View Full Version : Full-time 4x4 advantage

03-24-2001, 06:48 PM
Hi, some of you guy's seem to really know you're K-Trucks. I'm hoping you might be able to help me out with my research, any info would be greatly appreciated.

I'm planning on buying a 73-87 K10 LB 350/auto/NP 203 T-case

I want the full-time 4x4 because of the superior and safer road handling that comes with this system. I currently own a 98 Jeep Grand Cherokee with Select-Trac which gives me the option of full-time, part-time, or 2wd. I leave it in full-time 4x4 since it is very clearly more stable when cornering at speed, and there is little or no difference in fuel economy, just as the manufacturer claims.

However, I'm hearing that the NP 203 is a real gas guzzler. How bad is it? Are there any other problems with this T-case? does it give the same handling advantages that I get from my Jeep? Or, are there any other full-time systems available to fit the K-Trucks?


03-24-2001, 08:27 PM
I have a 74 k20 with the np203 case which I converted
to part time about 2 years ago. I presently get 13mpg on a good day 12 on a bad with sm465 4 speed and 4.10 gears. I drove it for a year before converting the case and the truck did stick to the road like flypaper but the best i ever did for mpg was 10.
The case is also VERY heavy weighing in at about 170 #s

hope this helps

03-24-2001, 10:27 PM
Thanks for your reply. Yeah 10mpg is very dismal, I could'nt live with that. Doing 20k miles/yr it would cost about $1,500/yr more on gas than what I spend on my Jeep which averages 18mpg. But I'll bet a Chevy K10 could squeeze at least 15mpg with a modern V6, 3.08 final drive in the lighter GM 10-bolts, even with the NP203?

mike reeh
03-25-2001, 12:58 AM
yeah i had a swb full time truck with stock sized tires.. it definately stuck to the road.. but gas mileage suffered.. I dont necessarily think its because of the case.. most people over the years have said the coversion for the 203 case does *not* make a significant dent on gas mileage. in other words that would not have been the reason to convert.. saving on drivetrain wear was the main reason...

i dont think 15mpg would be an unreasonable goal to set if you're putting together a truck, especially with overdrive.. but I dont know if Id even bother with a V6 :) also your choices are limited to less than 73-87 trucks.. it was not offered all those years (the 203). I forget the years exactly, but its well documented, if you do a search (on say, google). you could also not get a 6cyl (I6) in 4wd.. not while the 203 was offered, at least. of course you could put it together, but would it really be worth the $ and trouble for a few mpg? definately your choice, but thats about all I can add.. I definately like the 203 case though..

good luck-


03-25-2001, 08:56 AM
I had a '78 K10 with 350/350/230/3.08 and it got 12 mpg. Didn't seem to matter how hard I drove it, it got 12 mpg. Loaded or empty it got 12 mpg. I wish I had it back, I loved the fulltime system and am seriously considering finding another one and bringing the body back from the grave so I can drive it for a few years again.

03-25-2001, 09:05 AM
Mike is absolutely correct but there is one more thing i would like to add. I don't want to offend any Six cylinder people out there but the whole family of Chevrolet big and small blocks are the best choice for these trucks. There are literally books of parts and knowledge to change the personality of these engines to fit your needs. Anything from low end torque to high end horsepower and at prices that are easier on your wallet than other brands.
I owned a 292 "6" in a 67 c10 and a 235 "6" in a 57 3600. They were great engines but they were no where near as adaptable as the V-8s.

Just a thought

03-25-2001, 10:42 AM
You were right about the years Mike. I've since learned that 73-79 was the run for NP203's.And you both make a good case for the V8, these trucks are heavy and a '6' would have to work harder than it was designed for. I read somewhere here that an engine is most fuel efficient when it's operating near peak torque. Most of my driving is highway so I guess I need to select a V8 that peaks out somewhere between 2000-3000 RPM. I stumbled across this very useful formula: RPM=(336/tire diameter)x MPH x gear ratio. with the 35" tires I plan on using x 65MPH x (3.08 final drive x 0.70 overdrive)= 1345 RPM!!! Either this formula is bunk! or a 3.08 with 35" tires and an OD tranny aint gonna work. More like 4.10 or 4.56 even, what say you?


