One man's trash is another's treasure, and when it comes to bottom feeding in the world of Mopar engines the 318 seems like the biggest bargain in town. Know for efficient grocery getting in modest transportation, the musclecar aura never kissed the 318. Though the 318's long production run began in 1968, poised perfectly to participate in the height of the musclecar era, it was only available as a two-barrel economy until the deep dark days of the smog era in 1978. Without the cool reputation as a powerhouse like the bigger 340, 318s are routinely plucked from all manner of Chrysler products, and are practically handed out for haulage in Mopar circles. Be that as it may, there are some true 318 devotees in the Mopar world, impressed by the efficiency and long service this loyal mill provides.
So what's wrong with the 318? Obviously, it's a little light on displacement, but not compared to some more highly revered engines of the past-302 Chevys or Boss Fords, for instance. The most commonly heard objection is that the bore is small, creating shrouding that hurts flow and makes big power impossible, since at 3.91-inches it's not quite at the "magic" 4.000-inch mark. Yeah right-tell it to the LS1 crowd. Those engines are known for their breathing despite an even smaller bore. With Mopar's factory 18 degree valve angle, this is pretty much a moot point for most street builds. The fact is a 318 can be the basis for a nice small-displacement, high-rpm powerplant. With its short 3.31-inch stroke, high rpm are natural. A case can be made for a small-displacement street engine that's fuel efficient by virtue of its modest size, but piles on the power on demand when needed, by winding the revs up the scale. With the 318 Mopar as a base, such an engine can be built for a ridiculously low price. Better still, you will always be the underdog, and there is nothing like talking smack when putting the hurt on the big boys with a "lowly" 318.
Our goal was to build a hot street engine, using the attributes of the factory package to its best advantage, keeping the build as cheap as practical. All 318s use long 6.123-inch rods, though the 1973 and later engines had the stronger but heavier 340/360 forgings, which are better. These later rods are stout from the factory, and toy with engine speeds on the order of 6,500 rpm, and even a little more. Nearly all 318s came with cast cranks, and here again the factory stuff can take 6,500 rpm handily. In fact there is nothing in the stock engine that can't work exceptionally well in that rpm range, including the lubrication system, electronic ignition, and even the production valvetrain. I guess you can fairly say that these engines are all 6,500-plus-rpm screamers just waiting to be unleashed; here's how we did it.
The Build
Low compression is a drawback of any 318, with all of the factory pistons dwelling deep in the hole at TDC. The low-cost remedy here is a set of KB 167 flat-top pistons, which allow for a zero deck with minimal milling. We had the guys at Precision Speed and Machine, in Bakersfield, California, punch our virgin 318 block 0.040-inch over, to 3.950-inch (getting close to 4.000-inch there, aren't we?), and then zero deck the block. With this overbore, the 318 grows to 324 cid. This minimal level of machining prep will usually be all that is required to build a hot street 318. We used a 273 steel crank in our engine, only because we found it in the dirt at the local boneyard, but any of the factory cast cranks are more than up to the job. To complete the short-block, we had the rods reconditioned with new Pioneer bolts, and used a factory windage tray, as well as a homemade sheetmetal baffle in the pan to prevent oil slosh. A summery of bottom-end mods is a pretty short list, basically all we did was put the flat top pistons in, milled for zero deck, balanced it, and put it back together with all re-conned factory stock stuff. Moving on to the top-end, here is where you might expect to find the secrets to making the big power put down by this little engine. Well, normally a really trick set of heads can work wonders, but here we just used a boneyard set of plain old 360 Mopar smog heads. After all, few guys will cap a $500 short with a $1,500 set of heads. All of those '70s-80s 360 heads essentially carry the same port layout of the high performance 340, and as far as run-of-the-mill production iron of the era, they are about as good as it gets. In fact, these old-school heads have a big advantage over the later castings. The pushrod constriction is much less intrusive than that found on engines spawned in the hydraulic roller era, far less than even the Magnum heads of the '90s. The bottom line is that these 360 heads have plenty of intake port for a very stout 318.
All of the 360 Mopar heads came from the factory with 1.88-inch intake valves, 1.600-inch valves at the exhaust, and big open chambers, at 72cc. To step things up, we started by having the heads machined for 2.02-inch intake valves. Some weird urban legend has it that 2.02-inch intake valves will not fit a 318, or that bore notches are required. Actually, even a standard-bore 318 will swallow valves of this size, so no problemo for our 0.040-over mill. After the seat machining, the heads received a little porting, consisting of a basic bowl blend, and some work at the pushrod area. Far from an all-out effort, these heads got little more than a swizzle of the carbide cutter-not even the ugly guide bosses were cut. These modifications may not seem like much, but it was enough to tap well into the potential of the castings, and produce respectable flow (See: Flow Chart). To reduce the chamber volume for our small displacement engine, the heads were milled fairly heavily to 60cc.
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Even though most guys think 318s don't have what it takes to make power, we started with a production 318 block, and decided to build it on the cheap. Precision Speed and Machine (PSM) punched it 0.040 for a set of KB 167 flat-tops, and zero decked it. We gave it the customary clean before assembly. |
Federal Mogul bearings were used throughout the build. We used Childs & Albert assembly lube to coat the plain bearing at the rods and crank. |
Though most 318s came with cast cranks, we happened to run across a factory-forged crank from a 273. Don't feel bad if all you have is a cast iron piece, they are generally reliable at 6,500 rpm. We balanced the crank at PSM, and recommend you do, too. The odd-looking bolts at the No. 2 and 4 main caps are to take the factory windage tray. We used the stock main fasteners. |
Even though we went with the cheaper Federal Mogul moly pre-gapped rebuilder's rings instead of the better file-fit SpeedPros, we checked the gaps to get them in specs for the KB pistons, which run wider gaps. |
The KB 167 pistons have deep valve notches that will swallow all kinds of lift, even with milled heads. The pistons have to go in order corresponding to the valve layout. |
The rods are regular Mopar factory units, actually the 73-up 318 forgings, which are the same as the 360 rods, which were the same as the 340 high-performance rods, except without the pin bushings. These rods are long at 6.123", and you won't break them with your street 318 (or 360). The rods were reconditioned, fitted with new Pioneer bolts, and the pistons hung, at PSM. We just put them in their holes and torqued-up. |