408-Inch Mopar Stroker

We filled up with ordinary 91-octane Unocal unleaded pump gas and ran the small-block on the dyno. Loading it up against the SuperFlow, we found 607 hp @ 6,300 rpm, and 551 lb-ft @ 5,100 rpm. That’s plenty of horsepower and torque in our book.

A small notch at the bottom of the bore is all it takes for a Mopar 360 block to swallow a 4-inch-stroke crank.

The Mopar Performance stroker crank has large fillet radius journals requiring chamfered bearings for clearance. We used Federal Mogul Competition Series main bearings, which cleared without additional massaging.

We installed a set of Milodon four-bolt main caps, which required line boring the block. Milodon main studs were used to clamp the caps to the block.

Pistons are Diamond Racing flattops, rods are Eagle H-beam, rings are Total Seal gapless top rings, and rod bearings are Federal Mogul Competition Series, again chamfered to match the crank fillets.

The final balancing bobweight for our combo was 1,946 grams, which is lightweight in Moparland. The crank needed some Mallory heavy metal to be internally balanced, making the balance job cost as much as the crank. The production 360 external balance factor is too heavy, though a custom external balance factor could have been used for a cheaper balance job at the expense of future interchangeability of components like flywheels, flexplates, converters, and dampers.

Our buddy Roger Helgesen applied TechLine’s thermal barrier coating to the piston tops and an anti-friction coating to the piston skirts. Thermal barrier coatings help retain combustion heat in the cylinder during the power stroke where it does some good, rather than in the piston where it doesn’t. The skirt coating combats the increased skirt loading with the shorter rod ratio that comes with stroker cranks.

Total Seal’s gapless rings are a two-piece design with a grooved main ring and rail which eliminates the gap. Leakdown was less than 1 percent after engine break-in.

The pistons were installed with a $7 Lisle cam lock ring compressor, which works well and fits any bore size. The piston/rods were numbered to the bores when we assembled them to assure the rods and valve notches are in their required orientation when installed.

For a camshaft, we went with a Crane solid-roller that spec’d out at 260/266 @ 0.050 inch, and 0.420-inch lobe lift on a 108-degree lobe-separation angle. It was the smallest solid-roller in the company’s catalog. Traditionally, when installing a solid-roller in a Mopar small-block, the oil to the tappets needs to be blocked to prevent the lifter waist from uncovering the gallery, which results in zero oil pressure. To block it and still feed the mains required sleeving the gallery, peening the sleeve in the lifter bores, and drilling oil-feed holes down to the mains. Crane says its lifter design simply drops in and works without all that fussing. They did. Oil pressure was 30 psi at idle and 70 psi at high rpm. As pointed out in the Crane instructions, some clearance needs to be ground into the block for the link bars.

With big cams, don’t skip checking valve-to-piston clearance. A clay check showed we had a mile.

In a Mopar small-block, the right oil gallery supplies the No. 1-4 mains, and then the oil transfers back up from the No.1 main to feed the left gallery. We blocked the right side feed to the No. 1 main, and fabricated this 3/8-inch tube crossover, which supplies the left gallery, and sends oil down to the No. 1 main from there. This mod helps with main oiling at high rpm.

To drive the cam, we installed a Milodon fixed-idler gear-drive system, which fits under the stock timing cover. We couldn’t hear it over the open headers while it was on the dyno. The cam was degreed to an installed centerline of 104 degrees.

The windage tray, oil pump, and pickup are Milodon items. We found that the tray louvers need some minor dimpling in line with the rods for comfortable clearance with the stroker crank once the pan was on.

We used a Probe Industries lightweight elastomer damper and sealed off the bottom with a Milodon pan.

Our W-2 heads are Mopar’s PN 4529995 long-valve econo heads with Mopar 2.02-inch intake and 1.60-inch exhaust valves, all from Westoaks Dodge in Thousand Oaks, California. The springs are Crane PN 99876, shimmed to achieve 185 pounds of seat load, while the retainers are Crane’s racy titanium with corresponding locks.

We needed dual springs to run the roller cam, so the spring seats in the W-2 heads had to be machined down. We used a seat cutter from Powerhouse. We cut the tips of the guides for PC seals and took an extra 0.100 inch off for increased retainer-to-guide clearance.

To work the valves, we used a set of Crane aluminum rockers in a 1.6:1 ratio. We actually measured them as delivering a 1.63:1 ratio for 0.665-inch lift at the valve after lash with the cam. Up top, a Mopar Performance W-2 tunnel ram got the nod, because we like the brutal image and power it produces.

On top of the tunnel ram, we mounted a set of Demon 750TR carbs, which are specifically calibrated for tunnel ram applications. Hey, if they’ve got them calibrated it means less work for us.

Street headers have long been a problem when going to a W-2. However, Tube Technologies recently introduced a line of 1-7/8-inch headers that fill that gap. These were the first set made.

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.