Some engines have always been considered performance lightweights. You know, the pushovers that never get any respect. Count among this group the 307ci and 305ci Chevy, the 400M Ford, and the 318ci Mopar. Some guys would rather lie than admit to having one of these mills under their hood. In the case of the 318 Mopar, there are rabid fans, but more as admirers of its anvil-like durability than breakneck performance. For most of its production life, the 318 was a bread-and-butter two-barrel economy offering. The 318 debuted in 1967 but was not blessed with a four-barrel until over a decade later in 1978. By then, deep in the smog era, it was too late even for the 318, rated at a meager 175 hp, while its 2V cousins cranked out a lame 120 hp.
Sure the motor is displacement challenged, but the bore/stroke relationship at 3.91/3.31 is pretty good. Sure the bore is a little under the "magic" 4.00-inch mark, but not by much, particularly after a rebuild. Stacked in the motor's favor is an excellent rod ratio of 1.84:1, and like all small-block Mopars, it carries factory 18-degree valve-angle heads with an advantageous spark-plug position at the top of the combustion chamber.
The real hamstring on the 318 isn't the design at all, but rather the way the production engines were offered. Beneath the restrictive intake, small valves and port heads were the norm on the 2V engines, with valves measuring 1.78/1.50-inch intake/exhaust. Four-barrel 318s were simply assembled with the top-end from a 360 4V, but the larger chambers crippled the already pathetically low compression. Most 318s featured pistons deep in the bore, putting the hurt on building power by conventional bolt-on means. The final insult was a paltry camshaft, usually with 0.373-inch intake lift. It's no wonder 318s never flexed their muscle.
Our '68 Barracuda had a well-worn 318 in need of some attention. With a four-barrel and dual exhaust, we measured 196 hp at the rear wheels, but as witnessed by the plumes of blue smoke at the top of the chassis dyno runs, the engine was just worn out. We figured this tired 'teen could be built to show its true potential, but we had to pull this off as a budget deal. The package had to bolt together with common off-the-shelf components, providing a recipe that anyone could duplicate.
With a full rebuild, we had the opportunity to correct some of the little motor's deep-seated problems. At the top of this list would be a piston that we could zero-deck to add compression and make quench effect actually work for us. We addressed this with a set of KB 167 hypereutectic pistons and a judicious milling of the decks that set us up for a desirable zero deck clearance. We elected to go for a 0.040-inch oversize, opening the bore dimension to a respectable 3.95 inches (324 ci). The other side of a decent quench quest is the cylinder heads, but the majority of small-block Mopar heads are of the open-chamber design, limiting our choices in production heads. The notable exception is the swirl-port 302 casting from the mid '80s, but small ports and valves make them next to useless from a serious performance standpoint. The late Magnum head, however, offered an intriguing possibility, with a quench chamber, generous 1.92/1.625-inch valves, and respectable flow in stock form. Available at a bargain price of under $600 a pair, complete, from Mopar Performance (MP), we were game. As an added bonus, the MP heads are already fitted with performance valvesprings and quality stainless steel valves.