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What is the compression I should be getting from my 1998 Yamaha XL1200? I am in the process of rebuilding the motor. Is a variation in compression among the 3 cylinders acceptible?
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You should have compression between 110 and 130 at sea-level for that motor. The differenece between cylinders should be no more than 10%(11-13 psi), least to greatest, measured after break-in.
This is assuming you are talking about a new or freshly rebuilt motor - if compression drops during the engine's life, however much, you have damage and/or wear occuring.
[ February 22, 2001: Message edited by: Bryan Glynn - SBT Webmaster ] -
I had the same motor in a 98 GP1200. The stock compression is 125 psi @ sea-level. That's listed by Yamaha. I don't see how it can get to 135 psi in stock form.
I don't buy into that % thing between cylinders. My old 1131 Yami was light only 2lbs in the middle jug. I have a murdered Blaster 701 and it's dead even between jugs and so is my wife's 650 (1992). Dead even @ 142 psi.
My 99xpl is dead even at 150 psi @ 550 ft and it didn't move a bit from new.Comment
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Steve - I understand how you would be sceptical, but it is an industry standard, and well known, that a 10% variance (with the exception of Rotax due to ring design) is considered normal between cylinders. Do most, or even a large percentage of them come out that way? No, of course not. But that is the standard.
As far as your assumption of a set compression point, you are just mistaken. All manufacturuers have a range of acceptable compression for each engine - some higher than others.
[ February 22, 2001: Message edited by: Bryan Glynn - SBT Webmaster ]Comment
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Bryan, I certainly accept your info/knowledge on this subject as correct relating to the industry "standard".
But personally, I think it's a cop-out (by the industry) and I'll stick to the even or almost even compression "yardstick". Even both my sleds are almost dead even ( 1 Fugi/ 1 Suzuki)Comment
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Steve my 1997 raider had the weirdest compression when i got it new. #1 has 122 PSI #2 has 132 PSI and #3 had 125 psi. someone told me that is how yamaha keeps the heads cool by varying compression. I took the head in and had it milled. my compresison went to 138 all the way across, and currnetly it went to 130 all the way across. Dont know why it dropped , but im gonna have my gauge checked
RobComment
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Rob:
The higher compression in the middle on your stock raider may have more to do with the dynamics of the exhaust manifold runner. The back pressure from the exhaust is different in each cylinder of a ski that does not have symetrical exhaust manifold runners. I don't have an 1100 in front of me, hehe, but I bet the dynmatics of the all three exhaust runners are different. So Yamaha compensates for this by staggering the compression on the cylinders. When running at or near WOT is when the actual compression is in effect and so some cylinders would require either more head compression or less depending on the back pressure from the pipe through each particular runner. Sorry, best I can explain it here.
Consequently, having the head shaved equally throughout may cause difficulties.
One difficulty with this board's format is that you cann't refer back to the original post. So I cann't recall your numbers, but I would think that your compression bump should have paralleled your stock cylinder numbers. But, there again I could be wrong about your motor.Comment
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<BLOCKQUOTE>quote:</font><HR>Originally posted by Steve 99xpl:
One difficulty with this board's format is that you cann't refer back to the original post.<HR></BLOCKQUOTE>
Steve: It's actually quite easy. First, you can always use the "" button to quote a message. Second, if you look at the bottom of the page when you make a reply, you will see a window showing the thread, in which you can easily scroll to view any post in it. :DComment
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Ok Bryan, thank you.Comment
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