exhausted ponderings

[jape]

A lot of people look at exhaust mods as one of the first modifications to their 4 strokes. Usually for power, sometimes just anti-socially for more noise.

Given that exhausts are tuned for any engine at some stage in the original design at least, and measured and tested, I wonder how many of the modifications people rate after time has passed as a true improvement?

I can see that removing a cat convertor or other restrictions may improve exhaust flow but wouldn't this be at the loss of either bottom or top-end power depending on the original design? If the loss is of the bottom-end power which most dirt-bike types are aimed at then it may improve top end a bit. Is this actually the case, does one necessarily suggest the other? I know that shortening the pipe from header back is said to improve top-end power but I do not want to shorten pipe!

What effect does an overall increase in pipe diameter have?

I know also that a megaphone type, or even a funnel type tip is said to improve final exhaust speed. But this goes against what I know of venturi effects from restricting flow!

Is the 'bomb' type bulge helpful on anything but a properly tuned and emission measured exhaust system? I see them on 4 strokes now when they used to be just on performance 2 strokes.

Beyond subjective imagination and noise levels which only kids really want to make louder, has anyone really rated an after-market exhaust? I am especially interested in 200cc.

So if any engineers or really experienced modders and tuners read this - please explain!

[slabo]

or really experienced modders and tuners read this - please explain!

[moilami]

I have understood that you can't get more power out than you put in in four strokes. So if your current exhaust is designed correctly and optimised for the output the fuel system and engine makes, then changing the exhaust system only changes sounds with some marginal power improvements. Two strokes are completely different story and you can make helluva big differences by changing the exhaust system.

In four strokes the exhaust system only requires to be free flowing enough, afaik.

[Lao Jia Hou]

Very interesting topic. Regrettably, I am only an armchair-tuner, not an engineer.

Most recently (yesterday), I picked up my JH600 with the infamous new Laser exhaust can freshly installed. It is also a Produro (as others have installed on their JH600), but a slightly different looking model. The Jialing dealer in Beijing, who is also the North China distributor for Laser exhausts, insists that this is the correct exhaust can for the JH600. During my decision process, the dealer argued that I should also change the entire pipe from the head to the can, claiming that is where I would experience the peak performance improvement. I graciously declined, as I wanted to see if there was any difference with just the can, first.

Difference? Well, it sounds more like a motorcycle, I guess. Performance? Really hard to tell. I guess I am saying ... I don't see any difference. But, then again, I am a granny-rider.

I'd trust a dynamometer for rear wheel horsepower/torque to see if there was a difference. Frankly, though, I doubt there is much. Unfortunately, I have not found a MC shop with a dynamometer in town.

I grew up around muscle cars - Hooker Headers and free-flowing dual exhausts were the rage. Wild claims of performance increases were advertised by the manufacturers and, of course, anyone who installed the items. So, I did it to a 427 Corvette, a tri-pack GTO, and a 455 Trans Am ... and the difference was? Hmmmm ... I couldn't really tell.

I suppose that for true performance track riders, looking for that 0.01 second of an edge, performance exhausts might make a difference. For me, factory gear is probably fine, aside from the fact that the Produra sounds better (to my ears).

Pics of my new Produro ...

Laser 01.jpgLaser 03.jpg

[moilami]

The thing is that fuel = energy in your four stroke. Now how much you get that energy depends of the engine. Your engine burns the fuel and tosses the exhaust gases to a tube which have a muffler in the end of it. The muffler or tube does not make more energy of the exhaust gases nor help the engine burn fuel better (if it doesn't restrict exhaust flow). You will need to mod the engine to get more energy. Usually this is done by increasing the size of the engine, but there are other ways too to get more power of the engine. However changing the exhaust is not one of those in four stroke as long as the exhaust system is of proper design.

(In two strokes the exhaust plays a very different role, but we don't have to go into details of it now.)

[Barry]

Most recently (yesterday), I picked up my JH600 with the infamous new Laser exhaust can freshly installed. It is also a Produro (as others have installed on their JH600), but a slightly different looking model. The Jialing dealer in Beijing, who is also the North China distributor for Laser exhausts, insists that this is the correct exhaust can for the JH600. During my decision process, the dealer argued that I should also change the entire pipe from the head to the can, claiming that is where I would experience the peak performance improvement. I graciously declined, as I wanted to see if there was any difference with just the can, first.

Difference? Well, it sounds more like a motorcycle, I guess. Performance? Really hard to tell. I guess I am saying ... I don't see any difference. But, then again, I am a granny-rider.

I'd trust a dynamometer for rear wheel horsepower/torque to see if there was a difference. Frankly, though, I doubt there is much. Unfortunately, I have not found a MC shop with a dynamometer in town.

Any backfiring, like the others have?

[barnone]

Most people equate performance with loud noise. Seems like the performance gains with straight through mufflers (loud) are a little at very high rpm where I spend very little time.

[Lao Jia Hou]

You will need to mod the engine to get more energy.

To me, it seems that performance gains would have to come from a combination of three things:

1) increased fuel / air flow into the combustion chamber;
2) increased combustion power (e.g., more efficient fuel/air burning); and,
3) increased flow of exhaust gases

Modding only one, or two, of these would probably have no noticeable effect as the remaining one or two would limit overall power. Modding all three may require additional mods, though, such as stronger pistons / rings / valves ... fuel mapping ... etc, etc, etc.

Any backfiring, like the others have?

A little bit more popping when the throttle is backed off quickly, but not a lot. A little bit of backfire, but only one out of four or five times of backing off the throttle hard (not sure why - if it is unburnt fuel being exhausted out into the hot header, then maybe the reduced back pressure might be causing the combustion chamber to expel more freely?)

Most people equate performance with loud noise.

So true! I noticed ONE big difference ... the more sporty exhaust noise made me feel more sporty ... and that made me want to drive more sporty. So perhaps my performance increase was psychological!