700R4 Tranny Swap

Torque Junkie · September 2, 2001

I'm beginning the research of this project, and just wan't to gather thoughts on my idea:

I would like to swap my TH350 with a 700-R4, for 2 main reasons: the 3.06:1 1st gear and the 0.70:1 OD to replace the TH350's weak 2.52:1 1st and the lack of O/D.

What kind of MPG improvement might I see in highway travel by having a 30% O/D? at 60 mph, instead of running at 3.1K rpms, it'd be more like 2200.

Thanks

-Mike

(Edited by Torque Junkie at 1:59 am on Sep. 2, 2001)

76BBSub · September 2, 2001

I've been toying with the same idea. It is easy to estimate how much the RPMs will drop, but converting that to a realistic estimate of fuel mileage improvement is a bit tougher. I would guess that we could pick up 2-3 mpg on the highway, since the truck still has the same weight and drag coefficient. Thinking about my neighbor's '99 Suburban with a FI 454, he was getting like 13 on the highway, so I think that is about the best I could hope for. I would think that your truck could get closer to 15, depending on your gears and tires. I'm going to wait and research this more before I make the plunge, but please keep me posted if you find more about this.

Thanks!

Jeff

Torque Junkie · September 2, 2001

Quote: from 76BBSub on 11:52 am on Sep. 2, 2001
I've been toying with the same idea. It is easy to estimate how much the RPMs will drop, but converting that to a realistic estimate of fuel mileage improvement is a bit tougher. I would guess that we could pick up 2-3 mpg on the highway, since the truck still has the same weight and drag coefficient. Thinking about my neighbor's '99 Suburban with a FI 454, he was getting like 13 on the highway, so I think that is about the best I could hope for. I would think that your truck could get closer to 15, depending on your gears and tires. I'm going to wait and research this more before I make the plunge, but please keep me posted if you find more about this.

Thanks!

Jeff

I havn't got a real-world MPG for my truck yet, but I was told by the seller it gets around 10. When I took it off roading - 20 miles there, 20 miles back and probably 5 miles on the trails in mud and slow speeds w/ alot of throttle, and I burned up a ~10 gallons of gas. I am starting to keep a log-sheet of my mileage and fillups, but that'll take time for me to get some actual numbers on.

I calculated that a 12 mpg could save ?/month in fuel costs driving 12,000/yr. 13 mpg would yeild ?.5/month savings over 10 mpg at ?.58/gallon. a ? swap project would then pay for itself in 2.7 - 6 years in fuel savings alone, not to mention the gratification of having the considerably shorter 1st gear.

My dad's '87 'burban w/ a 350cid and 700R4 100% bone stock is getting around 13 average, and it's in DIRE need of a rebuild.

I'd be happy with 12-13 mpg in my rig. It's got 33x12.5" tires, and either 3.73 or 4.10 rear end gears (is there an easy way to find out? in the 1.00:1 3rd gear @ 60 mph it's revving about 3100-3200 rpms)

I plan on hitting a salvage lot and picking up a good-condition used one (if one exists), but this won't be til later.

I'll definately keep you posted on what I find out in my research. Thanks for your help.

-Mike

95 Silverado · October 14, 2001

My last truck was a 1983 full-size 1/2 ton, 402 big block, 700R4 with a shift kit, 3.42 rears and 32x11.50 tires. it consistently got around 12 mpg. city driving, and got 16 highway, and it ran great. I like the comfort of the new one, but I miss that big block.

Jack Boelte · October 15, 2001

I've had as high as 18MPG doing 55-60 on the interstate. When I first bought the truck, the lock-up torque converter was not locking. After I got it working I saw a 2-3MPG increase. Very nice.

Ken73 · November 4, 2001

Mike, I've been considering doing the same when I yank the crappy 700-R4 out of my '82 Vette - rebuilding it and putting it in my Jimmy. Make sure you look for an '87-up 700-R4 when you're at the junkyard, the '82-'86 ones just plain suck. If I rebuild my '82 one, it'll be with beefier components.

There's a thing or two you need to figure out though - is the intermediate length of the tranny (from the bellhousing to the output housing mount) the same? Also, is the output housing bolt pattern the same for the 350 as it is the 700-R4? I see you've got a K10 (4wd) - do you know which transfer case you have? I know I've got a NP203, but I don't know if the bolt patterns are the same.

