Exhaust Modifications, General By ellpee (Read 515 times)

[ellpee]

I was pleased to find, when we returned to Ruidoso NM for the summer, that a small business that last year was only about golf carts and the like has now decided there's money to be made maintaining motor scooters and ATVs and the like.

So anyhow, when I took my Magnum down there to show it off, I got a mini-lecture about how the design of the exhaust system is costing me performance.

The general situation is this: it's a typical "one-lunger," exhaust ports into one pipe. That pipe, however, eventually directs exhaust to two mufflers, presumably more about looks than performance, and along the way there are several 90-degree bends. The guy at the new shop says that the longer the path is for exhaust flow, and the more sharp bends it has to take on the way to the outside world, the more you are losing in the way of performance.

He was of course hustling to some degree for building me a custom exhaust system, short as possible and with as few bends as possible.

But I'm thinking he was probably right to some degree; it makes some technical sense to me that getting the exhaust out into the real world with as few impediments as possible is a good thing. However, my Magnum was designed to have two cool-looking chrome mufflers, AND I assume that fiddling with the exhaust might have implications for the valves, carburetor, air intake, etc., etc., etc. So anybody who knows a lot about this particular "modification scheme," please sound off here. When somebody tells me "you oughta do X" and is going to earn $$$$$ if I hire him to do X for me, I automatically want to get some second, and third, and fourth opinions.

[JR]

He is right as far as how the mufflers are built in general, if you cut one open you can see for yourself. But here's the thing, you have so much engine displacement, so much fuel, so much air intake and they all need to be in tune with each other.

For the most part one can add some air and fuel without messing with the muffler but you can cross the line where the exhaust has to be opened up too? But there is also the equation of you must maintain so much exhaust back pressure as to not heat up the exhaust valve too much too, it's all a fine balance and do remember you come to a point that the engine will only take so much, fuel, air and opened exhaust and to go over that threshold will not only be a waste but will hinder the engine instead of making it perform better.

I have raised the needle one notch on both of my linhais, didn't bother the air or muffler, it helped a little with top end and in the mid range, it's all I'm going to do and am happy with them as they are.

Unless you want this engine to perform at it's absolute high end part of the performance curve leave the muffler alone add a little fuel and air if necessary and keep on keeping on. But if you have welding skills you can easily modify this muffler and maintain the OEM look to boot.

JR

[ellpee]

Well, it's of interest to me because my Magnum's performance on hills -- man, do we have hills up here! -- is a bit wimpy due to high altitude HP loss, but it doesn't bother me enough to get into some daisy chain of modifications. I'm no welder. I can picture a custom header that comes off the exhaust port to a simple 'Y' and then to the right and left OEM mufflers, eliminating a lot of what seem to me like pointless 90 degree bends. But I imagine that would result in reduced back pressure, which I guess could be a problem. Sounds to me like I'd better just leave well enough alone.

[rapidjim]

JR is right in what he says. To add to this, experience has taught me, in both the automotive and motorcycle world, the last thing you want to mess with is the exhaust. Changing the back pressure of an exhaust system does in fact get the exhaiust out faster and that is what you want, however doing so also changes the air/fuel mixture requirements for an
engine. Just bolting on a set of headers and glass packs can accually cause you to lose performance, same with just changing to drag pipes. Every machine that I have modified the exhaust on has required carb mods in one way or another.

You have to remember that Ice Bear 300s used to have a single exhaust and the motor was tuned for that exhaust, when they modded the 300s to dual exhaust, they did not just bolt something on to look cool. There was design issues and also the dreaded passing of USA EPA standards. Also, Chinese Scooters are lean to start with to pass EPA standards.


Where this person is right about exhaust systems, what he fails to tell you is that the tuning and the power of the motor must match and that can get spendy.

[ellpee]

Yup, definitely sounds like an IIABDFI (if it ain't broke ...). I'll just avoid the 10-degree slopes when I can.

Interesting, though, that a scooter designed to run lean from the factory was running fat, fat, fat when I first fired it up here in the mountains. Wasn't enough mixture screw in the world to lean it out, had to have it re-jetted. Would never have guessed the air is that much thinner up here at 7500. Did find an on line table somewhere that told me my alleged 18 HP is more like 13 HP up here. All hopefully useful information for other mountain dwellers contemplating a scooter or motorcycle buy.

[rapidjim]

EPA standards are set for sea level. Of course a carb will run richer at altitudes because of less air. 7500 ft is only a mile and a half above sea level LOL. When I moved from Denver to Wi I had to rejet my Harley.