I have been reluctant to the 360 idea because of the bad rep they get when it comes to MPG. My car is my daily driver and it set up with that in mind; 727, 8 3/4 with 2.94. No 750 DP, No 3.91s, not a racecar.
Yes people blame the 360 for poor mpgs, but it ain't exactly fair, cuz if you gear two same cars identically one with each engine, then they will get close to the same mpgs ........... because it takes a certain amount of power to push your way thru the atmosphere and it don't much matter if you have 5.2 or if 5.9 liters
The thing is; a 5.9 can run less gear, have the same performance as the 5.2, but turn 10 or even 20% less rpms at steady-state cruise and so reap about half those numbers in less fuel consumed.
The bigger thing is this; If you are taking the engine apart anyway, then you can cure the principal reason for poor mpg of either engine, by pumping up the pressure. There is absolutely no good reason to run the paltry 130 or less psi that these engines run.
But;
geared identically, and around town, driven the same, the bigger engine will give up economy most of the time.
BTW 360s love 2.94s as DDs
And if you are shooting for a 300RWhp, DD, AND fuel-economy, then in a 2.94 equipped DD, you don't actually want 300RWHP, cuz with a 318, this comes at such a high rpm that you can't get to it until say 5600rpm/nearly 60mph in first gear; and in the meantime, unless you have matched the compression to the cam, you will have a low-performance bottom end. and forget about economy with that big cam.
When you say 300RWHP, you are saying 350 crank hp, and in a DD with 2.94s. what you really should be saying is that you want mega-torque so you can at least burn some rubber; am I right? Well that don't take mega-power; that takes low-rpm torque. And that is something the 360/5.9 is good at so it's a natural. Especially the 5.9 because it already has a full point higher Scr.
In your case I vote 5.9, hands down.
You can run 2 sizes less cam,than in a 318, and/or you can run at least one smaller gear; and
even tho you won't have 300RWHP,
with 3.23s , in comparison, at 65mph or less, it will feel like 400 crank.
It's all in the numbers; torque and gears.
Suppose your 318 somewhere in it's power band, peaks at 318 ftlbs. and you run 2.94s,with a 2.45 low gear; ok that calculates to
318 x 2.45 x2.94=
2290 ftlbs into the axles.
Next, lets say you build a 360 to the same 1 ftlb per cube then,
360 x2.45 x2.94=
2593 ftlbs
2593 over 2290 shows the 360 to be 13.23% stronger, so the 318 would need at least that much more gear to hang with it as to first gear performance, which would be 3.33s. With the next closer Mopar gear, 3.23s,the teener looks like
318 x2.45 x3.23=
2516 .
Of course cruising 9.9% faster with the 318 is gonna cost you ~5% more fuel-economy.
Conversely; you could run 2.76s with the 360, so
360x2.45x2.76=2434, still 6% stronger than the 318/2.94s, but now having given up 6.5% rpm, she's looking like 3.2% better fuel economy.
Another trick you can do is use the A999 ratios with the 360 which has plenty of muscle to pull the wider splits, and for the same second gear, you can give up one rear gear size. Here is how this works ;
First, the 318 with 3.23s, then the 360/A999.
3.23 x 1.45=4.68 second gear, and
2.94 x 1.54=4.53 with the A999. Low gear will now be
7.93/A904, versus 8.06/A999 . But the hiway is
3.23 versus 2.94; advantage of 9% to the 2.94s so ~4.5% less fuel consumption
If you start adding up the advantages of smaller cams and less rear gears and the A999, it should quickly become apparent which way to go.
And besides all that, if you have to rebuild a 360, it costs less for hi-compression pistons, and you don't have to cut anything. Throw on a set of small-chamber alloy heads and you can run 11/1 easy enough,and pick up better economy in street driving as well.
Whatever you decide, I am not a 318 hater. The numbers speak for themselves.
