Engine Efficiency + exhaust back pressure theory

[PaulO]

This is purely a question on theory

I would imagine an engine would run most efficiently at a certain RPM with the back pressure experienced from the exhaust. My 120HP exhausts from prop hub and the lower leg so I guess it has to turn over at mid to high RPM to build up the back pressure.

Does any one know if a variable (or fixed but of different size) exhaust outlet to water or to the free air would improve efficiency in these two strokes at lower RPM (or higher) reducing fuel consumption to what is currently considered normal?

 

[PaulO]

Re: Engine Efficiency + exhaust back pressure theory

Bump

 

[Frank Acampora]

Re: Engine Efficiency + exhaust back pressure theory

Force dual exhaust lowers were developed to decrease back pressure and increase horsepower.

The firing order on 4 cylinder Force engines is 1-3-2-4. The exhaust dumps into a common chamber. Port timing is open for approximately 120 degrees and crankpins are at 90 degrees. Thus there is a 30 degree open port overlap. Increased backpressure blows exhaust from 3 into 2 and decreases horsepower.

That said, when Merc redesigned the midleg and lower unit, the exhaust passage was reduced in cross section--I don't know, the engineers must know something. It may be like on auto racing engines: Port volume and headers, and thus gas velocity, are matched to the power band required.

Engine blocks are extremely old design--at least 50 years. Because they are dirt simple cross flow design with a heavy piston, massive baffle and short rods, they use more fuel to develop horsepower. Really not much can be done to improve them. That is, not without extensive modifications and gobs of money thrown at them. Cheaper to buy a newer used modern style engine.

 

[PaulO]

Re: Engine Efficiency + exhaust back pressure theory

I hadn't thought of the port overlap and now puts things into perspective. Thanks Frank.