Mopar 318 Engine - Cheap as Dirt

Tallying up so far we had a stock short with flat-tops, and a set of milled junkyard heads with bigger intake valves and very modest porting. It may not sound like much, but this is good stuff for a 318 build. The next consideration was the camshaft. Here we gravitated towards a hydraulic flat tappet, which may not be romantic, but is certainly budget conscious. A solid flat tappet certainly would have offered even more rpm potential, but a hydraulic allows us to use the factory valvetrain. While the OEM valvetrain is non-adjustable, and looks as low-tech as it gets, it is extraordinarily lightweight, and being a shaft system, it's as stable as it comes. With a hydraulic cam, the ugly stock stuff will out-rpm virtually anything from the aftermarket.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.

Mopars have another advantage, and that is the 0.904-inch tappet diameter, which translates to about 8 percent more intensity at the valve with a cam designed to make use of the large lifter diameter. COMP has just the cams for the job, with their Mopar-only XEHL series. This is a variation of the popular Xtreme Energy cam series, but built to provide the additional intensity the 0.904-inch lifters allow. We went with the smallest of this series, the XE275HL, specing out at 231/237-degrees duration at 0.050, and 0.525-inch lift, on a 110 degree lobe separation angle. This duration level is enough to give a noticeable chop at idle, but not temperamental enough to make you insane in day-to-day driving. To cope with the aggressive rate of this cam, we used a fairly significant level of spring load for a flat tappet, deciding on a set of COMP 972 single springs installed at 1.750-inch, for 140 lbs on the seat and 310 lbs over the nose.

The final aspect of the build to consider is the induction, and here past experience has taught us that there is no beating an Edelbrock Performer RPM AirGap in the street range up to 6,500 rpm. That much was a given, but motivated by ultimate cheapness we bolted-on a TransDapt adapter plate, which allowed us to mount a factory Mopar ThermoQuad carb. The TQ carb is actually a fine unit, and the one we used is the larger version rated at 850 cfm of airflow, though an 800-cfm unit was also OEM equipment. That may seem like a huge amount of carb for a 318, but an odd characteristic of the TQ is that it allows monstrous capacity to work on seemingly impossibly small combinations. Finally a set of TTI headers with 1 5/8- to 1 3/4-inch primaries were obtained to handle the exhaust, and we had our combo. On paper, it seemed pretty tame, a flat-top piston 318, with mildly reworked iron 360 smog heads, a hydraulic flat tappet, an Edelbrock two plane, a stock four barrel, and headers. We knew better.

Running The Numbers
When we brought our 318 to Westech Performance Group for testing, the attractively detailed 318 Mopar drew some interest, and naturally a few questions about the combination. We revealed it was a 318, basically a factory short-block put together with KB flat-tops and a hydraulic flat tappet, mildly ported ordinary factory 360 heads, and the four-barrel setup with the ThermoQuad visible to all. By the specs, it didn't seem like much, especially being "only a 318", so there was some serious doubt when we confidently stated that it should make over 400 hp. After all, mildly reworked 318s don't make that kind of power, especially with some junkyard heads. Well, it didn't take long to silence the skeptics, when the engine was broken in and then loaded against the brake for the first power pulls. With out baseline combination, the engine zinged-up the rpm scale to make 400 hp at 6,200-6,300 rpm.

We could see that the 318 was doing exactly what we expected-making lots of power and rpm. We could also see from the dyno sheets that the factory ThermoQuad carburetor was running at least a point too rich, and it was time to make a decision. There were a few things we wanted to try while on the dyno, and we only had one day reserved for testing, and about half of that was already eaten just setting the Mopar mill up on the dyno, and getting our baseline figures. Experience has taught us that playing with the ThermoQuad could easily soak the rest of the day in tuning, so we decided to substitute a Mighty Demon 750-cfm carb, primarily for its easy tuning changes. A 1-inch open spacer was used to mimic the 1-inch adapter that had been necessary with the ThermoQuad. With some quick jet changes of the Demon to get our mixture in the bullseye, we had 406 hp showing at the same 6,200-6,300 rpm peak.