High comp piston
 

Hey Kev, Freez, others...

Anyone know the effect of just a hight comp piston - now other mods?

- GF-

We used to race Supermono (single cylinder in road chassis - generally RGV chassis with XT600 motor) and we used high comp pistons, and bored out as well

Now, I don't know about the XT660X motor, but we had to run avgas (otherwise would blow head gaskets) and would crack crankcases fairly often - about every 18 months. But they went like you know what!

Also ran different camshafts.

As I said, don't know about the new engines, especially in regards to fuel injection over the carbs we used to run.

I reckon reliability would be compromised with a high comp piston.

That mod would be great & I have tried for sometime to find a piston that would work, I even had JE pistons doing some research for me on the dimentions I gave them.

Todate I have not found a straight drop in piston 100mm bore with a higher compression then 10:1.

I have been doing a lot of research into all kinds of performance parts and if the XT and raptor sleeve length is the same, then it might be possible to increase compression without replacing the piston. Taking the stock XT's 10:1 compression to around 11 and maybe a bit higher. That compression should be fine with a nice high octane fuel you can get from a normal fuel station and should add 1 or 2 HP.

If not, we can still increase compression with playing around with the head, but both options becomes a bit more complicated than just adding a new high compression piston.

Kev, do you know the combustion chamber volume on the stock XT head?

I was going to measure the swept volume when I stripped my motor but did not need to in the end to fit the Raptor piston.

It is easy to machine the cylinder base, to up the compression ratio. Then degree the camshaft to get the cam timing right. One would need a slotted sprocket & once the cam timing is set you would pin the sprocket to the camshaft.

Kev, I have started to loose a bit of faith in these off the shelf high compression pistons, especially the domed type ones.

If I work out mathematically, based on Volumetric efficiency, then I should be seeing massive power gains when you add a high compression piston.

For example on my XT, making 44HP at 5800 RPM on my dyno with a 10:1 piston , VE works out to around 101% If I do the same calculations with an 11:1 piston and assume VE stays at 101%, then I should be gaining around 4.5HP@ 5800 RPM with just the piston.

Now, I have never seen that big a gain from just a piston alone. Maybe 1 or 2 HP, but that�s it.

I think the dome pistons mess with the combustion process and also the flow in some way and therefore don't see the big gains.

My next bunch of experiments will be to see if I can use the stock, flat top pistons, push up the compression and hopefully gain a bit more power than the dome pistons.

On the raptor, you have a massive potential to play around with and I hope the XT has the same potential. The raptor guys claim you can push the stock piston compression up by 1 to 2 points. That�s a lot.

I was also thinking of measuring the cam degrees and then just having the cam and "mounting plate' split, turned to the correct degrees and then pressed back into place.

Otherwise I will see if we can slot the cam sprocket or even have a new, adjustable cam sprocket made up.

maybee you need some of this

http://www.gizmag.com/designer-fuel-offers-more-mpg-less-emissions-less-cost/9467/

????
:angel3:

Colin

OK so am I correct in understanding that:

1) We don't know of a high comp 'drop in' piston that is available for the XT660 motor?
2) A high comp Lets say (11.5:1) piston could yield anywhere from 1-2 hp though theoretically could be as high as 4.5hp?
3) No one has yet tried just upping the compression on this motor and measuring the results?

Both my Yamaha wr250 and wr450 had 11:8 compression ratios and worked perfectly on 98 octane pump gas. Why wouldn't the XT660 at the same compression or 11.5:1?

1/ We do not know of a HC piston of 100mm bore that will fit the XT.

2/ We do not know of a HC piston of a 100mm so the HP gain is theorey only if you are using a 10mm bore. We do know the HP gain with 102mm bore with a 11:1 piston.

3/ I have upped the compression from 10:1 to 11:1 using the Raptor 102mm piston & have full dyno results of Torque & HP gains.

I will make my next XT+++cc conversion 11,5:1. The Raptor boys run up to 14:1 with race fuel.

Is the 660 Raptor engine the same as the XT engine?
If so, these guys claim to have 100mm 11:1 pistons for the 660 Raptor engine.