Cylinder head porting can take many different levels; in most forms of all-out racing, the porting is among the key considerations of an engine program. This should come as no surprise, since, as pro engine builders are well aware, airflow is critical to making power. Opening the door for better airflow through the strategic application of a carbide cutter isn't something solely reserved for the pros. Even a basic street-performance build can be seriously improved with judicious carving of the ports. The problem for most beginners is not knowing where to start, or what to do, or, worst of all, not knowing how much the mods are helping. Take a late-model Magnum head, for instance. These heads are known to deliver good power in stock form, and, in a performance build-up, getting into the range of 400 hp is possible with a nicely thought-out street combo. With their bang-for-the-buck, Magnum heads have become popular with the budget small-block enthusiast. Just bolting the heads on, though, might leave you wondering what would happen with a little massaging.
We were in exactly this scenario with one of our built 318s. The engine had been assembled with KB flat-top pistons set at zero deck, a Comp Cams 280H camshaft (230-degrees at 0.050-inch lift, 0.512-inch lift, single pattern), a balanced bottom end fitted with stock components, and topped with Magnum heads pulling air through a single plane M-1 intake. The engine was good for 400 hp, but a flaw in the cylinder wall of one hole meant it needed to be torn down for a sleeve. While it was apart, we couldn't help but look at those Magnum heads sitting on the bench and wonder what a few simple mods could do. We weren't interested in making a set of full-house racing heads from the small-port (156cc) Magnums, but rather just a little tickle-up that could add some air flow and more power when this little engine goes back together.
What's a practical level of porting? For us, it was defined by the amount of work that can be accomplished in a heavy day of porting, or maybe a more relaxed weekend effort. It's difficult to swallow the practicality of going crazy on a production Magnum head looking for the last ounce of flow. Such an effort would definitely find substantially more flow, but consuming forty or more hours on maxing out the potential of these small cross-section heads is hard to justify. The larger Mopar Performance Magnum R/T (200 cc) or new Edelbrock RPM heads (186 cc) offer a greatly advantaged design to start with, and the potential is hard to ignore when contemplating $1,000 or more worth of porting to the factory stock heads. With this in mind, we set out to find the easy potential of the Magnum cylinder heads.
Examining the intake ports by starting at the valve end, the first thing apparent is a generously sized and nicely shaped bowl, and substantial valve area. The factory valve job measured undersized, and, under the machining, the normal rough transition into the as-cast portion of the bowl is found. Going into the bowl, the guide boss is much larger than it needs to be for a performance application and an obvious source of lost flow, but the shape isn't objectionable for a production head. Looking a little deeper into the bowl at the short-side turn, the height of the short turn is favorable for tailoring this area to a nice final shape. The intake port runner itself is quite small, with limited potential for enlargement. The major constrictions are a tight pinch point adjacent to the pushrod, and a large bulging mass of iron cast just behind the pushrod bulge in the upper portion of the runner, presumably for the rocker fastener, although we cannot really rationalize why it's there.
Critically analyzing our modification plan for the intake port, we started at the bowl area just under the valves, and we could see there were areas where large flow gains could be had. Blending the bowl and opening up the valve job to a spec closer to the full potential of the valve size will usually result in large flow gains. However, gains in these areas on the Magnum heads were modest. While there seemed to be plenty of flow left on the table in these areas, the bowls are proportionately large in comparison to the tight port runners. In contrast, opening up the pushrod constriction unlocked a substantial amount of flow, as did reworking the obstruction at the guide boss. It seems the Magnum's large bowl and small runner work together in kind of a happy harmony, with a balance of flow potential that requires mods at each end of the bowl and runner to significantly impact the overall port flow.
