Small-Block Mopar Engine - Junkyard Jewel 5.9L Mopar Magnum - Engine Buildup
Chrysler unceremoniously unveiled the 360ci (5.9L) LA-series small-block V-8 in 1971 as a low-compression engine with only a two-barrel carburetor. The 4.00-inch-bore-by-3.58-inch-stroke powerplant was originally designed to provide an economical alternative to the popular 383 two-barrel big-block, while the shorter-stroke and larger-bore (4.04x3.31) 340-cube engine carried the performance banner. With the cancellation of the 340 engine option after the '73 model run, there were a few performance variants of the 360 in the early and mid-'70s, though these were all low-compression engines with the further disadvantage of add-on smog-control equipment. There really was no love for the 360 engine in the early years, and they developed an unflattering reputation as a low-performance "smog motor." It took decades for this impression to lose its hold. Eventually the 360 earned acceptance from the Mopar crowd, and ironically its popularity has grown to become one of the favored powerplants for performance use, especially when the objective is high output on a real working man's budget.
The 5.9L Magnum engine was a big step forward in the 360's evolution. By the early '90s, the 360 was seriously dated, especially compared with engine packages offered by competitive manufacturers. Looking to gain an edge on the competition for a minimum of development cost, Chrysler gave the old warhorse a significant facelift, reintroducing the engine as the 5.9L Magnum in 1993. Changes were made to improve the engine in output, reliability, and sealing against oil leakage. Chrysler succeeded on all three counts with revised cylinder heads, modern MPI fuel injection, and greatly improved assembly gaskets and seals. These revisions were enough to put the 5.9L Magnum at a competitive power level and bought the basic engine platform another 10 years of production life in Chrysler's popular truck, van, and SUV offerings, until it was ultimately replaced by the 5.7 Hemi.
When considering low-buck performance for a Mopar, good power potential and ready availability make the 360 a winner. However, the days of buying a good used but serviceable LA-series 360 to swap are drawing to a close, and even good cores are getting harder to find these days.
The later Magnum-series engines open the door for a number of possibilities. It isn't a problem to find a decent used 5.9 Magnum engine from late-model used-parts sources, ripe for whatever performance plans you might have. In fact, Magnum mills seem to have survived the rigors of service much better than the earlier engines, owing to improvements like a better ring pack with thinner moly rings and effective MPI fuel injection with modern engine management, all leading to a reduction in bore wear. It's not uncommon to pop the lids on a 100,000-plus-mile Magnum and discover negligible bore wear, while the older carbureted small-blocks would have been spent. Good bores and a sound engine are essential to the foundation of a low-buck performance engine.
So what kind of power is lurking in one of these late-model Magnum 5.9 engines? Some time on the dyno answered that question, but first we needed a suitable test mill. The most common approach here would be to salvage a core or used engine from a late-model truck, van, or SUV, but we took the easy way out and shortcut the process by using a 300-hp Mopar Performance crate engine as our dyno mule. Before you cry foul, consider the origins of and equipment on this basic crate offering.
Mopar's Magnum 360 crate engines were offered in 300- and 380-hp versions, both based on the production Magnum 5.9. The higher-powered engine comes with higher-compression pistons, a long-duration camshaft, and a single-plane Mopar Performance intake. Far more basic, the 300-hp version is the same engine as in your typical Durango, van, or Ram truck. With the 300-hp crate you get a bone-stock production bottom end, complete with the stock truck pistons and cam. Changes are only the addition of a center-sump passenger-car oil pan, an LA engine-style front cover, a conventional distributor, and a two-plane carbureted intake to replace the factory "beer-barrel" MPI injection system. The fact that our dyno shop had one of these stock-based crates sitting around gathering dust sealed the deal. Even though our engine began life as a crate motor, our results would be representative of an effort that starts with any serviceable or remanned production 5.9 Magnum.
