[Teh_K]

From what I understand, it's the collector (the part that connects the end of the headers to the mid-pipe) that creates the correct back-pressure. So... I think you can do just about anything to the mid-pipe and/or muffler and not have it affect the pressure.

[Rok_Stars_Bars]

OK, Maybe you know something I don't. How would going from 3 baffles to 2 baffles increase backpressure. Im really not following. If anything u are decreasing backpressure. Which is proven by pics on the previous page and plus the muffler gets louder cause there is less restriction. And in the same part. I have put around 500 miles on the bike since this was done. There was no increase nor decrease in power. But also that by me driving it. And one more thing. As teh K said the backpressure in mostly from the collection into the midpipe. That is for the somereason you can stick a D+D slip on and not have to jet the carb. But Im not a muffler expert. Nor did I just come out the woodworks. I have been around and study engines alot. So help me understand your standing on it.

[FloridaKat]

OK....I drew something out by hand and debated doing a graphic, but let's see if this makes sense. Use Astro's pictures as a reference.

Chamber 1 (or first) has 1 exhaust inlet and 1 exhaust outlet. The airflow bounces around the chamber and then leaves for chamber 2 (or second). Pretty self-explanatory for chamber 1.

Chamber 2 (or second) has the exhaust outlet from chamber 1 pass directly through this chamber. Chamber 3's exhaust appears to go backwards into chamber 2. Chamber 2 then has an outlet to exit the entire exhaust.

If you look at chamber 3, if you cut the "smaller" pipe short, wouldn't some of that airflow hit part of the "larger" pipe and head backwards? This may not be much airflow, but that is where I thought there would be the potential of additional backpressure.

Does this make sense? I am no exhaust expert either, but I just want to think this thing out before I start a choppin'.

[Astro4x4]

Chamber one goes back to chamber #3 via small diameter pipe. Then via the short pipe goes from #3 to chamber #2,(if you named them 1,2,3 from mid pipe back) where the inlet of the long pipe that runs through chamber #3 is located. That long pipe carries the gases from chamber #2 which is actually chamber #3 in the progression of the exhaust, out the tail of the can. So by cutting the baffle you make a 3 chamber muffler into a 2 chamber muffler. Because the chambers are rather large, fiberglass lined and the fact that you are now pushing noise through just two chambers(instead of three), you get a deeper and slightly louder tone.
Gases won't (to any degree that affects performance anyway or is easily measurable) travel backwards in the pipe simply because of the pressure produced by the engine. Exhaust (and air, water etc. etc.) will naturally flow from a high pressure area( inside the can) to a low pressure area.( atmosphere behind the can)

[FloridaKat]

This confirms my understanding of the exhaust flow from the mid pipe, aft.

Since the exhaust airflow has been inhibited by a certain % of turbulent vortices formed by the difference in diameters of the pipes and the innermost pipe being cut, there is a loss of optimum airflow due to increased backpressure from the obstructed exhaust. Again, the losses are likely negligible, but I would venture to say the if I held a flowmeter to a stock exhaust and this modified one, for a given RPM, the stock exhaust would have a higher CFM, thus less backpressure.

[GregS]

Simply put, stock mufflers work by causing sound waves, pressure waves, to collide in the "can". By cutting out the end plate of the can you allow those sound, pressure, waves to escape without as much of the canceling collision. You have reduced back pressure and increase sound level.
Another clarification on the collector. It is designed to combine the pulses of the headpipes and, at its calculated rpm range, create a suction that will help draw the next air/fuel load into the cylinders. To spread this rpm range the stock header has a balance tube between cylinders 2 and 3 so at a non-ideal rpm range the reversion pulses have somewhere to go. All of this is greatly affected by headpipe diameter / length / step size and location / collector baffling / outlet diameter .... etc....

[FloridaKat]

Unfortunately, the innermost pipe is not directed outward to a clean exit. It is shorted by some small amount where the sound/pressure waves, through conical geometry, is reverberating back into chamber 3. If there was no end plate to the can, this would be a mute point.