Unlike pre-Magnum production...
Unlike pre-Magnum production heads, the Magnums' chambers are a modern closed-chamber design, with a generous quench area at both ends. Valve placement and angle are at the same advantageous positions as early heads.
We all know about 340 Magnums, 383 Magnums, and 440 Magnums. During the musclecar era, the "Magnum" moniker was hung onto engines that delivered the goods. It was a power and recognition thing. Plain-Jane engines received no special name, notoriety, or fame. There were no Magnum engines in the late-'70s or early-'80s, but in 1992, when the 360 engine was revised for higher output, Chrysler wanted the change to be noticed. They dusted off the Magnum name, and once again, Magnum Performance was alive.
The major revisions to the basic 360 small-block were in the cylinder heads and valvetrain, and also the addition of a hydraulic roller-cam-aided output. The roller cam had been installed in small-block 318s dating back to 1985, but that's it. As far as engine architecture, the heads and valvetrain marked a serious departure from the traditional design. Shaft-mounted 1.5:1 rockers were replaced with pedestal rockers in a more favorable 1.6:1 ratio. The higher rocker-arm ratio translates to quicker valve motion for a given cam-a plus in a street engine.
So, you wanna go Magnum? Here's...
So, you wanna go Magnum? Here's the hardware you'll need. Besides the heads, you'll need a set of Magnum-specific rocker arms, pushrods with oiling provisions, Magnum head bolts, Magnum valve covers, and a Magnum intake. Don't forget fresh head gaskets and Magnum-specific intake and valve-cover gaskets. Fortunately, most of the production-based Magnum components, including the heads, bolts, and rockers, are bargain priced.
Another primary change is found in the combustion chamber. Mopar did away with the recessed open-chamber design and moved to a modern closed-chamber layout. The open chamber is the biggest weakness of the old design. The old relieved chambers don't take advantage of the efficiency benefits of squish/quench effects in combustion. The open area traps residual exhaust gases and contaminates the air/fuel charge during overlap. It also provides a large volume of leftover gas to pre-ignite and detonate on the power stroke.
Port revisions are also major features of the Magnum heads. The exhaust port is improved, with the dead area at the floor of the runner leading to the valve seat, revised to a substantially improved form. The dogleg in the two center runners is diminished, and the roof of the port is given a less abrupt change in angle at mid-port. As a bonus, the exhaust-valve size is enlarged from the 1.60-inch used in 340s and 360s and the 1.50-inch used in 273s and 318s, to a 1.625-inch diameter. On the intake side, the restrictive hook in the guide-boss of the old heads is eliminated.
The floor is improved as it approaches the critical turn to the valve seat, and the valve size is 1.92 inches. That's between the common 1.88-inch size found on the older 360s and the 2.02-inch size of the early 340s. It's also much larger than the 1.78 inches of the 273 and 318. The large pushrod hole found in late 318 and 360 head castings is retained to give clearance for the hydraulic roller valvetrain. This cramps the port cross-sectional area at the port entrance; additional material allowing for the pedestal rocker's mounting-bolt bosses also impinges on the port.
A key improvement in the Magnum...
A key improvement in the Magnum design is in the exhaust port. Efficiency is increased, as the shape of the floor is improved from the earlier castings, with a nice radius in the shortside turn leading toward the valve.
The intake port squeezes around...
The intake port squeezes around a bulge adjacent to the pushrod. However, the port is capable of supporting over 400 hp in a streetable combination.
The Magnum head assemblies...
The Magnum head assemblies come equipped with one-piece stainless steel valves, each with a single groove for locks, and steel retainers with 7-degree locks specific to the Magnum. Valve sizes are 1.92-inch intake and 1.625-inch exhaust. This is quite an improvement over the 1.88/1.60-inch valves in earlier 360s. The valve stem is significantly reduced in diameter to 8 mm (or about .315 inch) as opposed to the older LA engine's 31/48-inch stem, which is .375 inch. This is a significant weight reduction at the valves. The single spring with a wound damper is similar to the early HP spring and more than ample for high performance street duty.
The Magnums are assembled...
The Magnums are assembled with positive guide-mounted valve-stem seals, quite an improvement compared to the umbrella seals of earlier engines. When measuring the valve-lift clearance, the retainer physically hits the seal at .540-inch lift on the exhaust side and .525 inch on the intake. This gives clearance for cams up to about .500-inch lift or so. This is plenty for lift ranges proportionally adequate for these ports in stock form.
Our subject is a slightly...
Our subject is a slightly used early 360 short-block. It's equipped with .030-over Federal Mogul flat-top hypereutectic pistons, a stock crankshaft and rods, and a Comp Cams street hydraulic cam. The addition of the Magnum heads will make for a nice street engine.
Although there are specific...
Although there are specific Magnum gaskets, regular early-style gaskets work just fine. We used FelPro gaskets, which have a compressed thickness of .040 inch, giving a perfect minimum quench clearance with our 360's zero-deck pistons. With closed-chamber heads, it's important to consider the piston-to-head clearance, and .040 inch is as tight as we'd go with a street engine. Early 340s had a positive deck and required .055-.060-inch-thick gaskets for minimum clearance.