Mufflers are particularly rare in VW engine installations. This situation is changing, however. Ultralight aircraft are appearing with a variety of clever lightweight muffling devices for their angry, hornet-like, high reving engines. Much to their credit, ultralight aircraft builders are trying hard to muffle the shrill while of their low-powered engines. As mentioned, in order to muffle engine noise, you must somehow smooth out the flow of the turbulent gases, or at least, modify the ordinary exhaust pulsations.
Longer pipes do, of course, help a bit.
So do some of the modifications of the tail pipe. Some builders add unusual treatments to the end of the tail pipes in order to effect some alteration in the exhaust pulsations. One such variation, is to flatten the last six to eight inches of the tail pipe and then drill random holes in it. The effect of the flattened pipe, is to help accelerate the flow of exhaust gases. Furthermore, with the pipes pointed straight back, a little bit of jet assist is also realized.