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Hi, quick question. I am looking at doing a motor and trans swap in my 67 convertible. One engine builder told me that the AOD is just for cruising and not good for motors with higher horsepower / torque. Here is what he said in an email:

AOD transmissions are used with cruiser. The reason being when you are cruising down the highway the engine is not turning very many RPM's so you need to run a mild stall speed. If you run a mild stall speed with a cam with a lot of duration to make more power then it will not launch very good from the light. Then if you go with a higher stall speed to match the cam your torque converter will not lock up because you now have more stall speed than your RPM cruising speed.

Thoughts from this group would be appreciated. I had asked about a 350ish HP 302 with an AOD. Car currently runs a tired 289 / C4.

thanks, Jeff
 

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I'm pretty sure there are several guys on here running some pretty strong motors with an aod. I was planning a ~450hp 408 with an aod, so I'd be interested in hearing what some of the folks around here think as well.
 

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Just some guy
67 coupe, 69 Sportsroof, 86 hatchback
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Johnpro is no longer with us but he use to laugh at such statements. He a stock AOD behind his stroker. It was even what is considered the "weak" pre 1985 version. He turned respectable 1/4 mile times and kept waiting for that "weak" transmission to give up.
That said, I wouldn't plan on beating on a bone stock AOD. At least beef it up a little. And if you're making good power and racing a bit, plan on breaking stuff, the transmission is just one component that may fail. Of course the AOD does make a fine cruiser transmission too. You can definitely get used to having overdrive. It ruined me. I can no longer stand to drive a vehicle without overdrive, be it a manual or automatic.
 

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AOD transmissions are used with cruiser. The reason being when you are cruising down the highway the engine is not turning very many RPM's so you need to run a mild stall speed. If you run a mild stall speed with a cam with a lot of duration to make more power then it will not launch very good from the light. Then if you go with a higher stall speed to match the cam your torque converter will not lock up because you now have more stall speed than your RPM cruising speed.
None of this really makes any sense to me he is trying to say something however I think he is confusing issues and not making any actual valid points. However, yes AOD are inherently designed to lower the cruising RPM of the motor to increase MPG. All auto transmission have a stall converter and depending on your needs it can vary from a stock converter which might stall 1800-2000 RPM to an 8 inch 4500 converter (or more). Older AODs had a two peice input shaft design which would 'lock up' to further decrease motor RPMs when cruising to eliminate converter slip. A performance build for an AOD generally removes the lock up feature and runs a solid input shaft and a c6 type converter with a stall speed to match your cam needs. I have an AOD behind my 410c with a 3500 stall. Car runs 11.4 starting at idle and cruises on the street :) If I am in stop and go or crawl traffic I don't use OD because the higher stall will generate heat but I cruise on the freeway in OD at 70mph. With my 4.11 rear gear the OD makes the car nice even if I am still technically at an RPM lower than the advertised stall speed of my converter.


With all that said...AOD cost more to build IME than a comparable c4 for use in a performance build plus if you aren't running a really high number rear gear (like if you have a 2.8 rear) it might not even make sense to run an AOD. It doesn't sound like you are really doing a big time performance build however so I wouldn't think you would have any issues with a simple AOD swap with a unit that was upgraded a little bit.
 

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Just some guy
67 coupe, 69 Sportsroof, 86 hatchback
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It doesn't sound right because AOD's don't lock up their converters. The lockup is in the transmission and happens when you shift into 3rd and shift into 4th. Now the later AODE/4R70W's do have a lock-up clutch in the converter itself which makes them rather different. An AODE will shift into 4th an THEN into lockup.
The shaft which accomplishes lockup is a bit spindly. They eliminate lockup in high horse AOD's primarily because that shaft breaks under a bunch of torque. Things hold up a lot better when the converter is allowed to slip old school style. Folks like Baumann and Lentech list HP/torque points where where they recommend eliminating lockup.
 

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It doesn't sound right because AOD's don't lock up their converters. The lockup is in the transmission and happens when you shift into 3rd and shift into 4th. Now the later AODE/4R70W's do have a lock-up clutch in the converter itself which makes them rather different. An AODE will shift into 4th an THEN into lockup.
The shaft which accomplishes lockup is a bit spindly. They eliminate lockup in high horse AOD's primarily because that shaft breaks under a bunch of torque. Things hold up a lot better when the converter is allowed to slip old school style. Folks like Baumann and Lentech list HP/torque points where where they recommend eliminating lockup.
I pretty much agree with everything said so far - I am running about 450 HP through a modified AOD with a 3500 RPM converter and I still use the lockup input shaft. Best of both worlds on the street - loose converter for acceleration and no heat build up on the highway.

I did opt for a hardened input shaft, but in reality the "safety fuse" for most people, myself included, is running street tires. Particularly with 3.70s or higher gears, lack of traction is what keeps my transmission together. If you race and run drag slicks, by all means go to a one piece input shaft; but seriously "I can't get no traction" and it is on purpose ;-).

Good luck,
-Rory
 

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The limits of an AOD transmission in stock form is about 450hp but I wouldnt chance it with an engine close to that. They can be modified to handle 1000+ hp, all it takes is money ;) You should be just fine with 350hp and an AOD transmission if it is in good working order. A aftermarket valve body would be an excellent idea here for better shifting characteristics and more durability. Silver Fox transmissions is an excellent place to look for a valve body.
 

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Discussion Starter #9
Thanks. I think the thing that concerned me was the concept that with a higher horsepower motor, that a higher stall torque converter would not work with an AOD. Logically, it makes sense, but I thought I had read plenty that the AOD was being used wiht lots of stronger than stock motors.
 

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I have a Lentech strip teminator with overdrive in my T Bird. It has trans brake, 1 piece input and a 31 spline truck output with a Neal Chance converter that stalls at 5,400 rpm. The 347 is making around 1,300 hp at the crank. It went 8.60 in a 3,400 lb car. You can cruise it all day on the street and it does not get hot. The converter drives like a stock one does. The overdrive is only good for 400 FtLbs of torque however and is only for cruising. You can pretty much build an AOD for whaterver you can throw at it.
 
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