swapping out a th400 to a 700r4

[dhurst29]

hi i have a 87 chevy c20 with a bad th400. so i want to make an improvement and gain overdrive due to the drive i make back and forth from home to school. i was wondering if this can be done? i dont see why no i figure i would have to change my steering column cut my driveshaft and move the transmission crossmember. is there anything else im missing and if so please let me know if there are any kits available to do this swap Thanks for your input

 

[89Suburban]

Welcome aboard dude, I am about 15-20 minutes north of ya.

These guys will help ya. Any chance you know your rear axle's gear ratio? Any pics of the truck?

 

[towjoe]

Steering colum is the same on all automatic trucks. Just the tin indicator on dash is different. What all do you use the truck for the big question. If you do a lot of heavy hauling the 700 may not hold up. Also what ratio rear axle do you have? For strength and durability may be best have your trans rebuilt and change out rear axle to a higher gear.

 

[dhurst29]

i only use the truck for a daily driver and if i ever do haul its very light im not sure its the stock 14 bolt underneath it currently

 

[dhurst29]

im really trying to make it better on the highway and be able to cruise at something faster than 60 and i figured this would be the time to make the change and swap the th400 to the 700r4

 

[bucket]

My C20 Suburban with th400 has no trouble at all cruising down the highway at 75 mph all day long. 3.73 rear end and tires that are around 32 inches tall.

 

[dhurst29]

ahaha im with you on suckin gas and haulin ass im just on a college students income and thats what i have my 79 camaro for

 

[bucket]

Lol, it's good for 13 mpg. For comparison, that's a 454 with no overdrive, 3.73 and 32's. My K20 Burb has a tbi 350 with a 700R4, 3.73, 33's, and the best I have ever done is 12 mpg, but it's usually just barely 11 mpg. I know it's comparing a 2wd to a 4x4, but I would call it a wash if all things were equal. Point is, a big heavy brick going down the highway doesn't always benefit from a weaker (and more $) overdrive transmission.

I think a 700R4 would only benefit you if you also swapped rear gears to keep from lugging the engine down on the hiway. Like 4.56 gears.

 

[dhurst29]

alright so i should just get a tranny guy with a good reputation to rebuild mine and yeah i have the 350 tbi and i get around 9mpg since when i rebuilt the motor i bored it to a 355 and put a little bigger cam with true dual exhaust with hi flow cats and the holley 670 cfm tbi and a tbi spacer.

 

[woodsrider250]

I'm interested in this swap info as well....not so much how bad of an idea it is. My junk turns 3k at 65, and that's unacceptable if I'm going to keep driving this truck on more than side roads. A built 700 isn't much more than a built 350/400....yeah don't tow in OD, blah, blah. Just a quick rundown of the specifics would be awesome. I can assume a 700 from a square would be a direct swap-in size-wise? Need donor crossmember? Kickdown setup? Shifter adjustment? This may have all been covered before.

 

[dhurst29]

from the research ive done ive gather i need to go with a late model 700r4 because they are stronger lockout the converter change the tin in the steering column and relocate the trans cross member back a couple of inches but can use the th400 crossmember. also cut the driveshaft and use the 700 yoke and spline and also if i do get the late model 700 i need to switch the speedo to mechanical. and also slightly change the geometry of the kickdown cable bracket also if yours is not tbi the kickdown bracket will have to be modded more from the info ive found.

 

[dhurst29]

and im at the same point with you mines prob about 3k at 65 i got a 60 mile drive to school so i burn almost a half a tank on the way to school and its not cutting it for me