In Revhead, compatibility between midpipes and mufflers isn't labelled. There is some compatibility between manufacturers, but there's no mention of that in the game itself.

This guide lists each midpipe from each manufacturer and which mufflers are compatible with it. To save space, only midpipes and mufflers that increase performance are listed.

In each table, the mufflers are sorted in order of how much performance they add. Where there's more than one muffler that adds the same amount of performance, those mufflers are further sorted by price.

The muffler compatibility is the same between standard midpipes and rotary midpipes of the same type (rear or side) from the same manufacturer, so the tables for muffler compatibility don't differentiate between the two. The same is true for single, Y and twin versions of a midpipe.

Vertical exhaust systems aren't included in the tables because it's very straightforward. There are only two manufacturers - Jingu and Narnoo. Jingu vertical mufflers fit both manufacturer's vertical midpipes and are better than Narnoo vertical mufflers, so the only reason to use Narnoo vertical mufflers is aesthetics.

[EDIT: Vertical mufflers and pipes added after feedback]

In some cases, side mufflers can be fitted to rear midpipes or vice versa. That will look odd but it will work.

This guide also lists all performance enhancing midpipes and their prices.

The general rule for compatibility is this;

The exhaust manifold must be compatible with the engine type - i4, i6, v6, v8, rotary2, rotary3, rotary4. Not necessarily from the same manufacturer, e.g. the Magura Turbo V8 manifold is a great budget option for any V8 engine if you want to use turbos on it.

The midpipe must be compatible with both the vehicle (it has to be from the same manufacturer) and the manifold. i4 and i6 require a single midpipe. v6 and v8 require a twin midpipe or a Y midpipe. For rotary engines, it depends on which manifold you choose - single or twin - and it must be a rotary-specific midpipe.

None. Jingu doesn't have any side midpipes.