Custom expansion chamber design for drags.

[Brett]

I have pretty much finished riding for the year due to summer heat and dust  and will be entering a dirt drag (1/8 mile) competition in the winter and thought i would like to make a custom expansion chamber with the intent of creating more mid and topend power   (probably more top end).  Has anyone made their own expansion chamber for drags and happen to have the design handy. (cone angles and diameters).  If not has someone got the port timings for the transfer and exhaust ports as this may be enough to plug into a expansion chamber design program.

[KXcam22]

Brett,
   Most tuning books will have the fomulas, which you can plug into excel.  I made a set once for my RD350LC roadracer.  They actually worked very well. I used the Jennings book. Cam.

[YUNGGUNNAZ]

maybe MUN will chime in on this also if he's feeling better.

[mun]

I try tell short how my only first and last MuN-pipe is done.
I have calculated pipes with many programs, but some year ago i purchased Mota-program. After long and busy working a planning was ready. I get lot help of my work (CAD-man) for cones. Then cut piece of 1mm plate and welding then together.
There is graph where you can see difference between Mota and real world. Anyway, i like Mota.



I think stock "modern" KX500 have exhaust KIPS close below 170 deg, KIPS open below 190. Transfers about 120 deg.
My stock old -86 main exhaust (no KIPS) was 181 deg and transfers 123. Now exhaust is about 195 and transfers 134. MuN-pipe needs so high transfers, so that is rideable.

[Brett]

Thanks for those timings mun i imagine with your fixed port timings the power must come on strong in the top end and probably have to be revved out more.  I imaging the "old" 500 exhaust passage is bigger than the modern due to not having the main powervalve, i know the cr's have a much bigger exhaust passage than the kx's.