Thin Gaskets for Higher Compression

dwyatt

Santa is bringing me a set of JE 77mm pistons and I'd like to use a thinner, copper head gasket to raise the compression ratio from the 10-1/2:1 these pistons give, to about 11:1. Anybody done this and can you recommend a gasket thickness?

yamaman

Just need to enter all the details in here, make up the gasket size till it suits:

www.csgnetwork.com/compcalc.html

dwyatt

Way cool program, will have to measure some volumes first though, but cool.

dpmphoto

I read in that 650 performance manual that too much compression on the xs motor can actually hurt both performance and obviously reliability. I don't know if the performance part is actually true but i just thought u might want to investigate

dwyatt

Too much and this motor can go ballistic, but I'm not aiming too high. The new pistons I just got claim 10-1/2:1, but the Yamaha factory tuners ran this motor at 11-1/2:1. I just thought I check into getting a bit closer to that, but may just leave well enough alone.

Srinath

Flow is far more vital for power than compression.
The second aspect of a head gasket is somehting that people forget about, partially because they have done so well at it for so many years. A head gasket is supposed to blow in order to save the motor. If you check with old school racers they always analyse their failures and so many cases they find that they had a head gasket failure instead of lunching the whole motor. Like detonation or pre ignition etc.

Your thinner gasket could in fact fail to do that and get your higher compression motor to fail in a more $$$ way.

Cool.
Srinath.

jayel

not much point to hotrodding a XS anyway about 35 years late on that, you want a hotrod go buy a Ninja

dwyatt

Good point I hadn't considered on the head gasket blowing! Just looking to raise the compression slightly over the off the shelf JE pistons to be closer to the CR run on the 70's racers. Probably overthinking this. I may reconsider this part of the plan.

As to the flow, it has a Craig Weeks ported head, racing exhaust and 36mm flat slide Mikunis, now waiting on Megacycle cam, think I have that covered.

Only point of the whole build was the fun of seeing what I could do with it. I knew from the start I could buy a slightly used modern bike for less than I would put into this sucker (nearing $7k). It's been a blast to build!

The fun of hot rodding it will be seeing all the young bucks wondering what the hell that damned yellow POS is.

Srinath

You want flow, try the backwards head that that one guy did.

But yea carbs in front and exhaust in the back looks absolutely awful.

I'm also building a yellow bike ... I'd put a similar yamaha race pattern on it but I am thinkign down the middle on top, not the side. Yours looks great, but I am running the stock side covers etc and rear fender, so tank and fender have down the center, side covers have smaller down their sides.

Cool.
Srinath.

xsjohn

The exhaust side ofthe thead is what you want to cool...........in the back it will get real hot.......

dwyatt

I'm running your top end oil cooler to help out.

Srinath

xsjohn wrote:

The exhaust side ofthe thead is what you want to cool...........in the back it will get real hot.......

Yes and ... well a very small no ...
The exhaust will flow and get out of there better, so it wont be heating up the head and ports and what not as much.
Of course the thing will get less air, though an xs wiht its side angle exhaust ports will be better off than others like a GS500 with a straight on exhaust, but it still wont be as nice and breezy as out in front. Maybe put some extra air flow directing vanes like they did on old air cooled VW's. Not very great again for looks. BTW there is one xs being built that way. Lets see how it pans out.

Cool.
Srinath.

dwyatt

I'd like to see that. I'm too lazy to try it, but would like to see it!

yamaman

It's right here mate:

www.xs650.org.au/offbeat.html

dwyatt

Yup, I was right, I'm too lazy to attempt that!

What Happened to 650rider.com?

Yamaha xs650