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73 k20 th350 to th400?

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bu2chlc

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Hi,
Does anybody know what I will need to know to upgrade my k20 from a turbo 350 to a turbo 400? My 400 small block has tons of torque but I am afraid my turbo 350 trans is not gonna be happy when I start doing some towing. The trans seems to be in nice shape, no slipping, crisp shifts, so maybe I will sell it when I am done. I have a 2wd th400 sitting in the garage, but I am not sure how to make it a 4wd trans to mate up with my np205.
Does anybody have any thoughts?
 
I'd just run your TH350,put a good sized cooler on it and keep the fluid & fillter changed often,and you'll probably have no issues..the TH400 is a nice upgrade,but how much of one is questionable--they waste more HP than a TH350 and they can fail too,they aren't always bullet proof..(I've had TH350's outlive TH400's in many older muscle cars!)...

To use your 2wd tranny,you'll have to gut it completely,and install the proper tail shaft needed for 4wd--then you'll need the right transfer case & adapter to bolt up to the Th400,they have larger input shaft splines and more of them (32 Vs 27 for the TH350)..then your driveshafts might not fit,if the length happens to work then the u-joints will be different!..see where this goes??..might as well swap in 8 lug axles too!..:doah:...
 
I agree, I wouldn't waste your time (and money) to do this.

Martin
 
rear output shaft on the bolt on yoke or short shaft slip yoke is the same as a fig 8 205 output shaft.
 
I agree with diesel4me, if your 350 is solid, put a nice cooler on it and run it. I have towed cross country with a big block/350 combination with no troubles, no worries. It is a great little trans when you build it right and keep it clean and cool.
 
It makes me nervous, but I respect the opinion of everybody here. I will get the fluid changed and upgrade the cooler this weekend. Did I mention it makes me nervous??? Since I already have 4.56's.... maybe I will just put 29in tires on it and feel much better! My truck will be at full speed before I get out of my driveway but at least the tranny will be happy :D
 
TH-350s are tough transmissions, I was going to up grade to one till I helped a buddy take apart both, if you are going to be towing heavy loads there is no point in getting a 400, even if you are making a ton of power. Maybe if you are making a ton of power and you launch the thing hard alot, but even then I would prefer the 350
 
blazinzuk said:
TH-350s are tough transmissions, I was going to up grade to one till I helped a buddy take apart both, if you are going to be towing heavy loads there is no point in getting a 400, even if you are making a ton of power. Maybe if you are making a ton of power and you launch the thing hard alot, but even then I would prefer the 350
Click to expand...

why are there so many th400s in service then? the military put em in all the cucv blazers and they don't tow much. they must be better in some way. I overheated one of my th400s til it blew atf out the dipstick tube. Topped it off at the next station and kept on drivin it. I thing they're good tuff trannys. of course i've never worked a th350 to failure either...

just put a sm465 in :thumb:
 
use to run built 400 sb and th350 /205 setup in 7klb 1ton turnin 38" tires on 4.10 gears. never one problem.
 
For what it is going to cost to put the TH400 in your truck you can rebuild the TH350 to handle more power then the TH400 in stock form.

The only real weak point in the TH350 is the intermediate sprag and that can be fixed for about $300. Spend another $250 on associated parts and the trans will handle anything short of a blown big block.

Put a decent shift kit in it and you will be fine. The most damage comes from those nice smooth shifts that everybody seems to like today. That is from the clutches slipping against the steel plates. Slippage equals wear.

The trans in my truck was built very similar to this and has no problem with a 500 horse 383 or towing a 7000 lb load. Without the trailer it will snap your head back though. The motor was enough to twist off a rear shaft doing a burnout.

You said you have an extra TH350. Buy a book, a few tools, and some parts and go to town. The TH350 is a fairly simple trans, now the 700R4 is another story.
 
thanks for putting the time into your response. Do you know anybody that makes a package that includes the parts you mentioned?
 
A TH700R4 isn't much harder than a Th350 to rebuild, they are very similar transmissions.

Martin
 
I ran a 2wd TH350 with a Rockwell divorced t-case in my Chevelle wagon that is in my avatar for 8 years,behind a 454 that was stock other than a mild cam and a Holley carb (750 cfm Q-jet replacement) --I whaled on that thing pretty hard,so did the guy who built the car,it had a 1969 350 4 bolt from the Suburban the frame was from in it before I swapped the 454 in it...never had any trouble with it..

I've got nothing against TH400's,though I have seen more of them fail than the Th350's in older GM muscle cars..the main drawback to a TH400 is the fact you must change not only the tranny,but the T-case and a bunch of other things to swap one into a 4x4..internally,the clutches and parts are not much larger than the Th350,both share the same torque converter,the output shaft is bigger on a TH400 ,but I cant recall ever seeing a TH350 break one myself....
 
oh ya and fyi if th400 over th350 say good bye to 20-30 hp to the rear wheels for bigger parts that eat more power. :eek1:
 
bu2chlc,

If I remember right this was the base kit I bought. https://www.700r4l60e.com/store/product.php?productid=34
My kit was modified a little from this one as parts were machined for extra clutches.

I have this manual and it is one of the best I've seen. Even details some of the more common mods, like extra clutches. http://www.amazon.com/Turbo-Hydra-Matic-350-Handbook-Sessions/dp/0895860511

You'll need a few specialized tools but nothing super expensive. A few step are so much easier with those. I have a cam bearing installation tool and I used it to put in the bushings. They can be found on ebay fairly cheaply.

In building mine, I acquired enough parts to build two more. There are two different sun shells that I know of. A standard duty and a heavy duty. The heavy duty is alot thicker and you know it if you can put them side by side. It really isn't necessary for what you want to do, but extra insurance never hurts.:D

The intermediate sprag, go here and scroll down to 13X. Went up a little in price since I bought mine, but still a good deal.http://www.transmissioncenter.net/350_or_400_high_performance.htm#350 Level 1

My trans in builod very close to the Mega raptor level with a few twists.
 
wow! you've done everything but rebuild it for me! Since you seem to know much more than I do, how about one more question... Do you have any thoughts on what temps I should be concerned about (both coming out of the tranny and going back in)? I am not sure what temps are normal, and what temps should make me pull over and let it cool off.
 
I'm pretty sure normal operating temp is around 200 degrees. Mine runs at 180 due to an aftermarket trans cooler. I got rid of the stock one when I put in a four core radiator from a diesel. Kinda necessary here in AZ, especially when the daytime temp can be well over 100 degrees.

I would be concerned with anything over 250 degrees, but it all depends on what clutches you put in it. Some will take alot more heat then others.

Take your time when removing and installing the bushings. A few extra minutes here will pay big dividends in the longevity of the trans.

Tear into it and let us know how it works out.
 
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