TH400 to 700r4 Swap questions

[BIGBLU 72]

so i am thinking about swaping a 700r4 into my 68 LWB. its a stockish 400 small block and a 3.08 rearend.

what will i need to do the swap, what problems might i run into?

the reason for the swap is my truck runs out of power around 55 mph and is getting terrible MPG. with a 3.08 rear gear ratio it should be performing much better so i guess its the trans thats giving me problems? what do yall think?

[timski69gmc]

Good time to bring this one up. I am looking at swapping out a TH350 to a 700R in my 69, and wondering what kind of fun I am in store for.
Hopefully get some good replies.

[s_nova69]

I would not think that your running out of power because of the TH400. Putting in another gear will only make the truck that isnt making enough torque at that speed struggle more.
On the other hand I put a 700r4 in a k20 I have and it is fun. It took a few tries at the right governor to get the shift points right. I bought the edelbrock throttle cable/tv bracket and it works good. The tv adjustment works together w the governor in dictating shift points. You'll love the first gear and the OD is nice on the freeway. But I would work out your low end power loss first because putting in another gear will only make it more pronounced. Just my two cents

[BIGBLU 72]

the 400 is cranking out 410 ft/lbs of torque so i dnt think that is the problem. the trans thats in there now shifts so lightly you cant feel it? is it possible that the trans isnt hitting 3rd gear? the converter is a 2100 stall too if that might have anything to do with it?

[BIGBLU 72]

another thing it does when it gets to about 55 the motor starts making an ungodly thrashin beatin sound

[westofb]

I agree with S nova, the overdrive will exaggerate your highway speed power loss. Your current set up should have plenty of power at road speed. To make the swap you will at a minimum the following issues: they are different in overall length, thus your driveshaft will be too long I think ( I think the 400 is shorter); the 400 tailshaft is too large for the 700, it the 700 is the same as a 350; the transmount will be different. They may be others these are the ones I thought of off top of my head. If you do the swap, you might consider a rear gear swap to a 3.73 or 4.11 to really take advantge of the overdrive by increasing your low end power, quicker lanches, etc.

[Mike Bradbury]

With a th400 you would know if it was not getting to 3rd gear. The rpm's it would take to get it up to 55 would be very noticable.
I am swaping to a 700r4 as well. I have already had it rebuilt and it is sitting in my garage waiting for me to get off my butt and put it in.
You will need to have the drive shaft shortened and balanced. Also you will need a new crossmember that fits the 700 (this is an over the counter type purchaces, the 700r4 is a very common swap). Also a o/d shifter on your steering wheel. several more odds and ends
I agree with nova, the 3.08 gear is more of a car gear, the 700 has a lower 1st gear and the rest are higher. The 700/3.08 combo will only make your loss of power at freeway speeds worse. You'll get great gas mileage but you will sacrafice passing giddyup.

[Mike Bradbury]

Originally Posted by westofb

I agree with S nova, the overdrive will exaggerate your highway speed power loss. Your current set up should have plenty of power at road speed. To make the swap you will at a minimum the following issues: they are different in overall length, thus your driveshaft will be too long I think ( I think the 400 is shorter); the 400 tailshaft is too large for the 700, it the 700 is the same as a 350; the transmount will be different. They may be others these are the ones I thought of off top of my head. If you do the swap, you might consider a rear gear swap to a 3.73 or 4.11 to really take advantge of the overdrive by increasing your low end power, quicker lanches, etc.

Wow I have never had an almost identicle post put up at almost the same time,
You are a faster typist than me

[BIGBLU 72]

Okay so i just rebuilt the motor and put a shift kit in the trans so where could my problem be? the parts i put in the motor claimed 350 horse and 410 ft/lbs of torque which i think would be plenty to go as fast as i want with my set up, right?

[pmpski_1]

Originally Posted by BIGBLU 72

another thing it does when it gets to about 55 the motor starts making an ungodly thrashin beatin sound

Just me, but I'd get to the bottom of that engine noise before I go thinking about a tranny swap.

[chevy_mike]

Originally Posted by BIGBLU 72

the 400 is cranking out 410 ft/lbs of torque so i dnt think that is the problem. the trans thats in there now shifts so lightly you cant feel it? is it possible that the trans isnt hitting 3rd gear? the converter is a 2100 stall too if that might have anything to do with it?

Originally Posted by BIGBLU 72

another thing it does when it gets to about 55 the motor starts making an ungodly thrashin beatin sound

I think you have a bigger problem than your trans or rear gears.

Is your 400 a SBC or BBC? If a small block, do you have the correct balancer installed? If you replaced it and did not get an externally balance balancer for a SBC 400, then there is your MAJOR problem. You are beating the engine to death.

