318 vs 340 Questions

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Running 318 has the advantage over rebuilding anything else.
Cam, headers and 4bbl on the 318 can get you close to 300 hp without tearing into the long block after that something like EQ's get you another 30 plus hp.

When it comes to short block biggest bore allows the most air flow there for the most hp so 340-4.04",360- 4",318-3.91 largest to smallest. When it comes to engine size the biggest engine in cubic inch will makes the most hp per rpm but the bigger bore can out rpm the smaller bores eclipsing the difference. Part 2 of this equation is gearing the bigger the engine less gear needed to equal the performance of the smaller engine (generally equal to the percentage of cid difference so a 3.23-418 would basically equal a 4.30-318 ). The best short block would be 340 block with the 3.58" plus stroke which would give you the most breathing and cid. Above 3.58" stroke you start compromising bore stroke and rod ratios if you care about such things but a 4" stroke requires a lot less rpm and gear which has its advantage on a street engine.

So if you dont mind running deeper gears build the 340' with the 3.31" stock stroke or 360's 3.58" stroke. If not drop a 4" stroke into it and if you want to save a bit of money start with a 360 block and sale yours but to me that doesn't make much sense since you already have it.

Depending what you are starting with I'd build a good foundation brake, suspension and good tires then gears and exhaust. Then cam, 4bbl like comp xe262h and performer with 650. While building the engine of your choice 340/360/408/416 etc.. you can sale the running semi build 318 before the swap.
 
Nothing wrong with a stroked teener for the street. That long stroke will make keeping the Dcr up,easy. Maybe Too easy!

Even a 3.79 stroker can make 370 cubes. That's a whopping kick in the butt. That's 16%.
And a 4incher can make over 390cubes. That's about 23%
A 367 is 15.4%
 
If I were going to sell the 340 I'd buy a 360 and a stroker kit.
 
yabut he already owns the teener, and the difference from 390 to 408 is, for a street car , um, well IDK..........
But to support either he's looking at so many other things, it's like; where to start?
 
One thing to keep in mind is, if fuel mileage in part of your goal, the smaller bore on the 318 will really win out. Even a stroked 318 will get better mileage than a stock stroke 360 if it's a mild build, because of the smaller bore.

If your power goals are no more than around 350 HP then I would stick with the 318 all day long.
 
Getting back more in line with the OP as I didn't see any mention of strokers or fuel mileage. If the performance of a 318 tickles your fancy by all means go with it, but if you want more rebuild the 340. The parts cost the same and the 340 will the 318 in every way. The 340s larger bore allows for larger valves but more importantly better cylinder filling.

Apparently I have a different take on performance than most on here. IMO when it comes to CI more is better. I've had 273 commandos, 318s, 340,s, 360s and can tell you this, 273s and 318 are anemic compared to 340s and 360s. There is a reason Chrysler NEVER put a 318 4bbl in a muscle car.

As far as fuel mileage is goes, if you are concerned with it buy a Prius. I built a bunch of small and big blocks over the years and never considered fuel mileage. My goal has always been to make them run hard for what they were. I guess I'm one of the dinosaurs that likes to build and drive fast cars so my perspective is skewed.

Good luck guys.
 
The OP asked if it's better to put hop up parts on a running 318 or spend the money rebuilding the 340?

He didn't say how power he's looking but I assume just a healthy ride. A 4bbl, cam kit, duals and gears can go a long way for not much money compared to a full rebuild.

And save the 340 for a rebuild down the road. And why dump a 340 for a 360 ? He's already got it. You can buy stroker 3.58" or 4" cranks fairly reasonable.
 
justinp61 said:
I've had two 72 340s, both had forged cranks. I've also seen cast cranks in late production cars.
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I've got a '71 build (early too, can't remember date) '72 340 with a cast crank and external balance. It's before the year's end of '71 for sure.

There was a minor change in HP for the '71 year with the switch to 2.02 J heads and IIRC the '71s got TQs. '72s had TQs, 1.88 J heads like the 360s and low CR pistons- most likely gonna see external balance on a stock motor. '73s got 587 heads with 1.88s and low CR pistons with external balance.

So then the final part- which to build. Ask yourself how sure you're gonna keep the car. If it's a given, doesn't matter- save the money and build a 318 or 360. If there's a pretty good chance you may let it go down the road- do the 340. A 340's worth a lot more to a car's value than the motor will sell for.

justinp61 said:
As far as fuel mileage is goes, if you are concerned with it buy a Prius. I built a bunch of small and big blocks over the years and never considered fuel mileage. My goal has always been to make them run hard for what they were. I guess I'm one of the dinosaurs that likes to build and drive fast cars so my perspective is skewed.
Click to expand...
Lots of minor changes can really change a car's fuel mileage. Sure, it's easy to say you shouldn't be concerned about fuel mileage, but the same 12mpg+ difference you're referring to between a boring econobox and a RWD V8 can be lost in a build.

Sure, it ain't gonna get fuel mileage if it's really built to run no matter if it's a 3G, SB, or BB- but in between there's a lot of compromises that can make a difference. A 3spd auto with steep gears rarely will.

AJ/FormS said:
yabut he already owns the teener, and the difference from 390 to 408 is, for a street car , um, well IDK..........
But to support either he's looking at so many other things, it's like; where to start?
Click to expand...
And that's why I think buying the cheapest, best motor is a sound idea. Which whenever I see is usually a Mag 318. The old teens never came with very good top end parts and that was their big downfall.
 
phase #1: Tune 318 aka cam headers and small 4 brl. If your happy with performance grate, sell 340.....if not............
phase #2: 340 stroked to a 410, moderate cam good 3 or 4 angle valve job on 340 head.......your 340 didn't come with head right?

Ok scratch valve job! buy some RHS Heads with 2.02s
 
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