mopar 318 400hp


318 Engine Build – A Parts Book 400HP 318

This One Just Bolts Together

writer: Steve Dulcich
photographer: Steve Dulcich

Putting together a 400hp engine is a lofty goal; doing it with a pump-gas street 318 is an accomplishment. Bolting one together without any custom grinding, porting, special machining, or other smoke-and-mirrors trickery may seem impossible, but it makes it that much more palpable for the average guy. we’ve featured plenty of more advanced engine builds, but the beginner may wonder how to get there with all the custom mods involved. Well, this one is for the Average Joe–a straight bolt-together deal even a first-time engine builder can put together. Ours made exactly 400 hp with a little tuning on the dyno. Yours will make the same, if the battle plans laid out here are followed exactly.

Why build a 318? Some may argue there is never a reason to build the Mopar 318. When the time comes to rebuild a small-block, the 318 should always be cast aside in favor of a 360. We won’t argue. A 360–built properly–will always have a torque advantage over a 318. However, 318 cores are free for the asking, and that offers some economic incentive. But the real motive usually isn’t a matter of cost. The bottom line is like it or not, 318s are being rebuilt by enthusiasts every day. Some guys have a certain sentimental attachment to these cute little engines. sometimes it’s just the desire to retain the car’s original powerplant. And some guys dig the idea of blowing doors in with just a 318. Count us in that last group. We were intrigued with working out a combo that shows how to really make one run. Our goals were straightforward:

  • The engine would be based on a regular 318.
  • The engine must run on 91-octane pump gas or less.
  • The camshaft must be daily-driver streetable, at least in a hot-rod sense.
  • The build will not include any exotic, rare, unusual, or excessively expensive parts.
  • The parts will be bolted on out of the box with no custom tricks, massaging, or porting.
  • The final output would break the 400hp barrier.

    The Build
    The subject of our build was the engine from our ’68 Barracuda fastback. The 318 was putting out almost 200 hp at the rear wheels with an antique Edelbrock Street Master intake and four-barrel carb, along with a dual exhaust featuring the stock exhaust manifolds. The output wasn’t terrible, but the blue smoke and blow-by certainly were. The engine was just worn out, as we found when the heads were popped to reveal well over .010-inch bore wear. It was the perfect candidate for our buildup. With a goal of 400 hp under the above noted constraints, we had to consider every aspect of the combo very carefully.

  • 318 Engine Build – A Parts Book 400HP 318

    Starting at the bottom, the contents of the block had to work together optimally in order to reach our goals. Piston choice is a critical aspect of any build, and in the case of the 318 the choices are limited. Our requirements were a flat-top piston at zero deck, which would provide a desired boost in compression ratio, while also ensuring that we would have an ideal quench clearance to make the most of combustion efficiency. Stock and stock replacement pistons place the pistons so far down the hole, that even with massive milling, the goal of a zero deck is out of reach. Fortunately, Keith Black offers a hypereutectic 318 piston that fits our criteria (KB 167). The assembly can be brought to zero deck with modest block machining, and the pistons are a commonly available shelf part at a practical price. To provide the combustion seal and desirable friction characteristics, high-quality moly rings are needed. We looked no further than Federal Mogul’s offerings, opting for their pregapped moly rings. Though file-fit rings can be had for our bore size, these rings are more in keeping with our requirement for moderate cost and a bolt-it-together assembly.