03-25-2001, 02:15 PM
4.10 sounds pretty good to me using your formula
thats about 1800rpm in overdrive and you still have low range for hauling (thats about 3600rpm in overdrive)

hope this helps

03-25-2001, 11:38 PM
I hate to be the bearer of bad news, but the OD tranny you speak of is the 700R4. Couple of things here...First, i dont think it was avalable til the early 80s, however im not sure, so if this is the case, you are going to have to swap a tranny and so on if you want to use the 203. Second, if you do get a 700R4, its only a matter of time before you need to rebuild it. They were updated several times on the old units, but they were always a problem. This would mean you have to settle for the good ol TH400 or TH350. I would go for the TH400, cause for the most part, she is bulletproof. It is up to you what to do, but if you want my opinion, here it is. I would go with the NP205 case, its cast iron and gear driven. They are the strongest out there i believe. Also, as Mike Reeh and reallyrusty pointed out, a six just doesnt have enough power. Even if they were offered with 4 wheel drive, just pulling the truck is a chore for the 6 cylinder, ESPECIALLY in 4 wheel drive. Good luck to you and i hope i have helped out a bit.


03-26-2001, 12:44 AM

Isn't the 700R4 also computer controlled?


03-26-2001, 01:48 AM
Thanks Rich.

But i'm sticking with full-time 4x4 and that means a 203. I'm determined to get at least 15mpg and worry free service, so I will be installing all new/rebuilt major mechanicals. I'm thinking a tranny with OD (th700r4) mated to a fuel injected 350 (TPi maybe) that makes peak torque close to highway cruising speeds, and geared to compensate for the 35" tires I want to use. You definitely have a lot of agreement on the 6cyl issue *thanks guy's, I'm over that now*. However there's mixed reports on the 700r4...I don't know, I have little expierience with GM/Chevy, that's why I'm checking with you guy's. But so far I'm not convinced that it won't get the job done...read John DiMartino's 2 replys to the post "700r4", last reply 01-29-2001 05:18am PT.

Unless of course, the 700r4 simply won't fit the 203 T-case?...then I'll have the mix-n-match blues.

I'd like to get more opinions if this is't getting old already. Just to give you more background; I'm planning on doing a frame up restoration with an alternate rebody, it's called a Badlands RT. You can see it at http://www.badlandsrt.com Hope this dosn't disqualify me from posting here though...after all it'll still be a tried and trusted Chevy underneath the skin, and the tail pipe will announce that fact.

03-26-2001, 07:32 AM
I know nothing about the 700r4 transmission or its bolt patterns but if rtallday is right and you still want to use the np203, you might want to consider useing the th400 with and after market overdrive unit. They cost about $2000 and they are aimed at the RV market.I can't tell where to look for one but i have seen them on the web.
hope this helps

03-26-2001, 07:39 AM
i don't know if this will work or if they have one for this application but it's worth a shot

03-26-2001, 07:56 AM
sorry for all the posts in a row but i'm not awake yet.

Talking again about the 700r4 didn't ford or dodge use the 203 case in a divorced application (shaft from trans to case) there might be another way to approch it.

03-26-2001, 08:27 AM
The late model 700R4's were good trannies. If you have one rebuilt it will have all the latest updates and should be fine. Just make sure that the TV cable is adjusted properly.
Also Advanced adapters probably makes an adapter to mate the 203 to the 700r4.
But if it was me I would be happy with the part time 208 or 205. The 203 just doesnt make any sense. Full time 4wd in trucks behave alot different than in SUV's.
If you want the full time 4wd just engage the hubs and lock in 4 wheel hi.That way you always have the option of using part time if you like.

03-26-2001, 03:16 PM
Great leads guys, thanks (reallyrusty, plowking35).

I did'nt know the NP208 & NP205 could be used full-time, I thought they were part-time only systems. Are you sure that they can operate reliably this way?

As far as differernce between SUVs & trucks, I admit my Jeep SUV handles more like a car. But I used to own a '78 Jeep Pioneer J-series p-up truck which had full-time 4x4 and it had that same unshakable feeling in the corners that my Grand Cherokee has. And I could be mistaken here, but I think the Pioneer had the AMC version of the NP203.

