Roger Halstead wrote:

Where have you been? There are several negatives. Speed and fuel
consumption for a trip. Lower RPM means less HP.

Why less RPM? Speed maybe, but why more fuel consumtion?

The idea is not to make noise, but to gain efficiency.

Noise is wasted energy. So theoretically, a system with less noise is
more efficient. Practically, there's nothing less efficient than
supersonic props. Mufflers, if well engineered, don't put back pressure
to the engine. The exhaust system is a comlex dynamic oscillation
system, and well tuned mufflers can even enhance the power of an engine.
Rip off the exhaust system from a modern car. You'll be surprised how
much power you loose! (Don't ask me why I know.)

Going from a 2 to a three blade prop greatly quieted the Deb, but it
lost about 4 knots top end.

Agreed, this is a trade off. But, frankly, I think a quiet airplane is
worth 4 knots.

I travel 500 to 1200 miles ... cruise is a160 knots true

Let's see.
1000 nm @ 160 kn gives 6 h 15 min
1000 nm @ 156 kn gives 6 h 24 min

So with the quiet prop, you loose 9 minutes on that 6 hrs trip.

The most efficient exhaust system is a tuned exhaust and they are not
quite.

They can be quite quiet.

No, this is an example of a muffler on a light, slow, airplane, not
something with speed.

Just opening the cowl flaps costs me 20 to 30 knots. Imagine what
something like that hanging out would do.

Agreed. But there are also exhaust systems that fit into a cowling. I
don't have a picture, though.

Stefan