    A zero-deck block is a waste of time without a closed-chamber quench head to go along with it. While practically all high performance aftermarket heads are of this configuration, we sought to reach our goals without the expense of aftermarket aluminum heads. Looking at the production options, the field is limited since the vast majority of stock small-block heads were of the open-chamber variety. The only exception in the LA-series heads are the closed-chamber, late-model, 318 two-barrel 302 castings. The ports and valves in these heads do not offer sufficient flow to meet our goals. With serious porting and larger valves installed, the 302 heads could get us there, but these mods are contrary to our goal of a bolt-together package. However, the later Magnum heads also feature closed chambers, and with their 1.92/1.625-inch valve combination, these heads do have the flow to support 400 hp in the right combination. The Magnum heads offer numerous advantages in this application. With a chamber volume of 59 cc, these heads will offer about 10:1 compression ratio when combined with a zero-deck short-block utilizing typical .040-inch head gaskets. That’s about where we figured we’d need to be to make the power number. The Magnum-head layout features excellent rockers with an added bonus of 1.6:1 in ratio, as opposed to the LA engine’s 1.5:1. This aids our effort to reach the 400hp mark. Other Magnum pluses are lightweight, 8mm stem valves and machined valve-cover rails with ten valve-cover attachment bolts to minimize the potential for oil leaks.

    318 Engine Build – A Parts Book 400HP 318

    With this, we had the basis of our build-up: a zero-deck 318, sporting Magnum heads and an MP single-plane intake. All that was left was to select enough cam to do the job. Getting to our output goal would be easy with a huge stick, but we were working within reasonable limitations. The objective was to put together a combo that would work well on the street. Another consideration was the retainer-to-guide clearance of the Magnum heads. The retainer in a stock Magnum head will physically hit the valve stem seal at .530-inch of lift, thus putting a ceiling on cam selection. Though the guide can easily be cut down for more clearance, we would be getting into custom mods, which we were seeking to avoid. In fact, the lift limitation would allow about as fat of a camshaft as we were willing to run. Keep in mind, nearly all cam specs listed for Mopar small-blocks are calculated at the LA-engine’s 1.5:1 ratio. With the higher 1.6:1 ratio of the Magnum engine, it doesn’t take a huge cam to use up all the available clearance. We selected a Competition Cams 280H Magnum camshaft, which is a single-pattern grind featuring 280 degrees of rated duration and 230 degrees of duration at .050-inch tappet rise. This cam is advertised as having .480-inch lift, but with the higher-ratio Magnum valvetrain, theoretical lift goes up to .512-inch. It seemed like the ideal cam for our combo. We liked the single-pattern profile as a compliment to the Magnum heads’ good exhaust flow ratio, and that the cam was maxing out the lift potential of the out-of-the-box Magnum heads.

    With our combination penciled out, we brought our 318 to Precision Speed and Machine in Delano, California, for machining. Since we were going to use new Magnum cylinder head assemblies from Mopar Performance, the efforts were concentrated upon the bottom end. We elected to go .040-inch oversize in the bores, and had the block bored and honed by Precision with moly rings in mind. We upgraded the connecting rods from the weak early 318 pieces, moving to the readily available later (’73-and-up) rod shared by the 318 and 360. These rods are virtually indestructible in an application such as this. Another upgrade was to swap the stock 318 crank for a forged piece, since we happened to have one pulled from a 273. The early 273 cranks have a different torque converter register, which creates problems in an automatic application, but it is readily compatible with four-speed components. The rotating assembly balanced fine.

    Precision inspected and reconditioned the stock rods and installed a set of high-strength ARP rod-bolts. Special care was taken in selecting the rods and resizing to ensure a set of rods with virtually identical center-to-center length. This is critical in setting up a zero-deck engine. Precision line honed the block to exact tolerances, and then square decked the block to put the pistons dead flush with the block at TDC. The machining checked out exact. The final assembly was a breeze, as should be expected with production-based pieces and top-quality machine work.

    Making the change to Magnum heads is a simple swap. All stock components can be used, however, Magnum engines oil the valvetrain through the pushrods as opposed to LA-style engines. Since all Magnum engines were equipped with factory hydraulic-roller camshafts, using the Magnum heads in conjunction with an LA block and a hydraulic flat tappet cam requires custom-length pushrods. Mopar performance sells a pushrod kit for this application (PN P5007477). We mocked up our engine combo (taking into account the block decking) and found that Comp Cam’s number 7960 pushrods at 7.650-inch length were an ideal fit. Another distinct feature of the Magnum design is a vertical intake bolt angle, requiring a Magnum-specific intake manifold. There are not as many choices in Magnum manifolds as with the traditional LA small-block. Mopar Performance offers a dual-plane, as well as a racy single-plane intake for this application. we opted for the single-plane. The single plane would undoubtedly cost precious torque lower in the rpm range, but we just didn’t think the dual-plane was the piece to make our target output. Word has it Edelbrock is introducing a Performer RPM intake for this application, and it should prove to be an excellent alternative.

