SBC guys:TH350/NP203 to 700R4/NP241?

[snowbored]

I have a fairly new to me FJ60 with a TH350/NP203. I haven't heard great things about the part time kits in the NP203, and I do not want to go back to full time on it. At the same time, it would be nice to get the overdrive in a 700R4, to drop the crazy ridiculous weight of the NP203, and to get the lower first gear of the 700R4. What do you guys think, would it be worth changing these out? Will I see any fuel gains from the switch? Do I run the risk since the 700R4 is wider of having to modify the tunnel? My engine doesn't seem to be as far forward as most of yours. I guess I would need to move the cross member back, but from what I've read it sounds like the TH350 one will work for the 700R, just back a bit. Anyone heard about using 200R4s? A shop up here has been doing fleet truck conversions to those for even better fuel gains and so far they have been awesome. Should be smaller than the 700R4, but I haven't seen anything about matching it to a transfer case. I work from home, so even though it's my daily driver it averages 0 miles a day, I don't wheel hard, just camp really remote places with my family. But I do want to take this up to Canada/Alaska in the summers, so something better than being able to actually see the gas gauge needle moving toward empty would be nice.

 

[brokenparts]

I'd say drop the chain part of the 203 and add a 205 for a doubler. If you dd the 60 it might be worth switching to the 700r4.

 

[lehiguy]

The 700R4, as you have alreasdy stated, has two huge advantges; the lowest first gear of any automatic used in a car or pickup, and an overdrive. They are everywhere, I have seen cores for less than $100 locally. The NP241 is what I run, Dodge put it behind the Cummins TD for eight years in case you're wondering how sturdy it is. The low range reduction is awesome, they are stupid simple to rebuild, parts are everywhere and they are cheap.

 

[ntsqd]

No idea what tire size you're running, but I'll give my experience with most of the combo that you're talking about, though in a different vehicle.

I've a '91 Suburban with a TBI350/700/241 powertrain, 285/75's, and 3.73 gears. So tire size and gearing are close to an FJ60 on 33" tires. The Sub's net weight is nearly that of the FJ's gross weight. What I've found is that the OD works against me at speeds below 65 mph. The mileage is actually better at those speeds when in direct rather than OD. Once I get to about 72 mph the OD works to my advantage. I've actually gotten worse mileage holding to 65 in OD than I have pushing as high as 80. That "pushing 80" trip I averaged 15.7 mpg for the whole trip! This from something that big and heavy with one serious long grade on the route and a couple smaller climbs.
I'm sure that a big part of this is my particular engine's combo of components (stock). I have worked out that a 4.11 axle ratio would put the truck at the same rpm vs. mph as it would be with the stock sized tires.

For a turn-key 700R4 I would consider bowtieoverdrives.com I've yet to hear a complaint about them that wasn't resolved to the customer's satisfaction. A 700 can be built for less locally, but perhaps not with all of the good parts that they use.

I also plan to run a 700/241 combo in my FJ, so I've been looking at what the whole combo needs to be. The only downside that I see in the 241 is the slip-yoke. I plan to install an SYE kit and modify both output flanges to accept a Toyota companion flange.

 

[lehiguy]

No idea what tire size you're running, but I'll give my experience with most of the combo that you're talking about, though in a different vehicle.

I've a '91 Suburban with a TBI350/700/241 powertrain, 285/75's, and 3.73 gears. So tire size and gearing are close to an FJ60 on 33" tires. The Sub's net weight is nearly that of the FJ's gross weight. What I've found is that the OD works against me at speeds below 65 mph. The mileage is actually better at those speeds when in direct rather than OD. Once I get to about 72 mph the OD works to my advantage. I've actually gotten worse mileage holding to 65 in OD than I have pushing as high as 80. That "pushing 80" trip I averaged 15.7 mpg for the whole trip! This from something that big and heavy with one serious long grade on the route and a couple smaller climbs.
I'm sure that a big part of this is my particular engine's combo of components (stock). I have worked out that a 4.11 axle ratio would put the truck at the same rpm vs. mph as it would be with the stock sized tires.

For a turn-key 700R4 I would consider bowtieoverdrives.com I've yet to hear a complaint about them that wasn't resolved to the customer's satisfaction. A 700 can be built for less locally, but perhaps not with all of the good parts that they use.

I also plan to run a 700/241 combo in my FJ, so I've been looking at what the whole combo needs to be. The only downside that I see in the 241 is the slip-yoke. I plan to install an SYE kit and modify both output flanges to accept a Toyota companion flange.

For the record, the Dodge 1500s up untill 1994 used a passenger side front drop NP241 with a fixed yoke out output. They also had a mechanical speedo drive. Unfortunately, the bolt pattern is a bit different from the Chevy tail housings. I welded up the original holes on my NV4500 and drilled new ones to match the Dodge T-case. I changed the input gear to the 32-spline unit to match the NV4500 and had my speedo cable modified at a local shop to screw onto the NP241. It was much, much cheaper than they SYE kit and I got it clocked right where I wanted it. My speedo if off by about 2 percent after purchasing a different driven gear from a local Dodge dealer. And no, you cannot make one T-case from two different ones on the passenger side drop versions. I tried that. You can do it on the driver's side drop models.

