Importance of exhaust on 4 stroke engine?

[AndyAndyAndy]

Silver exhaust is the original exhaust that came with the engine. But I run into a clearance issues. To solve it I will cut & weld two different exhausts together. Pipe on second one (black one) is a hair wider.

Am I looking at any problems here? Except like loosing 5% of power, louder sound, emissions. Some real problems? I can't really find any solid info on Google about DIY modifying 4 stroke exhausts.

 

[mckutzy]

In the end the motor is too small to really get a tuned pipe....
It can be worked on to gain points of improvements... but thats in a racing world.
Cut/weld at your content...

 

[madprofessor]

Excessive restriction usually occurs in 2 places, in the diameter of the "exhaust pipe" leaving the head, and in the amount of packing or baffling in the "muffler". Your pipes look so small already that the motor must not need a lot of pipe flow in the first place. How many cc's? The length of the silver horn makes me think it would just have baffling perforations inside and not any packing. Looks the opposite for the black one, but can't know anything for certain.
Regardless of what's inside, the simplest check for any added restriction would be to just make the change and see if it's any quieter or louder. Louder means there's no problem mechanically. Quieter means find a way to remove some of the internals.
Know this: If you replace the intake with a larger aftermarket performance air filter and adapter to get more power (and with larger carb main jet), neither one of those stock setups is going to be big enough. More mix in needs more exhaust out.

 

[AndyAndyAndy]

It's 125cc with silver exhaust. I changed silver exhaust entirely for the black one (after cutting and rewelding mounts), which come from 50cc.

I didn't try to run it yet as brakes are still stuck at customs in China somewhere. So I'm not sure how it's gonna run. But it starts and idle.

Black ones pipe seems to have a hair wider pipe, but has that really sharp curve. Which I'm little bit worried about.

 

[jamyers]

I wouldn't worry about the bend, unless it's truly a 90-degree elbow it won't hurt flow - look at car header primary pipes, some of them look like spaghetti.

Generally speaking, 4-strokes don't really care that much about the size / shape of their exhausts as 2-strokes do. As long as the pipe diameter is big enough (at least the same size as the exhaust port at the head) and the muffler isn't stuffed full of baffles or packing, you should be good.

 

[madprofessor]

Perfect call jamyers. Cool that you know those 2-strokes, that's where the "tuned expansion chamber" originated. Ran those myself on a bored oversize Yamaha 100 that raced at Iron City.
Andy, that "true elbow" he speaks of is like the screw-together ell you'd buy in the plumbing section at Home Depot, hugely restrictive to flow. A "sweep" is what's required on a 4-stroke. On my engines I just order a Ebay flange-only for my size engine, weld on a stub with enough room to run down the nuts, then enlarge from there.
Since you've already made up a header stub to your 4-stroke, I suggest sliding a larger size pipe over the stub, all the way to the muffler. With that muffler strangling you, that's the best you can do. Or get a bigger muffler.