    318 Engine Build – A Parts Book 400HP 318

    Putting It Up
    To see if this 318 would meet our lofty goals, we brought our fresh mill to Westech’s engine dyno facility to post some numbers. The assembled long-block received an MSD distributor and wires, a set of tti 15/8–13/4-inch step headers, and a 750 Mighty Demon carb–all top components we were confident using. The 318 cracked to life, and we ran the engine through the normal break-in cycle with the distributor set to provide 34 degrees of total timing. Idling down, the 318 sounded sweet commanding 10.5-inches of vacuum and exhibiting a mild loop. With the first real power pull, we recorded 383 hp at 6,200 rpm, stout output but a little shy of our goal of 400 hp. One thing was clear, the little 318 did love to rev. A timing loop followed, as we sought to zero in on the ideal timing and found the best overall curve with total advance set to 36 degrees. Power nudged up only slightly, now reading 385 at the same 6,200 rpm. A jetting loop followed, but there wasn’t anything to be found after making several jet changes.

    We noted the fresh engine was producing a maximum of 95 psi of oil pressure hot with the 20w-50 oil we used for break-in. It seemed as though a lower viscosity oil would be worth a couple of numbers, and better still, a nice synthetic may add even more output. We drained the sump for a fill of 10w-30 Royal Purple and found more than we expected, with power now up to 394 at 6,100 rpm. We were tantalizingly close to the 400 mark, and still had a trick or two to try. A Wilson 1-inch tapered spacer was sandwiched between the intake and carb, and we hit pay dirt–400 hp on the nose at 6,150 rpm!

    Dyno Results
    Superflow 901 Dyno
    Tested At Westech

    Mopar 318  
    RPM   HP   TQ  
    3,000   190   332  
    3,500   241   376  
    4,000   286   376  
    4,500   321   375  
    4,900   357   382  
    5,000   363   381  
    5,500   388   371  
    6,000   398   348  
    6,200   400   339  
    6,500   393   318

    318 Engine Build – A Parts Book 400HP 318

    Power Squeeze
    While 400 hp from a bolt-together 318 may seem like plenty, we figured with a little go-fast gas our little engine could go hunting for some big-block game. With the hypereutectic KB pistons, we decided to keep the nitrous moderate and sane, just enough juice to stalk some of the hardier street beasts we may encounter. Be aware that KB has very specific ring end gap requirements for nitrous use, and these must be followed during the build if any kind of life expectancy is to be had from the engine with nitrous. We selected a Nitrous Works jet-metered plate system, a quick and easy addition to any engine. The plan was to add 100 hp to our normally aspirated output, not an excessive amount, but enough to really notice.

    The Nitrous Works recommended jets for a hundred horse shot were installed. installing the plate system was only a matter of unbolting the carb, slipping the nitrous plate between it and the manifold, and then running the fuel and nitrous lines to their respective solenoids. The solenoids were wired to a remote switch, and we were good to go. With an aim towards being conservative, the fuel supply was switched to 100 octane, and the MSD ignition was set to provide 8 degrees of retard upon the activation of the nitrous system. Adding nitrous at a low rpm can lead to excessive cylinder pressure, so we planned on hitting the nitrous at 4,700 rpm, which would equate to just out of the hole on the dragstrip and above the shift recovery rpm going down the track. The nitrous system delivered on its promise, boosting output to just above the 500hp mark, and maintaining a flat power curve up to our self-imposed redline of 6,400 rpm.

    Nitrous Output  
    RPM   HP   TQ  
    5,000   471   494  
    5,500   483   461  
    6,000   509   446  
    6,300   500   417  
    6,400   497   408






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