Stroked For Power

Displacement is nice. It's a classic saying in the performance world that there is no substitute for cubic inches, and it's for good reason. Opening up to a larger displacement has a direct benefit in torque production, right from the bottom of the power curve. We like that, but the potential for horsepower is opened still further. Horsepower is a function of torque and rpm, so extracting big power from smaller cubic inches requires serious rpm. At some point, rpm begins to take a toll on reliability, durability, and the cost of achieving power. A comparable combination at a larger displacement can make similar peak horsepower at a lower rpm, making for a more reliable engine.

Looking at street/strip combinations, the benchmark rpm level for a nice small-block falls in the 6500-7000-rpm range. At this level, a reliable engine can be built without exotic parts and ultra trick valvetrain components. However, if building for a given rpm range, a bigger engine just provides the opportunity to build more power at an equal level of execution. We wanted to test this idea so we sought to take advantage of the benefits of a larger displacement small-block Mopar. The largest small-block Mopar engine in production measured 360 cid, with a bore of 4.00-inch, and a 3.58-inch stroke. With readily available 4.00-inch stroker crankshafts and the appropriate pistons, stretching the displacement to 408cid is relatively inexpensive, and simple.

BIGGER BREATHING
Flow capacity, piston speed, and valvetrain limitations are three characteristics that need to be acknowledged when looking at larger displacement engines, especially when the added cubes come via a longer stroke. The first of these factors, piston speed, is normally the standard by which a bottom-end's reliability and stress is gauged. Calculated average piston speed is the normal measure here. Large increases in stroke will substantially drive up piston speeds. Another factor with displacement increases, and one which may have an influence on piston speed, is the airflow potential of the engine. Increases in displacement will place a higher demand on the cylinder heads and induction, and if an equivalent top-end package is employed in either a large or small displacement combination, the larger engine will make more power at a lower engine speed. If, however, the airflow is increased proportionally to displacement, the rpm range will be comparable to the smaller engine, and the longer stroke engine will have a higher average piston speed. The quality of the bottom-end components requires consideration in any stroker combination.

The final factor is a simple truism, that the valvetrain has no sensitivity to the displacement of the bottom end. Most engines are rpm limited by the capabilities of the valvetrain, and extending this limitation comes at a penalty in terms of reliability and/or cost. Here, we return to the under 7000-rpm range we mentioned earlier. Building a reliable valvetrain with an acceptable level of power to this rpm range is easily achieved at a reasonable cost. If the rpm-capabilities of the valvetrain are the limiting factor for the larger engine, and if built to take advantage of the rpm range, will always outperform a smaller engine.

THE BUILD
Our plan was simple, we'd build an economical, and reliable bottom end centered on the 4.00-inch stroker crankshaft offered by Mopar Performance, and a production iron block. Mopar small-blocks were produced with two different main bearing dimensions, the 360-block used the larger journal, and a smaller diameter was employed in the other small-blocks. Naturally, our crank was the large journal spec to match the 360-block. To substantially add to the strength of the bottom end, a set of Milodon 4-bolt steel main caps were installed in place of the factory 2-bolt mains. All Mopar production small-blocks came with two-bolt mains, while the Milodon caps are configured with an additional set of splayed fasteners for the center three mains.

Rather than use production rods, aftermarket Eagle H-beam rods were installed, a popular and cost effective upgrade. The Eagle rods all but rule out the likelihood of rod failure in an application likes ours. The rods measure 6.123-inch in center-to-center, the same as any production Mopar small-block. The 4.00-inch stroker crank requires cutting the bottom of the cylinder bores for rod clearance. However, with the compact cap screw arrangement of the Eagle H-beam rods, the grinding required for the stroker combination was minimal.

Stroker pistons have a shorter compression height, which is the distance from the piston pin centerline to the top of the piston. For popular stroker engine combinations, stroker spec pistons are readily available. For the Mopar small-block application, Diamond Racing Pistons lists forged pistons in both flat top and dished configurations. We opted for the flat tops, though for a more street orientated combinations the dished pistons may be the better choice, depending upon the cylinder head cc specs and the desired compression ratio. Sealing the rings to the bore, we opted for Total Seal's gapless top rings, which incorporate a two-piece top compression ring assembly effectively closing the ring end-gap for a better compression seal.

