exhaust back pressure. do you need it?

Hey guys im looking to custom make my own exhaust, either a 4-2 or a 4-1. I see guys running baffles and no baffles. Im guessing straight pipe is bad for the engine and to freaking loud for my taste anyway. So ultimatly my question is, how much back pressure do these bikes need to run without exploding? Also i do know i have to re jet the carbs lol.

Cheers
Jason

They do need back pressure and the exhaust system has to have a "resonance" tuned to the right frequency to get the scavenging effect that gives increased power.

The baffle in the 2-1 system I used to run that made a lot of power was a tube about 1.5" diameter with a flat "wall welded in the center and about 100 holes drilled in the tube maybe 1/16" diameter. The exhaust came out the holes into the expansion chamber then went back into the holes to get out.

Bountyhunter +1

There's a real science to exhaust systems. In addition to the level of back pressure, the length and diameter of the pipes, where multiple pipes should be joined, a bunch of other stuff must be considered so the exhaust system matches the flow requirements of the specific engine. Unlike most other plumbing, exhaust systems must deal with the fact that the flow of gases are pulsed rather than a continuous flow. Tuning the resonance of the shock wave to match the engine is no small task. I suggest you do a bunch of research on the subject and be prepared for some tricky math if you want to get the best performance from your exhaust system. Just in case you're not familiar with scavenging, it's the ability for the exhaust system to create negative pressure at the exhaust port at just the right time to help suck exhaust gases from a cylinder. Ed

Thats the reason i was thinking of making a 4-2 exhaust off the factory pipes. Basically id just be making shorter mufflers with less baffles. So where the two pipes connect would be a bigger diameter collector into the new "muffler"

Cheers
Jason

Ed and Bountyhunter are spot on. There's more to it than just "backpressure." That said, I don't see how you'd blow up your engine just from a poor exhaust design. You'd lose some performance,and if you were to not adjust your carbs, you'd go lean and could burn valves or cause other "lean" related damage, but since you were already aware you'll need to rejet, you have that base covered. I can't think of any other damage that could occur due to the exhaust (maybe some hearing loss if you make em loud). Unforunately the exhaust world is full of people who don't understand the science behind all of it, and just use made up justifications and false science to explain what ammount to old wives tales, so it can be hard to find good information. I'd love to learn some more about all of it myself.

One thing is the bike makers tune the intake and exhaust as a set and then cut the taper of the carb's needles to match so you get the right mixture across the full rev band. You can change jets, raise and lower the needle, but you can't change it's taper. If you significantly change the intake/exhaust design, you may end up with an engine that runs OK only at specific RPM ranges but not all the way.

Thats the info im trying to gather. I know the exhausts need to be tuned. I was just curious, that if i didnt change the collecting points of the header tubes, if it would disrupt the meeting point of the exhaust gasses qnd make it go out of tune. Id love to be able to make a custom tuned exhaust myself, i just dont have the know how to do it yet.

The kz1000 I just bought has an non-descript 4-1 exhaust with no baffle, I can attest that it is a turd below about 3000rpm and just retarded loud(I can't stand it). I'm going to do up a baffle like bountyhunter described and see if that doesn't help. The PO dinked with the jetting when he put pods and the exhaust on but I think he just kinda blindy guessed what size jets it needs cause its still running pretty lean.

cherokeekid58 wrote: Thats the reason i was thinking of making a 4-2 exhaust off the factory pipes. Basically id just be making shorter mufflers with less baffles. So where the two pipes connect would be a bigger diameter collector into the new "muffler"

Cheers
Jason

This guy did exactly that, I plan on doing the same (I love the dual exhaust look). I wonder if he had to re-tune?

I might add to the fire here but of all the reading I have done on exhaust you need to have two step downs after the collector.

I run open exhaust. I just gas e'm up. I am deaf by the way. :pinch:

Since you are going to make the exhaust you might want to find the post that OMR started about exhaust. There was a lot of reading there. Roy

DesertKZ, thats exactly what i wanna do. Keep duals, just shorter, louder pipes. Id imagine you would have to re jet anyway. Because of less resistance in the exhaust gas flow.