I've had good success with installing a DSP SX header pipe and a shortened 426 muffler. The 426 muffler will slid over the o/p of the DSP header (using the gasket though). The 250f is a little choked and with this mid the bike will
hit hard down low and pull stronger in the midrange and rev out fairly well (not quite as well as the stoker). I've even installed an adapter (a friend builds the adapter for me) that accept the white bros disks which works well for off-road (lots of low end but doesn't pull quite as long on top as w/o the
disks). A little cheaper than a full blown exhaust system and a little quieter than the stock DSP muffler which is REALLY loud ( 105db?).
Yeah and I herd that if you take out the baffel and stick a solid rocket booster in the hole and light it, it goes a bit faster too! Oh and looping a tie strap to the header gains about 10 Hp if you hook the other end of it to a YZ 426, :) Sorry. :p
I discovered a very simple and inexpensive modification while watching a re-run of the movie Beverly Hills Cop. This mod also significantly lowers the sound output from the stock exhaust! I just stick a plain old banana in the outlet of the stock muffler and I get exactly five HP from my YZ250F!
So, I gather that by the responses there really is no reason to hope for more
performance out of the 250F since the stock exhaust performs well enough.
Well, it sure seems that the bike runs stronger with the full DSP S exhaust, but maybe not. Maybe it's just louder. But I don't think so. I can actually
pull a gear higher in most situations whereas with the stock exhaust I couldn't.
Banana? Sure sounds good and it might even provide a little more lube!!
Sorry to be a stickler, Jim, but I can't agree with this statement either. There are some high buck systems on the market that actually REMOVE a few HP along the way and at peak. Dirt Rider did a YZ426F pipe shootout in March I believe. They were as nice as they could politically be to these companies without losing credibility with the readership, but some pipes flat were a waste of money. Unfortunately this applies to two stroke pipes as well. For instance on my KTM, the FMF M-20 pipe is fine, but the M-28 pipe totally screws up the power per the testing Alan Randt did for Mike Lafferty's bikes.
In the words of Billy Crystal, "it is better to LOOK marvelous than to BE marvelous".
When I was at the Washougal race this past weekend, Larry Ward said the only mod you need to make to the 250F is change the pipe. Other then that the bike is awesome. He said with that change the bike is just as good as some highly modified 125s.
A guy from another team asked him how he liked his Yoshimura modified motor and Larry said its better but not any faster, then laughed. So engine mods will not add that much to this already awesome motor.