Thinking about a transmission swap...couple questions

[shiner2001]

In my Suburban, I currently have a TH400 trans and have been thinking about swapping to a 700r4 to get another gear and hopefully help my mileage out some. I've had a 700r4 rebuilt in and modified in another ride, so I know that's gonna cost me about $1500, which is more than I want to spend to replace a perfectly good transmission. So, I'm thinking of taking a chance and buying a used one and just throwing it in and seeing what happens. Obviously there are risks with doing so, especially if I'm buying from a stranger and don't have any history of them or the new transmission. Concerning the swap itself, I assume this should be a fairly straightforward swap, assuming I start with a 4wd 700r4 (my Suburban is a 90 V2500 4wd). I don't know what the differences in overall lengths might be, but I know that some of the Suburbans (not sure what years) came with then 4 speed (right?), so I don't know if I'll be dealing with any fitment issues or not. I'm still pretty new to this truck, so I figured you guys might be able to give me some general pointers on what I figure is a pretty common swap. From browsing around on Craigslist, it looks like I can have my pick of a 700r4 for about $100-200. I'll probably have the swap done by a shop I trust, but not knowing what all that will entail, I don't know what to expect for that bill.

In addition, does anyone have any real-world numbers about what kind of mileage differences you've realized going from a 3 speed to a 4 speed?

 

[diesel4me]

Swapping a good working TH400 for a 700r4 would be a downgrade in my estimation,though I'm sure many here will disagree...the chances of finding a GOOD 700r4 on CL for a few hundred bucks are slim,they were the most troublesome tranny GM made and often short lived..if you do buy one I'd plan on having it torn down and at least inspected inside,it would be worth putting a rebuild kit in it with new clutches first...........................................................................................................................................................................................................You would have to change the transfer case along with the tranny,since the TH400 has a larger output shaft,different adapter bolt pattern,etc too--so add another 300+ bucks for a good used transfer case...I am not certain but I think a TH40,700r4 and a SM465 are nearly the same overall length and the driveshafts will still fit,but yokes and or u-joints might need to be swapped....................................................................................................................................................................................................I doubt you'd gain much more than 3-4 mpg having overdrive VS a TH400--not worth all the effort and expense,IMO---the main reason my '85 Suburban still hasn't had its ailing 700R4 replaced is the fact I dont think I'd have ANY luck trying one of the "good used" CL ones I see listed around here--I'd rather put a TH400 and a matching T-case in it and ditch the 700R4,because I know it would be nothing but more grief...unless I spent the big bucks to go right thru a used 700R4 and at least know what condition it is in..I wouldn't use the truck enough to justify that expense..

 

[Deuling]

Just wanted to add, the length of a 400 is roughly 28", and the 700 is 23". I was just reading a book on these. Im gonna have my 700 built, but i know the guy and hes only charging me 150-200 bucks for labor

 

[Greg Ducato]

The 400 with adapter to 208 or 241 transfer case is less than 1/2 inch longer that the 700R4 and it's adapter so you can use the same crossmember, front and rear driveshafts, linkages etc. Make sure your carb or EFI has a tv cable connection provision. You will need a tv cable, dipstick and tube, tv cable to intake bracket and wiring for the Torque converter clutch that will work with whatever trans wiring your used trans has in it. Make sure your flexplate is drilled for a dual bolt pattern as the 700 has a smaller pattern than the 400. You can make your cooler lines work with a little bending in the right places. You wil need to change the input gear on your transfer case to the appropriate gear to match the output spline of the 700 which is smaller. The 700 has a 30 percent overdrive which translates to a 30 percent reduction in rpm. Ideally, you will want your engine at 2100-2200 rpm at 70mph for best performance and MPG as the engine will be happy at this rpm point pushing this barn through the air. Go too low in rpm and the trans will be constantly shifting between 3rd and 4th and milage will suffer as will the trans. As others have advised, be wary of the CL 700R4. Why go through all the trouble of this conversion relying on a complete unknown of the most critical element to the success of the project?

 

[Ned Kelly]

Will your diff gears be matched to the tire you are running and the 700r4, so you are in the optimal rpm range? Just another part of the equation that may add costs.

Do a fuel savings vs cost calculation with this formula:
http://www.fueleconomy.gov/feg/savemoney.shtml

I've seen people put big money into making their vehicle get better mpg, but for the amount of driving with the vehicle they do, they will never realize a savings. For some conversions they had to log 50-100k to get to the break even point, and then they would start to see a savings. That is assuming your 700r4 has no problems for those first 50-100k miles. Be very realistic about what mpg increase you will get, and all the costs involved, parts sourcing, and down time to do the swap.

I did have the same thought to do a 700r4 swap with our family camping 1980 Suburban with TH400. After some number crunching I didn't think it was worth it. And if the 700r4 has problems, all the savings are gone, and probably more, plus consider your time/labour. And if it dies somewhere on a trip - you may need a tow - maybe have to add shop costs to fix if you can't do the repair yourself.
My wife would flip if I did a 700r4 swap and it died on a family trip.

 

[Fixer]

My advice would be this:

Do a 700R4 swap, it really can be a better unit than the 400, but the right thing to do is wait and get a unit that you know will run and is built right.

 

[shiner2001]

The more I think about it, the more it makes sense to put the transmission project on the back burner and spend my money elsewhere on the truck right now. If it was a straight bolt in, then maybe, but as it stands, I think paint (gonna try that myself) may be next. Also perhaps the whole CFM TBI upgrade.