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Starter/generators are still used on almost all gas golf carts. And they are generators, not alternators (alternators generate AC, generators DC). They work in golf carts and are quiet because they are so low-powered they can be driven from a v-belt without too much problem. A generator powerful enough to start a high-compression V8 would have to be gear or chain driven and that's where the noise comes from.
Honda has developed under-flywheel starter/generators for single-cylinder industrial engines (the iGX440 I think?). It works by using the flywheel magnets as both a magneto generator and a brushless starter motor and since it's directly connected to the flywheel it's silent. It starts the engine with lower power requirements by first indexing the engine to TDC (either stroke, doesn't matter) and then applying starting current, and it starts in a fraction of a second with the push of a button. This technology is scalable to automobiles but generally requires the increase of system voltage from 12-36 (or from 14-42 to use prevailing high-voltage terminology). There is a reason that automotive system voltages stopped at 12VDC and it has to do with higher voltages arcing at the ignition but I'll have to dig to find the specifics...all I know is there were safety concerns with higher DC voltages and distributor ignitions. Most modern engines are distributorless so hopefully this hurdle is gone now.
metro, backpressure is bad for engines. The reason that engines don't do better exhausting straight to open atmosphere is that acoustically-tuned exhaust pipes can do *better* to improve performance than zero backpressure. When an exhaust pipe comes into tune with the engine's pulsing the exhaust port will actually see a vacuum near the end of the exhaust valve event that improves scavenging. Having an exhaust pipe to achieve this tuning inherently provides backpressure, but it isn't the backpressure that helps the engine it's the absence of backpressure during certain designed operating parameters that helps the engine.
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