Build a Hot Mopar 318 for Street or Strip
The 318 engine is absolutely the best all-around small block Mopar, in my opinion. It can easily make 400 plus horsepower and still has great street manners. If you're building for fuel economy, the 318 beats the Mopar 340 and 360, hands down. The 318 was produced in three versions: polysphere, LA series, and Magnum. Each are good engines, but for performance use you should stick with the LA or Magnum because performance parts for the 318 Poly are hard to find. This article focuses on the LA version.
Why Build a 318?
Why build a 318 when it's just as cheap to build a 360? For one thing, 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. 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 a great 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 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 street-worthy 318 can easily make 400 horsepower, about what a lot of street 360s put out.
When the engine in my 66 Barracuda needed to be rebuilt, I decided to replace the stock 273 with a modified 318. Forty five extra cubic inches equals lots more torque and horsepower, and the 318 is a bolt-in replacement for the 273. I'm running 302 head castings with 1.88/1.60 valves and mild porting, KB hypereutectic pistons (10.5:1 compression with my heads), Comp Cams 268H cam, stainless steel roller rocker arms, Weiand Action Plus intake, Edelbrock carburetor, and SSI headers.
This Book Tells You Everything You Need to Know
If you've never rebuilt a small block Mopar, you really need this book. How to Rebuild Small-Block Mopar Engines has all the information you need to know. It covers stock type rebuilds, so if you're looking for high performance info, it's best to consult with an engine builder who has experience with the Chrysler LA small blocks. They aren't the same as Chevys and you won't get maximum performance if you just copy what works on a Chevy. Please note there is another book available titled How to Hot Rod Small-Block Mopar Engines which I do not recommend, as much of the information is out-dated and a lot of the parts mentioned are no longer available.
Building a Strong 318 Engine
Overview: Even though almost all Mopar 318s came from the factory in low performance two-barrel trim, the 318 can be built into a great 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.
- 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 re-build. 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.
- 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.
- 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.
- Stock pistons aren't the right choice, even in a mild build. I used Keith Black KB-167 hypereutectic 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.
- 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.
- 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.
- 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.
- 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.
- 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.
- 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.
Mopar '302' High Swirl Cylinder Heads
The small chamber of these heads give your 318 a higher compression ratio, which is good for both power and fuel economy. If you need to build a low compression engine, these are still the best heads to use, but run them with dished pistons. Some builders report that for two otherwise identical engines (including compression ratio), an engine with dished pistons and small chamber heads can make as much as 20 horsepower more than en engine with flat top pistons and open chamber heads.
Ten Ways to Build a Mopar 318
The Mopar 318 in my opinion is Chrysler's most versatile engine. One time I tried to think of all the unique ways to build one, and I came up with over 20 different ways. Here are my top 10. Complete recipes for each coming soon.
- Stealth 2 bbl—Look stock but run fast.
- Performance with economy—What the 318 was born to do.
- 273 replacement—45 extra cubic inches and the only one who knows is you.
- Vintage drag racing—Dual Carter AFB's on an Offy manifold.
- Daily driver—Just your average 13-second grocery getter.
- Hot Street—Yes Virginia, a Mopar 318 can run 12 seconds in the quarter and still exhibit good street manners.
- Ultimate Stealth 2 bbl—Like option #1, but with a twist.
- Improved vintage drag engine—Option #3 with a twist.
- That 70's Show—Late 70s / early 80s hop-up tech.
- Post Modern—All the latest high tech hop-up parts.
The best intake by far for a 318 running "high swirl" cylinder heads is the Weiand 8007 Action Plus Intake Manifold. Running a large port intake with 318-style ports causes a "damn" in the intake path where the intake mates to the head because of the port mismatch. Most intakes designed for the 318 ports are economy type manifolds that aren't really intended for high performance. The Weiand 8007 Action Plus is the only high-rise, high performance intake designed specifically for the 318 sized intake ports. I have one on my Barracuda and I love it.
If you're running 340/360 heads, the best intake is probably the Edelbrock Performer RPM (don't run it with 318 heads though). I really interesting option would be the single plane 2-bbl intake manifold that Mopar put on the 273 and some early 318. I think that with porting and Extrude Honing this manifold could be made to flow pretty well. Combine it with a tuned over-size 2-bbl carb from one of the low end big blocks and you'd have one heck of a sleeper.
Intake Port Mismatch
The 318 4 bbl used 360 heads, so if you're thinking of using the intake from one of these to convert your 2 bbl 318, don't. The port mismatch will kill intake flow and performance. Instead, use an Edebrock Performer or Weiand Action Plus which are sized correctly for the 318 intake ports.
318 Recipes on the Web
For an engine that supposedly gets no respect, the Mopar 318 has been surprisingly popular with car magazines. Car Craft, Hot Rod, Mopar Muscle, and Popular Hot Rodding have all featured 318 build ups in their pages. The nice thing is the articles are available on line and make very informative reading. Be sure to check them out.
- 400 HP 318 build from Mopar Muscle Magazine
Very easy performance 318 build from Mopar Muscle Magazine.
- 406 HP from a properly built 318 by Hot Rod Magazine
I like this 318 the best of all the magazine build ups. They used the stock 318 bottom end and the "proper" heads for a 318, the high swirl "302" castings.
Blueprinting an Engine
Blueprinting an engine is the process of measuring, fitting, and balancing all the parts of an engine during the rebuilding. Blueprinting an engine improves its fuel economy, dependability, and most importantly its performance.
An engine is a complex piece of machinery, and explaining all the steps necessary to blueprint one is beyond the scope of this lens. In fact, it is a subject that could take up a whole book. As it turns out, there is one: Engine Blueprinting: Practical Methods for Racing and Rebuilding by Rick Voegelin. If you are interested in what it takes to blueprint an engine, I highly recommend this book.
What I like best is that it's written on a level easily readable by someone with even limited mechanical experience. Instead of just stating what should be done, it also explains the "why" and "how" of engine blueprinting. It is also very clear on what steps can be accomplished by the home mechanic and what steps are better left to a competent machine shop. There is a ton of good information in this book. I wore out my first copy and bought a second.
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.
Even More Mopar 318 Information
If you're into Mopar engines, you will like this book very, very much. It has over 30 articles reprinted from the pages of Hot Rod and Car Craft magazines. It covers small block, big block, and Hemi engines. It has two excellent articles on the 318. The first is very useful because they start out with a bone stock 318 and then add parts to it a step at a time to increase the performance. At each step they put it on a dyno so you can see the effect of that step on power output. Very educational. The second article related to the 318 is on how to stroke it to a 348. This is a very cool modification to the 318 that I'd like to try some day. It's even easier now since Scat makes a 3.58" stroke crank that drops into the 318 (as well as the 340) without having to grind down the main journals.
Are you a Mopar fan? Do you think the 318 is under-rated? Over-rated? Let me know what you think about the Chrysler 318 or small block Chryslers in general.