Why are the exhaust pipes design on the Ilyushin Il-2 not very popular?
This engine is about 10 times more powerful than a modern average piston engine. With such power you get some useful thrust from exhaust power directly. This design helps to recover it. With a small SEP, noise and keeping exhaust smoke away from the cabin probably is more important. And it's not true that it's not popular. It's popular with single engine turbines. Source: Wikimedia (click for link)