Yeah, I would never go higher than 12.5:1. Marvin @ customcraft said they supply a 12.5:1 in their 330 kit and some on this forum have said they have 11:1. That's why I asked and all other things equal I bet the 12.5:1 crackles nicer !

 

I just put my 331 kit last weekend and thought I'd show a comparison of the pistions. The height above the wrist pin is exactly the same.

a little less skirt and a lot more diameter

Sorry Deej, I guess that UPS didn't get your message.

 

So with the height above the wrist pin being equal, where does the compression ratio increase come from?
The subtle differences in the shape of the dome?

The impressions for the valve heads appear to be deeper in the stock piston.
I think maybe that's because the JE piston has a little "valley" on top?

 

I originally asked the question as analternative optionto looking for cams to increase HP. Sounds like there is more than one offering out there in the big bore kits. Folks are looking to cams to increase HP, which often results in a loss ofTorque at the bottemend (power is usually raised but peaks higher in the rpm range). Higher compression willgive youan increase in HP and Torque across the whole range. It would be interesting to know how much more the 12.5:1 will add to the increase in torque. It will still allow the use of pump gas and give more torque than 11:1. Note the dish in the piston, not only does the wider bore pushing a bigger volume of air into the same head area (naturally resulting in higher compression), but that dished area is what probably results in the 12.5:1. A flat top like the original would be lower.....

In short, unless you are going for an all out high rpm dirt burner, increasing your compression MIGHT be a more effective way to get what you want versus cams. I assume there is even a piston available in the stock 250 bore. Or you may want to do BOTH ! In any case, if doing a big bore you should consider the higher compression offerings, but don't go over 13:1 for reliability and gas reasons.

 

When increasing bore size but not increasing combustion chamber size, you have increased compression ratio, all else being the same. A domed piston in a stock bore keeps the piston swept area the same but reduces effective combustion chamber size, the reverse of boring. I know it can be confusing...but the principal is really pretty simple. Otherwise I'd be confused too.

Bill Dragoo
Norman, Oklahoma

 

Warning: Math Content

So I ran the numbers based upon Gary's photo and here's what I can up with.

The two pistons and/or the gaskets have a "residual" volume of 15.6 cc (JE) and 11.7 cc (stock). (this residual amounts to less than 3mm of height and is due to differences in the thickness of gasket materials, if any, and the shape of the dome of the piston) It certainly appears that the JE piston can give about 4 cc more of displacement with the dishing out.

The head volume is about 13.1 cc (except on Bill's bike )

Stock displacement is 249.1 cc and the KK displacement is 331.1 cc (Vdisp = stroke * (bore / 2) ^ 2 * pi) stroke = 61.2 mm, bore = 72mm and 83mm

If you use these numbers in a ratio calculation: ratio = (Vdisp + Vhead + Vres) / (Vhead + Vres)) You'll end up with 11.0 and 12.5

 

I just realized that Marty was the original author of this thread......I wonder what ever happened to him? Sorry didn't mean to thread jack, as you were.

 

What did he say????? [sm=smiley23.gif]

JK

 

No kidding, it's good there's people like mooredan around, doin the math n stuff, so that we don't have to!

 

OK, let's talk camshafts again. The list of things to do is getting shorter and I'm now interested in this. I need to educate myself in what's supposed to happen by changing overlap, centerline, duration, ..... (Sorry, just can't help myself, it's just the scientist in me).

Looked over this thread again and made an inquiry at Web Cams. I shared with them the details of my bike and the sort of riding I do. Faith (of Web Cams) recommended the #293 grind. As far as I know, no one here on the forum has gone with this grind yet. (If you have, let's hear about it!)

Others have reported great results with the #101 grind.

Can anyone explain to me what the effect would be between the #101 and #293 grind?

Also, does anyone know what the stock valve lift is?

Thanks.