Excellent post from Mr. Abraham
Anyone doubting the effects on 4-strokes, need look no further than Formula 1 racing cars.
Both inlet and exhaust tracts can be tuned, but without corresponding alterations in cam-profile and ignition-timing, the results are limited.
Experimental research engines have been built,where it's possible to alter valve-timing and completely stop it .
Thus, a fully tuned engine, running at the speed where the induction/exhaust tracts are at resonance, can continue to run with the valves permanently open!
Direct-drive Aircraft piston-engines are ideally suited to tuning ,because they are operated mainly in a tightly-defined rev-band.
As a generalisation, noise= energy = wasted power. the conundrum to the designer, is how much low power do you throw away,in order to generate higher-power........look at the enormous waste energy from a dragster!
Tuning is a balancing act between economy, efficiency, absolute output and longevity.
Your average car ,nowadays, is electronically controlled, "chipping" (reprogramming) the engine can make enormous differences. invariably, there are some risks. With Aviation, reliability is paramount. that's why the Lycosauruses survive. they are fairly well optimised to their intended usage-profile, but there is room for improvement
A quieter exhaust does not mean you have to suffocate the engine, contrarily, it can mean you do, inded , make usable power instead of noise.
The Merlin exhaust story is worth looking at....again, the stubs were more than just to stop the paintwork getting burnt.
They had a big effectfor such a seemingly insignificant item.