[freebeard]

I posted this for aerohead in another thread:

enginehistory.org: Waste Heat Utilization in High Output Aircraft Piston Engines by Robert J. Raymond

3b. Added Thrust

A significant increase in thrust can be realized by the simple expedient of pointing the exhaust stacks toward the rear of the aircraft....
A rule of thumb given by Hoerner [3] is that between 0.11 to 0.13 pounds of thrust per horsepower can be expected from properly directed exhaust stacks with the appropriate exhaust exit area.