2 Stroke Exhaust on 4 Stroke?

[gross392]

Scrappy dog scooters has a stroke scooter exhaust for 150$ here is a picture www.scrappydogscooters.com/images/jog-performance-exhaust.jpg . The question I have is will there be any benefit to putting that on a 4 stroke 72cc scooter? What makes an exhaust 2 stroke, is it back pressure? Is back pressure bad for 4 strokes?

[bolt]

back pressure is bad to an extent even 4 strokes use some back pressure to keep from burning up your valves...

P.S. not scooter expert mainly mess with dirt bikes but i see a lot of similarities take another opinion if you want. you want to decrease back pressure but not cut it out if u get what im saying

[thecableguy]

2 stroke exhausts are designed completely differently than 4 stroke exhausts. As bolt says, in the 4 stroke world back pressure is (mostly) bad. In the 2 stroke world it's essential to the operation of the engine. Read this Two Stroke Expansion Chambers Explained and it should clear things up...

Jim

[gross392]

I understand that 2 strokes need more back pressure than 4 strokes, but what would happen if it was put on a 4 stroke? Would it overheat the engine? Would it even have any benefit?

[calvo50sport]

You say a 72cc 4 stroke so u have the big bore right?? If so I what's your engine code if it's the 139qmb I bought an exhaust from scrappy bolted right on and gave me pickup... Hope I could help

[thecableguy]

It's a completely different design. It might work or might not. Heck, it might not even bolt up!

[ start bad sports analogy ] A football is a pointed leather ball, a basketball is a round one. Completely different designs. You wouldn't do very good trying to play basketball using a football, would you? [/ end bad sports analogy ]

P.S. Scrappy has a 4 stroke exhaust designed for a QMB139 for $129... looks like the one calvo got.
www.scrappydogscooters.com/139QMB_50cc_Performance.html

[90GTVert]

I put an expansion chamber (2 stroke) pipe on a 4 stroke once just to see what happened. This was on a 49cc 4 stroke pitbike engine, not a scooter. What happened was the 4 stroke sounded pretty annoying (and I like the sound of a 2 stroke with a pipe, but it doesn't work the same on a 4 stroke). Otherwise there was no noticeable loss or gain from a stock pipe. I didn't keep it on there for more than a mile, so that's about all I can tell you from experience.

Like these guys have basically said, 2 stroke pipes are designed and tuned to a certain RPM/resonant frequency to help a 2 stroke remove stale gases and draw in more fresh fuel and air. There are lots of calculations to be made just to make one work properly on a 2 stroke engine. Thinking one would just bolt up tp a 4 stroke (have fun fitting it properly btw) and give you any sort of gain would be wishful at best.

Here's an example that will show you some of what goes into designing a 2 stroke pipe... See what I mean?...

www.mh-aerotools.de/airfoils/javapipe_en.htm