Optimum exhaust system length?
 

About a year ago just after I bought my bike, I kept running into a post on some unknown forum about how a certain overall length was found to be ideal by some Australian race team or shop that was involved in racing etc

Of course I can not find it now.

Has anyone run across that thread before? I know my system's shorter than most of what I see...maybe there's some torque waiting to be gained by adding length to it

:D

On the KLX specifically? And what kind of racing? Header/exhaust tuning is an amazing technology, but optimum performance gains often don't fit with the shape, length, etc. of the exhaust and how the whole package fits the bike or vehicle...especially a street vehicle. And then there's the issue of how a given use...drag racing vs. off road riding as an example...is benefited from the exhaust being designed and shaped to work to compliment that specific use. I'm sure you're already aware of that, but many think a one-size-fits-all exhaust can cover all the bases. And of course I'm talking about performance at the extreme edge, and that may apply since the reference you're looking for seems to be connected to racing. It would be interesting to see the article. Maybe someone will remember where it is.

The optimum length will change with the diameter of the pipe and your state of tune and even the operating temperature which changes the speed of sound. Basically, there is some length that will return a negative pressure wave just when the exhaust valve opens, which can aid in pulling the spent gas out. During valve overlap there can also be some aid to filling. In theory the FMF Bomb series of headers tries to use this method to achieve this effect at some rpm/throttle openings. The bulge sends anotehr negative wave back in addition to the one that comes from overall exhaust length.

This is also why you'll see pipe crossovers and 4 into 2 into 2 headers on street bikes.

What tuners try and do is fill in some other dip in the torque curve, but that will vary with your specific tune.

I don't think this is really something worth fussing with, it's not a huge factor like a bore kit or proper jetting. In high level road racing this may show up as a couple of hundredths per lap but for us, your not going to notice the change. Maybe you'd see a couple of 1/10s of torque across a few hundred RPM band.

I take offense to that. You seem to be indicating that our KLX's won't produce earth rotating levels of torque and horsepower that can be maximized by exacting, precise exhaust and other tuning mods. What's the matter with your KLX?...someone stuff a freezer bag in your airbox?:D

+1 to everything TNC said .... and everyone knows that a gym sock is what you put in the airbox for maximum power.

There are formulations for calculating optimum exhaust diameter and length. Don't ask, I don't know. I do know that if someone is doing off road or dirt track racing the exhaust size and length for the bike would probably be good. That has been my experience with the SR 500. I got my numbers from an old Gordon Jennings article where they did a lot of mods and did dyno runs as they did them. The optimum, which I also found on a flat track pipe, was a 1-5/8 ID and 33" in length. There was also power gained by adding a megaphone too. As I said, when I did get a hold of a flat track pipe it measured 33" and was 1-5/8" ID. Dirt track requires a good range of power.

I know it's no help on the 250, but just an example that some "magic" numbers do exist.

Yeah, you leave one sock in an airbox and from then on you're known as a sock stuffer...sheesh!:D

i believe that Deej has the same nickname.... but the socks werent going in the airbox. :D

Exhaust Length,exhaust pipe,induction

Here it is, a pipe length calculator. I'd be curious to see what sort of results one would get if they took the time to fill it in.

The effect is pretty much destroyed when running a muffler/silencer. For gains, a full open exhaust system will give the most gains, with a megaphone or a expansion chamber (for 2 cycles) installed.
So if your running a muffler/silencer, you might as well forget about it.