Also, I contacted Advance Adapters and they do have a kit to mate the 700r4 to the NP203:-) AAAnndd they say they could help me mate the Jeep trans/full-time case assembly that I love so much, to a GM350. Only then I'd have to get Dynatrac to custom build a front axel since the shaft runs down the RHS, not to mention mounting, speedo, and other incompatabilities. But it could be done, probably get better gas milage too since its a lighter case with less friction...Oh boy! funny how these projects snowball, but I'm not commited yet it's just research at this point

03-26-2001, 06:59 PM
From what i remember about gas prices in San Fran, I would be running a gerbal powered truck if it was me.
The quadratrack system is probably the best out there.
What I meant about them behaving differently is, and I am not sure about the jeep, but most newer AWD systems use a viscous t case that spilts the power as needed to the slipping wheels. The 203/208/205 all split 50/50, and will cause increased driveline, steering and tire wear. Not to mention kill fuel mileage.
As for the right side off set front axel housing, why couldnt you just use an axel from a jeep ford or dodge. Some steering and leaf spring pad mods would have to be made, but might be cheaper than a custom unit from dyna-trac.

Chuck Smith
03-26-2001, 09:35 PM
Dino is correct. It would be easier to use the Jeep axle. Which, is most likely a Dana axle.

As far as the NP 203, the way it operates as fulltime 4wd, is that it has an internal differential. It allows the front wheels to turn at a different speed than the rear wheels. In fact, it typically uses the front axle to move the truck more than the rear wheels. When you move the fulltime 203 case into one of the "Loc" positions, it locks that internal differential in the t case. Then it behaves the same as the NP 205, or NP 208.

I learned a few fast lessons when my 77 still had fulltime 4wd. First of all, if you have it in 4wd Hi or Lo, with the truck in "park", and jack up one tire, the truck will roll!
You have to put the t case shifter in one of the Loc positions to keep the truck from rolling when you jack it up.

When the truck is in 4wd Hi, and you stomp the gas, the front tires try to spin before the rear tires.

Since the NP 203 uses no locking hubs, but instead uses a solid gear to connect the axle shafts to the hubs, your front tires will wear "scalloped" on the edges.

A few quick facts...

To convert a NP 203 to part time, a solid gear is used in place of the internal differential.

The NP203 is a cast iron, chain driven case.

The NP 203 is the heaviest case (out of the NP 205, and NP 208)

The NP 208 is chain driven, and has an aluminum case.

The NP 205 is a cast iron, gear driven case.

When you convert a NP 203 to part time 4wd, you gain an additional gear.
Normally, you have 4 Hi, 4 Hi Loc, N 4 Lo, and 4 Lo Loc.
When converted, you have 4wd Hi, 2wd Hi, N, 2wd Lo, and 4wd Lo.

You can get a ton more info on these trucks on my website.


Chuck Smith
03-26-2001, 09:37 PM
I forgot to mention running a truck with an NP 205, or NP 208 t case in 4wd with the hubs locked on dry pavement can result in SEVERE damage to the drivetrain.


03-26-2001, 10:50 PM
Heres some more ups and downs. The NP 203 case, as chuck smith mentioned, is the heaviest case of them all, and it is a chain driven unit. The NP 208 case is aluminum and is also chain driven. The down falls to this are first, the chains do stretch eventually causing the units to fail. The NP 208 is aluminum, great cause its lightweight, but less durable, if you were to maybe hit a rock on the beach, etc. The NP 205 is a great case, gear driven, and cast Iron. 4 low is not as low as the 208's but its still great. As far as automatic Transmissions go, the TH400 is the way to go and you should look into the overdrive unit as reallyrusty pointed out. Plowking35 also pointed out what i forgot to say earlier. The late model 700R4 tranny is a decent one, the earlier units are the ones that have to be rebuilt, trust me on that, i went through that headache with an 86 I had. Plowjocky, to answer your question, I am not sure if the 700R4s are computer controlled, im sure the late model ones are, but the earlier one has some wires and stuff inside when you pull the pan down. So if anything they are internally controlled or something, i really dont know. Good luck Houlihan.


03-26-2001, 11:44 PM
Yep, gas is expensive and gerbals are scarce. They have a different use for gerbals out here Plowking..(don't ask).

