72 K5 Blazer 700r4 upgrade

[PompUOP]

hey all,

My Blazer has a 350 w/TH350 trans and I’m looking to upgrade to the 700r4. Has anyone active on this forum done this already and what additional parts and consideration is involved, such as adapter plate, drive shafts, mounts, etc.?

Here’s my truck... just bought it recently

Steve

 

[shady]

Hello, nice truck, and welcome to the site.

I'm not much help on your particular question. Unfortunately. But some one will chime in.
You may get some fluff about the 700r4 because a few guys aren't fans lol. But hang in there if so. It's all in fun here.

You can try the search function here and may come up with something.

What transfercase do you have? That's a good starting point.

 

[shady]

One thing I do know is you will need to wire up a torque converter lockup circuit. There are ready made kits available, but it's not super complicated really.
Without it you won't be able to use the OD. It'll heat up and kill your trans.

And as an after thought, I do believe I remember the 700 being like 3" longer or so. Which means it's possible that drive shaft issues may come up.

 

[PompUOP]

It’s the np205. I figured there would be more to the upgrade, however I know it has been done on a handful of Blazers I’ve seen for sale. I’ve read some posts about getting the TV cable adjustment correct and risk of burning up the transmission. I’d like to get educated on the whole project so I don’t have any surprises. I’ll search the forums, too.

Right now, the TH350 in it has a 3200 stall convertor which I do not care for. Backing it into my storage with a 2” lip to the floor has me gunning it... imagine that!

 

[CK5]

hey all,

My Blazer has a 350 w/TH350 trans and I’m looking to upgrade to the 700r4. Has anyone active on this forum done this already and what additional parts and consideration is involved, such as adapter plate, drive shafts, mounts, etc.?

Here’s my truck... just bought it recently

Steve

Nice!
I have a 700r4 in my 72 but I bought it that way, PO installed TBI 350/700R4/NP241 out of a 91 Suburban.

I believe you will need an adapter plate and 27 spline drive sleeve like so: http://offroaddesign.com/catalog/700r4spacer.htm

 

[6872xtc]

Welcome!
I put a 700R4 between my big block and a NP205 back in the nineties. It's in my '70 K10.
The spline is the same between the 700R4 and TH350, just the length is different. The ORD website has a tech section which shows that.
You can get a shorter tail shaft and have it installed in the transmission, but this needs to be done with a full rebuild of the transmission because the output shaft is the first part to get installed. I used a spacer on between the trans and t-case so that I could use a 4L60E tailshaft which has better oiling that the 700R4 shaft. We did this to try and make it last better behind a big block. The cross member has to move for this swap anyway, so the extra 1 and 3/4" of the spacer wasn't a concern. I had to have driveshafts built anyway, so that was a given already. You will need to re-do both of your driveshafts.
The transmission cooler ports also move backwards slightly. I don't know if the hard lines for a TH350 will fit nicely if you bent them to fit.
The TV cable will probably require a different mount at the carburetor, but I highly recommend the kit from Bowtie Overdrives. It makes the transmission behave better, but I did reinforce the throttle cable mount of mine because my pedal stop in the cab wasn't quite perfect. I bent it and had to go back and fix it.
The factory shift arm needs to be redrilled to change the throw to get all of the gear positions. This isn't quite easy as the lever where the linkage pivot goes is hardened. I used a torch to heat mine up to drill. I believe that I had to move the hole up just above the original hole.
You will also have to get a way to lock the converter if you don't find a builder who knows how to make it lock automatically by modifying the valve body. A toggle switch will work, but then you have to remember to lock and unlock it.
And the last thing that I can think of is that the transfer case shifter will move rearward too. Unless you build a new one or modify yours to hit the original hole in the tunnel.

 

[6872xtc]

After watching the video, I know that it's a little difficult to tell for sure, but I don't know how much the cam in your engine will like a stock stall torque converter. That may be something to consider if you go with a 700R4. I wonder if you would be able to use a lower stall one in the existing TH350 and have less to change. I think that you would have to try it to see how it does. Not quite an easy experiment though.
Another concern of using 700, is if you spin the engine over 5500 rpm, a rev kit is recommended for the clutch packs to keep them from applying slightly at high rpm. It is a spring kit to keep the clutches from dragging, staying fully released.

