Drivetrain swap/ wheelbase

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chrissock

chrissock

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So I have a 71 dart swinger with /6, 727, 7 1/4 driveline. I am purchasing a complete driveline minus motor from a 68 barracuda fastback. It's a 904, 8 3/4 small block driveline. He is selling me super stock springs and the driveshaft. Is this a complete bolt in with the driveshaft being the same length?
 
The trans will bolt right in. The differential will bolt in but you need to swap the shock plates as the 8-3/4 has 3" tubes where-as the 7-1/4 has 2-3/4" tubes. The 68 Cuda is a 108" wheelbase car and the Dart is 111" wheelbase. The 904 trans. requires a longer driveshaft (not exactly sure but it's somewhere between 3-1/2 and 4") and the 8-3/4 requires a shorter driveshaft (I don't remember exactly but it's around 1-1/2" difference) than a 7-1/4" equipped car. Unless you get extremely lucky the driveshaft probably won't be a bolt in. Sorry I can't be more precise on the measurements.

Check out this thread and you might be able to figure out what length driveshaft you need so you can compare it to what your getting

http://www.forabodiesonly.com/mopar/showthread.php?t=139090
 
Good info. My understanding is that the 8 3/4 and the 904 trans would have come factory in this barracuda and that the only difference would be the wheelbase which I would think would be the 3 inches you describe. If that's the case then I would need to use the shaft that is in the car now and and possibly shorten it? Not really any local shops that do this so im gonna have to ship it out or order online.
 
chrissock said:
Good info. My understanding is that the 8 3/4 and the 904 trans would have come factory in this barracuda and that the only difference would be the wheelbase which I would think would be the 3 inches you describe. If that's the case then I would need to use the shaft that is in the car now and and possibly shorten it? Not really any local shops that do this so im gonna have to ship it out or order online.
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You may or may not be able to have your shortened. I'm no driveshaft guru but I've read stock shafts (at least some of them) have a rubber insert and due to that they can't be shortened. Considering the expense of shipping it somewhere, getting it shortened and the cost of return shipping you might be as well off ordering a new driveshaft from Dr. Diff.. Last time I looked they run about $320 for a complete new assy. It'll be a lot stronger shaft also
 
Try Inline Pro in Springfield, VA. It's about 100 miles from you.

http://www.driveshaftshop.com/

You may want to ask shops closer to you that do driveshaft repair work for big trucks or farm equipment. PTO shafts and jackshafts aren't that different from automotive driveshafts.
 
Still working through this. Just bought the driveline. Has 11 inch drums on the rear which I thought was cool. Also got a 273 in the deal. Just cause he wanted to get rid of it. I have to pick everything up this week. I found a napa that will shorten the driveshaft but they can't balance it. So that might be an option. I would like to get the rear in so I can change bolt pattern and get decent wheels. Thanks for the info.
 
fishy68 said:
You may or may not be able to have your shortened. I'm no driveshaft guru but I've read stock shafts (at least some of them) have a rubber insert and due to that they can't be shortened. Considering the expense of shipping it somewhere, getting it shortened and the cost of return shipping you might be as well off ordering a new driveshaft from Dr. Diff.. Last time I looked they run about $320 for a complete new assy. It'll be a lot stronger shaft also
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I am going to go this route but I feel like I can't order a final driveshaft without getting the motor/trans in. I know the rear changes the length but does if I change the trans from a 727 to a 904 does that change the length?
 
chrissock said:
I am going to go this route but I feel like I can't order a final driveshaft without getting the motor/trans in. I know the rear changes the length but does if I change the trans from a 727 to a 904 does that change the length?
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To answer your last question, yes.

I'd bolt everything up and measure for the driveshaft. Install the trans yoke all the way in and measure the u-joint c/l to c/l, then subtract 1". You need trans yoke clearance for suspension travel and don't want the yoke to bottom out in the trans.
 
pntastar69 said:
To answer your last question, yes.