Ken

Ken73 · November 4, 2001

Mike, here's a page I scanned from a hot rod mag a while back. It shows a comparison pic of all four main GM auto trannies for you to see. Looks like the 700-R4 is an inch or so longer than the TH350.

Tranny Comparo

Ken

Torque Junkie · November 4, 2001

Quote from Ken73, posted on Nov. 04 2001,08:03

Mike, I've been considering doing the same when I yank the crappy 700-R4 out of my '82 Vette - rebuilding it and putting it in my Jimmy. Make sure you look for an '87-up 700-R4 when you're at the junkyard, the '82-'86 ones just plain suck. If I rebuild my '82 one, it'll be with beefier components.

There's a thing or two you need to figure out though - is the intermediate length of the tranny (from the bellhousing to the output housing mount) the same? Also, is the output housing bolt pattern the same for the 350 as it is the 700-R4? I see you've got a K10 (4wd) - do you know which transfer case you have? I know I've got a NP203, but I don't know if the bolt patterns are the same.

Ken

The length of the 700R4 is longer than the TH350, I've heard by about 3".

I found a place to pick up a beefed up 700R4 4x4 version for about $1500 (after it's all said and done, shipping, tranny, TC and a couple accessories) and a OEM-Rebuilt for ~$750 from the Auto Salvage.

The Tranny shop in town says he has done this swap a couple of times, and it is a 'simple bolt-up with a catch'. The 350 & 700 should interchange without much convincing. However, the Transfer Case and crossmember will have to be moved back 3" to accomodate the tranny's length difference. Because of this, the front DS will have to be lengthened, and the Rear DS will have to be shortened by the equal amount. Becuase of my lift kit, my front DS looks aftermarket, and possibly is "easily extendable", but I'm not exactly sure. It's a job I have a feeling I could tackle and do myself, however I have no shop, no lift, and very limited tools, so I'm just going to have to pay to have it done. The above mention shop quoted me $700-800

It's gonna be at least 4 months before I get financially comfortable to do this job. However, my TH350 is slipping quite a bit accelerating from stops, and if it goes out before It's swap time, swap time will be changed to accomidate. I will not pay a dime to have that 350 worked on - if I need to have it worked on, I will spend the $2K to just get rid of it and get a decent tranny in there.

I'm not sure exactly what Transfer Case I have - Is there an easy way to tell? It looks OEM, but with a truck that's been modified it's hard to tell.

Thanks! If you get your swap done, please let us know what milage improvements you see. I'm guessing a real-world improvement of 20-25% is easily attainable.

-Mike

Cap'n Hook · December 17, 2001

I've been lurking for awhile. But this 700R4 swap interested me. I've got an '85 C10 with a GM Performance Parts 350 HO Deluxe, with the Holley 650 and Vortec heads, etc, in front of a TH350 (recently rebuildt by my friend, an auto tranny specialist for $100 plus Borg-Warner parts), and a 3.08 rear. Now, this thing sucks the gas! It's supposed to be rated at 330 hp and 380 tq, but I think I'm loosing alot to the tranny!

With my previous 305 and Quadrajet, I got about 15 mpg. This thing gets from 9-11 mpg, depending on how heavy my right foot gets.

I'm also curious how much my mileage would improve from a similar swap to the 700R4. Plus, would I have to have a special driveshaft made for the increase in the tailhousing length?? Or could I just find a DS in a junkyard off one with a 700R4??

I'm gonna start driving my truck about 425 miles a week, and the OD on the 700R4 would be kickass, compared to the 1:1 on the TH350!!

Torque Junkie · December 17, 2001

You would need to find a different DS for it, and I immagine you could find one off an OEM 700R4 at the auto slavage for pretty cheap.

I think I'm loosing alot of power to my tranny too... I've got a 350 with an Edelbrock Perofmer cam, intake and 600cfm Carb, headers and dual exhaust with flowmasters. I'm not sure what the HP is rated at, but it should be enough to move that truck to 60mph from a start faster than 12 seconds. I'm pulling identical mileage numbers as you too.... in the 9-11 range - depending on my leadfootedness. I'm also driving 400-450 miles/week. I also have 4.56 rear end gears helping the engine suck more gas then it should, and I think my fan clutch is sticking engaged so it's constantly sucking a full-load of air. My next spare $100 will go into an electric fan conversion.

