The vast majority of the cars being run were four-cylinder
Ford Model Ts or their successor, the four-cylinder
Model A. The cars were cheap, plentiful, lightweight, and easy to work on. They responded to simple "hop ups" like higher compression, ignition and timing adjustments, additional carburetors, and more radical cam grinds.
The drill was fairly simple: Buy the nicest roadster you could find (because roadsters were the lightest); strip off everything not needed to go fast, like the fenders, headlights, hood, and top; find some cheap used tires to replace your bald ones or to mount over your existing tires for a little extra tread; and go racing.