My converter uses a 15 hp idler, and the pony is a 3/4 hp single phase motor. Frankly, I can't tell the difference with the belt on or off. Most of the windage is in the big idler.
Because the running speeds of the two motors aren't exactly the same, there's a bit of a squeal when the big idler first gets power and the pony is switched off (I have the switches interlocked). But that's all.
Gary