Ok guys, this isn't about any quad or any particular machine in general. Here's the deal. My buddy got a machine w/an older yamaha snowmobile engine. The exhaust that came with it (original to the engine) is crappy and will not really work for his particular application. So i told him to try to "make/piece together" his own that wouldnt be so bulky and that he could make fit the way he wanted (he has the equipment and know how). Then just get a cheap muffler.

Well... here's where we see differently. He says that on a 2-stroke engine if you dont use the exhaust that came with the motor (ie stock) it wont have the right amount of backpressure and so forth. Thus making the engine not perform up to par.

I say, why do they make aftermarket exhaust for 4 wheelers and such. And if that was the case, what about the people who modify their exhaust?? I just dont see what the big deal is, if you put your own exhaust together and put a muffler on it.

Thanks for the help guys!!!

 

2 stroke engines need back pressure. When the piston is moving up on the compression stroke, the exhaust port is still exposed (open). Fresh, unburnt fuel escapes through the exhaust port. The back pressure of the expansion chamber (exhasut pipe), pushes that unburnt fuel back into the combustion chamber.


I just happened to find a video showing you how it works.

http://www.angelfire.com/md/mikesbikes/page8.html

 

Thanks mosher, I do understand how 2 strokes work. What i am trying to figure out, is why you couldnt use a say "custom or universal" exhaust system for a 2 stroke and it would work. I dont see why as long as you have back pressure (from a muffler for a 2 stroke) it wouldnt work?? But thanks the clip is very helpful.

 

obviously you need back pressure.. but you're going to get that from an exhaust pipe. now, the problem is "fabbing" on that can create enough back pressure. I'm sure there is a range the engine will operate in. one way to go is to use piping the same size as the old pipes, and if you're going to shorten the pipe, go with a smaller diameter pipe (which will restrict air flow, causing more back pressure)... you can always go back to stock if it doesn't work...

 

You don't "need" that much back pressure. My sled is a 78 Ski Doo Everest with a liquid cooled 444. When I got it, the exhaust system on it was a stock muffler with the back 1/2 of it just hacked off. No baffles, no tuned size, nothing. It was loud but it ran and drove fine. Soon after I replaced it with a good, stock muffler. Sound went back down to stock levels and I could ride without ear plugs. Power output and driveability was no different. Sound level was the only difference going from a hacked off, half of a muffler to a complete stock exhaust. I'm sure there would be a difference in power between a tuned pipe and a stock muffler, but there sure does not seem to be a difference between a stock muffler and a hacked open stock muffler other than sound level.

 

You don't have to have the factory exhaust, but whoever builds a custom exhaust needs to know what they are doing to make it work right. There are companies out there that make custom exhausts for 2 strokes, but I'm sure it's not cheap.

Think about a Banshee exhaust. There are TONS of different shaped and sized exhausts for them. The factory exhaust looks almost like a snail shell, then there's the in-frame aftermarket exhaust (like Toomey), the out-of-frame drag exhaust, 2-into-1, etc, etc..........

That is a cool link - I always wondered why the 2 stoke exhausts had the expansion chamber.

 

You don't need the muffler at all really, for it to run. But you need the pipe to be tuned to the motor. You can get pipes for more low range, mid and top range. Same motor, but the pipes have been tuned differently.

Get the wrong pipe, and you get a crappy running motor.

 

<div class="FTQUOTE"><begin quote>Originally posted by: Scootergptx

You don't need the muffler at all really, for it to run. But you need the pipe to be tuned to the motor. You can get pipes for more low range, mid and top range. Same motor, but the pipes have been tuned differently.



Get the wrong pipe, and you get a crappy running motor.</end quote></div>

Wrong. You can run a muffler or a tuned pipe. A muffler is not necessarily a tuned pipe. It can be, but does not have to be. If you want a tuned pipe, a longer pipe before the expansion chamber will enhance low end torq, whereas a shorter pipe before the expansion chamber will help top end power.

 

Thanks guys. Seems we were both kind of right. The thing was, i just knew that he could get away with doing some aftermarket exhaust and stuff but he swore that it HAD to be the stock exhaust! Then i talked to him last night and now he is going to fab up his own w/out any muffler just has to put a 90 in it and some smaller pipe. So who knows, we will see when he gets his machine done. Thanks for all the help guys!