When the average enthusiast thinks of the Mopar small-block, the engines that instantly come to mind are the traditional older series of engines, known as the LA series. The LA small-blocks were introduced in 1965 with 273 cubes. More commonly recognized are the legendary 340s of the musclecar era, the bread and butter 318s that seemed to have had a production run of forever, and the stalwart 360. The LA series of engines remain popular with hot rodders to this day, and even though they have been out of production since 1992, there were millions built ensuring a supply of parts and cores for generations. The more recent line of Mopar small-blocks were the Magnum engines, a group of powerplants that get somewhat less recognition, but served as the replacement for the LA series in Mopar V-8 applications for over a decade, until the introduction of the new Hemi. The OEM Magnums were produced in displacements of 5.2 and 5.9 liters, which not coincidentally equates to 318 and 360 cubic-inches. Perhaps one of the reasons these engines are not high up on hot rodder's radar screen is the fact that Chrysler did not field a V-8 passenger car during the entire production run of the Magnum. The Magnum engines were exclusively found in trucks, vans and SUVs, not exactly hot territory for mainstream performance fans, though there certainly are plenty of modified trucks.
Magnum engines did, however make some inroads into the traditional performance world, thanks in part to the direct interchangeability with the earlier LA engines on which they are based. Mopar Performance offered the Magnum crate engine for years in performance trim, and these engines were fully interchangeable with the earlier units. So, what distinguishes the Magnum from the earlier LA engines? The Magnum series engines were not dramatically different than the LA series, but rather a significant revision. Essentially, the blocks are the same as the earlier 318 and 360, with the major change being an additional set of mounting bosses on each side of the block to work with the truck motor mount system. The original small-block "ear" style lugs were retained as well, so there is no interchangeability issue there. Another change is in the rear main-cap rail that the oil pan seals to. The 340 and smaller LA engines used a rear main cap with a different oil pan sealing rail than the 360, so the oil pans and end seals did not interchange. With the Magnum, the 5.2 was standardized to accept the same pan as the 360.
All Magnum engines were designed with a hydraulic roller cam in mind, and there were modifications to accommodate the cam configuration. Bosses are cast into the center of the lifter valley, tapped to fasten the lifter-retaining spider. The lifter bores were cast much taller, to accommodate the tall hydraulic roller lifters, and the tops of the lifter bores were machined to accept the factory roller lifter alignment yokes. Although all Magnums featured hydraulic roller style blocks, these mods were also found on some of the later LA-series blocks, which also featured hydraulic roller cams, starting with the 1985 318 2V. The most significant changes that defined the Magnum was a new top-end package. Magnum heads featured larger valves with smaller stems, revised ports, and a new valvetrain style. The architecture was not dramatically altered, retaining an 18-degree wedge configuration, the same valve position, spacing, and length, and the same port layout. Again, the changes here were more of a refinement, rather than a complete redesign. Magnum heads featured machined valve-cover rails, with ten valve-cover fasteners, as opposed to the LA engine's five. This makes the Magnum valve covers unique. The intake manifold attachment was changed to a vertical bolt pattern, from the angled pattern used earlier. With this change, the Magnum and LA intakes are not a bolt-on interchange, though other than the pattern, nothing else was changed dimensionally.
The biggest change with the Magnum engines related to the valvetrain. Mopar wedge V-8s had always used shaft-mounted rockers, with the rocker and valvetrain oiling provided by an oil passage through the block and heads, into the shaft and then to the rockers. The Magnum system utilizes rockers that are individually mounted on paired pedestals. Oil to the rockers is provided through the pushrods via the lifter, much like a small-block Chevy. The Magnum rockers feature a ratio of 1.6:1, providing an increase in valve lift of almost 7 percent, compared to the 1.5:1 ratio used on the LA engines. Early Magnum blocks still came through with the oil passage for the LA-style valvetrain oil feed, which simply terminates at the heads' deck. Obviously, the heads did not have a valvetrain oiling provision. These blocks can accept either LA or Magnum style heads interchangeably. Later magnum blocks still carry the boss for the oil provision, but are un-drilled, precluding traditional heads from being installed without modification.
Retrofitting an LA engine to a Magnum configuration is amazingly straight forward, requiring no special modifications, just the right parts. Naturally, the Magnum heads are needed, and to attach them to the block, specific Magnum fasteners are needed. Mopar LA small-block heads are attached with 10 bolts each, with four short ones and a long one under the valvecover, while the Magnum bolt set uses five long ones in these positions. The stock fasteners are available from Mopar Performance, or uprated pieces can be had from ARP. The production Magnum head gasket is a unique design, however the changes from the LA series are primarily to corral the pushrods into position during assembly. Regular LA engine gaskets can be used here.
