Mopar 318 Small Block

Why even bother with a Mopar 318 when it's just as cheap to build a 360? If you already have a 318, it's not "just as cheap" to build that 360. First, you'll need to get a 360 core which will set you back $250 or more. The 318 and 360 engines are balanced differently, so you'll also need a new flywheel or flex plate and torque converter. Then there is fuel economy - being almost 12% smaller than the 360, the 318 should get about 12% better mileage, all else being equal.
When it comes to building for power, the 318 is also an excellent choice. I'm not an engineer, so I can't explain the details, but there are many aspects of an engine besides outright displacement that contribute to its power potential - bore to stroke ratio, connecting rod length, port volume, etc. Suffice it to say that the stars were aligned for the 318, and all the design specifications come together to produce an engine with excellent power potential for its size. A streetable 318 can easily make 400 horsepower, about what a lot of street 360s put out.

Overview: Even though almost all Mopar 318s came from the factory in low performance 2 barrel trim, the 318 can be built into a very good street performance engine. Blocks can be a little harder to find than 360 cores depending on where you live, but it's worth the effort to track one down. I recently had a 318 rebuilt to replace the 273 in my 66 Barracuda. Here is how it was done.

Block: I used a 1986 block. These have the advantage of having factory provisions for a hydraulic roller cam, but the disadvantage of thinner cylinder walls than earlier castings. Fortunately my block cleaned up with a .020" bore - most blocks require .030" which is the maximum you should go on these thin wall blocks. Another problem with the later blocks is that they're prone to cracking between the center head bolt and the water jacket. My block was cracked. It's easily repaired, but adds about $100 to the cost of the rebuild. Machine work included boring and honing with torque plates (very important), line honing the main bearing saddles, and decking the block. For a street engine, that's all you need.

Crankshaft: Since this is a street engine, I decided to use the stock cast crankshaft instead of a custom forging. Mine required grinding and cleaned up at .010/.010 on the rod and main journals. The stock Mopar 318 crank is actually a pretty good piece and is good for at least 6500 rpm, far higher than I'll be spinning mine.

Rods: I decided to go with Scat I-beam rods. They're probably not necessary in this application, but if a rod lets go it will ruin your whole day, as well as your engine. I figure they are cheap insurance. I should also mention they're only a little more than Eagle rods but made out of better steel. If you're putting after market rods into a small block Mopar, I'd recommend Scat over Eagle.

Pistons and rings: Stock pistons aren't the right choice, even in a mild build. I used Keith Black KB-167 hyperuetectic pistons. These are much stronger than stock pistons and give a higher compression ratio. On my engine, the compression ratio worked out to 10.1:1. For rings, I used KB's matching moly ring set.

Cylinder heads: I used high swirl 318 (#302) castings. These are by far the best cylinder heads to use on a mild 318 (I know, some people prefer the 360 heads). They have high velocity ports that give good flow with just a little work and a closed, high swirl combustion chamber that helps prevent detonation. I upgraded from stock valve sizes to 1.88" intake and 1.60" exhaust valves. I also had the shop do a 3 angle valve job and bowl blend. At this performance level, more extensive porting isn't necessary.

Cam: I'm running a Comp Cams 268XE cam that has 224/230 duration and .477/.480 lift on the intake and exhaust. This should give me good power without sounding too racy. In retrospect, I should have gone with their XE275HL cam which is designed specifically for Mopar's larger lifter diameter, but the cam I have should be OK.

Intake manifold: I used to think the Edelbrock Performer would be perfect for this combination, but after talking to several people, I've decided that the Weiand Action Plus is a better manifold, so that's what I went with. I had the shop block the exhaust crossover when they installed the manifold.

Carburetor: If I was going for all out performance, I would have run a Holley 750 vacuum secondary carburetor. Since I wanted some semblance of economy, I decided to run an Edelbrock 650 cfm Thunder carburetor. I also like the fact that I can tune it without pulling the fuel bowls as is required with the Holley carbs.

Exhaust: Headers are a great idea with benefits in power and economy. At this power level, though, it kind of depends... On most Mopars, headers are probably worth the money. On an early A-Body, it might take a long time to recoup the cost. However, since my engine looks so pretty, I decided to splurge on a nice set of TTI headers. Just don't ask what they cost, it's still painful to think about it.

