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When I raced MotoX we used what was termed a GP gasket or a very thin copper gasket which in turn would increase the compression in the bike. But we're talking single 2 strokes, now can a thin "GP" type gasket be used in a Ducati L-Twin to increase compression and would that be an alternative to a hi comp piston installation????
 

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It really depends on your application...

For race machines, I usually run my motors at .95mm squish. Anything below that and you run a bigger chance of ruining things quicker. High comp pistons, generally work better then setting squish less then .95mm.

For street machines, its safer to keep it OEM. The higher compression system will wear out your motor much quicker and you'll be doing rod bearings before you know it.



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I recently had some enginework which involved (amongst other things) machining 1mm off the heads - much cheaper than high comp pistons and equally as effective. ;)
 

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Which actually has the same effect as reducing the head gasket size. ;)
Almost but not quite, fitting a thinner head/base gasket will bring the head closer to the piston which will tighten squish and, of course slightly increase compression but machining the head reduces the size of the combustion chamber which increases compression but has no effect on squish, in a similar way that a high compression piston, with raised 'dome' would do.

As you know you can only go so far with the squish but having gone that far a 'skim' off the head will increase compression for much less $ than a set of high comp pistons.

My experience of 2-stroke MX bikes is that the inside of the head is piston 'shaped' so that changing gaskets increases compression because the piston better fills the combustion chamber. Until recently I had a KTM 500 MX for which many different thickness of base gaskets were available (no head gasket IIRC) and these were used to obtain different powerbands by increasing/decreasing the compression.
 

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If you've got 1.05mm of squish... And you lower it to .95mm via the head gasket, you're doing essentially the same thing as shaving metal off the bottom of the head. You're taking the combustion chamber and making it closer to the piston, hence increasing squish. The only number that really matters is the squish, because its a pretty decent barometer of the changes being made.

Now a lot of people machine the combustion chamber out a bit, which actually can increase squish just a very small about. I understand if you machine out the head a bit and then sand the mating surface down to compensate, that makes sense to me, but we're talking, .10mm or so.

On 2 strokes, the world is totally different, because you don't have to worry about valve clearance and such.



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If you've got 1.05mm of squish... And you lower it to .95mm via the head gasket, you're doing essentially the same thing as shaving metal off the bottom of the head. You're taking the combustion chamber and making it closer to the piston, hence increasing squish. The only number that really matters is the squish, because its a pretty decent barometer of the changes being made.

I think the point the OP is trying to make is that if you have 1.05mm squish and reduce it to .95mm your talking about a tenth of 1mm or .004". If you mill the head 1mm your talking about .040" so the volume reduction is MUCH more significant and you don't have to worry about banging the piston off the head. You need to verify you still have proper piston to head clearance if you're running a domed piston as well as valve to piston clearance and you may need larger belt idlers if you cut enough off the head but this approach will move the compression ratio a lot farther than tightening up the squish.
 

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I think the point the OP is trying to make is that if you have 1.05mm squish and reduce it to .95mm your talking about a tenth of 1mm or .004". If you mill the head 1mm your talking about .040" so the volume reduction is MUCH more significant and you don't have to worry about banging the piston off the head. You need to verify you still have proper piston to head clearance if you're running a domed piston as well as valve to piston clearance and you may need larger belt idlers if you cut enough off the head but this approach will move the compression ratio a lot farther than tightening up the squish.
Indeed and since stock 916 pistons are flat topped they won't hit the head if squish is OK.

1mm off the head gives a significant increase in compression over and above anything gained by tightening the squish by 0.2mm and the whole job; flow/skim etc cost less than a set of HC pistons and I doubt that fitting HC pistons by themselves would make 118hp/71lbft from a 916.

Work by Chris at http://www.cjsracing.co.uk/

;)
 

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Neither will a High Compression piston if the head hasn't been shaved! Piston designers take combustion chamber shape into consideration when designing a slug.

It's all Apples to Oranges. He flowed the heads as you mentioned below in you post.
Here's a link to a thread where Chris says he worked the heads.
http://www.speedzilla.com/forums/ducati-superbikes/54260-another-916-a.html

So it's not all about compression.

I would much rather have lighter, stronger well designed HC pistons in my motor making good power, than the stock heavy OEM slugs! Your rod and main bearings will thank you.

Those heads are now only good on that bike or another stock bike. Using Omegas or Pistal will require some head gasket shimming to get them to work with aftermarket HC pistons.

There are many ways to skin a cat, cutting cost usually isn't the way to go in the long run.
If I was looking for a 916 to build up I would steer clear of that bike and those heads or source another set. If I didn't want to touch that motor, I'm sure it would be perfectly fine as a street bike.

Indeed and since stock 916 pistons are flat topped they won't hit the head if squish is OK.

1mm off the head gives a significant increase in compression over and above anything gained by tightening the squish by 0.2mm and the whole job; flow/skim etc cost less than a set of HC pistons and I doubt that fitting HC pistons by themselves would make 118hp/71lbft from a 916.

Work by Chris at http://www.cjsracing.co.uk/

;)
 

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I think the point the OP is trying to make is that if you have 1.05mm squish and reduce it to .95mm your talking about a tenth of 1mm or .004". If you mill the head 1mm your talking about .040" so the volume reduction is MUCH more significant and you don't have to worry about banging the piston off the head. You need to verify you still have proper piston to head clearance if you're running a domed piston as well as valve to piston clearance and you may need larger belt idlers if you cut enough off the head but this approach will move the compression ratio a lot farther than tightening up the squish.
Yes, that makes sense, but it only works on a certain amount of heads. For instance, my 749R's heads already have a extremely thin gasket and are already putting out roughly 13:1 compression. You might be able to increase that slightly, but its not gonna be a whole lot without reducing squish to levels that would be too close.

Of course, with desmoquattro's, shit those things are built so loose, you could shave 1mm of the heads and still have room to play with. We shaved a whole lot off the last 748R I built (I forgot the numbers) and even then, the compression was still not very high. :(



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On my Guzzis and ST2, I removed the base gasket to get .80 - 1.00 squish, and just use 3-bond.

On a Ducati, shaving the head 1mm is the same as reducing the head (or base) gasket by 1mm, as the squish band is NOT equal to the sealing surface!

You will need to re-check you valve-piston clearances in either case.

Tom
 

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So it's not all about compression.
Indeed it's not all about compression but shaving the heads is an alternative to HC pistons, I wanted to make a a little more power and the cost of lightweight HC pistons, new rods, crank balance plus bottom end blueprint etc could be as much as (or more than) a 916 is worth and certainly more than I was prepared to spend on an 11 year old bike, I'd rather just buy an 1198.

Right now it runs great and for less than the price of a main dealer service I've got more or less stock 998 power, It's not a race bike so hopefully the engine will last but if it breaks it can be fixed, replaced or I'll get a new bike.

As for using HC pistons with these heads yes they would probably require a little work to fit but I have no plans to do so, I'm finished (yeah right :rolleyes:) spending big money and I just want to ride it now.

:)
 

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If I wanted to bore and put high pistons in an 848 (i have disposable money and like to play) is it advisable, would I get any significant gain...? Would be nice to mod it to be a sleeper...of its just an 848...wham gone...!!!
 

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848's are capable of doing 150rwhp with lots of mods. But high-comp pistons are very low down on the list. Full exhaust, cam timing, Nemesis ECU and of course larger air runners/air filter. After you've done those, then you can get into high-comp pistons, setting squish and porting the heads.
Why do all those mods and pay that money when you can just get a 1198?
 
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