What gears with OD A-833?

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MOPARJ

MOPARJ

What can I upgrade now?
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Running an overdrive A-833 in my Duster with warm 318. There is a 3.21 sure grip setup in the 8.25 rear with 255/60/15 tires. The first gear is pretty decent in the trans and pulls nice. Cruises at 2000 rpm at 72mph. Should I move up to 3.91s or stay where I am at? I do plenty of highway driving in the LA area to cruise nights and what not. Not unhappy with performance, just thinking if I should change things up or not to get a little more acceleration out of it.
 
I would go with 4.10 or even 4.30.

With overdrive, your final drive will be 2.99 and 3.13 respectively. Pretty conservative still.
 
Leave it. or up to 3.73s
I used 25.5 tires for all calcs
Teeners like a starter gear of 10 to 11.5, and a second of about 7, and any third and a fourth of around 2.5, to cruise at sub 2000rpm.
So how did I come up with those numbers? Well second is the workhorse gear. It needs to get you 30mph at a minimun of 2800rpm for brisk acceleration in a performance car. With 25.5 tires that demands a ratio of 7.07 Then third is used for just two things; cruising and going from 2nd to 4th. Fourth is the cruiser gear, and I like sub 2000 at the least, 1800 is better.Back to the starter gear; If this gets much more than 12 then I feel kindof silly cuz you drive like 100 ft and then hit second, making second to all the work from then on. The teener will want not want to pull from much less than 1800 in second,so first has got to hit at least 20mph.And anything more than 2800 in first,sounds like a Mustang, so that puts the starter ratio at 10.6
So the targets are;....... 10.60-7.07-any-2.54
And you currently have 9.92-5.36-3.21-2.28...65 = 1956
with 3.55s it would be.......10.97-5.93-3.55-2.52...65 = 2162
and 3.73s would make......11.53-6.23-3.73-2.65.....65 = 2274
and 3.91s are...................12.08-6.53-3.91-2.78.....65 =2385

So the snappy second is with 3.91s, but it has a terrible starter gear
The nice starter gear is 3.55s, but 2nd is a teensy bit lazy.But 4th is excellent.
So that leaves the 3.73s as a kindof default gear.

FWIW
I ran this tranny with both 3.55s and 3.91s. My lazy L/C-73 teener liked the 6.53 second for pulling it's 3650 pounds around. But I imagine a stout teener will pull the 5.93 second pretty good, in a lighter car like a Dart/Duster.
 
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For highway driving I didn't like my 4.10's even w/ the o/d. My tire was a bit shorter than yours though @ 205-60-15. It really got to droning and was uncomfortable at 65 mph for any length of time.

Now I have 2.90's (w,225-60's) and I believe I've gone too far in the other direction. First gear isn't bad but it falls on its face in second.

I need to work out what 65-70 mph cruise rpm will be with 3.23, 3.55 and 3.73 respectively, and make a choice. Like you I don't want highway cruise to suffer too much.
 
As already pointed out, the 3.09 1st gear in the OD trans will make a very short starting gear with anything over 4.10 rear gears.
Something else to consider is your driveshaft RPM. With deep rear gears, it may exceed the critical RPM for a stock driveshaft. You may need a 3" or 3.5" shaft...
 
I have had stock drive shafts to 6800 racing and constant cruise at 4K.
 
I'm in the L.A. area too, so I know our freeways. If there's no traffic and you're not doing at least 70 to 75, then you're just pissing off everyone behind you.

Seems like cruising at 2 grand at 72 mph is pretty ideal. I wish I could do that, but currently I have a 4.10 gear with a non-OD 4 speed. I would love to have an OD, but I'm looking at changing my rear end to a 8 3/4 with a 3.23 instead. Like you, I currently have a 8 1/4 rear end.

Anyway.... If you like how it drives and how it cruises, then why spend the money? Just stay with what you have. But if you really want more "oomph" then I'd say go no bigger than a 3.73 or 3.91 gear.

Here's a fun gear calculator for you. You can plug every thing in such as tire size, transmission gear ratio's and of course rear end gear ratios. Check it out here: RPM & Speed & Gear Ratio Visual Calculator
 
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that rear gear is not bad , i would swap out to the earlier trans gear with 3.09 first , and one to one 4th , like the 74-76 318 one . everything should swap over , and you can go back easier then a r/e gear change .
 
I was able to look at a trans, rear axle calculator that was on bigblockdart.com

I calculated my A833 O/D setup. In O/D gear, with 27" tall tires and 3.73 gears at 73.7 mph the engine will be turning happily at 2500 rpm.

