Your mixer probably came with a minimum of a 3/4 horse, 1725 motor.
I've got three round-drum mixers. Using one right now, doing my basement floor. Two are big and sound like your's. One is a Wards and the other a Gilson Brothers. Both have drums approx. 25" wide in the widest part, 25" deep, and the opening around 17". Both have drums that turn around 30 RPM. The Wards has a 3/4 horse/1725 RPM, and the Gilson a 1 horse/1725. With a full load, I wouldn't want a smaller motor than the 3/4 horse.
I usually put a 3/4 bag of Portland with the 1-2-3 ratio - and the full load is probably around 400 - 450 lbs.
I also have a large mortar- only mixer with paddles. It has a 9 horse Wisconin gas engine with a gear-reduction drive - and it gets a workout.
Also got a little Sears one wheel concrete mixer. Has a little 12" drum by 24" deep and even that has a 1/4 horse electric motor on it.
These old mixers are very heavy and built like farm tractors. To my surprise though, I went to Home Depot a few days ago to buy a pallet of Portland. They've got brand new mixers with tiny little motors on them. Tiny by physical size, anyway. Their small mixer that has a 225 lb. total load capcacity with a 1/2 horse electric motor, and their bigger model has a 500 lb. mix capacity and has a 3/4 horse, 1725 RPM motor.
Here are some photos of my old clunkers.
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