7/8" flex pipe for 4 strokes

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CrazyDan said:
I see you trollin :rolleyes:. If you honestly believe what you say then do more research. Google 4 stroke exhaust theory. Here's the link: https://www.google.com/search?q=4+stroke+exhaust+theory&sa=Search . Over and over you will see that best efficiency is no back pressure and proper diameter pipe. Length tunes what rpm the vacuum wave hits the exhaust valve during valve overlap, basically doing almost what 2 stroke expansion chambers do, except instead of shoving unburnt fuel back in, it helps draw the intake charge in. Pick your length for the rpm it hits on. Back pressure doesn't help 4 strokes.[/QUOTE
CrazyDan said:
"Backpressure caused by the exhaust system (consisting of the exhaust manifold, catalytic converter, mufflerand connecting pipes) of an automotive four-stroke engine has a negative effect on engine efficiency resulting in a decrease of power output that must be compensated by increasing fuel consumption."
Source: https://en.m.wikipedia.org/wiki/Back_pressure
Back pressure is a lie.
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When I used to race go karts I was in the Blueprint class 5 hp Briggs and Stratton on alcohol we ran a 12 to 14" straight pipe. That's been 25 years ago and I'm sure things have changed. We did wrap the pipe with fiberglass wrap and used Stainless Steel wire to hold it to the pipe.
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I did buy some SS wire from Jeggs to wrap my headers when I used to drag race. I would give them a try.
 
How does the flex pipe attach to the engine?
I'd love to see pictures of your builds with this pipe. I have a 4-stroke (49cc HS142) on the way but haven't much thought about the exhaust yet.
 
greenjon said:
How does the flex pipe attach to the engine?
I'd love to see pictures of your builds with this pipe. I have a 4-stroke (49cc HS142) on the way but haven't much thought about the exhaust yet.
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That's the fun part. Welding tends to be a good way but I did notice that the flex pipe doesn't like welding the heat makes it melt away a bit easily so you need to keep a cool pool, if you only weld about a square centimeter at a time and give it a moment to cool before adding to it you can meld them. On my welding helmet I wait for the automatic lens to lighten back up (maybe almost 2 seconds) then I continue the bead while it's still glowing.

I've used hose clamps too and just covered it all in muffler wrap, speaking of which the tube isn't great at contaning all the exhaust but with a wrap over it it doesn't excape. 4 strokes don't have much pressure in the pipe so they might deal well with plain pipe.
 
Okay cool! I'm not set up to weld, but I'm okay with just clamping it. Wrapping it sounds fine.
Does it make the outside of the pipe cooler too?
Does it reduce the exhaust noise, or just redirect the noise to the rear?
 
It would resonate less but the flex stuff doesn't vibrate much anyway. The outside of the whole thing is cool enough to put your hand on and not burn you. The actuac pipe though is much hotter but it's obvious why and doesn't affect it much, if anything it keeps the carbon from clotting up the pipe as much.

If you sorta flare the end of pipe that will go inside the flex pipe and wrap past the end of the flex then use 2 good hose clamps (not harbor freight brand, get something that costs more than 50 cents a clamp, cheap ones can't take the torque) to crimp the flex pipe on on the tube and sealsthes end of the wrap on the inner tube. Make a 7/8 hole in a block of wood and split it down the center (with the grain) and use that and a hammer to press the flex onto the pipe tight. Really tighten the clamps on that and you should be OK.
 
Very neat. A flexible, cool to the touch exhaust seems like the way to go! Thanks for the tips.
To be USFS compliant I need to stick a spark arrestor in it too. Not sure if I need a muffler or not.
 
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