Exhaust opinion
Truck has 30" tall tires at the moment but thinking on stepping down a size and moving the gears back to 3.55 for drive ability sake.
FWI;
3.91s and 30s is the same as 3.55s and 27.25s
With the former; 65=2848@zeroslip
With the latter 65=2846@ zeroslip
My 223* cam pulled double overdrive where 65=1588rpm (clutch-car) no problem. That would be the equivalent of a final-drive ratio of 1.97 (3.55x.71x.78) and 27" tires.
I did this with an A833od box plus a GVod behind it. Low gear was 3.09x3.55=10.97 about the same as the standard A833 where 2.66x4.10s (=10.91)
Your A904 and 2500stall is a nice combo. The TC acts like a super-low gear at zero mph, and automatically " upshifts" as rpm and roadspeed increase. And even at it's minimum ratio it is still close to our less than one gear size higher than the rear end would seem to indicate. That makes your 3-speed automatic act like at least a 4-speed and more like a five speed.
How is that possible you might ask?
Well the way the TC is built, it has an internal fluid multiplication feature that is approximately equivalent to 1.8 at zero mph. This turns your 3.91s to 7.04.
As the car begins to move out, this 1.8 ratio slips less and less, automatically, and eventually might couple to 1.1 or a little less .... and dare I repeat,it does this automatically.
So what does this mean to you?
At zero mph then your starter gear could be 1.8x2.45x3.91=
17.24. And whatever torque your engine makes at stall,2500 in this case, is gonna get multiplied by that amount. Say your engine made 290 ftlbs at 2500, then to the rear axles, this would be 17.24x290=5000 ftlbs.
Then as your vehicle accelerates, this 1.8 ratio begins to drop lower, while simultaneously your engine torque output is rising. So soon, say 3200rpm, your engine may be be peaking at 300 ftlbs, and the ratio may still be the same, so now you are at 4023 ftlbs. Go up another 1000 rpm and the torque may have fallen to 280, and the ratio to 1.5, so to the axles is now 4023.
Go up to 5200 now, and torque may be 240 and the ratio 1.4, so 3219 to the axles.
Now comes the shift into second (1.45 ratio) and the Rs fall to .59x5200=3070. Here your engine may be making 295 ftlbs, and the Tc ratio maybe 1.3, so to the axles is now 2174. Notice that is quite a hit from 3219. So now from this point on the TC ratio is gonna change only a little, and by 5200, again the torque might be 240 as before but the ratio may be down to 1.2, so to the axles is 1633.
Now into third/direct the Rs fall to .69x5200=3590rpm. Torque may be 290 and the ratio may be 1.1, and so to the axles might be 1247; another hard hit. But it's ok , 3590rpm with 30" tires is 75mph, and if you are a streeter, that is speeding in most places,lol.
If you had a manual trans, you would lose that Torque Multiplication feature, which is serious business. At zero mph, this is equivalent to one lower gear in the box or 80% more rear gear. By 75 mph, if the TC has dropped to 1.1, it is still equivalent to 4.30s.
So lets line up the overall ratios
17.24 at zero mph, diminishing to
13.41 at the top of first. Since this is a difference of .778,lets count this as
2 gears. then
7.37 at the bottom of second going to
6.80 at the top of 2nd and since this is only 7% difference,lets count this as
one gear. then
4.30 at the 2-3 shift,
one gear, and then
3.91 to cruise with,
the last gear
That's 5 gears by my reckoning.
Lets compare that to an A833 also running 3.91s. the box ratios are 2.66-1.92-1.40-1.00 so the rear axle ratios are
10.40
7.51
5.47
3.91
No matter how you slice it, the auto has at least one more gear.
Lets line them up to see how they compare;
17.24-13.41-7.09*-4.30-3.91 (* is averaged)
.......... 10.40-7.51-5.47-3.91
How nice is that
So whats the point?
Well, 5000rpm in second gear at 20% slip with 30s and 3.91s comes to about 65mph; that is a fairly nice place to be with that cam.
and 3.55s with 28s will be about 67mph, not much difference.
But
5000rpm in second gear, with 28s will be ~61mph with the 3.91s, or just about perfect. and
30s with 3.91s cruses 65=2850 at zero-slip, and up to 3130 as you roll into the throttle,at 10% slip.
So what you really need, IMO, is a loc-up ...... depending on just how much hiway traveling you do.
