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High comp. pistons vs. "GP" gasket ???

4K views 27 replies 11 participants last post by  748R_bb155 
#1 ·
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????
 
#2 ·
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.



 
#6 ·
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.



 
#7 ·
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.
 
#11 · (Edited)
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
 
#13 ·
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...!!!
 
#14 ·
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.



 
#22 ·
Ohh don't get me wrong, I feel the same way. I only race in classes with WSBK rules because I enjoy the "tuning" aspect of racing. Its kool to build a better bike then everyone else and win not just because your an OK rider, but because your bike is just that much better.

But the guys who cheat are mostly inline 4 guys. I have a buddy who's got a GSXR thats putting out 190rwhp and he's running in stock class! WTF? :eek:
 
#24 ·
No way! Its just like the rule that if people think my motor is over-bored, I have to pay for someone to take it apart and prove its not. Nobody is gonna go through that hassle, its just not going to happen.

Once you become a racer, you realize very quickly, lots of people ride the line of cheating, but only the best riders out there ARN'T cheaters.
 
#28 ·
No need to say you can't have both.

If you set squish to spec on the 748 R/995 its 0.95-1.05 mm.
One the 749R 1 to 1.1 mm.

the sqush about 1 mm is what the factory says it should be.... No need to go below to get nice "knocking sounds.

HC pistons is for minimising the cobustion chamber, which is also the effect gained from skimmng the heads.

Thats why : squish set to spec, and hc piston used and you would be a habby camper.

And the compression in the ducati spec when sad to be 11.5 is probably 11.2-11.8 dependent on the assemby that day.


a little cut and paste from sigma performance :
Squish setting is not a change to the compression ratio, although it does increase slightly as a result, it is getting the distance between piston top and head surface down to a point where at maximum revs the piston does not quite touch the head. Checked in static conditions the clearance closes up as revs rise and the rods stretch and the cases flex, it is important not to get it too tight as a touch will destroy the engine in fairly short order.


EO
 
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