Changing from a TH350 to a 700R4?

[ImpalaMan]

 

Well fellas, I recently bought a used 700R4 from a friend of mine and I really want to install it into my 84 Silverado. Mainly for the OD gear for freeway driving. Does anyone have any input on what I will need to change out before I begin this adventure? I know that I may have to get my driveshaft shortened, I think the yoke will fit in the 700R4 and I know I will have to relocate the transmission cross members for the 700R4 to mount up properly. What else do I need to know? Any input is much appreciated. Thanks.

 

-ImpalaMan

[Cheekster96]

I thought the 700R4s came stock in the 80s trucks??? I had an 87 GMC with one. My 87 K20 has the TH400 in it.

what tranny do you have now?

Lee

[ImpalaMan]

My 84 Silverado has a TH350 transmission in it, stock. That is what it came with according to the code sheet.

 

-ImpalaMan

[heavy4x4]

Couple of things:

 

The 700R4 is just shy of 2" longer than the TH350. You may need to shorten your driveshaft which isn't too costly if you have a driveline shop in your area.

The only other thing to worry about is torque converter lock-up on the 700. These trannies will burn up in OD when cruising if the TC isn't locked. There are switches that splice into the tranny's wiring that allow you to have full control over lockup. I highly recommend that.

 

What year is the 700R4? Post '87 are best.

 

-Steve

 

P.S. Afterthought: I know the 700's in 4wd vehicles changed spline count at some point. I'm not sure if they did in the 2wd versions, but you'll need to make sure the output shaft has the same number of splines as the TH350.

[ImpalaMan]

The 700R4 is out of a 91 Caprice. I was wondering about the converter lockup, My friend has a similar setup on his 77 Scottsdale, but he has a manual control lockup switch for the drag strip and he's running a 700R4. So, will the cross members be moved easily? I checked with my transmission guy and he informed me that the spline count should be the same at the TH350. So hopefully all I'm gonna need is to get the driveshaft shortened. What size torque converter is recommended anyway? The truck will be used for light duty hauling (no trailers) and as a daily driver to work.....so I can keep my SS in the garage.

 

-ImpalaMan

 

P.S. - Thanks for all the input/help.

[heavy4x4]

I would hook up the torque converter lockup with a manual switch as well. There are a fair amount of inputs that factor into the lockup electronics (vehicle speed, throttle input, etc). For your use, I would not activate lockup below 55mph. When you're cruising on the freeway in OD and are pretty confident your truck won't downshift to 3rd, leave it in manual lockup. By the way, what gears do you have in the '77?

 

As for the crossmember...if it does need moving, it's a matter of grinding off the heads of the factory rivets, drilling four new holes, and installing 4 new bolts.

 

If your '77 is a daily driver and you're not going for performance, I'd go with a torque converter that gives you close to stock, or slightly higher, stall rating.

 

One more thing...there may be a difference in the governor installed on a car 700R4 and a truck 700R4 (I'm not sure, I've only ever dealt with 4x4 trucks and there is definitely a difference between gas and diesel trucks). If you notice your truck reving higher than you would like during full throttle application, you will want to look into replacing the governor/servo.

 

-Steve

[batty54]

There is a great article in this months "November" Hemmings MUSCLE magazine that covers all your questions pick one up good reference too.

 

I forgot already but one of the 350 or 400 needs the distributor from the vehicle the trans came from that signals the lockup or shift points.