Magnum Cylinder Heads - Mopar's Magnum Maulers

Heavy-Hitting Flow From Magnum R/T

By Steve Dulcich

Mopar's Magnum series of engines were Chrysler's play at stepping up the venerable small-block to keep pace with changing engine technology requirements. This kept the series alive and succeeded in making Mopar engines more than competitive. Among the improvements were extensively redesigned cylinder heads. Magnum heads addressed more demanding requirements in terms of both power production and emissions by increasing efficiency. Compared to older head designs, the Magnums stepped up efficiency on a number of fronts. First, airflow was improved with higher-flowing ports, which picked the intake flow up significantly. Dramatic improvement of the exhaust port layout was also accomplished. Mechanically, the valvetrain was upgraded with the use of higher ratio rockers, stepping up the leverage from the traditional 1.5:1 ratio to 1.6:1. Perhaps the most significant step forward was in the combustion chamber. While the earlier cylinder heads carried a fully-relieved open-chamber layout, the Magnum design utilized a double quench closed-chamber configuration--a superior arrangement in both power production and efficiency. The icing on the cake was changes to the manifold and valvecover attachment, putting an end to the decades-old problem of oil seepage and frustrating leaks.

Standard Magnum heads offered great power at the levels demanded in OEM applications at the time. however, the head was conceived with truck production in mind, not all-out, gut-wrenching power. Be that as it may, with enough cam and compression, stock-production Magnum heads could achieve outputs of just over 400 gross STP horsepower at the crank. To allow output to move to the next level, Chrysler's Mopar Performance introduced the Magnum R/T cylinder heads. These cylinder heads feature all the benefits of the Magnum design, but step up the power potential with larger, better flowing ports. The Magnum R/T increases intake runner volume from the production Magnum size of 153 cc to 180 cc, and is a direct bolt-on replacement for the production piece. While a stock Magnum will flow in the neighborhood of 190 cfm on the intake side, the R/T--in out-of-the-box form--will pull flow in the range of 220-230 cfm--an improvement of better than 10 percent.

Now, we've already stated the production Magnum head can support 400 hp, and maybe that power potential seems like plenty for the average street mill. but the Magnum R/T's superior ports will make more power with the same basic engine combination, and will require a less radical combination to achieve the targeted output. While building a 400hp engine using the production heads will require a fairly nasty camshaft, getting there with the high-flowing Magnum R/T heads can be done with a more conservative and street-friendly combination. It's all in the airflow. Gaining big increases in output without relying on radical cam timing becomes a significant consideration in late-model MPI applications, where low vacuum and high overlap can cause havoc with the computerized engine-management systems.

Mopar Performance sells the Magnum R/T cylinder heads as bare castings in two variations: one machined to accept the production 1.92/1.625-inch valves (PN P5007140), and another cut for a larger 2.02-inch intake valve (PN P5007141). The head can also be purchased as a complete assembly fitted with the smaller valve configuration (PN P5007145), making the upgrade a bolt-on deal. We recently gained possession of a set of 1.94-inch valve-size castings and decided to run a series of tests to explore their potential. A preliminary examination showed generous and well laid-out intake ports with nicely shaped direct runners leading into a deep bowl, and a tall short-side turn to the valve. Unlike earlier production heads, the valve-guide boss protruded only minimally into the air stream. To accept the production valvetrain, a hump at the inlet side of the runner was required to clear the pushrods. the shape is highly defined, and the protrusion is minimal for an as-cast port. In short, the intake runner was a nice design, hitting all the marks in requirements for an excellent high-performance head casting. The exhaust ports also showed excellence in design, again with a deep bowl, straight runners, a nearly ideal short turn, and minimal guide boss intrusion. The exhaust layout is strikingly similar to the race W-2 port configuration, but retains an exit and flange pattern that accepts stock exhaust manifolds or conventional headers.

FLOW FASCINATION
We brought the cylinder heads, along with our porting kit, to Westech Performance Group to steal some time on their FlowCom-equipped, SuperFlow 600 flowbench. The goal was to run some tests to quantify the flow numbers in as-delivered condition, and run through some basic modification to gauge how these casting respond to porting modifications. The heads were first base lined as cast, with the production valve machining and stock production 1.92/1.625-inch valves. The results are shown in tables 1 and 2, column one. The intake port flow was right in line with our expectations, reaching the mid-220's with peak flow coming in at .550-inch, and then leveling off. These are good numbers for an as-cast iron head, particularly with the stock 1.92-inch valve--enough to comfortably support the 450-plus-horsepower range. The intake port flow was significantly better than any production iron small-block head. The exhaust flow was outstanding, reaching numbers out-of-the box that are rarely seen with fully race-ported early heads, and light-years ahead of any of the as-cast iron stock heads. It was interesting to note the exhaust port was nearly dead quiet on the flowbench at higher lifts--indicating very little turbulence.

