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Highcompression forged pistons

  • Thread starter Thread starter ashdricky
  • Start date Start date
A

ashdricky

Guest
The bike is a GS674, I would like to know how high I can raise the compression and still keep it kickstart, right now I have a large oil cooler fitted which should lower temps. what CRs do the aircooled race bikes usually run?

Thanks - Rob
 
What kind of fuel? The 650 has a more modern combustion chamber than the other 2V GS engines, but I suspect you may run into troubles using pump gas with a compression ratio much higher than 10:1.
 
ashdricky said:
The bike is a GS674, I would like to know how high I can raise the compression and still keep it kickstart, right now I have a large oil cooler fitted which should lower temps. what CRs do the aircooled race bikes usually run?

Thanks - Rob
Click to expand...

Even at 674 cc's keep in mind you are only kicking over 168cc's at a time. People have been kicking over way more cc's for a long time. Last week I sold my 580cc TT500 flattrack race bike. It was kick started by the new owner (with some coaching;)) even with the stroker motor and 11:1 compression. For the record, This bike was run it's whole life on 110 octane leaded race fuel.
 
A normal 550 kicks over with just a flick of the wrist, not even a foot required. A little more compression ought to be OK. Where are you finding these high compression 674cc pistons? I need some too.
 
tkent02 said:
A normal 550 kicks over with just a flick of the wrist, not even a foot required. A little more compression ought to be OK. Where are you finding these high compression 674cc pistons? I need some too.
Click to expand...

I would custom order them from wiesco they will custom make them to your specifications. Could I run 13:1 ?
 
Not on pump gas! Not on the street either as it will make WAY too much heat. That is RACE engine compression. Ray.
 
rapidray said:
Not on pump gas! Not on the street either as it will make WAY too much heat. That is RACE engine compression. Ray.
Click to expand...
Thanks ray, what Is the highest I could run on the street? I have a huge front mount oil cooler.
 
I am running 11-1 cr pistons on the street. I buy premium gas all the time for it and carry octane booster. That is a 4 valve combustion chamber though, with flat top pistons, allows higher compression without detonation. Compression tests show about 180 psi dry/cold.
 
is it not practical to run a 12-1 CR in an 8-valve motor for street use?
 
I'd recommend 11:1 as an outside figure for these bikes, the combustion chamber isn't designed for it. Burn governs your efficiency and a slower burning chamber like the 8 valvers will not tolerate a really high compression ratio easily - you're talking custom ignition, race fuel, extra cooling... the works.

I would recommend around a 10:1~10.5:1 as starters as I think you're only at about 9:1 now anyway. Any more than that and you really are talking serious modifications that the bike was not ever designed to take.

CHeers - boingk
 
A cheaper way of getting slightly more compression is to fit a thinner base and head gasket - talk to local tune shops about this possibility.

If you are just after an edge to what you already have then this may be the best way forward.

Cheers - boingk
 
boingk said:
I'd recommend 11:1 as an outside figure for these bikes, the combustion chamber isn't designed for it. Burn governs your efficiency and a slower burning chamber like the 8 valvers will not tolerate a really high compression ratio easily - you're talking custom ignition, race fuel, extra cooling... the works.

I would recommend around a 10:1~10.5:1 as starters as I think you're only at about 9:1 now anyway. Any more than that and you really are talking serious modifications that the bike was not ever designed to take.

CHeers - boingk
Click to expand...

If i did get new pistons I was planning on ordering new cams as well, I currently have a dynatyek 2000 so I am able to program my advance curves. Is that what you meant when you mentioned "custom ignition"?
also I have a large oil cooler mounted up front,

0099_zps6672e381.jpg


It's not a very good shot of the oil cooler but it dose give some perspective.
 
So again, I'd be leary of going above 10:1. Detonation is BAD, and can hole pistons and destroy engines. Pushing the limit is risky, for minimal gain.
 
Nessism said:
So again, I'd be leary of going above 10:1. Detonation is BAD, and can hole pistons and destroy engines. Pushing the limit is risky, for minimal gain.
Click to expand...

What Ed said above^^^^^^

Holing pistons on a racebike is one thing.... doing it on a street bike miles from home is quite another. Choose wisely "grasshopper!"
 
slayer61 said:
What Ed said above^^^^^^

Holing pistons on a racebike is one thing.... doing it on a street bike miles from home is quite another. Choose wisely "grasshopper!"
Click to expand...
haha apearently my father is not the only one who calls me "grasshopper" when I am getting wound up about a subject. Possibly it makes sense to get it running as it is, considering all of the variables that we already have.
 
Yeah , get her running as is before you go tearing her apart. Most likely you'll love it.

One saying to remember for performance, too, is that there's no replacing displacement.

- boingk
 
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