backpressure and exhaust

[bigdalk]

I've searched the forum but still haven't found anyone's success on using an automotive muffler on their machine. I have a Max 4 with the 18 B&S vanguard. I would like to make it as quiet as possible. I have a welder and would like to fabricate a header that dumps into the smallest car muffler that I can find. I've read on other sites about people doing this to quieten generators. In no way do I want to compromise the reliability of the engine. Has anyone done this or can anyone tell me if you think a small car muffler would have more or less back pressure than the stock inboard muffler on my max? Thanks in advance for any help.

[Roger S]

I wouldn't worry about "back pressure" as it's a misunderstood term for 90% of it's use anyway. I suggest you make primary header tubes an 1" or 1.25" ID dia and 12 to 16 inches long, join them into a collector, which could be a discussion of it's own, and then into any muffler you want. I can give you links for header flanges and tubing if you want. I used elect conduit, perfect size, but is galvanized so a health hazard when welding.

[bigdalk]

Thanks Roger. Do you think a car muffler will be more or less restrictive than the stock exhaust?

[thebuggyman1]

A car muffler will be less restrictive, because it was designed to pass a larger quantity of gasses. However......listen to Roger, make a header. Smaller dia. will give more low end, larger dia. will give better high end. In theory the exhaust gasses flowing out of the cyl. down the header creates a vacuum in the cyl. allowing more air/fuel mixture into the cyl.

[KrS14]

For your reading pleasure

Manifold (automotive) - Wikipedia, the free encyclopedia

Works the same in our engines. (Except 2 strokes)

[thebuggyman1]

"Works the same in our engines. (Except 2 strokes) "
Why is it different with 2 strokes?

[6X6]

Without going into great detail, Here's the basics behind a 2-stroke exhause (and why I love'em) A 2 stroke has no "Valves" (Yes, rotary valves in Rotax and reed valves in many styles But I'm talking about valves as used in a 4-stroke, to open the combustion chamber to the intake and exhaust and seal it up again.) A 2-stroke uses "Ports" in the cylinder wall, and as the piston travels down, they open up (Because the piston is below them in cylinder, and as it travels up, they are blocked by the piston again. The exhaust valve is ALWAYS highest, so it opens first and closes last. Because of this, raw fuel/air can and WILL travel out the exhaust. The header and pipe are therefore "tuned". The way they are shaped and the length, tune how it works, by using the pressure pulse of the exhaust: BASICALLY
1)As the pulse travels down the pipe and into the expanding area in the pipe(Divergent cone), it creates a vacume at the cylinder's exhaust port, drawing out spent gasses and drawing in a fresh charge; in the process some fresh charge is drawn into the pipe,
2) Now as the pulse hits the narrowing end of the pipe (Convergent cone), a pressure wave is sent BACK at the exhaust port, timed correctly it will push the fresh air/fuel that was drawn into the header BACK thru the closing but not closed exhaust port, into the cylinder, creating better efficiency and a mild turbo-charging effect, (Using exhaust pressure to bring the pressure in the cylinder above that of the Naturally Aspirated charge).

If you wish, I'll stay on my soapbox another minute. The Pulse is a soundwave, so the pipe is tuned for the pulse at the speed of sound, but also to RPM. Since at lower RPM you'd need more time before cramming the last of the charge back in, a LONGER pipe is suited for lower RPM, Shorter pipes work better at high RPM, Too Long and your inefficient, too short and you cram in too much hot exhaust gasses and can overheat the motor or damage pistons/rings.
The Pipe breaks down into 5 parts, in order from engine to tip: Header, Divergence cone, Belly, Convergence cone and stinger:
The Header matches the diameter of the exhaust port for best flow, and it's length is part of the overall length of the pipe, so it must work WITH the length of the rest of the pipe.
The Divergence cone is tapered to create a useful vacuum to draw out spent an in fresh charge. Too steep and it's not useful/too short. Too shallow and it's too weak. The Belly, like the header, tunes the length of the pipe. I haven't gotten into fooling with diameter of this so am least experienced here. (Heck, there are guys who are 2-stroke PIPE people, what they do is Pipes, everything else is secondary, they are Complex) The length determines the time between the vacuum ending and the pressure pulse beginning, again tuned longer for Lower RPM, shorter for high, in conjunction with the header
The Divergence cone at the end of the pipe is also critical. The more gradual it tapers down, the wider the RPM Range is, but you won't get AS MUCH Power at any RPM point. The sharper it tapers down, the more power you can get, but it will be peaky. The stinger is also important. It must allow the pressure to bleed down, but not be too large too. Larger provides broader powerband but at the expense of peak HP, a smaller stinger provides better HP, but is more peaky. All aspects are of course, to a point, step beyond the limit and you just loose power all around.
Maybe this graphic will help, I tend to ramble


If you feel like fooling with it, and can weld, AND CAN AFFORD TO BLOW UP WHAT YOU'R TUNING (Yes, get it wrong and it probably will just not run right, but you can also melt down a piston, just by fiddling with the exhaust.) Have fun!

[KrS14]

Thanx 6x6 That'll do nicely

[LarryW]

Awesome explanation 6x6!!! Thanks!

[Don]

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.

[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.