Well, to make 60 Hz the engine has to turn 3600 RPM if the generator is a two-pole generator. If it is a four-pole, which many are, 1800 RPM. If six-pole, 1200 RPM. I have owned several two-pole generators and a few four-pole, but never seen a six-pole.
Several years ago I bought (via Internet) a reasonably quiet Chinese copy of a Honda GX620 -- two-pole, 3600 RPM, 3500 watts. It had a Honda-style muffler and was actually fairly quiet. It had electric start, too -- a nice unit. I modified it for remote start and it served me well. I paid less than $400, brand new.
Of the generators I have owned, my favorite was an Onan JC 12.5KW unit with a four-cylinder propane-fueled engine and a four-pole generator, 1800 RPM. I put a car muffler on it, and it was quiet enough to hold quiet conversations while standing a foot away. These are easy to find on eBay, and virtually indestructible. They're big and weigh about 700 pounds. There are diesel-fueled JCs [called DJC], gasoline-fueled JCs, and propane-fueled JCs (which can also burn natural gas). Some JCs have two-way carburetors and can run on gasoline, propane or natural gas. Some JCs are 12.5KW and others are 15KW. At various times I owned one propane 12.5KW JC and two gasoline 12.5KW JCs. All were rock solid.
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-- Carl
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