Chrysler 318 Engine - A Powerplant To Brag About

Some engines get all the glory. You know, those legendary powerplants that you can't wait to brag about. "Yep, she's got a 427..." or how about, "I'm running the hi-po 351..." or, "Sure is nice, came stock with the solid-cam LT1!" Just calling out the engine type is enough to get some respect. On the other hand, there are plenty of engines out there that will actually tempt a street hound to start lying. You know the 307 in that Chevelle, but he's tired of making excuses, and everyone just seems happier when he tells 'em it's a 327. In the world of Mopar small-blocks, the 340 basks in all the glory, and in recent years, the 360 has gained plenty of credibility, but the little 318 is generally regarded to be fairly low in the automotive pecking order. Like boat anchor low.

The fact is, with a simple combination of parts, and not a lot of money, you can consider any 318 a high-winding and stout small-block waiting to be unleashed. There are plenty of naysayers ready to explain why this can't be. They say stuff like: "Those things don't have enough cubes." But it's actually considerably bigger than the popular 5.0 Ford. "A 318's bore is too small!" Tell it to the even smaller-bore LS1 guys. "The 318 was never a high-performance engine." Maybe true, but we'll show you how to make it one!

The Good And The BadIn terms of durability, the Mopar 318 is robust. The bottom end of most 318 engines used a cast-iron crankshaft, and though these are not as desirable as a forged crank, the cast-iron piece has proven to be very reliable. Early 318 engines (up to 1972) used lightweight rods with full-floating piston pins, while later engines were equipped with the heavier 340/360 forgings with press pins. The later rods are considered stronger, and this bottom end has proven very reliable in high-performance applications-especially in the street rpm range up to 6,500.

Before delving into an engine build, it's worthwhile to define the goals, and then evaluate the engine for its strengths and weaknesses to decide what will need upgrading to meet the build's objectives. First, let's look at areas where the 318 can use some help, as there are a few shortcomings that are common to these engines in stock form. Other than areas remedied by the usual bolt-ons, the factory setup fell short in three key areas: compression, camshaft, and cylinder heads. Two-barrel 318 engines were equipped with low-performance heads with small ports and valves. Later four-barrel engines, introduced in 1976, were somewhat better endowed, using the larger port and valve cylinder heads from the 360; however, these heads have larger combustion chambers that exasperate the second shortcoming of the 318-compression. A key weakness with the 318 engine is the low factory compression ratio resulting from pistons well down in the hole at TDC. Finally, all factory 318 engines were equipped with very conservative camshafts, delivering less than .400-inch lift.

So, is it all bad news? Actually, in the course of a rebuild, the lack of compression ratio and camshaft can be corrected for not much more than the cost of stock replacement parts. The compression ratio side of the puzzle is quickly solved with a set of flat-top KB pistons, bringing the piston up from the depths of the bore. A modest machining of the block deck achieves a zero deck height. The KB flat-top pistons for the 318 are available in either cast hypereutectic (PN KB167) or forged (PN KB844). For the purposes of our mild street application, we opted for the more economical hypereutectic pistons.

Camshaft choices for the 318 are wide-open, but for basic street use and low cost, a hydraulic flat-tappet cam offers the greatest performance bargain. Here, the Mopar offers a potential advantage, having a .904-inch tappet diameter, which is significantly larger than that found in other popular engine makes. The larger tappet diameter allows the cam to be ground more aggressively, with a higher lift velocity, giving a power advantage in comparison to a cam designed for a smaller tappet diameter. Many off-the-shelf camshafts are designed for a minimum tappet diameter of just .842 inch (Chevy-sized), leaving some of the Mopar engine's performance potential on the shelf. We know COMP Cams offers true .904-inch tappet camshafts with their Xtreme Energy High-Lift series of cams just for Mopar engines. For our street performance small-block, we selected the most conservative of this line of camshafts, the XE275HL. Specs for this stick come to 231/237 degrees duration at .050, and .525-inch lift with a 1.5:1 rocker ratio, all on a 110-degree lobe separation angle.

Heading For PowerThis leaves the final upgrade in our power trio of compression, cam, and cylinder heads. While good results can be had with 360-derived four-barrel 318 heads, there are some serious shortcomings. The main drawback is the large and inefficient open-style combustion chamber. Not only does this take down the compression ratio, even with the flat-top pistons, but the chamber design precludes building an engine with a desirable head-to-piston squish/quench effect. Choices in OEM cylinder heads with a closed chamber are limited to small-valve, small-port castings, or the later Magnum-style heads.

The Magnums provide some notable advantages, such as a compact modern closed chamber, generous valve sizes (1.92-inch intake, 1.625-inch exhaust), respectable port flow, and a revised valvetrain with a higher rocker ratio (1.6:1 versus 1.5:1). Converting to these cylinder heads has become a popular upgrade for earlier engines (see sidebar: "Magnum Head Conversion"); however, the main drawback with the OEM Magnum heads is a propensity for cracking. It is very difficult to find used Magnum heads that are not cracked.

Engine Quest has a viable alternative to the OEM Magnum castings with their iron Lightning cylinder heads. Essentially an OEM replacement casting, the Engine Quest heads feature refined ports and higher flow than the stock pieces, and just as importantly, these are all-new castings without the fatal Magnum head cracks. One of the traditional drawbacks of the original Magnum design was the revision with this cylinder head series to a vertical intake bolt pattern, precluding the use of readily available aftermarket manifolds designed for the earlier LA-series engines. Engine Quest addressed this compatibility issue by offering their Magnum-style replacement heads for either the Magnum-style intake (PN CH318A), or for a conventional non-Magnum-style intake (PN CH318B).