Hi there. Basically the difference was made for noise attenuation purposes. In Europe we had massive noise problems leading to the loss of nitro friendly tracks which had EFRA explore a lot of options into how to make engines quieter. The INS box as well as 3 chamber pipes were developed. Unfortunately the 3 chamber pipe is quieter at the expense of some power.

The 9886 2-chamber pipe with normal air filter used to sound fantastic but is much louder and now banned in Europe (and I think most other countries/blocs too).

So basically the 3 chamber pipe is quieter than the 2 chamber at the expense of a little bit of power.