Keep the th400, or go OD

[Kay86K5]

I just purchased a 1989 V3500 crew cab that I am going to build into my dedicated tow rig. Right now it has a TBI 350, TH400, NP205, d60, 14bff with 4.10 gears (I just started a build thread on this thing but I know the driveway section is getting pretty clogged up and most people don't look at every thread in there now, so thought I would ask this question here to try and get more opinions on it).

I am planning to swap in a 8.1 and I keep going back and forth on if I should take the time/money/energy to swap in a OD tranny like a 4l80e/4l85e. Here is what I have figured the pro's and con's are of both options:

Keeping the th400 means keeps my costs lower for the overall build, don't have to mess with anythings as far as crossmemeber (4l80e is a little bit longer of a case) or lenghthen/shorten drive shafts, or messing with shifters (I have read most 4l80e's have the same shifter location as the 400, but have found a few threads where guys said they ended up being different for them, not sure if it's a generational thing). The 8.1 will bolt up to the TH400, so that isn't a big deal. Not having OD will mean when I drive on the highway I will be pushing higher RPM's which will reduce gas milage, but this is not a DD and when I tow I don't really like to go above 65mph anyway. It will sit in front of my house and spend most of it's time hooked up to trailers when driving. I will drive it every once in a while just cause but I would assume that would be more in-town driving, maybe occasional highway. If I swap the OD tranny in then there would be additional things to set up such as VSS and a wiring harness for the tranny along with harness for motor. The OD tranny's have the overall benefit that is obvious of not having the motor scream around 3000+ rpm's at 70mph which is why people take the time to do it. But even with my current rig that has a 4l80e in it, I am rarely in OD while towing.

I am sure I am missing some stuff and sorry for the long read here, but if this was your truck and you were going through the process of building this and didn't want to go absolutly crazy with it (I don't want this to be a show truck or something I drive everyday, just a really cool, reliable tow rig), what would you do? Let's here it

 

[dyeager535]

What do the RPM's work out to be when you plug them in for 60-65MPH? If you can keep speed in that range, will give you a better idea if it will scream or not.

Honestly if the RPM's would be within reason, for not so frequent use, I'd probably lean away from the 4L80E or variants, unless something fell into my lap. I can't imagine that any way you look at it, a 4L80E can be retrofit either cheaply, or easily. Obviously a '90-91 V series with 4L80E would make the swap easier, but what are the odds of finding one of those nearly free, and then look at the time required to swap pretty much everything. At least that's how I'd want to do it...wiring harnesses, cluster, etc.

 

[76k5blazerr]

If you are rarely going to tow above 65mph and rarely use OD when towing then why not keep the 400. With the right gears your engine should be at about 2500 rpm max at 65mph, with a good muffler and exhaust system I think that would be a comfortable enough ride (noise level wise). Me, I am usually forced to "budget build," and from a durability standpoint, a th400 is just as good as a 4l80, therefore i'd probably chose to save my money and keep the 400.

 

[sweetk30]

all above info and

400 to 4l80 = same 1-2-3-r gear ratio's . so no gain there other than lockup and overdrive .

www.grimmjeeper.com/gears.html gear calculator for rpm/tire size / ratio info .

 

[skunked]

Drive it with the TH400 for awhile and see how you like it.

What are you going to run for a tire? 33"?

My truck had the 454/th350/np205/4.10/35" tires for awhile and I didn't think it was too bad on the freeway.

 

[454Sub]

Spending the much much larger amount of money on the 8.1 swap, then cheaping out on the tranny and sticking with a dinosaur 400 seems like an illogical idea. You are swapping over the 8.1s electrics for all its advances, why not get something with the same and possibly more strength than a 400 capped off with lockup and OD.

Not to mention allows you to swap to a 241 case, which is nicer, and can be had with mechanical vss.

 

[diesel4me]

My vote would be stay with the TH400...the old "K-I-S-S" theory--keep it simple stupid (no offense !)--why go thru the extra expense of buying a 4L80E,the computer it'll need to shift it,deal with possible shift solenoid failures in the future,and endless other mods the truck will need to finally get it driveable--a crew cab's mpg will never be stellar period,no matter what tranny you used also..

In my opinion the "gains" would be minimal,and the headaches multiplied if you went with the overdrive..
The TH400 would be less costly to rebuild also..

I dont see why a NP241 couldn't be swapped onto a TH400,the NP208's are practically identical fit wise and thousands of those are in use...