CYLINDER HEADS
To get the most out of a stroker combination, it's vital to provide enough airflow to make full use of the inches added to the bottom end. For this engine, we selected a set of Mopar Performance W-5 cylinder heads. The W-5 heads are a step beyond the familiar W-2, sharing the same offset rocker arm arrangement, but with rectangular ports and aluminum construction, in contrast to the W-2's iron castings. Unlike most of the subsequent aluminum "race" heads that are designed to work on the Mopar "R" race blocks with 48-degree lifter bank angles, the W-5 works just fine with production 59-degree lifter bank blocks. Headers and intakes, however, are W-5 specific; to work with the raised rectangular intake ports and raised W-5 exhaust port. With the correct intake, exhaust, and W-2/W-5 valvetrain, the W-5 top-end is a simple upgrade. Our W-5 heads were fitted with Mopar Performance 2.05-inch intake and 1.60-inch exhaust valves. As used with the race W-2 heads, W-5 requires the long-valve stem configuration. The W-5 heads can be ported for very good flow, and a full port job was part of our plan.

To supply the induction, we opted for Mopar Performance's excellent W-5 tunnel ram manifold, topped with a set of 750 cfm Demon TR carbs. To match the porting in the heads, the tunnel ram was also ported. The great heads and dual quad tunnel ram would provide all the flow our 408 cube bottom end would need to make great power right into our intended power range of 6500-plus rpm. Just where the power would come in, and ultimately the upper rpm range of our engine would be a matter of camshaft choice. A solid roller would provide the best service with high lifts and the required spring loads. With long term durability in mind, we steered away from some of the ultra-quick lobe profiles and outrageous duration levels available in some of the really radical race rollers, and selected the proven and reliable standard Crane race roller series.

We used the smallest race roller in the Crane book, which specs out at 260/268 degrees of duration, and 0.420-inch lobe lift. This cam is very stable and easy on the valvetrain, and does not require outrageous spring loads, and yet supplies the area under the curve to let the heads show their stuff. It seemed like the ideal combination of heads and cam to get the most from our large cube small-block up to the mid-6ks. To actuate the valves, our cylinder heads required offset intake rockers of the same spec as for the W-2s. We went with Erson's extruded aluminum roller rockers, which feature a unique design with reduced diameter shafts for less bearing inertia and greater spring clearance. The Erson rocker kit comes complete, assembled with rockers, shafts, spacers, and hold-downs. Erson has a very wide offering for Mopar applications. We stepped up the valve intensity by opting for 1.6:1 ratio rockers as an upgrade from the factory 1.5:1, which raised the gross lift at the valve to a calculated 0.672-inch. To complete the valvetrain, Smith Brothers produced a set of pushrods to ours specs, which we determined by mocking-up and measuring the length required.

Some of the other details were aimed at durability, with oiling components from Milodon, including the sump, pump, and tray, as well as Milodon studs to fasten the main bearing caps and cylinder heads. We performed some additional oiling mods, installing a bypass in the upper oil galleys, and rerouting oil to the #1 main. We also chose Speed-Pro's proven Competition Series bearings throughout, again with an eye towards reliability at high power levels.

ON THE DYNO
Once completed, we brought our 408 to the Westech dyno facility for testing. The engine certainly looked impressive with the Mopar Performance tunnel ram and dual Demon carbs, but the bottom line is always measured in twist at the crank. After the initial run-in to seat the rings, we reset the valve lash to specs with the engine hot in preparation for the coming power pulls. The ignition timing was set to 36 degrees total timing, and Westech's Steve Brule initiated testing with some static pulls to determine if the fuel mixture of our tunnel ram system was in the ballpark. A credit to Demon's calibration, the instrumentation showed that the carbs were delivering the mixture in a safe proportion, and we were ready to initiate some partial pulls to dial in jetting and timing.

Testing revealed that the 12:1 compression ratio engine performed optimally with only 33 degrees of total timing on 100-octane fuel, indicating very efficient and quick combustion. Jetting was found to be optimal with #86 jets at all eight corners. With the final calibrations complete, this 408-cid Mopar was truly a monster, delivering torque in staggering quantities, peaking with 583 lb-ft at 5200 rpm. That equates to a specific torque output of 1.42 lb-ft per cubic-inch, a level seldom seen. Power output came in just where we had planned, peaking at 6600 rpm, while shoving the dial to the 660hp mark. Considering the lack of exotic components in the bottom end, and that we were turning a stock water pump, the output was truly remarkable. It just goes to show what can be achieved when cubic-inches are combined with great airflow, in a well thought out combination.