Technical To AOD or not to AOD

Hi all,

Question, in the future I was planning to replace my Ford-o-matic with an AOD which would give me the overdrive and better hwy drivability. That was when I thought I had 3.56 rear gears behind my 312 Y block with a 4 barrell. I have since found that I have 3.10 or thereabouts in the differential. Is it worth putting in an AOD or will that mess things up gearing wise due to the 3.10's?

I don't know how to calculate/determine a good match between an AOD and the differential gearing. I am also running stock 14/15" wheels.

Thank you,

Mike

 

Stock 302 with an AOD in my 54 Ranch Wagon. 215/75/15's. After many different opinions I went with 3.50's in the 8 inch rear end. JMO

 

I am really pleased with changing out my Ford-O to an AOD many years ago. Also changed the rear to a 3:70 Tru-trac. Gear set with tire size is a itsy bit tall for around toe=wn but not an issue if I keep it in 3rd. Took a bit of a learning curve for me and my application, but it is awesome on trips on the highway or across town hiways. Much more comfortable to cruise at 70-80 now if I want to. Engine not working hard at all and much less engine trans noise. Previously it would sound like 3000 rpm it was wound up tight, I'm running a 292 that is stroked with lotsa headwork and an isky cam and some longtube headers. buncha other stuff to go with it all of course.

 

Do some measuring of tire height to help you calculate it all. I was also not savvy at the ratio accuracy since it a math thing! But other Hambers on here love that stuff and will be glad to help you I bet by chiming in.

 

That 3:10 ish gear will be too high in my opinion depending on tire size. AOD’s are quite different when they go into 3rd and 4th as they are direct drive without converter action and it can get a little “trailer hitchy “ feeling if it’s lugging. Also, it is super critical that the TVcable be mounted and adjusted exactly or it won’t make it around the block without burning up hi gear clutch. Just my 2cents.

 

Go for it you will like it

 

Yes, I agree with Kevin A., this kind of stuff all comes under the heading of read all there is thsat you can find on the AOD install. The tv cable is critical as said. Lokar provides a machined piece for measurement. BUT, with that I additionally prefer the use of a pressure gauge off the trans.

 

Thank you all, I am running a stock set up, nothing fancy. I am just worried that it will go into 4th gear and lug the engine with that low of a diff gear set. Is there a way to manually put it into O/D if it starts to lug around town?

 

drive the car as is and enjoy. don't fix things that are not broke . by the time you put a lower gear to compensate for the overdrive you will be about where you are now

 

Your shift linkage should allow you to shift between 3rd gear (direct drive) and 4th gear (overdrive) both up and down.

 

Use one of the rpm calculators that are available on the internet and input tire size and the mph you cruise at. When you see where the engine will be running at, you can decide on whether it will be happy or not depending on whether you have changed camshafts or not. Without tire diameter figured into the calculation, you are just guessing.

 

What P Prather said

 

I had an AOD in a 5.0 mustang with 308 gears and 235/65r15 tires.

the shifter had oD, D and Low. You couldn’t put it on 2nd.

I had to drive in D to lockout overdrive unless I was on the highway.
Otherwise it was a great combination.

 

I can't offer insight as to the AOD, but I have a 700R4, which has similar gears. The AOD has a similar spread though the first gear ratio is somewhat higher, and a deeper OD. I too am running a 3.08 rear with a 27.7" tall tire. The 3.08 is really too long, and the car would not only benefit in acceleration by going to something like a 3.73, but would put it into the power band better on the highway. As it is now, 80 mph is about 2000 rpm, and I need to be cognizant of where my foot is on the pedal because the car will start to take off if you're not paying attention.

That said, it will run cooler, quieter, and get significantly better fuel economy than without OD. I observed 21 mpg on a trip in mixed driving, running 80 mph on the highway, and at 160 degrees. It's magnificent.

 

More gears is more better, so long as they are paired with the correct rear end gear.

3.73:1 to 4.10:1 are are good with overdrive transmissions.

 

I replaced the 56 Ford-O with a 62 Cruise-O. The car had 3.21’s. I wanted a higher highway gear and replaced the rear end with an 8.8 with 2.74’s and could not be happier. It does everything I wanted. It took a bit of fabrication to get an extra spot on the column indicator but worth it.

 

Someone, B&M I think, makes a constant pressure valve body that, while you still need the cable, takes the gnat's**** adjustment out of the equation. Used to have a Crown Vic Police Int. with 351W/AOD/3.08s. Seemed to drive well & evidently good enough for the cops, of course quite a few more cubes involved.

 

I have a 302 with a AOD and a Explorer rear with 3.73 gears and it likes it at 75 on the freeway, in town it shift into overdrive at 40 mph. it is in my panel.

 

3.73 rear gear give you a 2.49 in overdrive which is on the ragged edge with down shifting for me. A 4.10 will give you a 2.74 which is way better. A .67 OD is a pretty big jump and can’t see using a 3.08 or a 3.25 at all.

 

When I swapped the 302 into mine, I used the AOD because I had it. I’m running a 8” rear from a 66 Fairlane, 3.00 gears. I can’t use OD unless I’m on an interstate, it is constantly shifting from drive to OD and back at speeds below 65 mph. I have a 3.55 I’m going to swap in once it gets cooler weather and I can dig it out from under the pile it’s under. Right now I just put it in drive and go. If the 3.55 still turns out to not be enough, I’ll hunt down a Ranger 8.8 with a 3.73 and drum brakes.

