Though Mopar's big-block has been out of production since 1978, their popularity never seems to fade. Unlike most other manufacturers that saved the "good stuff" for their high-end performance packages, Chrysler tended to use the same hardware across the board. Whether it was going in a high-performance package car, or just a run-of-the-mill sedan or truck, the hard parts were all the same. Chrysler's 400 big-block was a latecomer to the party, introduced in 1972 as a replacement for the low-deck 383. With an identical 3.375-inch stroke borrowed from its predecessor, the 400 came through with the biggest bore ever from a factory big-block, at 4.342-inch—that's .022-inch larger than the mighty 440. Even with its promising big-bore, short-stroke configuration, the 400 never earned legendary status during its production life, a victim of the repressive smog equipment of the time. For the most part it was just a basic middle to large displacement production engine, powering a wide range of Chrysler products.
Interestingly, the performance legacy of the 400 came many years after its production ended. With the popularity of stroker engine combinations, the 400 became worthy of another look. The big bore was definitely a plus, and the crankcase had the capacity to swallow a giant increase in stroke. The low deck carried the additional benefit of shorter and stiffer cylinder bores, in addition to the more compact overall girth of the assembled engine. Taken together, these factors made the 400 a posthumous winner in the eyes of performance enthusiasts.
Production Parts Bottom
The cheapest way to add displacement to the 400 is simply adding the 3.75-inch stroke crank from an RB engine. This addition, plus a clean-up overbore yields the popular 451ci low-deck combination. Randy Malik from RM competition was going to stick to that formula when contemplating building an engine for dyno competition. Adding the 440 crank, however, is not as simple as just oiling the bearings and dropping it in. All low-deck Mopar big-blocks feature a smaller main bearing diameter than their tall-deck cousins, and the low-deck counterweight diameter is smaller as well. The solution here is to cut the mains to low-deck specs, and turn down the counterweights by .300 inch for crankcase clearance. Randy had Performance Crankshaft perform the required machining to his used 440 cast-iron crank, and another production-based low-deck stroker was born.
While a stock block and a reworked factory iron crank may not seem like performance equipment, Randy's experience told him it would do the job. "At this power level the engine will live, but you can't detonate it. If you let it detonate, all bets are off." Randy reused the production main caps, with the only upgrade being a set of used big-block Chevy main bolts: "The Chevy bolts are about 1/8-inch longer, which lets me use a washer and keep the thread engagement." Keeping with the theme of using the existing stock hardware, the rods are production 400 units, held to the crank with ARP bolts and bushed on the small end to Chevy big-block .990-inch specs. Randy explained his choice of the shorter 400 rods versus the popular swap to 440-spec rods: "The shorter rods increase the piston speed away from TDC, and that seems to add torque to the curve below peak torque."
Filling the bores are a set of custom Race Tech pistons, which ride to a zero deck height. Wrapping the pistons is a Total Seal ring package consisting of a low-friction .043-inch top ring, followed by a 1/16-inch second ring, and a 3mm oil ring assembly. Lubrication is provided by a Melling pump drawing from a fabricated external feed pickup via a Muscle Motors pump cover. An old Mopar performance deep sump holds its fill of AMSOIL 10w40 synthetic lube, while a radically cut-down windage tray gathers the oil from the spinning crank. All in all, the bottom end is made up of regular old used production parts with a nice set of pistons and rings stuffed inside.
While most of the bottom end parts could be found in an old late '70s Dodge pickup, Malik added to those parts to make this Mopar come alive. The effort here centered on a set of ProComp cylinder head castings CNC ported by the late Jeff Kobylski of Modern Cylinder heads. The big-port heads were fitted with 2.250/1.810-inch valves, angle milled to 70cc, and moved toward the centerline of the block via a re-doweling. As ported, the heads flowed a peak of 362 cfm on the intake side, but Malik reworked the castings from there to reduce the runner volume. "My approach to intake port size was to reduce the volume right up to the point where it began to cost flow. Working on the bench I filled the floor until it began to affect the flow and that determined the port."
Looking into the crankcase of the RM Competition low-deck wedge, you will not find exotic racing parts. Instead, you'll see production parts, from the 400 block itself, to the stock main caps, and the used factory 440 cast-iron crank. Crank mods by Performance Crankshaft include turning down the mains to low-deck specs, reducing the counterweight diameter and lightening.
The pistons are custom forgings from Race Tech, featuring a .990-inch pin diameter and a .043/.062/3mm ring package from Total Seal. The rods are stockers from a production 400, upgraded with ARP bolts.
AMSOIL 10w-40 synthetic motor oil is pushed through the engine via a stock replacement Melling oil pump topped with a Muscle Motors cover plate to accept an external oil pickup line.
Holding the AMSOIL lube is an old Mopar Performance deep sump pan, modified with a custom fabricated pickup tube and bung. The tray started life as the standard-style Mopar unit, though most of it was cut away.
The piston/rod/stroke combination leads to a zero deck height, with the quench clearance of .038 inch determined by the thickness of the Fel-Pro gasket.
A key player in extracting serious output from this Mopar big-block is the pair of ProComp heads, CNC ported by the late Jeff Kobylski of Modern Cylinder heads. These castings can accommodate large ports.
The modified ProComp heads were fitted with 2.250/1.810-inch valves from SI. Mods include angle milling to achieve 70cc chambers, along with moving the heads on the dowel pins to present an improved valve position in relation to the cylinder bore.
Although the heads were fully CNC ported, Randy Malik fine-tuned the port volume by filling the floor to maximize low-end torque. Volume was reduced until a reduction in flow was noted.