Whoa; I just asked a question.
Can your slanty pull the gearsplits?
The 6-cylinder A903 came in two flavors;
2.95-1.83-1.00; splits of .62/.55 the usual unit;and
3.22-1.84-1.00; splits of .57/.54, compared to
3.09-1.67-1.00; splits of .54/.60 for the od
these are all VERY wide ratio boxes and have to be, to cover the useable rpm range.
As you can see, the A903 has a Wide 1-2 and a shorter, but still very wide, 2-3, and
the od box has a wider-than-A903, 1-2 and a shorter-than A903, 2-3. You can see it in the splits
Op currently has the Commando-box;
3.09-1.92-1.40-1.00 splits of .62/.73/.71 This has a deep low to get out of the hole, followed by 3 close ratios. From first to third is .45, which Op finds doable with 3.55 gears.
Ok so lets check it out as a final drive ratios.
with 3.55s we get 10.97-6.82-
4.97-3.55, and the jump from 10.97 to 4.97 is acceptable to him when being lazy. I currently have this same combo. I like it a lot.
Ok so with a 4.97 third gear; 1500 rpm is 23.25 mph and if his slanty can pull that gear at that rpm, at anything but very small throttle opening without detonating would be a miracle. But lets just say it does it. To get to 23.25mph in first gear would require an rpm of 3525. So what is Op saying? Is he reving first to 3525 to drop into third at 1500 so the dang car will continue pulling? IDK.
But what's gonna happen with the od box?
Still with 3.55s the gears are; 10.97-5.93-3.55-2.59 ..Now the 1-3 split is .32, and 1500 in 3.55 gear is 29.5 mph, (more than 6 mph higher so a longer pull), and to get there in first will require reving to 4445 rpm. So that is never gonna play out in the real world, except at WOT.
But look at the 1-2 shift; 5.93 is not very far removed from 4.97.. So lets take it up to 23.25mph as before.
this will be 2273 in second, and 3500 in first. Ok that's all right, except this is now a mandatory shift. There is no longer an in-between gear.
The split with any rear gear will remain the same when calculated as percent. The only thing that changing the rear gear does is change at what roadspeed the shifts occur.
Lets look at this a lil differently; With a 256*, I doubt that OP cares much about fuel mileage. And with the od trans and no other changes, I can almost guarantee the mileage will not change worth spit, and if it does, it will just be worse. So lets not even go there.
Lets say OP just wants Rapid Transit to 60/65mph, and that is the reason for the 3.91s. Lets set this up: first the 3.55s and the Commando trans.
10.97-
6.83-4.97-3.55, then the od and 3.91s;
12.08-
6.53-3.91-2.85
Ok so look at it; second gear is practically identical, but third gear is waaaaay out to the right, and first is waaay out to the left. Forcing you to rev waaay higher to be able to drop into the next gear at a reasonable pulling rpm. In other words the ET in that contest is gonna
increase with the 3+1, because ..... the average power delivered to the road will be less. For low ET, this od-trans does the opposite of what the slanty needs.
Ok
The 256 cam makes peak power around 4700, so lets call it a 5000 shift and go thru the gears.
First the current set up;Commando and 3.55s
First to 5000@
33mph,at the shift dropping to
3100 in 2nd, then pulling to 5000@
53.5mph. Going into third the Rs drop to
3645, and then 5000=73 mph. 65 will be 4430, a lil low for a 4700rpm peak, but all in all a good pull.
Next, the od box and 3.91s.
First to 5000@
30mph,dropping to
2700 into 2nd.Then pulling to 5000@
55.8mph.Going into third (direct) the Rs drop to
2995, then 5000=93mph; 65 will be5820 in second/3485 in direct; NOT looking good.
Furthermore; check out from what rpm the od box is forced to pull from, compared to the Commando box.
The 1-2 shift is down 400rpm; but the 2-3 is worse at 650rpm.
In a million years, for a performance slanty combo, I would not swap that Commando box out, for the od box. Not with any rear gear.
For all these calculations I used a 77inch roll-out, which is a 24.5" tire.
IMO, the Commando box is the best 4-speed for this combo, for rapid transit. Which is why I have one, after testing every other 4-speed box Mopar gave us in the 60s and 70s, and 80s.
Yes, I did run that od box, and besides breaking three of them, Even my 367cuber did not like those splits, even with 185 psi Cylinder pressure! So much so that I bought a GVod to split those bad-boy ratios. In the end, I found full-time splitting too much of a PITA, and swapped that od box out, in favor of the Commando box. Best move I ever made with my combo.
Now for a grocery getter or a cruiser, I would give a different answer. But the Howards 256 would likely have to go. It's great cam for performance, but has a narrow powerband, not very well suited to the wide overdrive ratios. But is very well suited to the Commando box.
On a 5000 shift,
the Commando has a powerband requirement of ~1400 rpm.
the od box is 2300 on the 1-2, and 2000 on the 2-3.
The wider splits are also harder for the synchronizers to match speeds, read slower.How much slower? Just enough to notice and be aggravating, in my combo, coming from the 2.66 standard box. I fixed that with the Splitter.