 

Below 40 MPH keep it in D to avoid the transmission from "searching" for the proper gear, that increases wear. Above 40 MPH use OD when not in hill country and curvy roads such as Interstate or long straight level stretches.

 

This may help

https://johnmaherracing.com/calculators/gear-ratio-calculator/

 

Depends on tire size. A lot of our cars are running small diameter tires so what looks like a tall rear gear acts like a normal ratio.

I run a 2.91 rear gear in my 9" with 215/75R14. That combo acts like a 3.55 with "normal" tires. I wanted the original 14" wheels to keep the car low and retain what I wanted for wheel covers.

I have a 3+OD manual out of a Granada. The 352 pulls hard in all gears. I can still cruise town in 3rd.

 

If you're replacing the trans anyway, it would always be my choice to go to AOD. It's easy to drive around in 3rd gear all the time at 1:1 ratio, and only shift it to 4th OD when on the freeway.
This is what I do all the time, and until I'm heading down the on ramp my car never sees 4th OD use. I do have a 3.73 rear gear because my tires are 29.5" tall and needed lower rear ratio. I cruise at under 2000 rpm's at 55 mph, and right at 2000 at 65 mph.

 

Much more goes into this setup than just the rear gear ratio. Torque convertor steups become very important with the AOD.

Are you planning on keeping the stock lock-up feature in 3rd and 4th gear? If so, an AOD will pull 3.10's just fine, especially if you have a smaller diameter rear tire.

However, a common mod is to remove the lockup feature (2 piece input shaft) and replace it with a C6 style single input shaft and use a non-lockup torque convertor. The 2 piece input shaft is the weak point of the AOD and this modification is often done to get them to live behind larger displacement motors, or high performance motors in bigger, heavier cars. I.E. AOD's behind FE's in early 60's Galaxies. In this case, you need to pay extra attention to the stall speed of the torque convertor as it becomes really easy to cruise at an RPM below the stall speed of the torque convertor which will lead to overheating of the******* fluid. This is where a 3.5 to 3.7 rear axle ratio will help.

I run an AOD in my '62 Merc behind the 352 FE (325 Hp, 375 ft-lbs) with a 3.6:1 rear axle ratio on 27" tires. That car is very happy running down the interstate between 60 (1800 rpm) and 80 (2400) mph. The torque convertor stall on this combo is right around 1800, so it works very well. This AOD has the lock-up delete modification

I also ran an AOD behind the 390 FE (375 Hp, 450 ft-lbs) in a OT '76 F250 with a 4.11:1 rear and 32" tires. This trans also had the lock-up delete mod, but due to the small diameter of the FE bell (limits torque convertor diameter to 11"), I could not get a torque convertor built with the stall low enough to match the ideal driving range. The stall in this combo ended up in the 2300 to 2400 range and after cooking the******* fluid twice, it got yanked, and a C6 went back in.

 

Unless you spend a lot of time driving on the Interstate at 80 mph, I don't see the OD working out very well for you with your drive train. At most speeds under 65, that OD trans is going to be bouncing between OD and the direct drive, never being happy. At 65, the OD will have the y block at the bottom of your power band and the slightest hill will lug the motor.

 

Just a couple thoughts on my past experiences. I don’t really know anything about the AOD except it was available in more than one gearset. Most seem have a 2.4x or so first.

I helped my buddy put a 350/700 combo in a 56 Nomad. Got to be close to your weight and a little more power. He says it was a 3.55 with the oem powerglide. It’s good around town with the 3.06 low, falls off a bit on the 1-2 shift from the wide gap. It tends to lug in the 35-45 mph range, especially after the converter locks up (no electronic control). He drives it in 3rd to avoid that, and overdrive is pretty much a 55 up gear. I know the right transmission guy could probably dial in a better valve body but he just lives with it.

I’ve had other vehicles with o/d autos, but lighter and more power than your y block. I’ve gotten by with 3.5’s, but 3.7 to 4.11’s are better.

While Lynden is pretty much a valley floor you’ve got some hills east and south. And lots of 35-45 mph roads. So you might consider that.

Since yours is a 57, you’ve got a 9” rearend (not the Dana used prior to 57) it’s pretty easy to change the rear gears, and lots of ratios available.

 

just put an AOD in my 47.
I run 225-70-15's and 3.25 gears. At 70 mph it is around 1900 rpm, and the engine is still lumpy on the cam. I need to change to 3.89 or 4.11

it is nice to have that 1000 rpm drop at freeway speeds though. I keep it in D around town, and shift to OD on the highway.

 

Since it hasn't been mentioned, just a reminder that you will need a special adapter to mate the Y block to the AOD.

 

ALSO........compare the 1st gear ratio BETWEEN the AOD and the Ford-O.....IF AOD is lower it will give you more jump when leaving the intersexual and not lug the engine. Once you are to 40-60 mph you're gonna be cruising anyway.....not TOO concerned with****-kick pick-up in 4th gear.
Try it first......if ya ain't happy.....slap in some gears or play with your tire diameter.

This ain't zac-lee apples to apples but...I run a T-5 with 3.08 gear. With the lower first gear I have plenty of scoot going into the next two gears. By then I'm at the legal speed limit.
I can shift to OD and its like I cut the engine off! It maintains the speed easily and will gradually accelerate. If I wanna pass a slow poke......I shift back one gear (or two if I wanna show out!) 3.25 -65-15 tires.
6sally6