 Magnum heads oil the valvetrain...  Magnum heads oil the valvetrain through the pushrods. We ordered Comp's 5/16-inch pushrods (PN 7960), which had the exact 7.650-inch length needed in our engine using the flat tappet hydraulic cam. |
 The Magnum rockers are ganged...  The Magnum rockers are ganged in pairs and provide a ratio of 1.6:1. These production pieces are cheap, sturdy, lightweight, and nonadjustable. |
 We went with MP's M-1 single-plane...  We went with MP's M-1 single-plane Magnum four-barrel intake--a great manifold for high rpm performance. |
 The Magnum's ten-bolt valve...  The Magnum's ten-bolt valve cover attaching rail, neoprene gaskets, and machined rails makes valve cover leaks a thing of the past. |
 A factory windage tray from...  A factory windage tray from a police 318 was added to the bottom end. These can be purchased new from Mopar Performance. |
With this, we had the basis of our build-up: a zero-deck 318, sporting Magnum heads and an MP single-plane intake. All that was left was to select enough cam to do the job. Getting to our output goal would be easy with a huge stick, but we were working within reasonable limitations. The objective was to put together a combo that would work well on the street. Another consideration was the retainer-to-guide clearance of the Magnum heads. The retainer in a stock Magnum head will physically hit the valve stem seal at .530-inch of lift, thus putting a ceiling on cam selection. Though the guide can easily be cut down for more clearance, we would be getting into custom mods, which we were seeking to avoid. In fact, the lift limitation would allow about as fat of a camshaft as we were willing to run. Keep in mind, nearly all cam specs listed for Mopar small-blocks are calculated at the LA-engine's 1.5:1 ratio. With the higher 1.6:1 ratio of the Magnum engine, it doesn't take a huge cam to use up all the available clearance. We selected a Competition Cams 280H Magnum camshaft, which is a single-pattern grind featuring 280 degrees of rated duration and 230 degrees of duration at .050-inch tappet rise. This cam is advertised as having .480-inch lift, but with the higher-ratio Magnum valvetrain, theoretical lift goes up to .512-inch. It seemed like the ideal cam for our combo. We liked the single-pattern profile as a compliment to the Magnum heads' good exhaust flow ratio, and that the cam was maxing out the lift potential of the out-of-the-box Magnum heads.
With our combination penciled out, we brought our 318 to Precision Speed and Machine in Delano, California, for machining. Since we were going to use new Magnum cylinder head assemblies from Mopar Performance, the efforts were concentrated upon the bottom end. We elected to go .040-inch oversize in the bores, and had the block bored and honed by Precision with moly rings in mind. We upgraded the connecting rods from the weak early 318 pieces, moving to the readily available later ('73-and-up) rod shared by the 318 and 360. These rods are virtually indestructible in an application such as this. Another upgrade was to swap the stock 318 crank for a forged piece, since we happened to have one pulled from a 273. The early 273 cranks have a different torque converter register, which creates problems in an automatic application, but it is readily compatible with four-speed components. The rotating assembly balanced fine.
Precision inspected and reconditioned the stock rods and installed a set of high-strength ARP rod-bolts. Special care was taken in selecting the rods and resizing to ensure a set of rods with virtually identical center-to-center length. This is critical in setting up a zero-deck engine. Precision line honed the block to exact tolerances, and then square decked the block to put the pistons dead flush with the block at TDC. The machining checked out exact. The final assembly was a breeze, as should be expected with production-based pieces and top-quality machine work.
Making the change to Magnum heads is a simple swap. All stock components can be used, however, Magnum engines oil the valvetrain through the pushrods as opposed to LA-style engines. Since all Magnum engines were equipped with factory hydraulic-roller camshafts, using the Magnum heads in conjunction with an LA block and a hydraulic flat tappet cam requires custom-length pushrods. Mopar performance sells a pushrod kit for this application (PN P5007477). We mocked up our engine combo (taking into account the block decking) and found that Comp Cam's number 7960 pushrods at 7.650-inch length were an ideal fit. Another distinct feature of the Magnum design is a vertical intake bolt angle, requiring a Magnum-specific intake manifold. There are not as many choices in Magnum manifolds as with the traditional LA small-block. Mopar Performance offers a dual-plane, as well as a racy single-plane intake for this application. we opted for the single-plane. The single plane would undoubtedly cost precious torque lower in the rpm range, but we just didn't think the dual-plane was the piece to make our target output. Word has it Edelbrock is introducing a Performer RPM intake for this application, and it should prove to be an excellent alternative.