With the preliminaries out of the way, it was time to do a little carving to further explore the head's potential. We would structure the mods in a sequence that would follow a progression of modification levels. We would record the flow improvements along the way, allowing anyone interested in duplicating our efforts the option of choosing how far they want to go. All of our mods would retain the production valve seat machining and stock valves. Since the stock valves and seat machining would be retained throughout our testing, port flow below .300-inch lift--which is primarily dictated by the valve and seat form--would be changed very little.

The first step is to perform a simple bowl blend where the valve job machining is blended into the as-cast bowl. With just a bowl blend, both the intake and exhaust ports picked up substantially (tables 1 and 2, column two). Intake port flow was up nearly 20 cfm at .500-inch lift, while the already outstanding exhaust also made significant gains. Blending the bowls is a simple mod requiring very little special know-how or skill, but improves the flow solidly. Bowl blending the heads can be quickly accomplished even by a beginner, and based on the return for the amount of work involved,we can see no excuse not to perform this level of modification.

Our next modification was to streamline the as-cast guide boss. As we noted, the guide bosses on both the intake and exhaust of the R/T heads are diminutive, particularly as compared to production heads. Paring them down to a race fully-ported shape takes some finesse, and is worth a modest improvement in higher lift flow (tables 1 and 2, column three). Next, we went in for a minor clean up of the short side turns of both the intake and exhaust ports. With the nice, as-delivered, short side shapes, our gains were minimal, though the intake picked up at the top of the curve, now breaking over 260 cfm (tables 1 and 2, column four). Focusing on the intake port, some checking with a velocity probe showed high airspeed over the peak of the pushrod hump. This indicated the port flow could be improved by paring down the pushrod restriction. The pushrod bulge in the port runner is cast to very close tolerance, so unlike many castings there isn't a large amount of excess material here. carefully gauging the metal thickness with a Helgesen E-tool, we were able to trim the material to a consistent .040-inch, enhancing the cross-sectional area available at this constriction. Opening the pushrod pinch in this fashion netted a good incremental gain in intake port flow (table 1, column five).

At this point most of the minor porting tricks had been applied. there was little left to do other than pursue a fully ported form. Both the intake and exhaust runners were gasket-matched, and the runners were cleaned up with a long carbide cutter. We didn't significantly alter the size or shape of the as-cast port, but rather eliminated the rough surface and created a flat, straight approach from the runners into the bowls. On the intake side, the flow was basically unchanged, although the exhaust gained substantially (tables 1 and 2, columns 5 and 6). The intake port shape was definitely better, but since we worked the runner from the manifold side only, the improvement in runner shape was largely negated by a less than ideal transition into the previously worked short turn and bowl. We would remedy this with our next mod. On the exhaust side, working the runner showed an immediate benefit, boosting the flow to over 210 cfm. as with the intake side, the runner was worked entirely from the flange side of the head, owing to the straight access to the entire port. the exhaust runner work made a nice transition into the previously cut areas.

The final mod was to fully polish both the intake and exhaust ports. While the physics of boundary layer airflow would predict little benefit to surface enhancement of the port in terms of flow potential, the reality is the aggressive carbide-cut surface of our porting efforts to this point left a far from ideal form. The subtle transitions from the runner to the bowls, around the short turn (particularly in the corners), in the vicinity of the guides, and from the bowls to the seat bottom-cuts were all simply roughed-out by the carbide cutting. Polishing would allow us to clearly see these flaws in the shape, and derive the maximum benefit from the mods made to this point. Finishing the ports with polishing improved flow significantly on both the intake and exhaust (tables 1 and 2, columns 6 and 7).

We found the Magnum R/T heads to be an excellent replacement piece box-stock with outstanding possibilities. In milder combinations, simply blending the bowls and bolting them on will provide a dramatic improvement in port flow over any production small-block head with minimal effort. Ported as illustrated here, the R/T heads will support outputs into the mid-500hp range in larger displacement engines. We achieved 270 cfm of intake airflow, which compares favorably with the better CNC-ported aluminum heads offered for competitive makes of engines. Is this the limit? We found all that air with the production Mopar valve job and stock production valves. Valve mods including a back-cut intake and radiused-margin exhaust valves would undoubtedly pick up the numbers at lower lifts, as would a competition valve job. Further flow enhancements could potentially be achieved with chamber mods, including a de-shrouding cut, which would be performed as part of the high-performance seat machining. Given the port layout, we consider the Magnum R/T as kind of a baby W-2, having a terrific port configuration that's just made to support high flow, while retaining the convenience of a bolt-on replacement.