 

[PompUOP]

Wow! Very concise on the modifications. Glad to hear that. I’m expecting that despite the amount of work involved it’s worth having the O/D in the long run. Sounds like getting a used 700r4 is better bc of the rebuild req on using the np205, which I think would be worth it. The shifter position updates will kinda suck just bc the tunnel has that custom rhino lining, so I’ll need to figure out how to make that work. I’d like to solicit some more experience on the O/D switch updates & options. Thanks for the info on all the mods. Could I still technically use the same 3 on the tree column shifter or go with a new one?

 

[PompUOP]

The 350 was bored .30 over to my knowledge, but I have no idea how much more power that provides. I could go back to a stock stall converter and see how I like that before going into this neck deep. Although for this truck, I think it would be worth the investment in the upgrade. I wouldn’t be ready to go full LS3 / 4l80e just yet but that also crossed my mind.

 

[imiceman44]

hey all,

My Blazer has a 350 w/TH350 trans and I’m looking to upgrade to the 700r4. Has anyone active on this forum done this already and what additional parts and consideration is involved, such as adapter plate, drive shafts, mounts, etc.?

Here’s my truck... just bought it recently

Steve

I am working on putting a 700r4 instead of the th350 in my k5 with a doubler.
I have a built 700r4 so I have to go with the spacer, but I have already the engine conversion to tbi from Howell so I should have the torque converter lockup.
The fact I have the doubler is making the job really hard and almost feel like doing the short shaft thing to eliminate moving everything.
I already have the front shaft good for both options but the rear needs to be done plus the triple stick shifter and the 3 crossmembers. I have it all out and now I am thinking.
I knew I had to do something with the shifter but now I know what thanks @6872xtc

 

[6872xtc]

I used the original column in my '72 C10, with the original shift linkage. That's how I found out about it. My '70 K10 has everything swapped from an '85 or so. Not exactly easy to do, but it was a full build at the time. So I learned things on both trucks which apply to what you are considering.

You will have to play with the neutral safety switch a little to get the reverse lights on at the appropriate spot too. No big problem.
The bore of your engine is only a small part of how it will pull in what rpm range. The camshaft is the biggest factor in my opinion. But if the tire size and gearing in the axles aren't low enough, then your engine may not be happy with the low rpms in OD until you get the speed up higher.
And you have to ask yourself how much you will be actually using OD and gaining on the long run. Especially if you end up wanting a LS /4L80E very soon.
My C10 and my K10 will both pull overdrive even at 1200 rpm because the cubic inches and camshafts work together. But you have to have good line pressure in the transmission at those low speeds to keep the lock up converter from slipping too. A good builder will make sure of that. It is just up to you to verify that the TV cable is set properly then.

 

[PompUOP]

Thanks again for all those who have replied today. Seriously the best kind of insights I was looking for!

 

[bassackwards]

I don't think you have to have a functioning lockup on the converter. They only lock up in 4th gear in cruising interstate type situations anyway. They don't lock up at all around town.

 

[6872xtc]

I don't think you have to have a functioning lockup on the converter. They only lock up in 4th gear in cruising interstate type situations anyway. They don't lock up at all around town.

That would depend on the gears and tire size. But you have to have lock up unless the engine is well above stall speed, otherwise the it will overheat the oil and risk damage of the transmission. I have 3 trucks that will all go into 4th above 40 mph or a little more. The '90 will then lock the converter, my old trucks I have to lock them my self, but they will do it. I have also had a couple of them that will lock the converter in 3rd when cruising.

 

[bassackwards]

That would depend on the gears and tire size. But you have to have lock up unless the engine is well above stall speed, otherwise the it will overheat the oil and risk damage of the transmission. I have 3 trucks that will all go into 4th above 40 mph or a little more. The '90 will then lock the converter, my old trucks I have to lock them my self, but they will do it. I have also had a couple of them that will lock the converter in 3rd when cruising.

The lock up is for fuel economy. The tranny won't get damaged if it never locks up or else every transmission in the world would get burned up in short order with the crazy fast-slow-fast-slow traffic we have around the cities. They lock and unlock continuously in most driving situations.

