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  #21  
Old 05-15-2010, 07:24 AM
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Originally Posted by snowman69 View Post
So if "no engine technically need back pressure" go and take the complete exhaust off a 2 stroke engine please. When you do that tell me how it runs.
2 stroke engines NEED back pressure to run properly.
done it to a couple engines, runs fine. You will have to re-jet.

Two strokes like the rebounding pressure waves created by expansion pipes, not back pressure. Any two stroke, or four stroke, will run fine with no back pressure if jetted properly. In fact, if you have a two stroke with a muffler (not an expansion chamber), straight piping it and re-jetting, will likely be an upgrade.
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  #22  
Old 05-15-2010, 08:25 AM
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The norm among RC pilots until the 1970s was to run an open exhaust port with no pipe or chamber. That's what the needle valve is for. As long as you have enough leeway in your needle valve to go from extremely lean to extremely rich and find what you need for each situation, an open port has an advantage over a muffler. At a specific RPM range, a tuned chamber or pipe has an advantage over an open port.

I wonder if you could say definitively whether there is any advantage to some length of pipe over an open port in a 4 stroke? Does a pipe have a better scavenging effect due to the momentum of the gases in the pipe?
  #23  
Old 05-15-2010, 09:56 AM
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edit on first post its big and red so you cant miss it.
  #24  
Old 05-15-2010, 09:59 AM
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I wonder if you could say definitively whether there is any advantage to some length of pipe over an open port in a 4 stroke? Does a pipe have a better scavenging effect due to the momentum of the gases in the pipe?
scavenging i believe only works right on multiple cylinder engines more then a single. scavenging works well on my racing mower engine because the 2 pipes go into 1 single pipe and that causes the exhaust moving out of the one pipe to add a slight vacum to the other pipe causing the exhaust to get sucked out

-this is my theory might not be right or it could be i dont know.. its what ive gotten out of sitting and thinking about the way my engine runs.
  #25  
Old 05-15-2010, 01:15 PM
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you kno what i have found works good and is easy to use, metal electrical conduit. its light steel but comes in the perfect size for exhaust
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  #26  
Old 05-15-2010, 03:03 PM
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Yeah, I built a new exhaust system for my brother's Subaru about 20 years ago using 2" conduit, welding it with a torch. Good stuff. How do you attach your pipes to the engines? Weld a flange to the end and bolt it on?
  #27  
Old 05-15-2010, 05:22 PM
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How do you attach your pipes to the engines? Weld a flange to the end and bolt it on?
yeah basicaly.
  #28  
Old 05-15-2010, 11:50 PM
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Quote:
Yeah, I built a new exhaust system for my brother's Subaru about 20 years ago using 2" conduit, welding it with a torch. Good stuff. How do you attach your pipes to the engines? Weld a flange to the end and bolt it on?
well on one engine i made a flange out of really thick matal strap (well thick for a srap, like 1/16") then on another one i took a 1" threaded pipe nipple and welded the rest from conduit
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  #29  
Old 05-16-2010, 11:31 AM
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I hate loud exhaust. I'll bet the conduit idea would work great for building mufflers and headers.
  #30  
Old 05-16-2010, 07:50 PM
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probally, cuz its really easy to bend but also really easy to kink if you bund it too much. but i think it would be perfect for building mufflers as well. i put a stack on my riding lawn mower after i found out that my broken muffler (the pipe broke out of it) would cost $225 to replace!
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  #31  
Old 05-16-2010, 07:52 PM
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probally, cuz its really easy to bend but also really easy to kink if you bund it too much. but i think it would be perfect for building mufflers as well. i put a stack on my riding lawn mower after i found out that my broken muffler (the pipe broke out of it) would cost $225 to replace!
i doubt anything with a muffler on a lawn mower would cost 225 to replace... you can buy running working mowers for that much.
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Old 05-16-2010, 10:03 PM
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He was just looking for an excuse to put a super-cool stack on his mower.
  #33  
Old 05-17-2010, 12:49 PM
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well i'm 17 and live with my grand parents. my grandpa loves this kind of stuff so i didnt have to convince him but my grandma would've fought me on it. so since a brand new muffler for our mower (not used, hehe) would cost that much it almost seemed nessecary to put a stack on it
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  #34  
Old 05-17-2010, 03:42 PM
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no way a muffler for a lawn mower would be that much.. show me the link that says it was that much and i will belive you... you can get 2 brand new flowmaster 40 serise for that much.
  #35  
Old 05-17-2010, 11:11 PM
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Quote:
Originally Posted by Kaptain Krunch View Post
done it to a couple engines, runs fine. You will have to re-jet.

Two strokes like the rebounding pressure waves created by expansion pipes, not back pressure. Any two stroke, or four stroke, will run fine with no back pressure if jetted properly. In fact, if you have a two stroke with a muffler (not an expansion chamber), straight piping it and re-jetting, will likely be an upgrade.
I have read the exact opposite and have conducted tests of my own. With a straight pipe the waves do not bounce like they do with an expansion chamber, this was discovered by walter kaaden in the 50's.
  #36  
Old 05-18-2010, 05:27 PM
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Quote:
Originally Posted by snowman69 View Post
With a straight pipe the waves do not bounce like they do with an expansion chamber, this was discovered by walter kaaden in the 50's.
Obviously. But not every two stroke has an expansion chamber in stock form. no lawn equipment does, most 70s and early 80s sled didnt, many 2 stroke golf carts dont...

If you remove a MUFFLER from a two stroke that was not equipped with an expansion chamber, and re-jet, there is a good chance it will produce more power than stock.

Like i said before, straight back pressure is not needed or wanted, if that was the case everyone would run 1/4" stingers, really restrictive mufflers.

Also, you can remove an expansion pipe, run a straight pipe, and do nothing but re-jet, and the engine should still run fine, but it will not produce peak power.
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  #37  
Old 05-19-2010, 01:28 PM
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So on my little 5 h.p. flathead Briggs, if I want the least amount of restriction, just remove the exhaust pipe and muffler completely?
I'm gonna try it and see how loud it is. The neighbors aren't too crazy about all our loud toys anyway. But hey, it can't be any louder than my YZ 450f.
  #38  
Old 05-19-2010, 03:26 PM
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go ahead... you might want a pipe just to get the sound to project away from you instead of all around.
  #39  
Old 05-19-2010, 04:26 PM
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My clone shoots a 8" flame without a pipe. Its cool to say the least, but i think it needs rejetting
  #40  
Old 05-19-2010, 05:51 PM
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That would look pretty awesome at night.
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