I do not know.
But I'm always willing to guess;
A four inch bore, give or take a lol, makes enough room for typical valve sizes, and for them not to be shrouded. Any smaller and you soon have to give up either valve diameter, or inter-valve support, or they get to be shrouded.
Long strokes allow more power-stroke time, at cruise rpm, to extract energy.
An old 318 had a cam of 240/248/112. In at 2* advanced this was 130 degrees for compression and 122* for power.
Now, 122* for power extraction is a stinking long time in terms of crank rotation. By the the time the exhaust valve opens, and with an SCR of sub 8/1 , the pressure of those hot exhaust gasses are almost back to atmospheric pressure, having release all they had to give. This is great for fuel economy. Even for making torque.
But those same 122* of power-extraction, if you want to make power, steal a lotta time from the exhaust cycle, and so those are usually the first to get sacrificed./ A typical power stroke for making street-power will be close to or less than ~105*..
If you think about this;
When the piston gets to 122* ATDC, it is slowing down and is just 58* from the Bottom of it's travel. My guess is that in a 3.315 stroke engine, this is less that 1" from the bottom.
But say you had a 3.58 stroke engine, this same 122 degrees would put the piston further from the bottom..... Which means, you can run more exhaust duration, and still have a similar amount of power extraction clock-time. And if you think about that long enough;
you might get to thinking about comparing the fuel economy potential of two same engines except one with a 3.315 stroke and the other at 3.58.
But if you did think about that, rightaway you might think what if I put a one-size bigger cam into the 3.58 engine. With say 7* less power-stroke and those 7* given to the exhaust stroke.
Rightaway the 3.58 stroke would make more power and torque at low-rpm, by virtue of the greater cubic inches. But with the bigger cam, it will also make more midrange and hi-rpm power.
but with a similar amount of clock-time spent in the power-stroke while cruising, should make similar fuel-economy.
And if you think about this long enough, there's a good chance you will conclude to never again build a 3.315 stroke SBM, but if you did; it sure as chit would not be at 8.0 Scr, lol, Nor with a pizzazz 240/248/112 hydraulic cam.
From the decks down, there is nothing wrong with a 318LA. I mean spec-wise it is almost the same as a 340, save the overbore. And it is almost the same as a 360, save for a bit of of bore and stroke.
What the 318 has lacked since 1972 has always been compression.
People say it doesn't matter that much, and under racing conditions they are sorta right. Once the rpm is up, cylinder pressure, or lack of it, becomes minor compared to the operating rpm.
and, while cruising, it is again almost a non issue, at typically posted speed limits.
Where/when cylinder pressure matters is exactly where a streeter operates. Namely; on/off the throttle, banging back and forth between stall and shift-rpm in First gear, and especially when getting into second.
And it's not just the pressure I'm talking about. It's about the activity that happens in the intake manifold.
The higher your cylinder pressure is, by deduction, the smaller the total chamber volume has to be. And the smaller it is, the quicker it will respond to the falling piston on the intake stroke.... because..... a big chamber acts like an Air-spring; you can stretch it a long way before the plenum even knows it's there because it is busy listening to 3 other air-springs on every revolution. It be like, the plenum is saying" yeah yeah, I'll get there in a sec." Whereas with a small chamber, the falling piston puts a good pull on the plenum, demanding attention like, right NOW dammit!
This translates to an engine that is always ready to rock, just stab the pedal and hang on! As you can see, this has ZERO to do with the Compression stroke! Yet the only way to get this kind of throttle-response, Normally-Aspirated, is with a hi-compression ratio.
IDK anything about emissions, and pretty much don't care,
cuz IMO, it ain't cars that are a problem.
IDK anything about what factories think.
or IMO, even if they think at all. lol.
Oh almost forgot; B/S ratios;
273 is 3.63/3.315= 1.095, baseline
360 is 4.00/3.58= 1.117, this 1.02% bigger than 273
318 is 3.91/3.315= 1.179, this is 1.056 bigger than the 360
340 is 4.04/3.315= 1.219, this is 1.034 bigger than the 318
If a bore to stroke ratios have anything to do with fuel mileage,
a 360 should, when similarly equipped........ make BETTER fuel economy than any 318 ever built, and even rival a 273, the difference going to internal friction, which we can do something about. And you may know this; my 367 was once combo'd to exceed any factory 273 fuel economy spec ever published, for a car at 3650 pounds (me in it); I mean smashed it.
So there is that, .......... but it wasn't similarly equipped, lol.