 

[snowbored]

Well, it looks like I'm putting my play money towards a new gas tank, so I guess this is a mute point for now. The Dodge sounds like the way to go, but probably beyond what I can do. From ntsqd's comment, sounds like I won't get a great boost anyway. I'm on 31s, but plan to go to 33s come spring. Plus, I think I might get a little bit better gas mileage now, not having a hole in the gas tank. I've heard swapping tanks for one without a hole is good for 1 MPG at least, right?

 

[ntsqd]

I think replacing the 203 with something lighter is a good idea. To give you an idea of how much they weigh in comparison to a 241:

My '79 Suburban came with a 203. I swapped it out for a 208 and the 203 was put (by forklift) in the bed of my Xcab 4wd mini. When I got it back to my then shop I put a dead 33-12.50 under the tailgate on the floor. I could only-just-barely lift the 203 by myself. It was at the very outside of what I was willing to do and beyond what I probably should have done. No way was I going to attempt to bend over while carrying it. I brought it out off the tailgate and dropped it into the tire.

Contrast that with I was recently in Central Oregon at my Grandparents place when the 91's 241 decided that it wanted to be resealed, again. The gasket sealer that I'd used when I put new lip seals in it wasn't up to the job of sealing the case halves. I pulled the truck into Grandad's shop and pulled the 241 by myself with no lift & using a floor jack. The first time I had it out I put it back in by myself using the floor jack. I can carry a 241 around with care but without strain.

A 241 for a 700R4 should bolt to the rear of a TH350, but I think that the output shafts may be different. They are in a TH400 so I'd investigate this. Could be a simple swap if the trans output shafts are the same.

A holeless fuel tank should be good for umpteen mpg's improvement.

For my own deal, the $340 for an SYE isn't too big of a deal since I won't have any adapter costs (unless I do opt for the 5.0 Ford).

 

[lehiguy]

I think replacing the 203 with something lighter is a good idea. To give you an idea of how much they weigh in comparison to a 241:

My '79 Suburban came with a 203. I swapped it out for a 208 and the 203 was put (by forklift) in the bed of my Xcab 4wd mini. When I got it back to my then shop I put a dead 33-12.50 under the tailgate on the floor. I could only-just-barely lift the 203 by myself. It was at the very outside of what I was willing to do and beyond what I probably should have done. No way was I going to attempt to bend over while carrying it. I brought it out off the tailgate and dropped it into the tire.

Contrast that with I was recently in Central Oregon at my Grandparents place when the 91's 241 decided that it wanted to be resealed, again. The gasket sealer that I'd used when I put new lip seals in it wasn't up to the job of sealing the case halves. I pulled the truck into Grandad's shop and pulled the 241 by myself with no lift & using a floor jack. The first time I had it out I put it back in by myself using the floor jack. I can carry a 241 around with care but without strain.

A 241 for a 700R4 should bolt to the rear of a TH350, but I think that the output shafts may be different. They are in a TH400 so I'd investigate this. Could be a simple swap if the trans output shafts are the same.

A holeless fuel tank should be good for umpteen mpg's improvement.

For my own deal, the $340 for an SYE isn't too big of a deal since I won't have any adapter costs (unless I do opt for the 5.0 Ford).

The Chevy NP241s use a 27-spline input for the 700R4 and a 32-spline input for the TH400s and SM465s. Dodge and Jeep applications use a 23-spline. Swapping them is a piece of cake.

 

[snowbored]

If I just swap the NP203 for a NP241 or NP208, I would need to swap the cross member, get the 700R4 to NP241 adapter plate(but use it on my TH350), mess with the linkage, have my drive shafts redone, and change the yoke, right? I don't know, if I'm going to do that, I might as well bolt in the 700R4 it seems? Especially since every CL add around here has the transfer case mated to a 700R4. The Dodge idea sounds nice, but above the tools I have available right now. No matter what as of today I have to live with the 203 until spring (the extra weight will help with snow is what I'll tell myself, because, you know, the problem with the 60 is that they're too light, right?). If as ntsqd says, not having a hole in your gas tank helps with mileage (I have some doubts) I might just stick with the TH350.

 

[ntsqd]

The Chevy NP241s use a 27-spline input for the 700R4 and a 32-spline input for the TH400s and SM465s. Dodge and Jeep applications use a 23-spline. Swapping them is a piece of cake.

There was a length difference btwn the 203 specific output shaft and the 208 specific output shaft. This was in a TH400. I've no idea about the TH350's. If it's a bolt-on, then it's just a matter of drive shafts.