Supermouse3: If you look at the KW/L or NM/L figures of a lycoming or conti and compare them to even a modest non turbo passenger car engine, you will find a lyc or conti produces less than half.
Hmmm, you need to compare apples with apples.
Kw/L is partly tied to reliability. How many automobile engines will produce 100% rated power every time the car gets moving and then will run at 75% of their rated power for the whole of every trip.
Some aircraft engines are rated a 100% max continuous for the whole of their 2000 hour TBO life. Show me any automobile engine that will do that out of the box.
The Kw/l is also because of the low RPM a non geared aeromotive engine needs to run at to keep the prop tip speeds down. Successful geared aeromotive engines of more than 100 HP with less than 9 cylinders are pretty thin on the ground. Have a look around the see how much money has been spent (wasted) on developing automotive engine for aeromotive use.
Nm/L is also an invalid comparison. An automotive engine in an aircraft would have similar Nm/l as an aircraft engine.
The only correct comparison is to use BSFC figures.