The stock mopar O/D is a .7 overdrive as opposed to a 1 to 1. If you went with 4.10s or 4.30s as Rusty suggests it may be a lot of fun around town, but may not be on the engine if you decide to take a trip with it on the freeway for any length of time. Otherwise go with the 4.30s if you decide to take a long trip and want your engine running close to 3k for several hours just to keep up with traffic.

These numbers are very close approximations to what your gonna get with a 27" tall tire which is approx what you have 255-60R15 and you O/D in 4th gear

3.73 + 73.7 mph = 2500 rpm
3.90 + 73.2 mph = 2600 rpm
4.10 + 75.0 mph = 2800 rpm
4.30 + 74.2 mph = 2900 rpm
 
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I don't know about that.
Stock O.D. ratio is .67, (or so since auto/manual & year of trans can change this ratio) multiply by the gear ratio to get the final drive ratio feel. We get....

4.10 X .67 = 2.74
4.30 X .67 = 2.88

Even with the allowance of converter slippage ....
Manual trans are a direct drive, so no slippage ....
These make excellent Hwy. gears.
The manuals 3.09 (IIRC) is a bit deep. Nothing a larger tire can't help with.
 
Seems to be some confusion on all the transmission gear ratio. 833OD 4th is .73 (cars) or .71 (trucks)

All 833 ratio's
"thru midyear 1966 A-body 3.09:1 - 1.92:1 - 1.40:1 - 1:1 = B/RB and LA engines. (These all used a 4.35" IBR.)

1964 thru 1970 B/RB engine 2.66:1 - 1.91:1 - 1.39:1 - 1:1 = (The "close ratio" V8 gearbox)

1970 T/A & AAR 340s 2.47:1 - 1.77:1 - 1.34:1 - 1:1, = 1970 thru 1974 high performance
(A 2.44 first gear was also produced in this time period and some units had 1.91:1 second gears.)

1975 thru 1987 Overdrive 3.09:1 - 1.67:1 - 1:1 - 0.73:1 (Trucks used a 0.71:1 fourth "

Info from http://www.slantsix.org/articles/4-speeds/ODA833fourspeed1.htm
 
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Had an 833 O.D. 3.91 in a truck and always felt like I needed one more gear when in 4th.

Switched to a 3.55 and was perfect for me.
 
Thanks for clearing that up. I listed the auto overdrive ratio.

4.10 X .73 = 2.99
4.30 X .73 = 3.139

Still not bad.
 
The tire diameter makes a big difference too. The numbers i quoted were using a 27" tall tire. 275 60 R 15.

If you do the same calculation using a 225 60 R 15 , a 25" tall tire the numbers are off by as much as 1000 rpm for the approx same mph.

Once your sure of the diameter of the tire your going to run, or have to use diameter wise then figure your rear gear.
 
...but remember the lower you go over about 3.55...the more useless that 3.09 first becomes.

I couldn't imagine a 3.91 with a 3.09 first spending any time at all in first.
 
YY1 said:
...but remember the lower you go over about 3.55...the more useless that 3.09 first becomes.

I couldn't imagine a 3.91 with a 3.09 first spending any time at all in first.
Click to expand...

I have a 3.09 in the 833 in my Duster with 3.55's and 26" tall tires. First is a little short but it's not terrible, it's still not as low as I would like to have when I'm stuck in traffic and have to work the clutch to crawl along. But it's a little lower than I'd like to have when I get to rip freely through the gears. With the cam I've got in my 340 though I need a fairly steep first gear as it's not making a ton of power below 2,500 rpm (which is why I got one with a 3.09 first anyway).

Personally I think the 3.73 would be the best of everything. First would be a little short but manageable, and 4th would still be good on the freeway. Of course it's all a compromise.
 
if only that second was just a little steeper........
 
Thinking same thing on my 72 duster is currently 318 3 speed but plan swap 340 and od and 3.55 275/60/14 tires?
 
The Borg Warner T18 in my Ford F250 has a first gear ratio of 6.72:1 with a 3.73 rear gear. I use first gear every time I pull of. Not in it for long of course, but it sure helps get it going.
 
LOL, even more so when it is loaded up real good!
 
just trying to get a feel for tubed cars , never ran one , street or strip . but i've only had one of this series cuda's one 67 cream puff fb , this would be my first 69 . had a few 69 darts , still got one 69 gts stocker , 64 barracuda 273 4spd car .
 
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