 

[dyeager535]

You are swapping over the 8.1s electrics for all its advances,

That's a good point, if the 8.1L is going in with the later electronics, vs. a TBI retrofit, if the 4L80E was offered with the 8.1, I'd be inclined to agree that running the 4L80E would make a lot more sense.

If the 8.1L is going in with the newer electronics, you are going to need to get the 40 pulse VSS anyway, might as well swap in a 1990-91 V-series 241 case as well, which would take care of that problem.

But if it's a TBI retrofit, the electronics work necessary simply to run the 4L80E would be nearly as much work as running the 4L80E with the newer wiring.

 

[76zimmer]

my 76 CC has just over 31" tires, with the 454/T400/4.10's combo it runs a touch over 3Krpm @ 70mph. And gets 5.5mpg doing it. I doubt it would stay in OD pulling my load of 10-11K lbs. even on flat roads.
my 99 2500 has about 30" tires, 6.0l/4L80E/3.73's and runs 3K rpm @ 70mph....can't keep it in OD towing.
So my first thought is are you going to have enough power to run in OD regularly? I've towed with an 8.1 and on the flat pulling 7K lbs it would handle OD (Allison), but in the hills we just pulled it back to D (1:1)

 

[Kay86K5]

That's what I'm talking about!! I was hoping to get more responses by putting this in here . Thanks all for the replys, I really appreciate it.

Everything that is being discussed is exactly what has been going through my head. I will be towing every bit of 10K+ with this thing too. My best guess is my K5 weigh's right around 6k, my trailer is 2k, and I plan to get a in-bed over-cab camper for the back of this thing to go camping and wheeling with my family. So the likelyhood that I would even be in overdrive while towing is going to be minimul, so that adds to the decision. Getting bad milage isn't that big of a deal for me b/c as I already said, this will not be a DD, only a tow rig with occational use on the side just cause.

As far as the "effort", "complications", and "much much more money" of swapping in the 8.1 is actually false from my research. Swapping in an 8.1 is actaully simpler then swapping a normal 5.3 or 6.0 LS into these old trucks. The most expensive parts of the whole swap are actaully purchasing the motor and getting a full stand alone harness and computer tune for it (~$600). So to say I would be "cheaping out" by keeping the TH400 I think is not a fair statement, considering the 4l80e is no stronger of a tranny than the TH400, only the added benefits of OD, but also more complicated as it is electronically controlled.

I plan to put either a 2-4" lift with 33's on this thing, and the RPM calc says with 4.10's at 65mph that I am right at about 2700rpm's. Now, if I pump up to 70 or 75mph then we start getting to 3000-3300rpms. That is where I start wanting the OD for sure. At 65mph I don't think 2700rpms is that bad

 

[1-ton]

Towing with an OD trans can suck sometimes. They are always shifting into-and-out of OD when towing, and then you just end up pulling it into manual 3rd any way.

 

[Kay86K5]

Towing with an OD trans can suck sometimes. They are always shifting into-and-out of OD when towing, and then you just end up pulling it into manual 3rd any way.

Yeah, in my current truck with an 4l80e I always just keep it in 3rd while towing unless i'm on the highway on complete flat, but even then when towing at 65mph, it's kinda a weird speed for the trans where it would want to keep going back and forth between OD and 3rd

 

[1-ton]

Yeah, in my current truck with an 4l80e I always just keep it in 3rd while towing unless i'm on the highway on complete flat, but even then when towing at 65mph, it's kinda a weird speed for the trans where it would want to keep going back and forth between OD and 3rd

Congrats on the new buy! I am an owner of one of those Crew Cabs too. Mine is a 1991 Chevy Silverado V3500. I am putting a another engine in right now. I bought a new GM Goodwrench direct replacement TBI 350 for it, and just got done buttoning back up after installing a Pro Comp Computer Controlled cam into it.

 

[Kay86K5]

Congrats on the new buy! I am an owner of one of those Crew Cabs too. Mine is a 1991 Chevy Silverado V3500. I am putting a another engine in right now. I bought a new GM Goodwrench direct replacement TBI 350 for it, and just got done buttoning back up after installing a Pro Comp Computer Controlled cam into it.

That is awesome dude! Thanks so much. I have been looking at different tow rigs for such a long time and I, for some reason, just love these square body crew cabs. So when it came down to a decision on what to do, I thought it would just be so much cooler (and cheaper) to build one of these into a tow rig instead of just buying a used diesel with a ton of miles on it. For some reason these things get me excited!!

