Highcompression forged pistons

[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

[Nessism]

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.

[slayer61]

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

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.

[tkent02]

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.

[ashdricky]

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.

I would custom order them from wiesco they will custom make them to your specifications. Could I run 13:1 ?

[rapidray]

Not on pump gas! Not on the street either as it will make WAY too much heat. That is RACE engine compression. Ray.

[ashdricky]

Not on pump gas! Not on the street either as it will make WAY too much heat. That is RACE engine compression. Ray.

Thanks ray, what Is the highest I could run on the street? I have a huge front mount oil cooler.

[tkent02]

Thanks ray, what Is the highest I could run on the street? I have a huge front mount oil cooler.

And while you're at it, what's the biggest overbore that works with those 650 cylinders?

[gearhead13]

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.

[ashdricky]

is it not practical to run a 12-1 CR in an 8-valve motor for street use?

[boingk]

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

[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

[ashdricky]

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

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,



It's not a very good shot of the oil cooler but it dose give some perspective.

[Nessism]

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.

[slayer61]

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.

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!"