408-Inch Mopar Stroker

When we heard that Mopar Performance had introduced 4-inch-stroke replacement cranks for its small-block engine family, we took notice. When we found out these cast-steel gems can be snagged for under $300, we had to have one. The cranks are available for either the 2.500-inch small main journal size used in the 273/318/340 series or a 2.810-inch 360-size main. We leaned towards the 360 crank, since 360 cores are still easy pickings and come with a standard bore of 4 inches, yielding 408 cubes of Mopar muscle with the stroker crank and a 0.030-inch overbore. With that in mind, we yanked a 360 from a ’72 Dodge van at the boneyard and were in big-inch small-block business.

Stroke’s The Word

With a block deck height of 9.6 inches and a cam location up and out of the way, the small-block Mopar’s crankcase is plenty roomy for a big 4-inch arm. Block mods for this combo are limited to some minor notching at the bottoms of the bores for rod bolt clearance. In contrast to other more modestly endowed small-blocks, all factory Mopar small-blocks came sporting long 6.123-inch rods. The combination of lofty decks and long rods enables the stoker combo to come together with a reasonable rod ratio of 1.53:1, while keeping the piston pin clear of the oil ring in a standard ring package.

Besides the stroker crank, putting together any stroker project requires specific pistons with a correspondingly shorter compression height than a standard slug. Although a long-stroke engine often evokes images of a lazy, low-rpm torque brute, we planned on building ours for big torque and ample top-end horsepower. To this end, we elected to fill the bottom end with premium components. Diamond Racing, a company long known in Mopar circles as a pipeline for serious race hardware, supplied lightweight forged stroker flattops with a factory pin diameter of 0.9842 inch. With the long stroke, a flattop piston can be dialed in to quite a wide range of compression ratios depending upon chamber volume.

While adding cubes is easy, getting them to pay off requires catering to the engine’s increased craving for air. We looked no further than the Mopar Performance catalog and ordered a set of W-2 iron performance heads, which were first introduced in 1976. Though the W-2 is a performance upgrade over any production head, we pulled out our grinder and gave them a basic home porting job (see “Basic Head Porting,” Feb. ’02). The W-2 responded well to our efforts, yielding a 315-cfm intake flow, and 230 cfm of exhaust (with flow pipe) compared to 245/140-cfm stock, as tested on Westech’s SuperFlow 600 flowbench.

Why use an “exotic” head like the W-2? Essentially, the W-2 and other similar aftermarket heads address and correct several bottlenecks to performance inherent in the small-block Mopar head architecture by offsetting the intake rockers to move the pushrods away from the center of the port for increased intake-port width. With the factory valvetrain, the intake port’s cross-sectional area is limited adjacent to the pushrods, while the W-2’s layout makes room for a wider port at the pushrod pinch. The W-2’s wider oval intake ports require a corresponding W-2 intake manifold. At the other end, the W-2 has raised exhaust ports and a spread bolt pattern at the header flange.

The bolt pattern change allows easy installation of large-tube headers, whereas the stock layout causes the header bolts to be crimped against the tubes—even with 1-5/8-inch street headers. Finally, the W-2 is available in a long-valve version, which increases the valvespring installed height from the factory1.65 inches to 2.00 inches, making room for the stouter springs needed to go with high-lift cams. W-2s come in a number of different chamber configurations. Our W-2 has the standard 70cc production-style open chamber. After the porting work, we had 75 cc’s for a compression ratio of 10.4:1. Milling the heads can bring the ratio near 12:1 with the same flattop pistons.

With lots of cubes and plenty of airflow, the next choice is the camshaft. We wanted to open the valves enough to take advantage of the good flow of our W-2 heads, but we didn’t want the huge duration that a flat tappet would require to meet our desired lift goals. Fast valve action and reliability with high lifts at the accompanying high spring loads steered us towards a solid-roller. We went with the smallest solid-roller in Crane’s lineup, a 260/266 @ 0.050-inch grind.

This Mopar 408 is a rowdy little thumper with enough lope to know it means business, but docile enough to idle nicely at 950 rpm—although with less than 5 inches of vacuum it’s no pussycat. Wind it up, and there’s over 600-plus horsepower on tap, and it’s all Mopar.

By Steve Dulcich

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