I'd bolt everything up and measure for the drive shaft. Install the trans yoke all the way in and measure the u-joint c/l to c/l, then subtract 1". You need trans yoke clearance for suspension travel and don't want the yoke to bottom out in the trans.
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Cool, funny how plans change so quickly. talked to a local guy about trans. Basically said ditch it and get a 727. He is a Mopar guy so I asked him why and he started digging through parts bin to show me. Very cool guy. So now I guess I have a 904 for sale! But I think in the long run this may work out better. If I swap the 727 /6 for a small block 727 it should be the same drive shaft length and yoke correct?
 
Sounds like the guy you talked to is an old timer. When I was young we ditched a lot of good 904's just to say we had a 727. What a dumb move. Unless your building a "balls against the wall" sb your better off with a 904. Why, because the 727's rotating mass is heavy and it consumes about 20 hp more just to spin it. If you don't mind giving away 20 hp go for it. Nowadays since we all learned that 904's aren't the great evil of the world guys run them in their race cars. Have a buddy running one in his 67 Cuda that goes 9.90's

Don't take it I'm knocking a 727 cause I love them. Have been rebuilding them since I was 17 and I'm 50 now. They are a great heavy duty trans and in a work application hard to beat. But in a street application of no more than 500 hp all it takes is a properly rebuilt 904 to do the job and do it more efficiently. Jus sayin
 
727 behind a 6 is like putting a Dana 60 behind a 2.2. Parasitic loss on a 727 is ~45HP! 904 is ~25. That 45 is about 50% of a real world slant six output. Now THAT is a crippler!
 
fishy68 said:
Sounds like the guy you talked to is an old timer. When I was young we ditched a lot of good 904's just to say we had a 727. What a dumb move. Unless your building a "balls against the wall" sb your better off with a 904. Why, because the 727's rotating mass is heavy and it consumes about 20 hp more just to spin it. If you don't mind giving away 20 hp go for it. Nowadays since we all learned that 904's aren't the great evil of the world guys run them in their race cars. Have a buddy running one in his 67 Cuda that goes 9.90's

Don't take it I'm knocking a 727 cause I love them. Have been rebuilding them since I was 17 and I'm 50 now. They are a great heavy duty trans and in a work application hard to beat. But in a street application of no more than 500 hp all it takes is a properly rebuilt 904 to do the job and do it more efficiently. Jus sayin
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I hear ya. I think the same as you. I guess his overall point was not that a 904 cant be made to handle it. but that starting from 0 you SHOULD do more the 904 vs the 727. I know me.... and I know at some point this thing is getting a bad A** small block. So Im willing to deal with the loss for now. AND as a bonus I can go ahead and swap in the 8 3/4 rear and get the drive shaft cut and it doesn't change when I do the motor. I have to go 100 miles to get drive shaft done correctly so that is a plus.
 
pishta said:
727 behind a 6 is like putting a Dana 60 behind a 2.2. Parasitic loss on a 727 is ~45HP! 904 is ~25. That 45 is about 50% of a real world slant six output. Now THAT is a crippler!
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hahah I hear ya. Came that way. Go figure. And I have the window sticker showing it as an option! I assume it was option over the straight gear.
 
chrissock said:
I hear ya. I think the same as you. I guess his overall point was not that a 904 cant be made to handle it. but that starting from 0 you SHOULD do more the 904 vs the 727. I know me.... and I know at some point this thing is getting a bad A** small block. So Im willing to deal with the loss for now. AND as a bonus I can go ahead and swap in the 8 3/4 rear and get the drive shaft cut and it doesn't change when I do the motor. I have to go 100 miles to get drive shaft done correctly so that is a plus.
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Check out the threads here and you'll find that Mad Dart is running over 600 hp and hitting it with a 200 shot of NOS and running a 904 he rebuilt with very few upgrades. Their a whole bunch tougher than most old timers think and they CAN be built fairly inexpensive to hold a lot of power. One of the biggest problems with 904's was back in the early days most only had 3 disc clutch carriers and the friction material wasn't all that great in the early 70's so back then they wouldn't hold a lot of power. That's why they got a bad reputation. In the mid to late 70's they came out with clutch carriers that held 4 and 5 discs and with todays much improved friction material you can make them hold a heck of a lot of power.

BTW: I was one of them old timers that used to say 904's weren't worth building. Time and seeing has changed my mind. I have a buddy running 9.90's with one in his 67 Cuda. Been doing it for several yrs. with only 1 refresh
 
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