On the exhaust side, we find a situation very similar to the intake, with a large valve and a nice ample bowl, combined with a rather small runner. As with the intake, the guide boss isn't overly objectionable in shape for a production piece, but it is much too chunky if high flow is the goal. The large flat ledge left by the production machining at the tip of the guide boss is the equivalent to a brick wall as viewed by the exiting air. The short-side turn of the Magnum exhaust is something that we welcome just for being there, since earlier LA heads were notoriously lacking in material in this critical area. The tall short turn of the Magnum offers plenty of material to create an excellent form in this area, dramatically improving the flow potential of the port compared to earlier heads. As with the intake port, the actual runner is quite small, but in contrast to the situation with the intake port, the exhaust runner is easily opened up with a little carving. We found the exhaust ports of the heads responded progressively and predictably to our modifications, achieving very high flow without esoteric or excessive reworking. Even with a basic effort, the exhaust flow improvement was dramatic, gaining a staggering 41percent.
The accompanying photos illustrate the modifications we performed, and the flow chart quantifies the results along the way. The only question that remains is whether it was worth it. For the level of work involved and the power potential gained, we'd have to answer that with a resounding yes. Thirty-four cfm of intake flow has the potential to add a solid 50-60 hp to a combination built to take advantage of the additional flow, and we'd expect a nice extended power band with the increase in port cross-sectional area and substantial exhaust flow. With gains like that up for grabs, I'd spend a Saturday making cast-iron chips.
Chart 1 PORT FLOW
Production Magnum Head
Superflow 600 Flowbench
Tested At 28-Inch Water Depression; Tested
By Steve Dulcich
Magnum Swap
Although we covered the topic in detail a few years ago, it's worth a recap of what's involved in making the conversion to Magnum heads on an earlier LA-style small-block. For openers, all that is required to bolt the Magnum heads to an earlier block is a Magnum head bolt or stud kit, since the bolt length is different. The major difference in configuration to consider is that the Magnum utilizes an entirely different valvetrain, oiling the rockers through the pushrods. To oil the valvetrain, the lifters installed in the engine must have a provision for pushrod oiling. Most aftermarket hydraulic lifter kits for Mopar small-blocks are already set up this way, though most solids are not. A quick check on hydraulics can be made by simply inspecting the center of the pushrod seat in the lifter for an oiling hole. If it's there, you're set, though if the seat is solid, the lifters will need changing. Flat tappet lifters listed for an AMC V-8 application will have pushrod oiling and fit the Mopar engine. In fact, most cam manufactures list the AMC and Mopar lifters under the same part number.
Once pushrod oiling is confirmed, you'll need pushrods set up to carry the oil. Stock Mopar pushrods were solid, while the Magnum needs hollow rods with oiling holes. Mopar Performance sells conversion pushrods under PN P5007477, which match the length needed for hydraulic flat tappet cams, with the stock Magnum valvetrain. Any custom pushrod supplier can provide the required pushrods for special applications where modifications require a custom length. The valvetrain, of course, needs to be the Magnum style. The OEM setup is very good for street performance hydraulic cams, and it's cheap. Crane or Comp can provide kits to convert to roller rockers, if you wish to step up.
Bolting an intake to the Magnum heads is another complication, since the intake bolts holes were revised to a vertical arrangement, making the Magnum intakes specific to Magnum heads. There are ways around this interchangeability problem, including redrilling and tapping the Magnum heads to the LA pattern or modifying the intake, but there are several good choices in specific Magnum replacement manifolds. Edelbrock can supply a Magnum pattern Performer RPM AirGap, while Mopar Performance has both single and dual-plane carbureted intakes available. Other suppliers also market manifolds, so there is no problem finding a bolt-on Magnum four-barrel manifold. Remember to get Magnum-style intake manifold gaskets to match the revised bolt pattern. Another bolt pattern change with the Magnum is in the valve covers, where the number of fasteners have been doubled to ten. Stock Magnum valve covers are an obvious option here, while Mopar Performance offers Magnum cast aluminum covers that make oil leaks a thing of the past. Finally, Magnums have a unique head gasket, however, the changes only help to align the pushrods during assembly, so the older LA-style gaskets can be used. Converting to a Magnum top end is really not a problem at all.
