Had the bike dyno tuned this past Friday and here is the best we could come up with. Not bad but I was hoping for a little better between 2000 and 2700. You can't see it here but in this range, the torque plummets about 15 ft/lbs before starting back up again. Then it comes on strong: 5th gear run 87.27 hp and 82.60 ft/lbs @4800rpm (STD 1.02). Could have had a couple more at the top end but we adjusted the exhaust to get the best torque numbers possible. The pipe was the real variable in the tuning session. Came with three shims. All three were removed to get what we thought was the best run.

http://xlforum.net/photopost/data/500/DynoMarch_28_2005.jpg

By the way, I run a stock CV40 with a 185 main jet and SE AC (K&N). All runs were done with AC on so I don't know if its costing me anything.

Excellent results! Good numbes for such a small amount of modifications.

Congrats on the results, bet it's fun to ride!

The torque dip down low is being caused by the pipe ... notice how the a/f is rich in the same place as the torque dip. At that rpm, there's a positive pressure wave arriving at the chamber during overlap, i.e. both valves open. Since both valves are open right then, the pressure wave pushes back on the intake charge that's sitting in the intake tract. A tell-tale sign of this is when you see this a fog out the mouth of the carb (aka stand-off). Then the piston goes down and sucks it back in. The intake charge ends up triple carbureted, as it got pulled in, shoved back out, and then pulled in again, picking up fuel each time, which is why the a/f goes rich on you. We call this condition a "reversion".

Pressure waves of both the positive and negative type are reflecting back and forth in the pipe. Since the waves travel at a constant speed (the speed of sound) regardless of the rpm of the motor, at different rpm's you may have either a positive or a negative arriving during overlap, or something in between. A good street exhaust times and diffuses the negative wave such that it arrives over a wide rpm range. Diffusing it makes it broader, although weaker. That gives a parabolic torque curve.

Your exhaust is doing a reasonable job of it, it's fairly broad and centered around 5000rpm (I can tell because that's where the torque is peaking, the shape of a torque curve is essentially a map of the cylinder fill). It's just not reaching quite low enough in the rpm range for you. But if it was designed to broaden the powerband it'd also make a little less across the area where the pipe is working. Nature of the beast. Drag pipes are the classic example of this, they cause a huge reversion in the mid range and then pull like gangbusters a couple thousand rpm higher. No diffusion, you're getting the raw, strong & narrow versions of both positive and negative waves.

The rich area caused by the reversion can be addressed, with a Thunderjet kit (not to be confused with a Thunderslide kit). It's essentially a high speed circuit for the carb. Let's you lean out the main and/or needle and then replaces the fuel up top. But it's a crutch, and it won't help the cylinder fill down there, it'll only mask the problem.

As it is, the bike could benefit up top from a little bigger main jet, see how it's creeping toward 14:1 as the rpm climbs. But it'd aggravate it some through the reversion.

But anyway, congrats again, and enjoy it.

Thanks for the compliments and thank you Aaron for the detailed explanation on whats happening with the pipe. This bike is fun to ride and it fits my riding style very well. I'm very happy with it but like alot of you out there, never satisfied. When I'm not riding, I'll keep searching for ways to improve performance.

Awesome results! Great bang for the buck.

Awesome results! Great bang for the buck.

Cool..Thanks for the heads up gw.