 

[shady]

Not true. In a 700 they don't lock until 4th gear. Part of the circuit you wire when installing one as a swap is a switch that activates when it's in 4th. When that switch deactivates so does the lockup.
Also in that circuit is the brake pedal switch and sometimes a vacuum switch.

 

[6872xtc]

Yes the lock up is for economy, but it will kill the trans if you run it unlocked at lower engine speeds. And @shady if you don't believe me that they won't lock in other gears, come out here and I will show you on 3 different trucks. My '70 and '72 will lock them in 2nd, 3rd and 4th. Because I have them on toggle switches. The 4th gear switch in the transmission is easy to bypass and some already have switches to lock in 3rd, it depends on the year of the valve body. My '90 will lock in 3rd on its own. Both the current trans will do it and the previous one did it before the doubler. The '90 never had a vacuum switch for the lock up because the ECM took care of it. That truck now no longer has the factory ECM because I put the Edelbrock Proflo on. No ECM trans controls.

I question how many 700R4 transmissions have you guys been into or owned?
Have you ever seen one torn down after it got burned up? Were you ever personally involved in building one of these, let alone several? How many miles have you put on a truck with a 700? I know that you don't known my history and what I have done or witnessed, but don't start your post by calling me a liar.
Try going back to the 80's and 90's when these non electronic transmissions came out.

 

[shady]

Didn't say they wouldn't. They don't in stock form on a 700 is all.

Newer, other, applications they might . No idea. But when hooked up correctly in an after market setup, the 700s usually only lock in OD.

 

[6872xtc]

Didn't say they wouldn't. They don't in stock form on a 700 is all.

Newer, other, applications they might . No idea. But when hooked up correctly in an after market setup, the 700s usually only lock in OD.

The OEM WILL lock in 3rd on later years. There is also a switch to lock the converter it the oil temp gets too high. And no, mine aren't locking because of the oil temp, I can get it to happen just after leaving the house. The aftermarket ones usually change switches and make it so that the guys wife can get in an drive the car and no have problems.

 

[AJs72K5]

Welcome!
I put a 700R4 between my big block and a NP205 back in the nineties. It's in my '70 K10.
The spline is the same between the 700R4 and TH350, just the length is different. The ORD website has a tech section which shows that.
You can get a shorter tail shaft and have it installed in the transmission, but this needs to be done with a full rebuild of the transmission because the output shaft is the first part to get installed. I used a spacer on between the trans and t-case so that I could use a 4L60E tailshaft which has better oiling that the 700R4 shaft. We did this to try and make it last better behind a big block. The cross member has to move for this swap anyway, so the extra 1 and 3/4" of the spacer wasn't a concern. I had to have driveshafts built anyway, so that was a given already. You will need to re-do both of your driveshafts.
The transmission cooler ports also move backwards slightly. I don't know if the hard lines for a TH350 will fit nicely if you bent them to fit.
The TV cable will probably require a different mount at the carburetor, but I highly recommend the kit from Bowtie Overdrives. It makes the transmission behave better, but I did reinforce the throttle cable mount of mine because my pedal stop in the cab wasn't quite perfect. I bent it and had to go back and fix it.
The factory shift arm needs to be redrilled to change the throw to get all of the gear positions. This isn't quite easy as the lever where the linkage pivot goes is hardened. I used a torch to heat mine up to drill. I believe that I had to move the hole up just above the original hole.
You will also have to get a way to lock the converter if you don't find a builder who knows how to make it lock automatically by modifying the valve body. A toggle switch will work, but then you have to remember to lock and unlock it.
And the last thing that I can think of is that the transfer case shifter will move rearward too. Unless you build a new one or modify yours to hit the original hole in the tunnel.

This covers most of what I was gonna say. Keeping the 205 is a more involved swap. The 700 is longer than the TH350. With a 241, I had to move my transmission crossmember back about 3 inches. Keeping the 205, you can likely expect more.

Regarding the stall on your converter, 3200 is pretty big. Find out what cam is in the engine and get the appropriate stall converter for the cam. With a healthy cam though, a "standard" vacuum lockup switch WON'T be happy. You'll either need to do a physical switch (as mentioned), or get a computer based controller that takes into account RPM, speed, and TPS.