Upgrading from a turbo 350 to a turbo 700

[imgtx]

I have an 84 C10 truck I use as a parts runner. The turbo 350 tranny is going out. A friend gave me a TH400 tranny in good shape and I am going to swap. Upon first inspection I found the dipstick different, No Problem.

It appears the truck frame has multiple places to mount the crossmember and the length is the same (both long tail housings). So far, So good.

The TH350 has two electrical connectors. One two wire on the passengers side and one 4 wire (3 terminals used) connector on the drivers side. The TH400 only has the 4 wire connector on the drivers side. Uh oh?? It that gonna be a problem?

What else am I going to find? Is the yoke the same?

Right now the truck runs and moves but it won't for long. I want to go into this swap all parts in hand so it will be less painful.

Thanks for the help.

[nailhead425]

Originally Posted by imgtx

I have an 84 C10 truck I use as a parts runner. The turbo 350 tranny is going out. A friend gave me a TH400 tranny in good shape and I am going to swap. Upon first inspection I found the dipstick different, No Problem.

It appears the truck frame has multiple places to mount the crossmember and the length is the same (both long tail housings). So far, So good.

The TH350 has two electrical connectors. One two wire on the passengers side and one 4 wire (3 terminals used) connector on the drivers side. The TH400 only has the 4 wire connector on the drivers side. Uh oh?? It that gonna be a problem?

What else am I going to find? Is the yoke the same?

Right now the truck runs and moves but it won't for long. I want to go into this swap all parts in hand so it will be less painful.

Thanks for the help.

Your current trans is a TH350C with a lock-up converter. Converting to the TH400 will require a TH400 yoke for the trans and the driveshaft will need to be shortened. The trans crossmember will slide back to the correct position, but that and the bellhousing pattern (it is Chevy style, right?) are the extent of the bolt ins. You'll need to make sure you have a dual or large patern flexplate for your engine. The TH400 requires a switch for kick-down and this must be wired up and in place. Not doing so will nuke it in short order.

In all honesty, just rebuild the TH350C, you'll get better mileage, won't spend as much as the swap, and you know it fits.

[imgtx]

OK I feel stupid now.

I found out that it is not a 400 but instead is a 700.

Is this a good or bad swap.

It came out of a Van so I think I can get the driveshaft out of the Van if needed but Is the yoke and length the same as what I have got?

I wouldn't mind rebuilding the 350 but I am in a budget crunch so I need to save money if possible. This tranny was free, a rebuild will be at least 250 once I get a rebuilt TC. I wouldn't dare use this one again.

Thanks again guys for the help.

[nailhead425]

The 700 is even longer than the TH400. It will also require the drive shaft to be shortened. The good news is that it uses the same yoke as the TH350. You'll need the correct bracket and linkage on the carb for the 700 TV cable. This is critical to proper operation. You can use your existing computer to lock up the 700, by dropping the pan and connecting the o/d lock up wire only. Normally, it will lock in 2nd, 3rd, and o/d. It isn't needed, but unless you cruise at 2000rpm or higher with a stock converter, you'll need the o/d. Also make sure the cooler lines and trans cooler are very clean.

My advise? Sell the 700, rebuild the 350C.

[2005 Equinox LS]

I wouldnt mess with trying to make the 700R4 fit when you have a 350 to rebuild. It will likely cost the same either way. And the 350 will last longer. It will be much easier.

[avro206]

Originally Posted by 2005 Equinox LS

I wouldnt mess with trying to make the 700R4 fit when you have a 350 to rebuild. It will likely cost the same either way. And the 350 will last longer. It will be much easier.

These trucks came with 700R4s--maybe not your year but your generation did.

You'll get better accleration AND mileage. Win-win situation

[nailhead425]

Originally Posted by avro206

These trucks came with 700R4s--maybe not your year but your generation did.

You'll get better accleration AND mileage. Win-win situation

Not true. The correct gears w/ a TH350 will generate better acceleration because the spacing between first and second gears is closer.

A 700 will pop you off the line faster, but unless you have a very strong low-end pull, it will fall flat on the 1-2 shift. The reason the 700 has a 3.06:1 first was to give the feeling of a strong engine, with one that had very little torque.

The mileage difference between his 350C (lockup) and a 700 aren't as great as you might believe. Plus, 700s are weak.

[avro206]

Originally Posted by nailhead425

Not true. The correct gears w/ a TH350 will generate better acceleration because the spacing between first and second gears is closer.

A 700 will pop you off the line faster, but unless you have a very strong low-end pull, it will fall flat on the 1-2 shift. The reason the 700 has a 3.06:1 first was to give the feeling of a strong engine, with one that had very little torque.

The mileage difference between his 350C (lockup) and a 700 aren't as great as you might believe. Plus, 700s are weak.

not true.

Yes there is a more pronounced 1-2 rpm drop but it won't matter that much because you'll be so far ahead due to the stronger off the line. As for mileage---a 30% drop in rpms will give better mileage period. 1:1 to 0.70.
Lower rpms = better mileage.

Better tell GM to drop the 4 speed autos and go back the dark ages!

Early 700R4s were not that strong but after 87-88 they are. Besides if he's putting it behind a realtively stock engine there won't be any problems.

[nailhead425]

Originally Posted by avro206

not true.

Yes there is a more pronounced 1-2 rpm drop but it won't matter that much because you'll be so far ahead due to the stronger off the line. As for mileage---a 30% drop in rpms will give better mileage period. 1:1 to 0.70.
Lower rpms = better mileage.

Actually, it is true for many reasons.
1st) Without changing the final gearing, he will get to 35-40mph before the 1-2 shift, and the RPM drop is 47%, which puts a greater load on the engine. Most acceleration advantage is eliminated on the 1-2 shift.
2nd) Again, without changing the gearing, in O/D the RPM might not be high enough for the engine to operate efficiently. Typically, 3.08s came in trucks without O/D and 3.42s for those with. Too much load in O/D can hurt mileage, especially if the tires are tall (like on a truck) and the vehicle is rather heavy (like a truck).
3rd) Without the benefit of the correct PCM controls of the lock-up, the O/D benefits will be negated if/when the converter doesn't unlock when load increases. Especially at lower RPM.
4th) Even with all the best parts, 700R4, 4L60,-E, 4L65E, cannot be built to take the same level of torque as a built 350 or 350C. The reason is the lower first gear reduction.
5th) for the amount of time, agrivation, money, and modifications that must be made for the 700 to operate correctly, it is simply not worth it.
BTW, I am speaking from extensive experience.
For this person, in this case, his money is best spent on the existing TH350C.

Originally Posted by avro206

Better tell GM to drop the 4 speed autos and go back the dark ages!

The 700 was born of the low-torque engines of the early 80s, that WAS the dark ages. A time when performance was laughable, mileage was poor, and quality was lousy. It is horribly outdated, and evey though there have been many improvements, it wasn't until the electronic valve body that it became a much more reliable transmission. The PCM control over the operation of the transmission far out weigh the strength and durability of the TH350.

[2005 Equinox LS]

I know about how the early 700-R4's and the 200 4R's were in the 1980's. Dads 700-R4 and my 200-4R both needed rebuilding after just over 100,000 miles. And if you dont get the TV cable adjusted just perfect you can ruin the transmission or at the very least it wont shift right. Computer control is the best thing to ever happen to modern transmissions.

[allenengel]

How do I post a question to this site like the "Upgrading from a turbo 350 to a turbo 700" exchange.
Allen

[pimp_squeak]

Read this post, then hit "back" at the top of the forum, below the title bar, but above where the threads are listed on the left side is a little button labelled "new thread". Click and have fun ^_^