Is this a bad idea?
Thank you all for your input! Sound like there are a lot of mixed reviews on this subject.
My dart runs at 2350 rpm at 55 and 2780 at 65
verses a little over 2000 rpm at 65 with the OD tranny (calculated). I have 25.2" tires and 3.21 rear on the beast. I'm not sure what the power range is for the 318 4 barrel. Maybe 2000 rpm will lug the engine on a hill at 65 mph.
Both of your first examples points to 3.21s
This is a common gear and very good general gear for a 318.
Here are the ratios of your various Mopar transmissions;
2.95-1.83-1.00 A903-V8
3.22-1.84-1.00 A903-slanty
3.18-1.83-1.00 A250
3.08-1.70-1.00 A230-slanty
3.09-1.67-1.00-.73 A833od
2.55-1.49-1.00 A230V-8
As you can see;
all of them except one have very similar starter gears, and
all have a 1.00 direct.
The difference among these is mostly where the Second gear is positioned; I lined them up with the Second gears progressing from slanty-friendly to V8-friendly.
And also as you can see, the A833 od is practically identical to the A230/slanty but with one additional gear.
Now, in my 68 Barracuda, I have run that od box behind a 1973 smoggerteen, with the only non-stock parts being a big TQ, and headers/free-flowing exhaust. The long block was stock but freshened. AND
I ran it with 2.76s, 3.23s, 3.55s, 3.91s, 4.30s and 4.88s..... cuz with a GVod sometimes behind it, you can run anything.
The 140 psi 318 pulled them all, and complained about nothing.
I really liked the 2.76s, and stuffing it into too high a gear, then flooring it and grooving to the roar of the mighty Thermoquad. So as to cruise-power, the 318 don't much care;
With 4.30s x.73od x.78 GVod, the final drive comes to 2.45s ; and 65mph with 25.2 tires, that would come to 2120 at zero-slip, and a good 318 near to sealevel, can easily handle that. With your 3.21s; 65mph would be 2030rpm(in overdrive)/ no problem.
I ran it with 2.76 and the .73 od, for 65=1750/no problem. The starter is 3.21 x3.09=9.92/ no problem .
If you want to learn where your 318 likes to run; put a vacuum gauge on it, and put it into neutral and slowly rev it up until the gauge plateaus. The lowest rpm, that the gauge reads the highest, should be your
minimum cruise rpm.
Now, while it's there, start advancing the ignition, keeping the rpm always the same. Keep pulling in more timing until the rpm no longer increases. Now read the timing at that rpm. Finally subtract 3 degrees for load compensation, and that is your proper timing at that cruise-rpm.
Now; GOOD LUCK building a timing curve to meet that need. But if you can, you will get fantastic point to point fuel mileage.
Finally, the short-tail A833od can be had with two different mainshafts; Either the 904 sized one or the 727 sized one. So when you go shopping, make sure you get one to fit your driveshaft, so you don't get an added surprise cost. It's also best to get the matching bellhouse, TO fork, and shift levers. Not to mention you will need a H-pattern shifter with reverse on it. The Z-bar may need to be modified.
Don't forget to put oil in it.lol
BTW-1
When guys talk about efficiency, we are talking about stock timing at low rpm.
Your 318 if still the Hi-compression variant, might like 48 degrees or more of cruise timing at 2400rpm
A lo-compression 318 might like 56 degrees
Those numbers are impossible to get with the factory parts. You'd be lucky to see 35*@2400.
When you cruise at ever slower rpms, the timing gets ever harder to find. By 2000 you cannot still get there no how, with the factory type parts, you will need a stand-alone timing-box, to make up the difference.
One combo I tried cruised at 65= less than 1600rpm/ 85=2100, and made 4-cylinder EFI type fuel-mileage...... because I gave her the timing she craved.
BTW-2
If you do not or cannot give your 318 the timing she wants or needs, your fuel-economy can easily go backwards when cruising at a lower rpm. The factory figured out how to make 2.45s work, and eventually even 2.20s. I ran the equivalent of 1.97s, with a clutch! Ok I'll admit I cheated lol, using a double overdrive with 3.55s. the starter gear was still 10.97/ funtastic, off the line, lol.