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Discussion Starter #1
Hey guys. Cometic are now offering a multilayer headgasket that is .018" inches thick. They make custom gaskets with no minimum production run. I don't know if the construction of this gasket would preclude it from the air-cooled twins or not. If we knew the volume, or at least the thickness, of the OEM gasket, we could derive the change in the (nominal) compression ratio with an ultra-thin gasket such as this with some straightforward math. Obviously, the piston deck height and piston to valve clearances would deserve a look. The clearances are likely to be cavernous, anyway. It seems that I've mentioned this around here before, but I'm far from my bike for now and ideas like this get stuck in my mind like a booger...just gotta get it out there :-D
 

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The trick is to get a used headgasket and measure that. Then find out what the thin gasket ends up at after torquing. Best to try one out and see. Thickness when new isn't what is important, it's the thickness as it's fitted. I suppose there is a little "spring" in it, but it should be negligible.
With the old bikes, we'd use the 8.5 to one pistons and eliminate the headgasket, using copper wire to "o ring" the cylinder. Ended up at about 10.5 to one, with better spark propagation than the TT pistons gave, so needed less spark lead. Can't do that with all the oil holes and chains in the new heads.
 

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I would say .028 would be good for .5 compression increase, I suck at math but that's the rule of thumb with a SBC.

Jojje may be able to provied a better educated answer off the top of his head.

Greg
 

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Discussion Starter #8
I did the math.

It works out to be a (nominal) CR of 9.278 assuming a compressed thickness of .018" on the Cometic gasket. This is a very loose estimate, as there are alot of variables that need to actually be measured. The calculations were made using a swept cylinder volume of 865cc and a static CR of 9.2:1 as assumptions. The benefit of this would be found more in the tightening of the quench area(s) of the combustion chamber, rather than the absolute inrease in static compression ratio. This benefit would be difficult to qualify, as it would manifest itself in increased combustion efficiency and throttle response, rather than raw dyno numbers. The butt dyno might notice it, an inertia dyno would never notice it. This is also often times the case when the ignition timing is advanced a few degrees from stock.

I would find the uber-thin gasket a viable course to pursue if the head was to be removed for porting anyway, but not so much as a stand alone action.
 

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Is there room to, as we used to say, shave the head. If, as I believe some have said, most of the hp increase with 904 kit is due to raised compression. Then wouldn't shaving the head produce similar results without the expense of new pistons etc?

Darcy
 

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A stock gasket is 0.8mm, using a 0.018 gasket will give you 9.55:1 CR. You can shave the head but if you want to use bigger valves in the future you might have valve clearance problems. I shaved the cylinder 0.3mm on my Wiseco and now have 11.1:1 CR, 75hp and 88NM with stock carbs, stock mufflers (one baffle removed) and 3 deg ignition advance (not a ProCom)
 

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Discussion Starter #12
Drat! I read Jojje's post and say to myself, "Hmmm, Self, that's not what you calculated." So I calculate it again, and my numbers agree with nine and a half to one. TDC volumes of 105.488cc stock, 101.727 w/ .018" gasket.

Since several cats here have had their engines apart now, does anyone recall what the piston deck height was on their particular engine?
 
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