78 xl250s exhaust suggestions

[BoJordan]

G'day,

I am in the process of fixing up a 1978 XL250s. I have plans for a larger carb, megacycle cam, high comp piston and to replace what ever other engine components seem a bit old and worn. My question is what kind of exhaust could I fit for optimum performance. It is a single cylinder, but has twin exaust ports the twin headers then go into a single muffler. My original plan was to switch to two single pipes going down and under the bike, but after thinking about it more I have a feeling this will be bad for performance as there will be bugger all back pressure. Does anyone have any suggestions for designing an exhaust that will compliment the other performance upgrades and still look cool.

Regards,

Beau

 

[BoJordan]

ok, I'll change the question a bit. Has anyone done a custom exhaust for any twin exhaust port single cylinder 250, or does any one know someone who has? Any one with performance exhaust knowledge and experience would be appreciated.

 

[Brodie]

I just took a photo of my XL250 exhaust then had a look at your bike which is an XL250S. They are a bit different though haha.

Anyway as for yours as it has two exhaust ports (and by reason 2 valves) it will send out two exhaust pulses.If you wanted to do two single pipes you would just have to use a smaller diameter pipe to compensate for the "back pressure". The reason you want a smaller pipe is so that the pulse that the exhaust valve makes when open does not dissipate too quickly and will help by pulling the next pulse along. You will also have to run two mufflers for that kind of setup. Keep in mind mufflers are needed to stop the pulse from just turning around at the end and heading back up the pipe. If that happens you end up loosing a fair bit of midrange power.

If you wanted to do a 2-1 setup in theory you will want there to be a small amount of difference in the length of the pipes so that one pulse will follow the other not smash into each other when the pipes meet. If you plan on running the 2-1 directly into a muffler than pipe length does not become too much of a problem and they should be near the same length.

Hope that helps you out some. Now start a build thread!

 

[BoJordan]

yeah cheers, that re-enforces what I was thinking! thanks for the help