Bonneville America Forums Forums Technical Forums Exhaust Are we making this too complicated?

Ok, folks, a potentially stupid question for y'all:

What happens if you just take the pipes off altogether (an example of htis would be http://img212.imageshack.us/img212/8821/sbtthruxton90010cv.jpg, posted just yesterday)? I mean, I've seen plenty of Trumpys that appear to have no "can" whatsoever, and the pipe just ends under the case. It'll run, I know, but would that be optimal?

Another thought: what's to prevent some unscrupulous fellow from, say, simply machining some lightweight steel pipe to direct the exhaust further down the bike? It is just a pipe, after all... Yes, I realize it would be VERY loud, but considering how loud some of the bikes I've heard have been, why the hell not, y'know?

I ask all this because I know some (all?) engines require some backpressure in the system to run properly. Where's the sweet spot there?

--Jaeger

Jaeger, While changing out pipes, I couldn't resist, didn't try much to resist, fired it up with just the exhaust pipe. It was pretty loud and sounded really crappy with a popping noise. My TOR's with all the baffles is louder and sound much better. It has some growel. Runs much better with that and the snorkle removed. Now the SS long slash cuts sound great and perform really well. Anyway the straight exhaust pipes s@#k.
Ps The engineers claim that a well designed exhaust system removes the exhaust better than open exhaust.

I used to run straight drag pipes on my old harleys, and I would screw different sized sheet metal screws into the pipe to add a little back pressure. Seemed to work for me!

Back pressure is only needed at certain rpm levels, where the cam is "too big" to allow the engine run properly.

It's more important for the pipes to be on the long side, with or without mufflers, because that length will enhance bottom end and mid range power.

Open exhaust systems make the most horse power, and if they didn't, then virtually every racing vehicle would have mufflers on them. (Yes, there are muffled race vehicles, but that's because the rules reqire the mufflers to keep the noise down, not because there is a performance benefit.)

A collected exhaust system, such as 2-into-1, or 4-into-one, or 3-into-one (for the big Triumphs and also 3 cyl. Geo Metros <chuckle>) provides the best broadened power band. If that weren't so, then Formula 1 cars would have individual pipes, not collectors.

There are many design features that must be considered when building a high performance ex. system. One of the, if not the most important item, is the diameter of the primary pipe! The other measurements are secondary, though important (i.e., primary pipe length, secondary pipe length and diameter, collector length, taper, ect.). The diameter sets the basic power peak rpm point, and the other other measurements fine tune the power by moving the power peak up or down from that main point.

Exhaust design can be complicated, but when done right, it really enhances the overall engine performance.

Remember: any engine (bike, car, airplane) is like an orchestra. All the various "sections" (horns, violins, cellos, percussion) must be in harmony for the orchestra to sound great. And so it is with an engine. All its components must be in harmony with the others for the engine to perform its best (cam timing, compression ratio, valve sized, intake system design, exhaust system design cylinder head ports, ect).

Wow, I went through all that without mentioning exhaust gas volume and velocity, and the separate, but equally important, sonic pulse fequency and amplitude. Gettin' old, I guess.....

Hrm. Interesting. So what you're sort've implying would be that it's optimal to, say, run a 2-into-one on the bonnie engine with a straight pipe.

Hrm.

Might look really cool too, especially with a sportbike-like high-mount... moo hoo ha ha...

--Jaeger