Current Transmission: 1982 C6 k code

Other Transmission: 1985 AOD T code

The rear end on my truck is a 13 code- which is a 9" 2.75 ratio, and the AOD is coming out of a 19 code rear end- which is an 8.8" 3.55 ratio. What kind of effect will this have on the driving experience?

Is an AOD a C6 with another gear... or the gears maybe a little different?

Will I run into linkage problems?

I will also get the shift indicator assembly out of the instrument cluster.
Is there any other changes/problems?

I don't want my truck to have a hard time getting going. I know I am going to have to do much homework. Is this even a good idea?

 

I've been looking into the same thing. Depending on your truck you have to find a way to attach the TV cable, which controls an AOD transmission. Without it you can burn up the tranny .

The gear ratios for the aod are as follows:
First: 2.40:1
Second: 1.47:1
Third: 1.00:1
Overdrive: 0.67:1
Reverse: 2.00:1

The c6s'
First: 2.46:1
Second: 1.46:1
Third: 1.00:1
Reverse: 2.20:1 (this may be wrong)

This website here enables you to compare speeds and such of two transmissions. Best way for you to see your speed at a given rpm

 

Effect should be lower rpm/slightly better economy in 3rd gear than the C6, because of eliminating converter slippage; but OD will probably be too high geared to realistically use with the 2.75 axle.

Ratios are effectively the same as the C6 (except OD obviously).....otherwise there is no connection between the two transmissions.
Be aware that the AOD is not anywhere near as strong as your C6.........but it does have the economy advantage of direct drive (no converter slippage) in 3rd & OD.

The truck's shifter linkages will all work. The selector levers on the side of the transmissions can vary in length, so may need altering for your shifter & shift indicator, to travel correctly.

The AOD wants all the cooling you can give it, so I would get the auxilliary cooler & lines if the donor truck has one.

I would also get the drive shaft so you have it to use or adapt.

Some flexplates will interchange between AOD's & C6's - some won't. If you use the donor truck's flexplate, the balance also needs to match your engine.

The NSS & reverse switches/plugs are different on the two transmissions, so some adapting is needed.

This swap (in both directions) is common, so a search will bring up a lot of info for you.

 

I also have taller tires (which scares me).

What I don't want to happen is, be driving and hit a slight incline and the transmission constantly be kicking in and out of od.
I am assuming very very low rpm's can be harmful/inefficient?

Also, the transmission is not exactly coming out of a "donor truck", we are swapping. I consider my truck more of a show truck, and the owner of the other truck wants the proven strong C6 for drag racing.

I guess I won't know 100% until I do it.
I guess I was kind of hoping for a: yes, good idea- you'll get much better economy or no, that's dumb why don't you do this...

Thanks for y'all's help

 

You can always swap the rearend out later if you don't like it. But yes, I believe you will have it kicking in and out of overdrive with that 2.75 gear. If that aggravates you, in certain situations you can just pull the lever down out of overdrive.

 

I thought about that, but the shift indicator assembly only says
1,2,OD.
No 3. We're they all like that?
Thank you

 

Although there's not a '3' position showing, it is a 4 speed trans. The later shifter showed P R N (D) D 1. The earlier indicators read slightly differently, but the function was the same.

Selecting D, the trans will only shift as high as 3rd, which prevents repeat up/down shifting between 3rd & 4th (OD), & damage to the OD band.....one weak point on AOD's.
Selecting 1 from a stop prevented any upshift; selecting 1 while driving caused a downshift from 3-2, then 2-1 once your speed allowed it.

One aspect I found frustrating with them was - if you selected 1 with the intention of holding in 2nd, but let your speed drop too much, the trans will dive down to 1st.........unlike a C6 which will hold any lower gear you select.

What type of driving do you typically do, & what do you want to gain from the swap?

Without changing your axle gearing as Dave mentioned, I think you'll find OD pretty much unusable (more so with oversize tires), so the swap wouldn't have gained you anything except to have eliminated torque converter slippage in 3rd.

If you have 33" tires, 70mph in OD would be about 1300 rpm; even on 28", it would still only be 1550 rpm - low for a 302.

 

A 302 is really NOT going to like OD and the 2.75 rear gear. I think you are going to find OD very limited in its usability and will, like Ken said, effectively be gaining only the lock-up torque converter for all your pains. And, as 86 said, you can burn an AOD up if you don't have the TV linkage adjusted properly.

