I entered my first race in 1963 in Detroit. I mentioned that the practice of the day was six-up rat racing. Some people questioned that, but with the assistance of Dan McEntee, I have a photo of the one-page rules of that time.
Detroit's first race had 24 entries of which only the winner had any racing experience. Most of the entries were flown by combat flyers, of which there were plenty.
Please note that the rules called for an engine-running start with two minutes of start time off the clock. That rule was set aside in even the first contest in favor of a cold start.
All the engines were combat 35's with the exception of the 5th place Dave Keats entry powered by a factory special pre-production Fox 40. 100 MPH was a goal which was not yet achieved.
As other engine makers: OS, K&B, Supertiger, HP, etc, got into the racing 40 business, the speeds went to 160 MPH and the number of flyer went down to two.
Since 1962, the performance of model engines has increased so much that 60's, 40's, 29's, and 25's are hardly needed at all. The 15's (2.5 cc) of today are all the engine we need for any CL competitive event.