That tranny just seems like a big turd connecting my crank shaft to my transfer case. It slips, it leaks fluid, it has poor gear ratios, and since I about fried it 2 months ago (See this: http://www.gm-trucks.com/cgi-bin....15;t=68 it's got alot worse. I'm not fond of 3 speed transmissions - never have been and never will be.

I will keep everyone posted when I can get the swap underway, and how everything goes. It, unfortuantely, wont' be until spring at the earliest, i fear.

-Mike

76BBSub · December 18, 2001

I've been thinking alot about tranny swaps and OD stuff again, too. I had gotten a price on a Gear Vendors Under/Overdrive unit, which is a direct bolt-on to my TH400, but it will cost about $2000 for the unit, and I think I need to have my driveshaft shortened slightly. They claim their unit is good for something like 1000 HP, and I've seen cars at shows and in magazines running them, should last forever. But I think a built-up 700R-4 would be alot cheaper, and it has a lockup converter to help the gas mileage even more. I also think it would have less internal drag than the TH400. Since I don't do much towing it should hold up OK, although I do have a lead foot, and with the HP upgrades I'm planning for the 454 one of these days it might be a bit much for the 700R-4.

There are a few places doing high performance 4L80E trannies, which are comparable to TH400 trannies, but they have an overdrive and lockup converter. One of those is probably a little less than the Gear Vendors unit, but would still cost some serious $$$$. I don't drive that many miles so it'll probably be awhile.

Your slipping trannies are probably costing both of you quite a bit of power and gas mileage. The slippage also creates heat, which in turn will accelerate the death of your tranny...

Torque Junkie, cool signature pic!!

Jeff

Cap'n Hook · December 20, 2001

I frequent LS1.com and 3rdgen.org alot, and I remember a link to a site with built 700's for about $1700 USD with a warranty!!!! I'll have to do a few searches and see if I can locate this cat again. })

gslim · January 11, 2002

Ok I know its an old post but I thought I'd share my thoughts. The 700R4 swap is pretty simple with a few minor glitches. The driveshafts will need to be shortend and lengthend, the torque converter lockup will need to be wired in, you will need a special tailshaft installed, and the 700R4 doesn't like heat. If your gonna wheel it like it appears you do save your pennies and get this BAD BOY Now I know it will be expensive however several of the people over at coloradoK5.com have them and they abuse them extensivly. If you look at most of the 4X4 mags over the past year there trucks were in several issues doing moab and such. Besides they can build it with the special tailshaft already installed. Then get yourself a GOOD cooler. Route the trannsmission lines so that it goes from the tranny to the cooler, back into the radiator and then back to the tranny. This will keep your tranny at the optimum temp. Not to cold or to hot. Then get a tranny temp gauge. Then you will know for sure that you won't over heat it. Then you will need some sort of vacum set up to get your torque converter to lock. THIS IS WHERE THE BETTER GAS MILAGE COMES IN. Several companies make a setup that will make the torque converter lock for you so its not bad its just something that you will definetly want to do so that you get the good gas milage. So bacically you get the tranny with the tailshaft, then you drill new holes in the frame 3" back. Now you buy some new drivshafts (or have yours modified) then you wire in a vacumn lockup. Now you are good to go. Just so you know that is the tranny that is going in my blazer as soon as I get my 400 rebuilt.

gslim

76BBSub · January 16, 2002

Torque Junkie,

I had to have the 700R4 rebuilt in the Corvette I just picked up for my dad. It was $1450, total, including installation, which was about $300 I think. So, if you get a good core from the boneyard you can probably get a good performance rebuild on it for $1200 or so, depending on the parts. The gearsets in my dad's were in good shape, so I didn't have to buy new ones, but it needed a new torque converter and flexplate which added a little cost. Also, since it was an '86, which is a bit weaker, I had him install the newer style heavier duty parts, which cost a little bit more, but you'll definitely want to do the same for your truck.

Just an FYI...