Before Dyno: 319 hp/424 lb-ft
Our game plan was to equip the engine with the same basics any Magnum would require for a retrofit into an earlier application. We'd start with a dead-stock Magnum long-block engine assembly, adding a carburetor, a conventional distributor, and headers for the baseline. This is the minimum level of trim needed to shove a Magnum in place of the slant-six in your old Dodge Dart or Plymouth Duster. For the induction combination, we used the same intake that came stock on Mopar's 300-hp crate engine—the M1 dual-plane—topped by a 750 Speed Demon carb. To handle the ignition, an MSD distributor was dropped into place, although an alternative here is the excellent Mopar Performance electronic ignition system. A set of Hooker Super Comp 13/4-inch headers completed the engine package, and we were ready to run. Testing on Westech's SuperFlow 901 engine dyno revealed an output of 319 hp at 4,400-4,500 rpm and 424 lb-ft of torque at 3,700 rpm. Not exactly a high-rpm screamer, but it had the very good low-end torque you'd expect from a truck motor.
After Dyno: 448 hp/435 lb-ft
Our next level of modification would be to step up the combination significantly with traditional hot-rod modifications like cam, induction, and cylinder-head changes. Our cam choice would naturally be a hydraulic roller, since these late-model Mopar small-blocks are thus equipped. We wanted to keep the cam specs moderate and streetable, so we decided on a 230/236-degree duration at 0.050 Comp custom grind on a 110-degree lobe separation, using Comp's XFI hydraulic roller lobes No. 3016 and No. 3037 on the intake and exhaust, respectively. These lobes provide very quick valve action, giving 0.576/0.571-inch lift with a Magnum 1.6:1 rocker ratio. The stock Magnum hydraulic roller lifters and lifter-retainer spider assembly were kept.
While normally we like to make such changes on the dyno one piece at a time, the production Magnum cylinder heads are limited to just over 0.500 inch of lift before they encounter disastrous retainer-to-guide collision. This precluded shoving our hot high-lift Comp hydraulic roller in place while retaining the stock heads. To keep the stock heads, the valve guides need to be machined down and new seals, valve-springs, and matching retainers installed. Fortunately, a set of Edelbrock Performer RPM Magnum cylinder heads were also part of our upgrade plan, and these are capable of swallowing 0.580 inch of lift as equipped from Edelbrock. While the head swap is certainly possible without changing the cam, it seemed ludicrous to run the upgraded heads with the 0.385/0.401-inch-lift stock truck cam. With the Magnum's stock, dished pistons, the compression ratio was 9.0:1.
Under these circumstances, swapping the cam and cylinder heads together was the most realistic combination. The Edelbrock heads require stud-mounted rockers and are designed to accept any aftermarket Chevy rocker arms for 3/8-inch studs. To fulfill this requirement, we went with a set of Comp 1.6:1-ratio aluminum roller rockers. Because replacing the pedestal OEM Magnum rockers requires hardened pushrods to work with the Edelbrock heads' guideplates, we finished the valvetrain package with a set of Comp Magnum pushrods.
The final item in our combination to be changed was the two-plane M1 intake. Clearly designed more for low-end torque than horsepower aspirations, this intake manifold is wholly out of place with our cam-and-head combination. The solution here was the Mopar Performance M1 single-plane intake, a long-runner manifold that has proven to make good top-end power while retaining high levels of torque through the midrange. In fact, this is the intake design Mopar Performance runs on all of its higher-output crate engines. With that, we had our Magnum power package—the Comp rockers, pushrods and XFI cam, Edelbrock heads, and Mopar single-plane intake. Each of these components substantially raises the bar on the hardware they replace.
