The longer, straight through system performs far better in the mid range than the U.S. shorty does.
That statement may influence my choice of muffler when I decide which one to fit to my WR450F (I have four Yamaha
systems in stock here!). Shame, because that "shorty" is so
light compared to anything else!
Anyway, back on subject......I had a quick look at the '09 YZ450F (long) muffler I bought when I got it home last night....
Interestingly, looking in from the back it has at least one of those perforated cones inside, sounds just like yours Lorre79, so Yamaha
must think they are on a good thing there! I'm assuming there is another one inside that (surgery will confirm this I'm sure.)
The entry pipe of the unit is around 43mm diameter but the exit tip of the muffler looks about 25-30mm diameter, so it is quite flow-restrictive in stock form. The gases would leave at a higher velocity, but still have to be pushed out under increased pressure.