Reply With Quotenice write up
Okay, I'm writing an article and I need your input/proof reading/corrections. Sorry it's a long read, but it's got some good information - I'm going to be adding calculations to an appendix along with all my sources.
Thanks, Ben K. :)
Informational Paper
XB9 Piston into XB12 Motor
In attempting to gain more horsepower and/or torque to the rear wheel, intake and exhaust modification along with tuning are the first steps. To take the bike further, there are 3 basic methods.
Turbo/supercharging - forcing more air into the motor via an impeller driven by the engine through a belt (supercharger) or exhaust pressure/velocity (turbocharger) to mix with more fuel.
Chemical - spraying highly flammable/explosive chemicals (Nitrous Oxide) into the intake manifold to mix with air, effectively raising the “octane”
Efficiency - increasing the efficiency of the motor so that less power is lost due to friction, inertia, etc.
All three methods can be very expensive and involving modifications. The focus of this informational paper
is raising the efficiency of the XB12 motor through increased compression through cost effective means - a set of stock XB9 pistons
A comparison between the two engine specs shows that 1) both engines have the same compression ratio and that 2) the bore of the two engines is equal. Additionally, the heads of the two are nearly identical along with the head gasket.
The major difference between the two pistons is that the XB12 is a flat top piston with valve relieves and the XB9 has a dome of around ~12.1cc. When the XB9 piston is installed into the XB12 motor, the added volume of the XB9 piston will decrease the combustion chamber volume of the motor. The stock combustion volume of the XB12 motor, per cylinder, is 66.8cc. With the XB9 piston in place, the combustion chamber volume is decreased to 54.7cc, which is also the stock combustion chamber volume of an XB9 engine, an 18% change. The result is a new compression ratio of 12:1. This setup has been tested by many people, including Eric Buell and is proven to increase horsepower.
The dyno graph below is a comparison between stock and XB9 pistons and is courtesy of Matt McCullough. Modifications include K&N filter, open air box, micron exhaust, and coated header. No additional modifications made between the two runs with the except of an “off-the-shelf” tune from americansportbike.com running with 91 octane pump gas. The map is “tune” for the following modifications: K&N filter, micron exhaust, and XB9 piston. It is not 100% specific to the motorcycle through dyno or data-log tuning.
http://www.badweatherbikers.com/buel...777/527608.jpg
Analysis of the run shows that horsepower is down after 6000 RPM, a 3.5HP loss in max power and there is no increase in maximum torque. More interesting is the 3-4 ft-lbs torque increase from the start of the run until 4.5k RPM, along with an overall increase in torque all the way to 5.5k.
Track Day Magazine did a track bike build-up using a Wiesco XB9 piston which is lighter and has a better side skirt although with higher cost than the Buell piston (Wiesco p.n. VT1737). The magazine reported very good results - lots of useable, increased torque below max torque. The build-up also included some light port and polishing and using the exhaust port sealing ring from Cometic as it has a bigger inner diameter to prevent the ring from getting in the path of the exhaust.

With the increased compression, there was been talk of pinging that can occur at high RPM, however Matt has stated that this has not been happening on his motor and is riding his bike with no issues. Track Day Magazine did not report of any issues either.
With this modification, data-log tuning is highly recommended. No two bikes are alike and therefore the best results will be achieved from custom tuning. The likely hood of pinging will also be greatly reduced once tuned.
If you are uncomfortable with the 12:1 compression ratio, but still desire some of the increased torque due to higher compression, consider placing a slightly thicker head gasket on the motor to reduce compression slightly. Cometic has a large variety of gasket thicknesses available, including a 56 thousandths gasket that would bring compression down to ~11.55:1.
nice write up
Nitrous is not a fuel it has slightly less than 10% oxygen which helps combustion but requires more fuel.
Hm...did I call Nitrous fuel? I reread what I wrote. I guess I might need to research Nitrous a little more. So it doesn't raise the "octane" rating. I put it in quotations since I know that it clearly doesn't do that, but acts like it?
What exactly does the nitrous do? More in depth than your post^^. I'm knowledge hungry!
EDIT: according to Wiki, Nitrous has more oxygen than the atmosphere - what would explain why more fuel can be mixed to maintain the correct afr. (almost like forced induction)
Any idea what the actual displacement is after you put the 9 pistons in the 1203cc motor?
And just so I'm reading the graph right, it increases low end but has a drop in hp in the top end, correct?
The actual displacement is the same, because the piston still sweeps the same amount of volume (as per the crankshaft). Yes you are reading the graph correctly.
I'm wondering if there is still more to gain through 93 octane instead of 91 octane and datalog tuning.
Too bad that I personally do not see this mod in the near future as I love riding my bike too much on a daily basis - and next winter I'm likely to be in the sandbox. But if I don't deploy, then I might. Never work on engine internals before, let alone removed a motor.
Thanks for the pointer to that Wiseco piston. That may come in handy. I'm hoping that $450 is for a set :D
Very pricey.
It raises combustion pressures. People that build nitrous engines usually lowwer their C/R run different cam profiles and build a stronger bottom end. Nitrous bottles for bikes are small so after a couple of hours of fun it needs a refill. The bigger systems that wouldn't be used on our bikes need their own dedicated fuel systems.
^
@Livers - that's for a set.
@twobuells - makes sense to run a lower CR.
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