The Magnum head is a simple conversion but requires pushrod oiling for the 1.6:1 valvetrain. Most aftermarket hydraulic lifters have this covered, so the only custom component required is a set of pushrods at the required 7.650-inch length. The closed 59cc Magnum chambers with our zero-deck KB pistons yield exactly an ideal 10:1 compression ratio with the appropriate 0.040-inch-thick head gasket. Intake choices are limited, but Mopar Performance offers either a dual- or single-plane manifold to cater to induction needs. We favored the high-rpm potential of the MP M-1 single-plane, since our track-bound A-body already had stout gears and a converter. With that, we killed the 318's three major demons, blessing it with compression, quench, and airflow.
Delving deeper into the works, we found the camshaft was primarily determined by the heads. Magnum heads offer an excellent intake/ exhaust-flow ratio, which favor a single-pattern cam. The stock valvetrain is cheap, but highly effective. It's also nonadjustable, so we went with a hydraulic grind. Retainer-to-guide clearance becomes a limit at 0.525-inch lift, so we opted for a Comp 280H, coincidentally part of their "Magnum" line of camshafts. With a stock 1.6 rocker ratio, gross lift worked out to 0.515 inch, and duration at 0.050-inch tappet lift was a generous but not outrageous 230 degrees. Lobe separation is 110 degrees, promising a drawn-out torque curve for prolonged high-rpm pull. The standard Comp 822 lifters allow for valvetrain oiling.
With the power plan settled, we chose Federal-Mogul for rebuild components and shuttled our block to the craftsmen at Precision Speed and Machine for machining. Assembling the little Mopar was textbook simple, and we went from a pile of parts to ready-to-run after a single day. Next it was on the truck and off to Westech's dyno facility where the numbers would finish the story.
Dyno DerbyCompleting the package, we trimmed out the 318 with top-quality components throughout. An MSD distributor and wires handled the spark, while an amply sized mechanical-secondary Mighty Demon carb administered Unocal pump gas to the captured air. We bolted TTI stepped headers to the port exits, because we know that they make power, fit, and don't leak. With the 318 complete and bolted to the dyno, we wagered over how much power it would make. One guess was 385, while dyno operator Brul was less generous at 370. Our hopeful estimate of 400 was seen as easy pickings.
With the ignition timing set at 34 degrees and 84 jets all around, Brul loaded the engine against the brake for a partial pull, and we saw 374 hp at 5,600. Upping the rpm a couple of hundred at a time to find the peak, we found the top at 382 at 6,200 rpm, and we hadn't yet started tuning. Pulling the timing back hurt power, so we added advance to 36 degrees and found 385 at 6,200 rpm. Another 2 degrees was only good for a 1hp gain, so 36 degrees it was. Running through a jetting loop next, we found a better curve leaning the jets to 82s, but no higher a peak number. We noted that the oil pressure was quite high with the 20-50W oil sloshing in the sump at more than than 95 psi. It looked like a power drain to us, so we changed to a 10W30 Royal Purple synthetic that netted us a fat 394 hp at 6,100. We were so close to 400 hp that we could taste it, so we shuffled a Wilson 1-inch tapered spacer under the carb and the 318 responded by dealing up a 400 hp dyno curve, with peak output at 6,100-6,200 rpm.
Satisfied? We know we were, with a production iron-head 318 at near 1.23hp per cube with a street hydraulic cam. The engine had over 10.5 inches of vacuum at a 900-rpm idle, but just enough rump to get noticed. With 400 hp on tap, it's got some bite, and comes with the satisfaction of dishing out the hurt with just a little old iron-head 318.