3.08 gears are more then fine at highway speeds. That is what my '98 has, which has a 4L60E (electronic 700R4) and around town I can't even use 4th gear. It totally bogs it. Only use 4th on the freeway. I can still cruise in 3rd gear at 65mph and only am turning about 2800rpm's. Your larger tires (assuming the 4x4 in avatar) are not helping low speed power but at 55mph and above, you should be okay.

Another thing, "putting in parts" that claim XXX amount of HP or torque is BS. They claim these numbers on an exact build, using very specific parts and tuned for max power on a dyno. Real world is very different. Unless you built the EXACT engine combo, tuned exactly, you'll never get those numbers. It's all about the hype to sell parts. Only way to know what you are putting out (unless you are buying a name brand crate engine) is to run the engine on the dyno.

Good luck but fix the engine problem before anything else.

[BIGBLU 72]

its a small block 400 and i got the right balancer. what is confusing to me is that i can stomp it and have it floored up to 55 and have no problems at all but when it hits 55 it sounds like its gunna come apart. I dnt have a tach but i would guess its turning upwards of 3500 rpm??????

[chevy_mike]

If you just push the gas in park or neutral and let it rev up, does it make any noises like that?

Before you do ANYTHING else, figure out what the noise is. I could be a rear end problem, u-joint/driveshaft problem, tranny or engine problem. Heck it could be something simple like a broken motor or tranny mount.

Find this problem FIRST!!!! If you have to buy a freakin' cheap tach at Pep Boys or something. Something is better then nothing and spending $30-$40 to keep from spending a few hundred or thousand dollars is smarter.

[BIGBLU 72]

Nah it dnt make the same trashin noise when its in park or N but i thought that might be because its not under any load from the trans?

but i agree that im gunna find the motor noise before i anything else

[chevy_mike]

TH400 are about the strongest trans out there and it takes a lot to break one. Usually it is lack of maintenance that kills them.

With the engine off and the truck in Park, crawl under and see if you can rotate the driveshaft. Check the U-joint and pinion areas for slop.

You know what just dawned on me, I am assuming it is for the '68 LWB in your signature, my guess is going to be the carrier bearing. It will make a thrashing noise, especially as the speed goes up. This will also rob power and cause other damage to things. Grab the driveshaft at the carrier bearing and see if there is any movement in any direction. It should be fairly tight and not move.

Willing to put my two cents on that part.

[westofb]

Do you have the right flywheel on the motor? I believe the flywheel for the 400 sbc differs from other sbcs. I am grasping at straws, maybe you put the shift kit in wrong, you say it sounds like motor is turning 3500 rpms at 55 mph, sounds like trans is not shifting into drive, that is way too many rpms unless you rear tires are like 15" in height.... LOL. Good luck.

[bamabaxter]

Do you have vac advance hooked up? When you get up to speed and start letting off of the throttle, this is when vacuum starts to rise due to engine rpm and the butterflies closing. Your vac advance could be pulling too much timing and causing detonation. If that is the only time that it makes that noise, when you reach cruising speed, it seems like a good place to start. Only way to really check the amount of advance from the vac canister is with a vac pump.

[bamabaxter]

I guess you could unhook the advance, plug the vac line, and take it for a cruise down the road. Just a suggestion.

[westofb]

Do you have the right flywheel on the motor? I believe the flywheel for the 400 sbc differs from other sbcs. I am grasping at straws, maybe you put the shift kit in wrong, you say it sounds like motor is turning 3500 rpms at 55 mph, sounds like trans is not shifting into drive, that is way too many rpms unless you rear tires are like 15" in height.... LOL. Good luck.

[olyeller70]

I just changed my 350 tranny for a 700R4 three weeks ago. All I can say is do it, 4th gear in 700R4 is usually a .7 versus your 3rd gear in a 350 tranny that is 1. I am enjoying the gas milage.
My suggestions:
Go to www.700r4.com and put in your tire size and back end ratio to get the drive and driven gear numbers for the speedo, changing these is not fun after you install the 700R4. Have builder install these for you.
Also if you have a 6" tail on the current tranny you will have to shorten the driveshaft about 3".
You will have to move the transmission crossmember or get an after market one that is made for this swap (about $55 on EBAY)
I used the TCI tourque convertor lockup through the brake switch, wiring inside the pan and vacuum to the carb. Do this wiring in pan before installing into truck also.
I had to get a longer speedo cable, mine was the original and a little short.
And connect and install the TV cable correctly, if you do not you will have to but another transmission.

Good luck

[CUSTOM_1O]

Its the carrier bearing ive have to replace 2 of them myself my truck is a longbed with the turbo 400 as well and one thing i would never do is change that i love it