So when I found this one I had to jump on it as it's 100% rust free. I plan to repaint it, make the interior nice and have a huge 8.1 in there for towing. I think it will be just a awesome truck to always have around.

 

[jtk26]

You say you only tow at 65 mph with your current tow rig, but isn't the whole point of switching to a new one to have a better set up?... To tow faster, haul more, with more power. The speed limit on most western freeways is 75mph in open areas, and 65mph in hills/turns. So only cruising 65 puts you at the same speed of the weighed down semi's, maybe less depending on the road. That turns an 8 hour trip into a 10+ hour trip. And 65mph is brutally slow on open freeways. Plus you end up being that super slow guy who slows the whole group down during caravans. A one ton truck should be able to pull 10+k around flat in OD without even feeling it. If your truck can't do that then it's not a freaking one ton truck (even if the badging says otherwise IMO). My 2000 3/4 ton burb with a lq4 6.0L (has some basic motor mods), 4l80e, and 4.10 gears pulls my 10k trailer in OD at 70mph (2200 rpm's) super smooth and easy. The tranny never heats up in OD with my 10k trailer...EVER. Holding 3rd gear on flats kills the mileage, especially in an older tranny with no 3rd gear tc lock up.

A healthy 8.1L (400hp and 500ft/lbs) will give you plenty of power to run in OD with a 10k trailer (17k combined). Could probably pull 2-3% grades in OD without downshifting. Throwing a th400 behind that will you get your where your going, but will probably do it worse than your current vortec454/4l80e truck does. To get even moderately comfortable freeway driveability you will have to run at least 33's which definitely hurt takeoff/acceleration. Not to mention stock th400/4l80e's are only rated to 450ft/lbs and a 15k combined load rating. All of the above reasons is why chevy went to the Allison and had to make the 4l85e (22k load rating). The Allison gear splits are way better. Lower first, better gear selection, and still has overdrive. Most 8.1L's came with them and the harness is already incorporated to run it. In fact to not run one you will have to rework the harness and get a custom tune. 8.1L/4l80e combos are rare, are usually in burbs/avalanches and they had weak tunes with really restrictive torque management to keep the tranny alive.

To me... that truck deserves nothing other than a 8.1/Allison. It would be a KILLER towing machine with that setup.

 

[jtk26]

Towing with an OD trans can suck sometimes. They are always shifting into-and-out of OD when towing, and then you just end up pulling it into manual 3rd any way.

Only when you are short on power, requiring more throttle, causing the tranny to downshift more often.

 

[jtk26]

My vote would be stay with the TH400...the old "K-I-S-S" theory--keep it simple stupid (no offense !)--why go thru the extra expense of buying a 4L80E,the computer it'll need to shift it,deal with possible shift solenoid failures in the future,and endless other mods the truck will need to finally get it driveable--a crew cab's mpg will never be stellar period,no matter what tranny you used also..

In my opinion the "gains" would be minimal,and the headaches multiplied if you went with the overdrive..
The TH400 would be less costly to rebuild also..

I dont see why a NP241 couldn't be swapped onto a TH400,the NP208's are practically identical fit wise and thousands of those are in use...

I would never put a 208 in a one ton truck. That is barely a half ton t case.

 

[jtk26]

Spending the much much larger amount of money on the 8.1 swap, then cheaping out on the tranny and sticking with a dinosaur 400 seems like an illogical idea. You are swapping over the 8.1s electrics for all its advances, why not get something with the same and possibly more strength than a 400 capped off with lockup and OD.

Not to mention allows you to swap to a 241 case, which is nicer, and can be had with mechanical vss.

Agree with above...

However I think it would be cooler and stronger to get an electronic speed sensor on the np205 and keep it

 

[1-ton]

I would keep the T400 and put a stroked 383 small block into it. A decent 383 stroker can be built out of a TBI engine. It just needs a set of edelbrock center bolt heads, a bigger TBI unit (bore out manifold to go with it), and some custom PROM tuning. Must be careful with cam selection though with a TBI/ECM engine. The T400 with a 205 transfer case may be a dinosaur, but it is a proven...bullet proof combination.

 

[campfire]

Only when you are short on power, requiring more throttle, causing the tranny to downshift more often.

This is a function of gearing. If your gearing matches your load & conditions, you won't see this hunting and pecking problem.