I'm not sure I'd do the swap without planning to change the rear gears. A quick calc says that going with 4.10 gears in the rear and dropping into the .67 OD would put you exactly where you currently are - 2.75 effective gearing. If you are currently happy with that, and I suspect it is already too high-geared given your tires, then swap for the AOD and install the 4.10's. You will then be turning ~2000 RPM at 70 instead of the 1300 that Ken indicated. For a reference, my 351M is turning 2000 at 65 with the ZF in 5th and it has plenty of power to use that gear on the highway. But, I wouldn't expect a 302 to like anything less than 2000, and it might not like that.

 

To answer one question, no, the AOD is in no way related to the C6, it is an overdriven FMX with a lockup converter and an aluminum housing. Stout enough if you don't abuse it, but it won't like 2.75 gears with tall tires. 302 won't much care for it either.
The C6 is stronger, but the AOD can hold up. Re-gear to about 3.55:1 and you will turn fewer revs in OD than the C6 does currenty, while making life easier on the 302/AOD.
You didn't mention two or four wheel drive...

 

With your current gears, I wouldn't bother doing this swap. OD will be pretty much useless to you.


In other hands, it would be even better, if you swapped to 3.55 and the AOD, but that's another thing $$$

 

I don't know where he lives, that will make a difference. I knew someone who had a 86 monte carlo with the 200r4 overdrive tranny and high 2's for the rearend, and it was a little aggravating around where I live(hilly with mountains). But take it on a trip to the beach, and once you got down where it was flat, it did fine. Of course it wasn't a truck, but if he lives where it's flat, he might get by with it using the od some.

 

I have 31" tires, and don't plan on going larger or smaller. I love the look!

Also, it's important that I add that I have a few performance parts: .447 lift 260 dur. Cam, Holley 650, and I am about to put gt40p's and shorties on. So my 302 can handle more than the average 302.

My driving consist of about 50% in town and 50% highway. I live in Texas, so there is plenty of open road, however I do constantly stop and start in town.

Highways are going to 75 mph (but I would rather calculate off 70) and west Texas the limit is 80 mph (ha ha though I might brag a little, even though I don't live in west Texas).

My truck is 2 wheel drive. So that calculator did not work for me. However I did find a calculator and I played with the ratios. Looking at the ratios I could get something between 3.55 and 4.11, I was thinking 3.70.

Because gears are relatively cheap, I don't mind changing them.

I really think changing the rear end will be great. I will be able to stop and start in town with the torque, and a little better economy on the high-way. What do you guys think about the rear end ratio?

I am not worried about the TV cable, NSS, and reverse switches/plugs, flexplate I can figure out that kind of stuff.

 

Unless you have plans to go to 3.73 or taller gears at the same time as the trans swap don't bother, the 302 makes no usable torque below 2000rpm so it'll be downshifting into a headwind never mind going up a grade.

 

I will probably go with the 3.89's or 4.00's from summit.

 

Over a 25 year period, I had a bunch of FOMOCO full-size cars with the 302 and AOD, including a wagon that was heavier than your truck. They had 2.73 and 3.08 rears (8.8" axles) with 15" tires. There was one thing about the AOD that always bugged me. To understand it, you have to understand the AOD's unique design:

The AOD does NOT have a lock-up torque converter. And yet in can run without torque-converter action. Unlike all other transmissions, the AOD has TWO input shafts. They are concentric, one inside the other. One shaft is after the torque converter, like a C6, C4, FMX, THD 350/400 whatever.
The other input shaft connects THROUGH the converter, bypassing converter action completely.
Full torque converter action is available in 1st and 2nd gears, and is always there, like a C6 for example.
However, 3rd and 4th gears use the direct input shaft, so there is absolutely NO torque converter action in 3rd or 4th, no matter what. This is good, for gas mileage. And the AOD is simpler, since there is no clutch in the torque converter nor need to control one.

And now the bad. Driving along at speed, somebody ahead of you decides to suddenly make a right turn, and you brake, and when you clear their bumper as they turn, you get back on the gas... and nothing! You slowed down to 25-30 or so, so the trans is now in 3rd, but with NO torque converter action, the engine is low on its torque curve. And it's not going to spin up fast, because it is also directly coupled to the load. So no benefit of torque multiplication, AND the engine is held back by the direct coupling. So the engine pulls slowly slowly faster. And the faster you finally go, the faster it finally accelerates because now your moving up the engine's torque band. But it takes a while, irritating.

To avoid it, you have to mash the pedal to force a downshift into 2nd, where torque converter action helps out, besides the lower gear ratio.

I found it aggravating in cut-throat traffic here in TX.

Just thought I'd mention that, FWIW.

Oh, and don't underestimate the importance and touchyness of the TV rod/cable (rod for carbs, cable for EFI - push a rod, pull a cable) adjustment. Can burn up clutches and bands real quick with insufficient TV pressure in the trans. Too high a TV pressure then all shifts are delayed to real high RPM.