Another big difference comes in the engine being used. The Midwest Modified is required to run an engine that can be used in "conventional passenger cars." Modifieds will use a spec engine up to 362 ci or an engine up to 410 inches that can be claimed. Midwest Modifieds are required to run compression of 9.5:1 or less, and there are specific car weights tied to engine type. The weight must be listed on the side of the car. Stock production crankshafts, OEM stock steel rods, and stock cast-iron cylinder heads must be used on Midwest Modified cars. There are also some Chevrolet casting numbers prohibited by the rules. Rules for the Midwest Modifieds are very specific in terms of parts allowed.
Modifieds can run any four-barrel or two-barrel carburetor while Midwest Modifieds can only use a Holley 4412 two-barrel. The promoter or drivers in the class may claim the Midwest Modified carburetor. While Modifieds can run gasoline, ethanol-enriched gasoline, or alcohol, there are restrictions on the Midwest Modifieds, contingent on engine size. Alcohol is not allowed on any Midwest Modified.
Engine claiming, protest, and pumping rules are similar with one notable exception. The Midwest Modifieds are divided into "small" and "large" engine classifications within the rules. The driver of a "small" engine car can only claim another "small" engine car. The same rule holds for "large" engines.
Sport Modified The International Motor Contest Association (IMCA) has developed its own economy Modified. The Sport Modified uses a spec Pro Topline head and contains a few more differences. This class has been somewhat slow to catch on at member tracks, despite the obvious intent to save cost to the racers.
Economy of Scale Racers will run the class of their desire. The creation of Economy Modifieds can be as misleading as the term "stock car." The intent of the creator may be best, but not all Modifieds are economy classes in the truest sense of the word. The world of racing is large enough to make room for all. The success of a class depends upon its ability to withstand challenge, whether that challenge comes from rules or economics. The effort in the 21st Century to keep racing above water in times of tighter dollars is laudable. In the end, organizers and competitors will work together to help the division grow, and this growth depends equally on all parties to do their part.