hell no, you already have the 340!
If you had an 727 based overdrive, and 3.91s this gets you a first gear Torque multiplication of
2.45 x 3.91=9.58, and a cruiser gear of
1.00 x3.91 x.69=2.70,
and 65mph=2185rpm
If you had a 360; to get 65mph=2234rpm, about the same cruise-rpm, would require a rear gear of 2.76 and so first would be 2.45 x 2.76 =6.76
Lemmee tell you this:
With the same stall, your
360 with 6.76 will be a pitiful dog compared to the
340 with 9.58
Ok, but, you say, you don't care about hiway rpm.
Ok then, let's say you build your 340 to put out a modest 300ftlbs at 2800@WOT
Thus the 340 will put down ;
300 x 9.58=2874 ftlbs into the rear axle at 2800rpm, at WOT; right on the starting line. and the 360, to equal that, will need;
2874/6.76 =425 ftlbs. Is this doable? Yes..... but not at 2800 rpm; it will need either more stall or more gear, so again the 360/A727/2.76 combo. is a loser, to the 340/A518/3.91s.
I can manipulate the numbers all day long, to make either combo do what you want, when equipped with the 4-speed overdrive.
Ok then lets say the 360, set up the same, will get you an extra 30 ftlbs at 2800, then;
2874/330= an 8.70 first Roadgear, and 8.70/2.45=3.55s. So now they both start out at an even-steven performance capability at Zero mph.
But the 360/3.55s will cruise at 65=2873 @zero-slip, say pushing up to 3000 on the tach without a Lock-Up...... so again, the 360 loses.
The point here is that
the stinking 4-speed auto, makes the combo.
Why not then just install the overdrive behind the 360?
Well sure but;
1) that's more money? and
2) what would be your plans for the 340? and
3) in an A-body, that 9.58 starter gear is ridiculously huge for a 360.
The 3.91s are way overkill. But to run less, the 360, with a bit of a cam, is rapidly gonna lose fuel efficency at below 2200rpm, due to not being able to give it enough ignition timing. Like with 3.23s for example, 65=1800, and the First Roadgear is 3.23x 2.45=7.91. Normally this is acceptable with a modest cam, you just stall it up until it performs adequately at take-off. To get to 2873ftlbs, as before, requires;
2873/7.91=363 ftlbs so that is quite doable with a 360 .
What I'm saying, I guess, is that the 4-speed/overdrive makes the impossible, possible.
Now, let's talk about making your 340 a serious tire fryer.........
Or, we can talk about making a 360/overdrive into a stupid tire-frying combo, yet she may get fuel-mileage into the mid 20s or more. This doing of two things, with the same combo, is something that the 340 may not be able to keep up with ............
BTW
If you have a Dart, Scamp, Valiant etc, with the small tubs in the back that max out at 255 sized tires, the 360 is doubly overkill, cuz with a starter gear near 10/1 the tires will just go up in smoke. The 318 is about the right size for this A-Body ....... but the 340 makes it just that much more fun.
A fun streeter starts at a weight to power ratio of somewhere between 11 or 10 to one. So then a 318 is good to 3180 pounds, a 340 to 3400, and a 360 to 3600 pounds. If /when the ratio goes higher than ~10/1 you have to start making compromises, with either gears, stall, or cylinder pressure.
And below 9/1, is just bragging rights as to how often you gotta replace the rear tires.
The 67>69 Darts are about 3200 pounds loaded up with a V8/auto/8.75 power everything and A/C; therefore, with the driver, say a total of ~3400 pounds, this is right on 10/1 with a 340.
The power/torque of the 340 (or the 360) can be shifted down or up, as may be necessary to satisfy your need for either acceleration, or towing, or hiway fuel-efficiency. But the long-stroke 360 does not have to compromise quite as much, and has the potential of being a lil more relaxed in the doing.
Happy HotRodding.
