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 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.
Those are the pros and cons. So how does it wash out? In practical terms, the Magnum heads, as delivered, certainly aren't race heads, but they make for a nice performance street piece. Out of the box, the potential is there to make 400-plus-horsepower with a well-thought-out street 360 combination. While those power levels were possible with the early 340 heads, the chamber design of the new Magnum-when coupled with the appropriate quench clearance-provides a noticeable torque advantage and greater detonation tolerance as compared to the older design.
So what does it take to bolt a set of new Magnums on an old block? Though the heads have been around for years, we've heard urban legends about required changes, when really, all that's required is to select the correct compatible components and bolt them on. We consider it a simple conversion. Follow along as we add a set of Magnums to a hot 360 short-block and see if you agree.
