backpressure and exhaust

[Roger S]

I've seen that graphic on a 6x6 web search too..

The original post about a quiet max IV.. wouldn't be as effecient as a header, but a guy could just stick some kind of silencer on the end of the stock muffler.

This is my favorite page on exhaust systems, it's about harleys, and besides the basics, it discusses unequal length primary tubes for unbalanced firing engines which the harley and briggs vanguard both have. I never understood exhaust fully, until I built my vanguard and looked around for header design. I used to think "just make it big, so there's no restriction and the exhaust can get out"

That's so wrong, if you care about low and mid range power. The smallest pipe that doesn't limit your engine's rpm is what you want. Bolting up a header pipe that's twice the size of the exhaust port does at least one thing wrong, it slows the velocity and causes turbulence. It turns out that the whole intake/exhaust system, from the air filter or inlet, to the point where the exhaust meets the atmosphere is very important to performance, or lack of it. Briggs makes some terrible exhaust header pipes, like the one on max II's where the front cylinder goes a couple inches and then makes a 90 degree tee. Can't get much worse than that.

RB Racing LSR 2-1 Exhaust Technology

They talk about vacuum in the exhaust.. I had a shop rag in my hand covering the tailpipe on my response, it was flapping back and forth.. I let go of it and it and it started floating back and forth, slowly moving up the pipe. I had to shut it off and fish it out with a coat hangar

[6X6]

Yes, great job putting the nearly unexplainable into words. Love the graphic, too. It's been around for a while- had it as my desktop wallpaper a couple years ago.

yea, I've had it as sig photo on a couple forums, but I couldn't get it to load animated here

. Briggs makes some terrible exhaust header pipes, like the one on max II's where the front cylinder goes a couple inches and then makes a 90 degree tee. Can't get much worse than that.

Yea, a lot of MFGs just kinda toss something "that will do" in. Just matching the exhaust flange opening to the opening on the block can do significant good. And on 2 cylinder 2-strokes, ditching headers that are non-equal length Like this:


for equal-length like this:( will allow better tuning as well.

Edit: Roger, that's a really nice link. Lotsa cool info. I like their approach to comparing to what's never intended to be marketed, especially since it's on a 18,000+ RPM motor I will be reading that more.