Other Stuff: Since I was kind of going all out anyway, I replaced the stock rocker arms with Comp Cams stainless steel rollers. I also replaced the stock oil pan with a Milodon pan and matching pickup, and bought a nice set of polished aluminum valve covers. Ignition will be a Mopar Performance electronic distributor with an MSD-6A control box. Overall, it should be a good combination. If I was starting over, I would have had the heads done by Shady Dell Speed Shop and taken the time to find an earlier block.

Well, there you go, a recipe for a Mopar 318 that should give great performance and decent economy. I can't wait to get my Barracuda back and see how this thing actually runs. If you like it (or not), leave a blurb in my guest book.

In the Good Old Days, piston selection was simple. Cast pistons were for Grandma cars and forged pistons were for everything else. Today there is a third option, hypereutectic pistons. Not all engines have all piston types available, but the Mopar 318 is one of those engines that has been blessed with a decent assortment of pistons to choose from. By selecting the right piston for your application, you'll ensure that your car runs at its best, you'll be a hit with all the babes, and the envy of all your rivals.

Cast pistons are the cheapest pistons you can buy. They are also the weakest, so you need to really consider what you're going to use your engine for before choosing to run cast pistons. Cast pistons for the 318 are made by Federal Mogul. A set will cost you around $100 from Summit Racing. Regardless of the year of your 318, use the Federal Mogul Z526P pistons which are listed for the 1985 through 1989 318. These will give you about a half point higher compression ratio than the Z285NP pistons listed for the 1967 through 1984 318 and will fit the earlier engine just fine.

Cast piston advantages: Inexpensive, tight piston-to-wall clearance, quiet when cold
Cast piston disadvantages: Low compression, limited selection, not as strong as other choices

Hyperteutectic pistons are most likely your best choice for a high performance street or mild (no nitrous) racing engine. Hypereutectic pistons for the 318 are available from Speed Pro and Keith Black. The Speed Pro pistons cost $147.12 for a set from Summit Racing. They are more of a stock replacement style piston, but will give you a little higher compression ratio than either of the cast piston choices for not too much more money. They are also much stronger than regular castings and have a friction reducing coating on the skirts. If I were building a budget 318, these would be my pistons of choice. For a high performance street 318 I like the KB167 piston from Keith Black. It's still a flat top, but with a higher deck height than the Speed Pro hypereutectics. It will give you a compression ratio between 9.2:1 and 10.4:1 depending on what heads you're running. They're also heat treated to T6 hardness, making them very strong. They cost $310.69 from Summit and are in my opinion the best pistons to use in a street engine.

Hypereutectic piston advantages: Much stronger than standard castings, tight piston-to-wall clearance, quiet when cold, better ring and oil control than forgings
Hypereutectic piston disadvantages: 5/64" ring package, shouldn't be used with heavy shots of nitrous

Forged pistons are the strongest pistons available, and therefore the best, right? Probably not, at least not for a street engine. While it's true that forgings are the strongest, they are also the most expensive. Also, because forged pistons expand more than cast pistons, they must be installed with a larger piston to wall clearance. This allows the piston to rock slightly in the bore when the engine is cold, which is noisy and also affects oil control. There are some very good reasons to run forged pistons, but most of them don't apply to a street engine, not even a really hot Street 318. Unless you're running big shots of nitrous or lots of boost, you'll probably be happier with a set of KB167's. If you DO need forged pistons for you application, I'd suggest skipping the less expensive offerings from Keith Black, TRW, etc., and going with a good set of custome forgings from Aries, JE, Mahle, Ross, Wiseco, etc.

Forged piston advantages: Extremely strong, (potentially) much lighter weight
Forged piston disadvantages: Expensive, large piston to wall clearance, noisy when cold, poor oil control (compared to cast pistons)

When buying a set of pistons, you should always go with the smallest oversize possible. Some people think the more you over-bore the better, but with a thin wall casting like the small block Mopars use, boring too much hurts power because it lets the cylinder walls flex more under load. You should also have your block bored and honed with torque plates installed to help ring seal and power.