I'd like to reply to some of the great suggestions you guy's came up with, but I gotta go make a call right now. Catch ya latter.

03-26-2001, 11:48 PM
I thought I saw an ad in one of the car or truck magazines for some place that was rebuilding TH400 with a fourth gear. Not sure about that or if they are available with the correct tailshaft to hook to a transfer case but it might be worht browsing a few magazines. Worst thing would probably be that you could never get one repaired locally if it went bad.

03-28-2001, 04:58 PM
Thanks for clearing up the confusion about the NP205/208. I've always understood 'Part-Time' to mean SLIPPERY CONDITIONS ONLY.

As far as the difference between SUVs & trucks, I admit that my Jeep SUV handles more like a car. However, I used to drive a '78 Jeep Pioneer J-series truck with full-time 4x and it had that same unshakable feeling in the corners that my Grand Cherokee has. Now I might be mistaken but I think it had an AMC version of the NP203. So I think I have good reason to believe that a K-truck with a 203 should deliver the same Pitt Bull grip on asphalt. But apart from that I think your right, it won't steer, brake and track the same as some car-like SUVs.

I agree that the Quadra-Trac and the more recent Quadra-Drive set-up are probably the most sophisticated systems out there. You could have 3 wheels buried to the axels and the one remaining wheel could pull you out of it. But there is one flaw to these AWD systems (from my perspective anyway); It's an automatic system, and on dry pavement your in 2wd like it or not. When things get slippery it senses wheel-spin ( not wheel slide, as in sideways) and that's when it becomes a 4x4.

Am I the only one who appreciates the wet or DRY highway traction advantage of a TRUE FULL-TIME system?...I guess it's because I'm a sports car driver at heart. I drive 4x4 truck-like vehicles because my work and lifestyle dictate that choice (construction, skiing, and the occasional bout of mud-plugging). So I'm gonna try to build a rigg that can do a little bit of everything. It won't be as strong as some of your riggs on the trail, but with care and religious maintenance I think it could handle some of the intermediate trails around the Rubicon at Lake Tahoe where I hang out. And when I get back on the highway, I'll stay safely in control when I swerve to avoid that Ding-Dong in the Honda who inexplicably brakes in front of me while doing 70mph. And I'll be getting my sports car fix as I wind along hywy 1 on the way to the beach.

So, I think I'm gonna pull a stock, post '87 350EFI/ 700r4 package out of a donor and have Advance Adapters help me mate it up to an NP203. That way I'll get the ECM, harness, sensors and all the other doo-da's and avoid some of the mix-&-match headaches. I just need to make sure I select an engine with a torque/power curve to suit my mostly highway cruising and get respectable MPG.

The th400 with aftermarket OD is useful info, though I'm erring on the side of a late 700r4.

By the way Chuck, your dead right about the tire scalloping. My Jeep Pioneer did it and my Grand Cherokee does it, but it's not severe. I got 60k miles out of the original tires. I think it's just an unavoidable consequense of the steering wheels being driven. I've noticed that FWD cars are prone to that too, especially if driven hard.

Well, I think I've talked this thing to death. Thanks to all for the input, it's really helped me to think it thru and narrow it down. I promise not to bug you guys again for...aaahhh at least a coulpe of days anyways.

mike reeh
03-28-2001, 09:11 PM
interesting what you guys have said about the tires not wearing correctly, in full time setups.. the full time truck that I had, was pretty much all stock, esp. regarding suspension and tires, etc. and that thing had the most perfectly wearing tires you could ask for.. it was *noticeably* good.. even without regular rotation/balances.. i thought it was probably just an added benefit from the fulltime 4wd. i guess not. maybe I was just lucky ;)


03-28-2001, 09:16 PM
Sure sounds like it, Mike.

My first 4X4 was a 1975 K-5 Blazer with locking front hubs and it still cupped front tires like crazy, but then again I drove it like a bat out of hades. Maybe the young and dumb driver (me) was the one that cupped front tires like crazy.


03-28-2001, 09:27 PM
Well Houlahan, good luck with whichever path you choose, and the one you choose doesnt sound too bad. Keep us posted and let us know where you decide to go. Advance Adapters makes good stuff, a friend of mind has dealt with them. So good luck to you.