a small motor is the hose used. A long, flexy, large diameter hose will soak up some of the compression pulse so it doesn't reach the gauge and stiff valves will add to that.

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Put 3-4 squirts of oil from a standard re-fillable oil can in each cylinder and re-did the compression check.

Cyl. #1 = 270 psi, #2 = 270 psi, #3 = 250 psi, #4 = 270 psi.

I guess an 80 psi jump is pretty severe? The valves seem to be holding well. What do you think Stan? Can I run her another year or should I re-build?

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Rering and 600 hoNE is what I would do. Replace the bearings.

I tied the pair system straight to the valve cover and plugged all the holes in the intake box. Pulled the valve out of the pair valve, but you probably have already ditched that. Made a big difference in the feel of the power.

You will want that vacuum with that light a hone job.

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250-270 is crazy high unless you have built an insanely high compression engine with mild cams which would't make sense. Any more oil than needed to wet the walls is going to yield invalid numbers as the oil doesn't compress. My guess is too much oil giving false readings.

Do it right, warm the engine up and then do the test. At the very least run it for a minute so the walls and rings are wet to their normal running conditions.

Edit: I didn't pay attention to the "3-4 squirts of oil". That's way too much and the reason for your high numbers. Run the engine and re-test or you'll never know what you actually have.

Edited by superhawk996
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Sea level at 14.6 BP and 13/1= 189.8

10cc oil gonna be a lot higher psi.

Numbers are not so important as stopping leakage at rings and deciding valves or rings, valves good, now 10% difference in dry to wet numbers. Hot would be good too.

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Since #3 remains the low one something's up. Maybe valve leakage, maybe adjustment, maybe head gasket, or maybe a build inconsistency. If you have previous numbers where #3 wasn't low it rules out a build issue. Check valve adjustment and do a blow-by test. If you don't have a tester you can rent one or just set #3 to TDC compression and use a rubber tipped blow gun to pressurize and see if there's a valve leak. A leaking valve or gasket will get worse if it's not tended to and could cause other damage. With lots of oil in the cylinder it's very unlikely to be a ring issue, but the leak-down test will tell.

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Hard to do a warm test when you have to remove the tank, air-box, coils, plugs, and wires. Maybe it wouldn't cool off that much, but.............

Put 3-4 squirts from the oil can on a paper towel and estimate that to be a little more than a teaspoon.

I'm not surprised #3 has less pressure. Last time I was in the motor (2008), that was the only cylinder that had a groove that I could catch with a finger nail, just barely.

That is also the piston that was originally put in backwards (2005) by the guy who did my big bore kit. Raced it two seasons that way. He made it right by replacing for free the valves, guides and seals for that hole, as there were two small divots in the top of that piston. I just smoothed the divots, and re-installed piston the correct direction when I did all the other work described in my first post. I know the head gasket is fine, and 99% sure the valve seats are good. It starts and runs too well for those to have a problem. If I do take it apart, I'm willing to bet the crank and rod bearings (factory original) still show little wear. I was amazed in '08 how good they looked. One of the reasons I'm sold on Amsoil.

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Just for a comparison reference Jon,my 91 600 had 13.5 JE pistons in it,had 205 psi in each when built and still over 200 20,000 kms and a few hundred passes later.

I also used Amsoil most of the time.

Coincidently my XX motor is apart right now too, going 81mm JE 11.5 and will shave the head,just not 100% sure yet how much,ball parking .015-.020,got a top guy doing it so not concerned.

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It doesn't necessarily need to be at full temp, tho that's ideal, but freshly run so that you eliminate the changes due to over/under oiled cylinders. Have everything prepped as much as possible so you can tear down and get in there in the least amount of time after shut-down.

The loss of some of the piston may be more to blame than the scratch, especially with a lot of oil in there as it would have to be more than a little scratch to really change compression. When he replaced those valves be probably ground the seats adding a little more combustion chamber volume. Since it ran well and you already have another build planned I say run it.

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That is helpful info, Partsman. I think I'm just going to run it.

Before you shave the head, install the new pistons and check to see how many hundredths of a mm they sit in each hole. Basically, you're checking the "squish area". Tomek told me that .70mm was a good number, and that is what I used. You may be better taking some off of the block as well, or instead. A .018" head gasket automatically takes .012" off the block.

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Been down a similar road already.When I put rings in 10 yrs ago we pulled 1 layer out of the head gasket,when it came off next about 5 yrs after that I found signs of a leak between #2 & #3 .Since I'm going 2 mm bigger now it made more sense to me to maximize squish area by shaving the head instead of going to the thinner gasket.I have a friend with his own bike repair biz who has a dyno and has built numerous race bikes and his own turbo ZX14 who confirmed this was the better way to go.

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My problem was using Wiseco pistons. I was advised on here (long after purchase) that they open up the squish area a lot, which I confirmed a couple years later.

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