As noted earlier, the major difference with a Magnum package is the valvetrain. Naturally, Magnum specific rockers are need. The OEM rockers are actually a very good design, suitable for milder street performance applications. The aftermarket also has rocker kits, available from sources such as Crane and Comp Cams. Since the oiling to the rockers is provided up from the lifter to the pushrods, the lifter needs to be designed for pushrod oiling. Nearly all hydraulic flat-tappet lifters sold in the aftermarket for Mopar applications have already been designed for pushrod oiling, so if the lifters or cam have been swapped, chances are you're already covered here. A check can be made of the pushrod seat in the lifter. If there is a oil hole in the seat, you're golden, but if the seat is solid, the lifters will need to be changed. AMC lifters all have provisions for pushrod oiling, and will interchange in the Mopar application. In fact, most aftermarket suppliers list the AMC and Mopar lifter under the same part number, providing all with the provisions for pushrod oiling. Solid lifters, however, most often do no come with pushrod oiling for traditions small-block Mopars. Here, AMC replacements will usually be required.
With lifter oiling confirmed, the pushrods need to be hollow, with oil passages at the tips to transfer oil up to the rockers. Since all Magnum engines were equipped with tall-body hydraulic roller lifters, the factory pushrods will be too short to work with the lower flat tappet lifters. Mopar Performance has a pushrod package designed to mate the standard Magnum valvetrain with a hydraulic flat tappet cam. Competition Cams provides pushrods in their retrofit valvetrain kit, PN 1425-KIT, so you are covered there. Any custom pushrod supplier can provide the required length pushrods where modifications require a special length. In fact, once the length is determined, the pushrods can probably be found in most aftermarket cam companies catalog as a stocking part by length listing, since the "ball/ball" end configuration in an "oiling" pushrod is the most common type used in the aftermarket.
With the Magnum's vertical intake mounting pattern, a Magnum-specific intake manifold is required. There are those who have gotten around this problem with custom modifications to change the bolt pattern on the Magnum heads, or revising the intake bosses. There are several good aftermarket choices in Magnum carbureted intakes, making bolt-on manifolds readily available. Mopar Performance has both dual- and single-plane intakes available for the Magnum heads, and Edelbrock has recently introduced their excellent Performer RPM Air Gap for this application. Professional Products has a manifold that has a dual bolt pattern, which fits both Magnum and traditional LA heads. You'll need to get intake manifold gaskets for the Magnum application, to match the revised bolt pattern.
Magnum valve covers are needed to work with the Magnum heads, since the number of attachment bolts are doubled for better sealing. OEM valve covers are a decent choice, but for a little more flash, Mopar Performance carries nice cast aluminum covers. The swap to a Magnum top-end is really a bolt-together deal on an LA engine, which shouldn't be too surprising, since that's the way Mopar essentially did it in the first place.
EDELBROCK'S MAGNUM HEAD
The big news surrounding Magnum engines is the recent introduction of Edelbrock's new Performer RPM head for the Magnum engine or LA engine retrofits. Edelbrock has been very active in the Mopar aftermarket, and the addition of a Magnum head to their line is a natural progression. As is established practice in the RPM series of heads, these new aluminum cylinder heads are designed as a bolt-on replacement, retaining compatibility with most of the stock engine components, making installation simple. The heads are cast in Edelbrock's own California foundry of A356 aluminum alloy, and heat-treated to T-6 specifications. Thanks to the lightweight material, swapping to Edelbrock's heads is worth a weight savings of 25 lbs per head, a worthwhile mass reduction from up high in the front of the vehicle. Measuring 176 cc, the intake port is a good size for a balance of flow, velocity and cross sectional area in street performance use. The intake port features a straight runner form leading to a nicely open port bowl with minimal guide boss intrusion under the valve. On the exhaust side, the deep open bowl shape and ample runners lend to the good flow from the nicely shaped port's 75cc volume.
