Gabe Tapia via CarCraft.com: I'm in the process of planning how to rebuild my small-block 318ci Mopar engine. It is completely stock except for headers and mufflers I added. What I am planning to do with it is to make it a daily driver/street/strip engine making around 400 hp. I plan on using 360 cylinder heads with 1.88/1.60-inch intake and exhaust valves, heavy-duty valves and springs, and 1.5:1 roller rockers. That's all I have planned so far for my engine.
I was wondering if you could help me out with my plans? I want to know what kind of cam, pistons, and carb, to use. Can I still use my stock crank? How do I measure the stall speed of a converter and do I need one? What rear gear ratio should I run?
Jeff Smith: Wow, we've got a lot to talk about. We'll try to reel you in, Gabe. First of all, let's start with 400 hp from a 318ci engine. Simply dividing 400 hp by 318 ci equals 1.25 hp per cubic inch. The problem is that you'll be lucky to make 1.1 hp/ci (which is 350 hp) from stock iron heads, even if they are from a 360. In comparison, a larger displacement 360 at 1.1 hp/ci is right there at 396 hp. It's an old saw but no less true that displacement is the easiest way to make power. If we can talk you into starting with a 360ci short-block, it'll make more torque and horsepower. But assuming you want to build the 318 because it's easier to bolt in, that's what we'll deal with here.
You can certainly make more than 350 hp with your 318, but that means you have to put a longer-duration camshaft and more compression in the engine and spin it much faster to get there. This also makes the engine less streetable, murders the fuel mileage, reduces its durability, and generally makes for a miserable engine to drive on the street. Since you said this is a daily-driver engine, our parts selection will need to be somewhat more conservative and also focused on making this affordable. This may not be romantic, since this means no talk about mechanical roller cams, aluminum cylinder heads, and 13:1 compression ratios, but your street engine will be much happier without all that Rickey Racer stuff.
A large cylinder bore helps airflow, but your 318 is somewhat limited with a 3.91-inch bore compared with a 327 Chevy or even a smaller 302 Ford, which both benefit from a 4.00-inch bore. The 0.090-inch difference will affect airflow by shrouding the intake valve. Your choice of the 360 head is good, since it offers much larger valves at 1.88/1.60 than the stock 318 1.78/1.50 diameters and will flow better. But keep in mind that the smaller bore will hinder flow somewhat. The 360 head's chamber volume is also the same at 70 cc, so there is no loss of compression. Yes, you can keep your stock crank and rods. Have a good machine shop grind the crank and rebuild the rods with ARP rod bolts.
We'd suggest aiming for a compression ratio of around 9.0 to 9.5:1 so you can run pump gas. There's not a ton of selection in stock replacement cast or forged pistons. The attraction for cast pistons is they are inexpensive, and if you don't plan on engine speeds above 6,000 rpm, these will work fine. PAW sells a stock replacement cast 318 piston and a moly ring set for less than $170. That's hard to beat. Forgings will cost more but are stronger. Speed-Pro offers a stock replacement forging for around $400 (PN L2329F 30) from Summit Racing, but it can only muster 8.25:1 compression with a 68cc chamber; this will hurt power as well as mileage. If you can mill the heads enough to reduce the chamber volume to 60 cc, this will bring the compression up to 9:1.