IMCA Modified Spec Engine Guide | Everything You Need to Know
IMCA Modified Spec Engine | What You Need to Know
For the 2025 season, IMCA has announced a new engine option for Modified racers. The new spec engine is an alternative to the GM 604 sealed crate engine and the open claim engine, and it answers several racers’ requests. Previously, the only options available for Modifieds were the unserviceable roller cam GM crate engines or the flat tappet claim engine. The new spec engine offers the best of both worlds for IMCA Modified racing. It’s an unsealed engine that racers can work on themselves, it uses the same low-maintenance hydraulic roller lifters as the crate engine, and unlike the claim engine, racers are still able to run the same rear spoiler as with the crate.
IMCA Spec Engine Rules & Build Requirements
The new IMCA engine rules state that all spec engines will be based on an OEM GM cast iron roller cam block, part numbers 14011148, 14088526, 14093638, and 10243880. You can read up on Chevrolet small block engine identification in our handy guide. No lightening, additional water lines, or other machine work is allowed on the outside of the block. Spec engines are the same IMCA Modified engine size as the 604 crate, displacing between 350-361 cubic inches and using a 3.480”-stroke stock-appearing steel crank. Stock appearing steel 5.7” or 6.0” steel rods are required and all pistons are flat-top or dished with a maximum compression ratio of 9.0:1. A 1” inspection hole is required in all oil pans, all of which must be steel wet sump pans with internal pumps.
Any hydraulic roller cam can be used, but all engines must use the stock-style roller lifter carrying GM part number 5234890 or Melling part number JB2079 riding in unaltered lifter bores. Beehive valve springs are required on all engines, either GM part number 12713265 or Melling part number VS2254. Spec engines use un-modified cast iron cylinder heads, either Dart IMCA-Approved heads from Speedway Motors, part number 91624360, or EQ part number CH3501. All spec engines will use an Edelbrock Victor Jr. intake manifold, part number 2975 and all will run methanol fuel.
The 2025 IMCA Modified Rules go into greater detail about what is and isn’t allowed in a spec engine.
Developing the Spec Engine for IMCA Modified
IMCA worked closely with Zach Woods at Speedway Motors Racing Engines to develop an engine that would answer what racers were asking for. Crate engine racers wanted an engine that they could work on while still being able to run a rear spoiler, and claim engine racers were growing tired of the hassles associated with the required solid lifter camshafts. The spec engine was designed to be a “best of both worlds” solution for Modified IMCA racing.
Zach and the crew at Speedway Motors Racing Engines have worked hard to develop their version of the spec engine that will retain the drivability that their customers are used to with the 604 crate engines, but will make a little more horsepower and torque than the crate. Speedway Motors Racing Engines spec engines reliably make 20 or so more horsepower and foot pounds of torque than a crate, but in IMCA Modified horsepower and torque are only part of the equation, so instead of all-out power, the Speedway crew’s focus instead was on creating an engine that has very drivable horsepower and torque curves. The valve springs are an obvious limiting factor in the rules, but the Speedway Motors engine shop knows how to get the most out of a limited spring from their extensive work with 305 Sprints. It revs to 6800 rpm like the crate and spec engines also burn methanol so crate racers don’t have to change fuel or cooling systems to run a spec engine. The Victor Jr. intake is slightly lower than the 604 intake, so minor modifications may need to be made to fuel lines and throttle linkages when replacing a crate.
Speedway Motors Racing Engines will continue to develop their spec engine using feedback from racers across the country who are running them. This kind of R&D at scale allows them to dial in an all-around engine that will work well across a variety of tracks and driving styles.
What This Means for IMCA Racing Modified
Obviously, the introduction of another engine option for IMCA Modified racing opens yet another door for racers. It ensures that multiple engine options will continue to be available regardless of supply issues. But the spec engine also reintroduces some individuality and personalization into IMCA modified racing. These days, everyone is running very similar cars using similar dirt Modified parts, and the ability to customize and work on the engine allows racers to reclaim some additional ownership over their car’s performance.
Speedway Motors Racing Engines spec engines are available now, and you can read up on 602 crate motor specs and 604 crate motor specs on our blog to help you make your plan for the coming season. In addition to engines, Speedway Motors has all the dirt track parts you need to finish up your racecar in preparation for a new year of racing!
