Swap from a 4-speed to an Auto

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twstd340

twstd340

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I have a 71 340 Duster 4-speed that I'm considering swapping to an Auto w/od. My reasoning is so I can put lower gears in it (currently 3:23) and still be able to cruise down the interstate with lower rpms. Am I way off base? Also, if I did and put the console in, will I need to cut the tunnel more or will the current opening work?

Thanks in advance
 
I'd pop an O/D "A" or "F" Body 4 speed into it. Pretty much a bolt in and change the 3-4 shift arm and rod. At .71 or .73 O/D you can run 3.91's and cruise like it has 2.78 or 2.85 rear gears.
 
The current opening won’t work with the factory linkage. IF you put in a console, IMO, your best bet would be an aftermarket shifter and run a cable. Which could very well use the factor 4 spd hole.

For a final drive ratio in the OD gear, multiply the gear ratio by the OD ratio for the final drive ratio.

3.23 X .69 (?) = 2.2287
That is mighty low! I hope your tires aren’t to tall! LOL!
3.23 X .71 = 2.2933
 
Final drive with a std 904 or 727 non OD is 1 to 1, just like a normal non OD manual.
 
GV O/D on an auto will give you 4 gears plus the convertor torque multiplier. You may end up with what you could get with an OD trans without the $2k GV unit. I remember seeing a diagram of a street rod that had 2 transmissions in series, one 3sp was behind the other 4sp and was used to lower the "rear gear" by using 2nd for racing(?) And 3rd for cruising. Dont know what first would give him, maybe a tree stump puller.
 
rumblefish360 said:
The current opening won’t work with the factory linkage. IF you put in a console, IMO, your best bet would be an aftermarket shifter and run a cable. Which could very well use the factor 4 spd hole.

For a final drive ratio in the OD gear, multiply the gear ratio by the OD ratio for the final drive ratio.

3.23 X .69 (?) = 2.2287
That is mighty low! I hope your tires aren’t to tall! LOL!
3.23 X .71 = 2.2933
Click to expand...

Tires are 275/60x15
duster 15.jpg
 
Was that in the trailer when you got rear ended? Gives 'back half' a new urgency.
 
You can buy either the complete gear set and have someone install it or he sells transmissions already converted. You would have to call him for what is all included with the gear set. A lot of people suggest using a Mopar OD trans, they have a horrible gear spread and are not that strong, in my opinion. I've seen two of them that were behind Slant 6's that looked like they had a stick of dynamite go off inside the case.
 
lemondana said:
You can buy either the complete gear set and have someone install it or he sells transmissions already converted. You would have to call him for what is all included with the gear set. A lot of people suggest using a Mopar OD trans, they have a horrible gear spread and are not that strong, in my opinion. I've seen two of them that were behind Slant 6's that looked like they had a stick of dynamite go off inside the case.
Click to expand...

I did not know they were THAT weak. I reassembled an oddball for one guy, it was an iron case with an OEM adapter IN A CHEV 1/2T 4x4!!!! They made the GM ones both with Mopar bell pattern and a special GM bolt pattern case
 
I never heard of them as weak. Never seen a complaint or one blown up.
 
I know someone running the stock O/D gear set in an A833 behind a 440 6 pack. Not a problem. I would not power shift that into O/D, the O/D is for cruising on the highway. I have been running an early O/D for years behind my HP 273 and it runs like new. I also run 90 wt gear oil. I like to use the gear set and main shaft with a standard cast iron main case, 64 or 65 if I can find one. It is stronger with no "float" and uses the standard front bearing retainer so I don't need any special bellhousing or adapter rings. Some of the later O/D's have cheaper cast gears and funky side covers and internal linkage. If you are making a lot of power, 3 speeds are enough + an O/D cruising gear. You can get an O/D main shaft and gear set relatively cheap so they are economical to put together.
 
I've never owned one, so the two that I saw grenaded might have just been exceptions. I knew a guy years ago that had an Aspen R/T with a 318 and the OD 4 speed. It was a gutless pc of crap, When he shifted from low to second it felt like he went from low to fourth. Passon has all of the information on those on his website, true, he's trying to sell his product.
 
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