|
|
Adding Magnum To A 360 Short Block - Magnum Head ConversionHeading For A Transplant From the August, 2003 issue of Mopar Muscle By Steve Dulcich Photography by Steve Dulcich
|
|
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. 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.  Pre-Magnum LA engines oiled...  Pre-Magnum LA engines oiled the shaft-mounted valvetrain via these oil-feed passages in the block deck. The Magnum heads' valvetrain gets oil through the pushrods. If you're starting with a bare block, the oil passages in the block can be tapped and plugged. If you're replacing pre-Magnum heads on a short-block, the factory oil hole will seal against a dead end when meeting the head's deck surface. A light smear of silicone here wouldn't hurt, particularly if extra-thin head gaskets are used.  The bolts are torqued in the...  The bolts are torqued in the conventional pattern to the spec of 95 lb-ft, the same as early LA engines. Although early LA engines all have dead-end head-bolt holes, some later small-blocks have head-bolt holes that lead into the water jacket. With those later blocks, use Teflon thread sealant. Note the revised intake-bolt angle and absence of exhaust crossover on a Magnum head.  Magnum heads use bolts of...  Magnum heads use bolts of different lengths. These new Mopar bolts feature reduced 51/48-inch heads and a built-in shouldered washer-and they're dirt-cheap.  To oil the valvetrain, the...  To oil the valvetrain, the lifters must have provisions for pushrod oiling. Fortunately, 90 percent of aftermarket hydraulic lifters for Mopar applications are universal, fitting Mopar and AMC; thus, they have provisions for pushrod oiling in deference to the AMC requirement. Check the catalog part number from the cam company that supplied your lifters. If Mopar and the AMC lifters carry the same part number, you're golden. An easy way to tell is to view the pushrod cup. Nonoiling lifters (left) have a solid seat with a hole in the center that indicates the lifter has provisions for oiling.  Magnum heads carry a valvetrain...  Magnum heads carry a valvetrain totally different than older LA small-blocks. In the Magnum system, rather than a shaft mount, the rockers pivot on individual fulcrums bolted to the head and are ganged in pairs for each cylinder by a guideplate.  Stock Magnum engines used...  Stock Magnum engines used hydraulic roller lifters, which have a much longer lift body than a flat-tappet hydraulic lifter. For the Magnum head with a flat-tappet cam combination, a unique pushrod is required to make up the lifter length difference. Mopar Performance sells a 7.625-inch pushrod, PN P5007477, for this combination. Stock Magnum rockers don't have a provision for valvetrain adjustment, so with variations in milling, cam-base circle, gasket thickness, and the replacement lifter's height, the best way to be sure is to measure. An adjustable length-checking pushrod can be set to the required length until it depresses the lifter plunger .020-.040 inch with the cam on its base circle. The pushrod's length can then be measured and the appropriate length pushrods ordered. The pushrod should feature 51/416-inch ball-ends and have an oiling passage through the center.  Installing the rocker assemblies...  Installing the rocker assemblies is simple once the correct-length pushrod is obtained. Just bolt them up and it's done.  Mopar Performance carries...  Mopar Performance carries two aluminum four-barrel intakes for the Magnum heads, a dual-plane, PN P5249500 (pictured), and a single-plane, PN P5249501.  Magnum heads use a unique...  Magnum heads use a unique intake gasket, which comes from Chrysler as part of a kit, PN P4876049. The kit includes stock-type torque-to-yield intake bolts and valve-cover gaskets that are leak-proof laminated with ribbed rubber over a steel core.  We chose MP's cast-aluminum...  We chose MP's cast-aluminum valve covers, PN P5249343, to dress up the engine. With the cast-aluminum covers, the Magnum's 10-bolt attachment system, machined rails, and trick gaskets, there's no way leaks will develop.  Conventional small-block headers...  Conventional small-block headers fit with no modifications or problems. We tried our TTI step headers on for size.  To Magnum or not to Magnum?...  To Magnum or not to Magnum? For low-buck bolt-on power, the Magnum heads are the quickest way to go.
|
|
|