Speaking of nice shapes, we were especially enamored with the combustion chamber, measuring a tight 58 cc. The small chamber makes it easy to achieve a higher compression ratio when retrofitted to LA-style engines, even at the smaller stock displacements. The chamber is a true closed design, with ample quench pads at each end, and retains the small-block Mopar's great central spark plug position. It has the appearances of a very efficient and fast burning chamber, especially if the piston is set-up for an effective quench. Filling the chambers are 2.02-inch intake valves, with the exhaust measuring 1.600 inch. This is an increase of 0.100 inches on the intake, with the exhaust 0.025-inch smaller compared to the production Magnum dimensions. The valves' diameter matches the sizes used in the legendary 340 engines of 1968-1971, though the port flow far exceeds what was delivered from those cylinder heads.These new heads feature top-quality hardware throughout, including valvesprings good for 0.580-inch lift in a flat-tappet application, and quality steel retainers and locks. The actual specifications on spring loads measure 120 lbs on the seat at an installed height of 1.800-inch with a spring rate of 350 lbs/in and an over-the-nose load of 295 lbs at 0.500-inch lift. The valvetrain arrangement departs from the production Magnum theme, as the heads are fitted with 3/8-inch studs and guide plates to accept common small-block Chevy rocker arms. This may seem strange, but actually the spacing in the stock Magnum makes the valvetrain interchangeable with Chevy rockers. Stock Magnums are converted to aftermarket Chevy rockers by removing the stock paired pedestals and rocker attachment bolts and substituting studs and guide plates. The complication in OE heads is that the heads are tapped for 5/16-inch fasteners, rather than the 3/8-inch specifications required for typical aftermarket studs. Special studs with reduced thread diameters to match the heads are used for this conversion.
Edelbrock's Magnum heads are already converted, making this an upgrade than is already taken care of. The heads are delivered with studs and guide plates installed. Hardened pushrods are a must with any valvetrain using guide plates.The Edelbrock Magnums offer substantial airflow for a replacement-style cylinder head, topping out at 260 cfm on the intake port. That equates to an improvement in peak intake flow of nearly 29 percent, which is a tremendous gain in flow capacity and power potential. On the exhaust side, the peak flow of 190 cfm also represents a dramatic improvement of approximately 26 percent. The numbers put the Edelbrock Magnums in the thick of the flow range for better aftermarket aluminum heads when compared to other engine types. The Edelbrocks seem like an excellent source for the brawn to build a manly Magnum.
The potential of Mopar's Magnum small-blocks took a giant step forward with Edelbrock's ne
Magnum engines replaced the familiar LA series when the 318 became the 5.2 Magnum in 1992,
One of the major differences in the Magnum family of engines was the change in valvetrain
Oil flow upstairs must be metered by the lifters. Most aftermarket hydraulic lifters have
The older LA series of engines oiled the rockers through a block passage that fed into a p
Another design change to be aware of when considering Magnum possibilities is the Magnum-s
Magnum heads have a specific head bolt set that differs from the LA engine. The LA takes f
All OEM Magnum engines used hydraulic roller cams, with a long tappet body. Retrofitting a
An interesting and fortunate circumstance with the Magnum heads is that small-block Chevy
Edelbrock's heads are already set up with standard aftermarket style studs as used in smal
A survey of the intake port of Edelbrock's Magnum heads reveals a port of substantially hi
A cut-away view of the intake port shows the smoothly contoured shape of the runner, a cla
Viewing the Edelbrock Magnum's exhaust port, the improved design is very apparent. To enha
A cut-away view of the exhaust port reveals the shape of the port as a nearly continuous r
One of the features we really favor with the Edelbrock Magnum heads is the compact chamber
An ideal complement to the Edelbrock Magnum RPM head is the Performer RPM AirGap intake. T
One of the best features of building a Mopar small-block based upon a production Magnum bl
MAGNUM HEAD FLOW
We were interested in seeing the flow of the Magnum heads for ourselves, and while at Edelbrock we were treated to a demonstration of the flow capacity built into these heads. We also had the opportunity to test the flow potential of a production set of Magnum iron heads, to set a benchmark. The Edelbrock heads definitely showed their worth, as the flow results attest.
|INTAKE PORT CFM |
|LIFT ||STOCK ||EDELBROCK |
|0.100 ||60 ||62 |
|0.200 ||120 ||119 |
|0.300 ||168 ||175 |
|0.400 ||195 ||225 |
|0.500 ||202 ||251 |
|0.600 ||194 ||260 |
| || || |
|EXHAUST PORT CFM |
|LIFT ||STOCK ||EDELBROCK |
|0.100 ||59 ||52 |
|0.200 ||113 ||105 |
|0.300 ||142 ||142 |
|0.400 ||148 ||169 |
|0.500 ||150 ||185 |
|0.600 ||151 ||190 |