I agree with Darth.
That od-box already has a 3.09 low in it,and a .73overdrive. (3.09-1.67-1.00-.73od)
All you gotta do is marry that to the right rear gear for your use and driving style, and get the exhaust out of that head; and yur away.
To cruise at 65=2200, you could run 3.73s, and that would get you a starter gear of 11.53, which is quite a bit. So taking off will be a breeze.
The problem comes when you get into second gear. The Rs fall to 54%, so you gotta rev first out pretty good, if you want to pull hard in second.
Here's how they stack up. Lines 1 and 2 are autos and I grossed them up 5% to allow for the TC, just to make it a fair comparison. the asterisk denotes in loc-up
1) A998/
3.55; - 9.13-5.40-3.73-3.55; 65=2870*
2) A999/
3.23; - 9.29-5.22-3.39-3.23; 65=2740*
3) A833/
3.23; - 9.98-5.39-3.23-2.36; 65=1900
4) A833/
3.73; 11.52-6.23-3.73-2.72; 65=2200
5) A833/
4.10; 12.67-6.85-4.10-2.99; 65=2420
6) Commando/
3.55; 10.97-6.82-4.97-3.55; cruizes at 55=2430
7) Commando/
3.23;- 9.98-6.20-4.52-3.23; cruizes at 60=2410
Notice that the first three all have about the same second gear ratios. They're all gonna pull about the same, in the first two gears.
But look at the fourth line; first gear is 25% bigger/ second is 17% bigger/ and third doesn't really matter cuz now yur getting up in speed, and mostly cruising.
What those number mean is the bigger they are in each gear, the more powerful your slanty will feel in that gear.
But look at the cruising rpm!
If you wanted to you could run even more rear gear. But there will come a point that first gear feels just too low. That comes at around 13.29 with 4.30s,and the cruise goes up to 2530rpm
But check out the 4.10s, and compare that to the A999; plus 36% in first, plus 31% in second, and passing gear is plus 21% yet the cruise rpm is down 12% or 320rpm.
Yur gonna need a LSD with the 4.10s,lol.
So how would that compare to a 5.2Magnum?
IDK but here's a guess.
Say that 5.2 makes 240ftlbs at 1800, and the car currently has 2.76s in it . Lets do second gear only. Ok then;
240x1.67x2.76=
1106 ftlbs@31 mph into the axles
Next; say the slanty can muster 120ftlbs@1800 (I'm really guessing), and the 4.10/A 833 combo; and I get;
120x1.67x4.10=822 ft lbs, at 21mph. But by the time you get to 31 mph, the rpm will be 2640 and the torque will gave climbed to near peak, and I'll guess 161 ftlbs, lol, so the torque into the rear axles would be 161x1.67x4.10=
1102ftlbs@31mph. Ok I'll admit it; I might have fudged the numbers to prove the point; but if I did, it wasn't by much.
But, this match-up only occurs up to 31mph or so. As the speed increases the Magnum is gonna climb up the torque curve, while the slanty is soon gonna be sliding down it's curve.
Let's try passing-gear at 55mph; the Magnum in second gear,will be at ~3700, probably a bit past peak torque say 300 ft lbs(guessing again) so I get
300x1.67x2.76=1383 on-road ftlbs@55 mph. BTW; 300ftlbs@3700 is 211hp
And the slanty will be at 4700 in second, too high. So lets try third (direct) and I get 2810 rpm; so again close to peak torque. So lets call it 200 ftlbs, and I get
200x1.00x4.10=820 on-road ftlbs@55mph. BTW; 200ftlbs @2810rpm is 107hp
So, against the Magnum it is clearly a loser.
But let's compare it to itself with 2.76s. 55 will be 3150, sliding down the torque-curve now, so lets call it 190 ftlbs, and I get;
190x1.67x2.76=876 on-road ftlbs @55 mph.
Huh; 2.76s suck
And so does trying to pass in direct gear, even with 4.10z,lol.
My torque numbers may not be accurate, but as you can see the point is made even so.
So, altho I agree with Darth;
with a stock slanty you just can't expect miracles after 30 mph. However, for the buy-in, it ain't too bad. And you don't have to be a mechanic to swap a complete rear end. And if you pay a guy to install new gears, it's still is pretty do-able.
As for the Magnum swap;
Yur also gonna need mounts,a bellhouse/etc, and a new exhaust system, and I would recommend bigger T-bars. And I guess an intake and carb, plus fudging with the serpentine system, maybe some hoses and a fuel pump, and, on and on.
And don't forget;
if the crank is not drilled for the trans input shaft,then you gotta deal with that too.
And the serpentine waterpump turns backwards. So if you eliminate that stuff, you also get to buy a new pump and front cover.
Parting shot; if this is mostly a city car only; then the 3.09Commando box and 3.55s is awesome;
6) A833/3.55; 10.97-6.82-4.97-3.55; cruizes 55=2430rpm
Now you got a passing gear; 55will be 3400rpm in third, again sliding down the torque curve but now you horsepower on your side and that 4.97 gear is wickedly fast.
In fact; if you already have 3.23s(or better); that Commando box is hard to beat.
7) Commando/3.23; 9.98-6.20-4.52-3.23; cruizes at 65=2600
and passing gear is 3100 so just a tic over peak torque.
The buy-in for a Commando box is likely a lil less than the rear-end deal; plus it's a straight swap in. On a 4-post hoist I've taken mine down in 17 minutes; including the GVod and the front exhaust.
As always; Happy HotRodding
