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Is there any way to safely lower compression?

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  #1  
Old 07-17-2010
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Default Is there any way to safely lower compression?

OK so I popped one of my engines apart to make sure everything is OK. It is. But the engine has over 200 psi in each cylinder. No, the gauge is fine... trust me... I know it's over 200. I'm sick of being worried about it, I'm sick of it going too fast (seriously), but mainly I'm sick of the battery having to be charged 100% to turn the damn thing over. It's a Rotax, I've already put the thickest base gasket you can buy for it on it, so what are my other options? Take a die grinder to the domes and evenly grind some away?

I'm not 100% sure why it's so high - either the top case was milled, the bottom of the jugs were shaved, the top of the jugs were shaved, or the head was cut. Honestly they all look factory so I couldn't begin to guess which one it was.
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Old 07-17-2010
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anyone make a copper head gasket for it?
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Old 07-17-2010
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I would think you could have the bowls machined out a little. I don't know about a die grinder though unless you have talent.
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Old 07-17-2010
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Use your base gasket as a template and cut out a steel gasket, sandwich it in between two OEM gaskets and you've got yourself a jug spacer. This way also allows you to pick how far you want to lift the cylinder based on metal gauge.
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  #5  
Old 07-18-2010
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by moving the cylinders up and down you are slightly changing port timing, on a stock sled it might not matter to much other than you might want to run 100 rpm higher than before? but testing will find that out.

usually ski doo heads have some engraved numbers on the bottom side of the head somewhere, so if they have been shaved you wont see that number.

its not to hard to have the dome cut a little.

cometic can make you a gasket any thickness you want alot cheaper than you can take the time to make your own.

I would double check timing, there might be a good chance that if they dropped the heads and then by rights they should have rolled the timing ahead.
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Old 07-19-2010
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^^hes talking about his 95xp, not his sleds....


so you not sure what alls been done to motor/top end?

thought you wanted it 180+psi w/race gas to go ubber fast....

post pic of head, probably milled. swapn head is cake
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Old 07-19-2010
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Quote:
Originally Posted by stagesrt4 View Post
^^hes talking about his 95xp, not his sleds....


so you not sure what alls been done to motor/top end?

thought you wanted it 180+psi w/race gas to go ubber fast....

post pic of head, probably milled. swapn head is cake
Master of all intelligence, ski doo and sea doo engines have the same characteristics, such as the marking on the domes or head. Like the madheifer said, if there is no marking on the dome then there should be some work done. As he also said you should be able to find an engine shop that can recut the domes to lower compression if you have the necessary measurements.
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Old 07-19-2010
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im aware....who coulda put that together....skidoo and seadoo are the same? lol


then why is he borrowing my head today?

stock heads are soooo cheap


especially a free one from a local member
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Old 07-19-2010
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Quote:
Originally Posted by Madcow View Post
by moving the cylinders up and down you are slightly changing port timing, on a stock sled it might not matter to much other than you might want to run 100 rpm higher than before? but testing will find that out.

I knew I was forgetting a minor detail lol

When I was looking to get stock domes bored out for my PSI cylinders the shop rates I found were about $50 a dome for re-configuration.
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Old 07-19-2010
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I'm going to snag Stage's stock head and see what happens. I really hope that lowers the compression, the only thing I'm worried about is the really thick base gasket I put on last time in an attempt to lower it.... if it's too low I won't be happy.
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Old 07-19-2010
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when was the last time you used this thing?

we can always just take out mine.


Ive just gotta put on new needle bearing, driveshaft, pump, and carbs on my 96 to finish to whole swap.



if you wanna swing by my place later i can give you garage code, i wont be home....
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Old 07-19-2010
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I had it out last fall. It's fine, just needed to lower compression.
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Old 07-20-2010
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any luck with new head?
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Old 07-20-2010
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Yep. Looking at it, it has the same model # but is different in a few ways. First, the deck height is different - second, the cooling route is slightly different, and third - the combustion chamber has like a chamfer in it that mine doesn't. Definitely will lower my compression - but how much We'll see....
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Old 07-20-2010
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get a syringe and CC it.
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Old 07-21-2010
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yea thats off the SBT rebuild, so they may have tweeked it a lil....


is your engine to more rare X model?
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Old 07-21-2010
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Yes it is. This is the 95 XP800 that only 500 were built.
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Old 07-21-2010
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yea im aware its the 95xp race model but is it the X model?


if it is, i wouldnt put anything but specific parts on that engine
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Old 07-21-2010
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I'm not 100% sure, but I believe it is for the following reasons:

-It has the setback pump/longer driveshaft
-This machine really was raced, and when I bought it the guy said only 500 were made. There were in fact about 2500 XP800's built in 95 but only 500 were 787X
-My pistons are marked .25 over (the .25 over gives it the 5cc's to make it a true 787 rather than the 782 of the rest of the '800 cc X4s
-The difference in the head - this is definitely the stock head from this engine. I'll take a picture of the bottom of it and see if anyone can spot the tiny difference (besides the chamber shape) between it and the one you gave me.


The only thing that makes me think it ISN'T the X, or the crank has been replaced is that the PTO coupler has what appears to be coarse threads and is huge like later model 800 CC x4's. I'd have to compete side by side to figure it out, or maybe the crank and PTO was swapped at some point.
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Old 07-22-2010
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if its the "X" model itll be stamped right next to the VIN on motor FYI



gosh, how do i know more about your ski than you lol


there were far less than 500 X models made....i can find out if you wanna know the exact #
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Old 07-22-2010
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i just picked up another 96xp hull last night for free if you got any ideas....?

951 swap maybe?
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  #22  
Old 07-23-2010
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lmk if ya need any part bryan

http://www.greenhulk.net/forums/show...p5859-part-out
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