How to Build a 300 Horsepower 318 Mopar
Whereas the "meep-meep" of a Roadrunner's horn may stir memories of 440 and 426 performance, the fact is that most Dodge/Chrysler muscle cars of the 1960s and 1970s received the company's 318 Under-Dog engine. There's nothing inherently wrong with the 318 -- it has larger bores than an LS1 small block Chevrolet -- but the fact that it was never offered as a performance option in any chassis meant that the motor was perpetually stuck in a state of corporate eclipse. However, close examination will prove the 318 (and its later 5.2-liter Magnum descendant) just as performance-capable as any 5.0-liter Ford, 302 Chevrolet or 340 Chrysler.
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Instructions
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Begin with any 318, preferably one manufactured after 1973; these blocks had far stronger forged rods than earlier units. Disassemble the engine block and send it to a machine shop for a standard freshening, including a 0.030-inch overbore, align boring the mains and hot-tanking to clean the oil passages. Purchase a set of cast or -- preferably -- silicon-alloy hypereutectic pistons designed to deliver a 9.5-to-1 compression ratio with a 72 cc combustion chamber.
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Instruct the machine shop to balance your rotating assembly (crankshaft, rods and new pistons). While the block is there, you might as well pay them to install the rotating assembly into your machined block. You can do this yourself, but you'll already be into the machine shop for about $500 to $800, at 2010 prices, so the extra $200 or so you'll spend to have the block assembled is money well-spent if it comes with a warranty.
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Purchase any set of 1970s era Chrysler 360 heads; they should be nearly free, since these heads are far from desirable for larger engines. Have the machine shop install 2.02-inch intake valves; this step isn't completely necessary for an engine of this power level, but will allow you to use a nearly stock cam to help maintain idle quality. Install the cylinder heads on the engine.
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Slide into your block any stock, non-rollerized camshaft from a 1980s truck 318, as well as the matching valvesprings and lifters. This very mild cam will allow your engine to maintain a nice low-end power delivery and idle as smooth as glass. The high-flowing heads, better induction/exhaust and higher compression will make up the power that you'll sacrifice with such a small cam.
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Bolt an aftermarket, dual-plane aluminum intake manifold and a set of block-hugger headers with 1-inch primaries to the heads. A set of long-tube headers designed for an RV or truck would be best to maintain low-rpm torque, but block-huggers will probably fit better in your chassis.
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Bolt a 450-cfm, vacuum-secondary carburetor to the intake manifold and install an aftermarket distributor. Utilize a high-powered ignition coil, a multiple-spark ignition box, 8 mm spark plug wires and platinum-tipped, single-strap spark plugs. Install the accessory drives using an aftermarket under-drive pulley set and cool the radiator with an electric fan.
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Tips & Warnings
Building a mild 318 to this spec should net you at least 300 horsepower and 350 lb-ft of torque -- probably a bit more -- while providing a very smooth idle, reliable running and good fuel economy. You can boost power and efficiency even more by coating the piston tops, the insides of the intake/exhaust ports and the inside/outside of your headers with thermally-insulating ceramic powder-coating. This coating will keep heat inside the combustion chambers and exhaust where it can make power instead of allowing it to leach into the coolant and oil as waste heat. Expect at least a 10 percent gain in power and fuel efficiency when incorporating a full ceramic-coating job like this.
References
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