BTW
as to the 4bbl thing:
the 318 has a bore and stroke that make one cylinder have a capacity to contain a maximum of ~652 cc or about 3/4 of a US quart. With the engine off and the piston at the bottom, at least two cylinders will have that much air in them, plus whatever fits in the chamber.
But as soon as the engine is idling, the closed throttle blades together with the rpm controlling the time-factor, there is no way for this much air to enter the cylinder.
Now, when you open the throttle blades as far as they go (WOT), only the rpm and upstream restrictions can limit how much air can actually pass thru the engine on every revolution.
If the heads are a restriction, or the cam events are, or the exhaust piles up; then all the carburation in the World, will not increase the power after the engine hits that wall.
The TORQUE curve is a good visual indicator of at what rpm your particular combo hits the wall.
Below is an example in which the wall is hit at 3200rpm. You can see the sharp drop-off in torque after that. The only reason that the engine continues to make additional power is by virtue of rpm. But by 4400rpm, it is completely choked and the power begins to fall.
Getting back to the one-cylinder capacity of 652cc; In one revolution, of a 4-stroke 8cylinder engine, 4 cylinders will attempt to be filled at WOT. So the maximum capacity is 4 X 652 cc=2608cc which converts to .09 cubic feet. At 4400rpm when this example hits the wall, that would be 4400 x.09=404.8 cubic feet. So your carb has to be that big. Or does it?
However, this engine stopped being able to ingest even close to the max, at 3200rpm. By 4400 it might be down to less than 70% so
70% x405cubic feet is 283cubic feet, and since we are talking revolutions per minute, this equates to 283cfm.
Back at 3200,say if this engine was able to fill 100% of its capacity, this would math to 100% x .09 x 3200=288cfm.Therefore, anything bigger than 288 cfm is just bragging rights...... on this example.
If you want this particular example to make more power, it will first need to be able to process more air; and that usually starts with a bigger cam. But, almost right away, the heads will become the restriction.
Therefore, in the search for absolute power, when starting with a stock low-rpm 318 engine, you need to cam it up AND get rid of the low-rpm heads.
Lets say you built it, and it torque peaks at 4000. Your new carb requirement at 100% is; 100% x .09 x 4000=360 cfm. With good heads, say the power peaks at 5300 and the VE ( Volumetric Efficiency) has only dropped to 82%. Now your carb requirement is; 82% x.09 x 5300=391cfm.
Now, your stock 2bbl might be rated at 250cfm. But this is based on it being measured on a lo-performance engine and uses a different measuring system. When your 250cfm 2bbl-rated number is converted to a 4bbl number, it becomes about 177cfm; so you want to get rid of that thing.
However. whatever 4bbl carb you put on there, your stock engine cannot pull hard enough on it to even tickle it's rating.
The simple conversion factor is 1.414; so
For example a 500cfm 4-bbl rated carb, rated as a 2bbl is a 500/1.414=707... and that means the front half is about 707/2=353 ........ which is already 353/288=1.226 or PLUS 22.6% bigger than your stock 318 can ever pull no matter what and
353/391=.903 or Short only 10% of what your heavily modified 318 can pull.
Now here's the clincher;
with 2.20 gears you can't hit 5300 until 64 mph in 2.74 low gear with 27" tires, and 10% convertor slip..
to restate it another way, and
IMO,
Installing a 4bbl on an otherwise stock 318, and with 2.20 gears, is a big fat waste of time and money. Your 4bbl will wail and moan and sound terrifying, but the stock 318 will only pull what the combo will allow, and with 2.20gears, this is way up in the roadspeed range.
Ok now what about with 3.91s? Well now yer talking; the big-cam/big-headed 318 with 3.91s and the 2.74 low gear will now hit 5300 at 36 mph oh yeah. And 5300 in 1.54 Second comes to .......64 mph about as perfect a combo as it gets.
So now, the 4bbl is gonna wake it up at favorable roadspeeds, BUT she still don't need as much as a 500; remember the math points to just 391 4-bbl rated cfm.
Having said all that;
You might have noticed that the 318 makes only .092 cubic feet per revolution, at 100%VE.. All calculations are based on that number, and on rpm.
There are three ways to make more absolute power 1) more rpm, and 2) more VE, and 3) a bigger displacement
The fastest way by far, is more displacement, which comes with the bonus having more inherent power at low-rpm, and in the midrange. The 360, for example has the potential to displace .104 cubic feet at 100% VE which is 13.22% more than a 318. So for the 360 to make the same absolute power as a 318, you can rev it lower,which means a smaller cam, smaller carb, smaller everything, and including a smaller rear gear. And the additional displacement means, that for the same take-off power, the 360 can use less stall. And the additional inherent midrange power means you can build it to a lower compression ratio . That's a lot of wiggle room.
I agree with Bud who said;
I am not against 318's nor against building up 318's - for they have their purpose. I just think for modifications needed, money spent and total HP achieved per buck spent, a 360 will get you further and a tad cheaper than what you can spend on a stock 318. Also, you can build your new engine while still driving your car - which is a big issue for some people.