With the jetting dialed in, the power pulls confirmed that this parts selection put major firepower in the basic Magnum package, with 448 hp recorded at 5,800 rpm and 435 lb-ft of torque coming to bear at 4,900 rpm. The cam, heads, intake, and valvetrain package was worth a staggering 129 hp on the stock Magnum mill. For us, that is a righteous gain for a well-selected assortment of out-of-the-box aftermarket parts on a stock production engine. Far from a raucous and unruly combination, the modified engine idled easily at 850 rpm with 12 in-Hg of manifold vacuum, a level that is right at home in a street-driven machine. With output like this ready for the taking, it might be worth considering giving Magnum power a home in your Mopar machine.
Mopar Magnum vs Mopar LA-Series
The introduction of Mopar's Magnum series of engines was the most significant revision of the Mopar small-block since its introduction in 1964. There are some points to consider when deciding whether a Magnum engine is the right choice in an engine buildup and when mixing and matching components or applications.
Heads & Valvetrain
Magnums utilize 1.6:1-ratio pedestal rockers in place of the earlier LA-series engine's shaft-mount 1.5:1 arrangement. The LA engine provides oil to the rocker shafts via an oil passage in the block and heads. This was deleted in later Magnum engines, which rely on pushrod oiling via the lifters. Magnum heads can be retrofitted to an LA-series block, requiring lifters with oiling provisions (nearly all aftermarket lifters are so equipped) and the appropriate oiling pushrods. A Magnum block cannot accept LA-series heads without custom-machining the oil passages or creating a custom bypass oil feed. For practical purposes this swap is a nonplayer, except in the case of very early Magnum blocks that were reportedly manufactured with the LA-style oiling provisions intact.
Magnums have a slightly revised head-bolt package requiring a Magnum-specific bolt set for use with Magnum heads. The head gaskets are also unique to the engine, although LA-series gaskets can be used in all applications. More choices in performance head gaskets are available for the LA-series design.
Intakes & Valve Covers
The Magnum intake has vertical fasteners, in contrast to the angled LA-series arrangement. This precludes swapping LA-series and Magnum intakes without modification, though it is possible to modify the intakes to work either way. Some aftermarket intakes are double-drilled to fit either engine. The fasteners and gaskets are also unique to the engine series.
Magnum engines have 10 valve-cover fasteners compared to five on LA-series engines and utilize a superior steel-reinforced rubber gasket. LA-series valve covers will bolt to a Magnum head, though this arrangement does not take advantage of the improved sealing of the later design.
Accessories, Cam & Front Cover
Magnum engines featured a serpentine-belt accessory drive, a reverse-rotation water pump, and utilized a timing-case cover with no provision for a mechanical fuel pump. The Magnum camshaft nose was also changed and will not accept a mechanical-fuel-pump drive eccentric. An LA-series timing case will bolt directly to a Magnum block, allowing mounting for the fuel pump, conventional accessories, and a standard LA-engine water pump in a retrofit application. The camshaft will also need to be ordered on a core with the LA-series nose to accept the fuel-pump eccentric.
Oil Pan Interchange
At the bottom end, the Magnum engines all use the LA-series 360 rear oil-pan relief. Any 360 oil pan will bolt to a Magnum. However, a non-Magnum pan cannot use the Magnum one-piece oil pan gasket, as the early LA-series pans are notched at each side of the rear relief, while the Magnum is not. A non-Magnum pan will require the LA-series 360 oil pan and gasket set.
Max Power Potential
In terms of ultimate power potential, the LA engine has a clear advantage over the Magnum. With the shaft-mounted rocker arrangement of the LA series, offset intake rockers can be readily utilized to dramatically increase the available intake-port cross-section at the pushrod restriction. This is a major consideration in very high-output applications. The offset rocker arrangement was used by Chrysler in the '70 T/A cylinder heads and subsequent Mopar W-series race heads, and is used by aftermarket manufacturers such as Indy Cylinder Head. The stud-mounted Magnum heads are not supported by available offset-intake valvetrain arrangements. This limitation becomes an issue at approximately the 500-hp level, though fully ported Magnum heads are capable of exceeding this output.