Boring out a 318

[CKH]

Okay here's my situation. I've got a 1972 Coronet with a 2(soon to be 4)-barrel 318. I'm hoping to get it bored out in the foreseeable future. What I was wondering was how much to bore. Right now it has the stock 3.91" bore X 3.31" stroke. I was also considering getting an aftermarket crankshaft. I'm very new to the whole cars thing so these are my questions: 1) How much is too much to bore out on a stock engine? 2) If not the bore job and only the new crank, how big of a crank? 3) If both, how big of both?

[JB400]

Machine shop will tell you how much to bore it out.  It could only need a honing, or it could need to be bored out 30 over.  It just depends on how far out of round the bores are or if there's any scratches.  60 over is generally the maximum, but it really depends on the block you have.  Some blocks can be bored more, some less.  Your machine shop will have to do a sonic test to find out how thick it is.

As far as crank goes, do you really need a new crank, or just wanting to step up to a forged crank or do a stroker upgrade?  If your wanting to do a stroker upgrade, it'd be easier to buy a 360 engine and build it up instead.

[greenpigs]

Yep, that's  0.030 or 0.060 of an inch just to clarify and a piece of notebook paper is about 0.003 of an inch thick for comparison.

[Cooter]

I have heard of a Mopar 4" stroker crank in a 318, but most of the time the 360 LA small block gets all the attention. As for how far to overbore it, only as far as you need to. Squaring up the cylinders for a good piston ring seal is the ONLY reason to overbore an engine. Contrary to popular belief, it is NOT to "Gain" any Cubic inch as mentioned in MANY songs, old wives tales, etc. Boring over cylinders to many times the MAXIMUM of .060 only nets you around 12 Cubic inches.


STROKING and engine with say a 4" Mopar stroker crank [Actual stroke measured from centerline of main bearing to centerline of connecting rod journal], many times yields a MUCH bigger Cubic inch displacement.

[Troy]

Sounds like a lot of work for a 318. I love the engines but, basically, I wouldn't do much more than clean it up and maybe add a 4bbl. I do have to say that on the only 318 I've ever had a 4bbl on I think I lost power. Those things are actually pretty impressive with a 2bbl - especially a larger one than stock. Mine were mostly 68-70 which were higher powered than the later versions but my 73 wasn't terrible. You can bore it way out and add a stroker crank but you can do the same thing with any of the bigger small block for the same money but with more improvement. The common stroker crank will give you 392 cubic inches but then you'll need head work, intake, exhaust, and (probably) other drive train upgrades. A stock 904 automatic transmission and 8 1/4" rear axle won't handle much more power than what you're already putting out.

It's much easier to start with a 360. You get 40 more cubic inches right off the bat and they are dirt cheap. If you spend a bit more and get a more modern 5.9 Magnum you get a roller cam and much better heads in the deal. The 360 has a different balance than a 318 so you'll need a weighted flex plate. Otherwise it's a direct bolt in. A Magnum will have a different weight yet and you'll have to swap the timing cover and accessories from your 318. If you stroke one of these you'll end up with a 408 but then you're putting out big block power and everything on the car has to be upgraded to match.

I left out the 340 just because they are typically more expensive just to get in the game. On the other hand, (if you get the right year) you'll have less in it than boring/stroking a 318 or 360 but still have a powerful motor. Plus, these things came stock in a 3rd gen Charger and look nearly identical to a 318. Sleeper!

Troy

[CKH]

Thanks everyone, really good stuff here. The reason I don't upgrade to a 360 or any other bigger engine is just because of money. I'm only 17 and don't have tons of spare cash, I was just mostly curious. Still, thanks. Any other suggestions (preferably not more than a few hundred bucks) which could yield more power, aside from the usual stuff?

[greenpigs]

  If it runs good and doesn't blow smoke I would go for bigger gears in the rear. The size depends on how much you drive it and if it will see a lot of highway use. I would go with a 3.23 as they should be your best bet for the price and gearing it way up like a 4.10 is not what that stock 318 will like long term. For $200-300 nothing else will match it for the price.

Check proven engine combo section here and pick parts for the 318 with something like a 5,000 redline, you want to build it for torque or low end.

Also I would build it on paper first before buying one single piece.

[Troy]

You can buy a running 360 for that much. You can't even think about a stroker crank (and installation) in your price range. You might be able to get a machine shop to bore it some but then you have to find oversize pistons to fit. More money. On my first 68 Charger I had headers and a BIG 2bbl carb on the stock 318. It was a blast! My 70 Challenger had a practically stock 318 and it was still more than enough for a teenager. The 351 in my 70 Mach 1 currently has a BIG (390cfm) 2bbl with stock exhaust manifolds on it and it's surprisingly fast. Yours will have a lot less power to start with than those because of the year but that's mainly due to the weak compression. With the gas crisis, insurance rates, and smog laws in the early 70s the simplest solution for car manufacturers was to "tune down" the engines. You biggest "bang for the buck" might just be having the heads milled or finding some pop up pistons. I'm sure there are a bunch. Headers are cheap as well. I've heard good things about the fit on the Summit brand and, if they aren't exact, they're cheap enough that beating on them with a hammer doesn't make you cringe. Again, it may be cheapest just to find an earlier 318 or 360 somewhere. Everyone (seemingly) wants a big block and practically give away the common small blocks (ie, NOT 340s).

I'm sure there are other small block threads here but I can't think of any off the top of my head.

Troy