[Davisca455]

Originally Posted by M3 Number 86

Does that thing mess with back pressure and horsepower?

Like most questions, the answer is "It Depends".
The more discs you have, the more it flows, but the louder it is.
Fewer discs, reduces flow, but the quieter it becomes.
I know it looks like a cork, but you can actually stack enough discs onto the assembly to equal the flow of a straight thru pipe diameter (the gap between each disc is tapered so there is some effect on velocity that isn't really apparent to the naked eye)
So if you check the flow of a 2" diameter straight pipe with a manometer, you can then add discs to equal the flow of the open pipe, and the assembly will be measurably quieter, because the 360 degrees of exhaust diffusion reduces the noise that a sound meter would read.
However, there are flow limitations within each disc diameter family, for instance, the 3" disc family will only flow up to a certain point no matter how many discs you install.
In that instance, you'd move up to the 4" family of discs, and eventually you may need to move up to the 5" family.
The 3" disc family was designed for smaller engines.
The 4" disc family was used on larger displacement motorcycles and some cars.
Examples for the 4" disc for motorcycles would be 750cc V-Twins (100ish HP) used in AMA Class C Dirt trackers.
Each cylinder had a 1-3/4" OD headpipe that connected to a reverse cone megaphone that finished with a 2-3/4" ID exit with 35-38 discs stacked on the end.
The AMA Superbikes (1000cc back in the day) would typically have 4 headpipes of 1-1/2" OD that fed into a 2-1/2" OD tail pipe capped off with a glass pack muffler with 35-38 discs.
The 5" disc family was designed for larger engines and was used by some sprint car teams. One set of 5" discs onto each header collector.