M1008 with 4.56's what RPM is safe

[btrapr]

I have been reading here and there trying to figure out a safe RPM range for sustained runs on the Interstate. I am trying to figure out what combination of parts I need to make this drivable on the Interstate since it is the only truck I own at the moment!

The RPM calculators are showing around 2260 RPM's with 30.5" tires, 4.56, auto trans A 45MPH________I believe that is the max speed rating the military gave them.

If I add a 700r4 and leave my gears with the 30.5-31" stock military tires it looks like I will be around 2286 RPM's at 65mph...WHICH is within the above listed range! I do not really want to do a lift kit and large tires.

WOULD this be a safe range for like a 2-3 hour drive? OR would I be better off switching to 4.10's OR 3.73 (which adds a another part list)? I am trying to keep the cost to a minimum! Transmission swap vs. gear/trans swap!

Is anyone running the 4.56's and a 700r4 on pretty much the stock drive train?

thanks in advance!

 

[Alucard-HD]

I have been reading here and there trying to figure out a safe RPM range for sustained runs on the Interstate. I am trying to figure out what combination of parts I need to make this drivable on the Interstate since it is the only truck I own at the moment!

The RPM calculators are showing around 2260 RPM's with 30.5" tires, 4.56, auto trans A 45MPH________I believe that is the max speed rating the military gave them.

If I add a 700r4 and leave my gears with the 30.5-31" stock military tires it looks like I will be around 2286 RPM's at 65mph...WHICH is within the above listed range! I do not really want to do a lift kit and large tires.

WOULD this be a safe range for like a 2-3 hour drive? OR would I be better off switching to 4.10's OR 3.73 (which adds a another part list)? I am trying to keep the cost to a minimum! Transmission swap vs. gear/trans swap!

Is anyone running the 4.56's and a 700r4 on pretty much the stock drive train?

thanks in advance!

If it makes you feel better i ran my m1008 to NJ from PA, bout an hr trip. and i was at 70 and it was screaming, i dont plan on that trip again until i get larger tires. You can put a 700R4 in there but you loose a lot of strength. The over drive is nice but when you start throwing lift kits, big tires and more power through it, it tends to get angry. I love the turbo 400 because of its strength. i plan on putting larger tires under mine to lower the RPMS a little, we'll see. Gear swaps are expensive if it was one or the other i would swap the tansmission.

 

[319]

I'm not the expert but if you're doing an occasional 2-3 hour drive I'd leave it the way it is and just keep it 55 or under. If you're driving a fair amount above 45 I'd change the gears and maybe the trans. These use a lot of fuel above 45 MPH with 4.56 gears.

 

[Alucard-HD]

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[Cucvnut]

i run mine at 65 all the time im sure its nor uber great for the trans mission and motor. but then again a 6.2 is a ticking time bomb anyways.

 

[Alucard-HD]

i run mine at 65 all the time im sure its nor uber great for the trans mission and motor. but then again a 6.2 is a ticking time bomb anyways.

Really? you dont like the 6.2? I dont think its complete junk, but still lol.

 

[wallew]

Just plug and play your info into this calculator...

Gear Ratio Calculator

I ran my M1028A2 (not the A3 that I originally thought - it has the NP205 transfer case) 1800 miles from Scranton, PA to Denver, Co over four days.

I CONSISTENTLY ran 65 mph. Which according to the Gear Ratio Calculator link above, I was going 2844 rpm. at 70 mph you are turning 3063 rpm. NOT that I'd want to run that high all day, but at 2900 rpm mine ran great. STILL DOES. OK, TH400, NP205, 4.56 gears, tire size is 235/85R/16. Plug those numbers in and check what I've just said.

Unless I'm mistaken the 6.2 does not redline until 3200 rpm, though I could be wrong on that.

OH, at 65 mph I got 10 mpg. At 60 mph, I got 15 mpg. Read about it here:

http://www.steelsoldiers.com/cucv/37101-1800-miles-no-license-plates-half-pack-smokes.html

Kind of an FYI, I sold my M1031 to a friend of mine who changed the gears out to 3.53's and he runs 75 mph ALL DAY LONG and gets at least 15 mpg - makes sense, he's only running about 2600 rpm. So, there are a lot of choices out there. I THINK he paid his mechanic like $750 installed, so that's WAY cheaper than buying a rebuilt 700R4.

 

[btrapr]

How did yuor firned manage to put 3.53's in there...I was told that with the Corporate 14 bolt rear and a Dana 60 front that the highest gears that match up are 3.73's and to got hat route with my tubes...I need the change the carriers and all to the tune of aorund $1200 in parts..

I wished I could do some 3.42's...that would give me an all around gear BUT not able to unless I give up the one ton axles!

 

[jatonka]

I have very little experience with CUCVs as compared to M35s, but I run one CUCV as my daily driver and I never drive it over 55 MPH. Mostly I run between 45 and 50 MPH. That gets me good fuel mileage and doesn't make me think engine parts are going to exit of their own accord. If I needed higher speed, I would change axle ratios way before converting to another transmission with over drive. There is no killing a TH400 if you maintane it. Even 4.10s would be an improvement over the 4.56s. IMHO. JT out

 

[91W350]

Governed to hit 4000 rpm with no load and 3600 with a full load, I must say, that is absolutely screaming for a 6.2. They peak on torque at a relatively low 2000 rpm, anything beyond that rpm is just beating on the engine. Sure, you can run 3000 and may get away with it for 30-40-50 thousand miles, but that extended high rpm will cause engine failure. A lot of people consider the 6.2 a throw away diesel, but in the civilian market, it is not unusual to find happy owners with over 300,000 miles. The key to long life though, is engine rpm. Those high mileage trucks are almost always 1/2 ton pickups, Blazers or Suburbans. The big four wheel drives and Dually trucks were geared too low and span the engines to failure. Normally it is a constant pounding of the block, causing cracks in the main bearing webbing. Once cracked, they flex enough that you cannot keep main bearings in them.

I always liked the 6.2, sure it has a poor fuel distribution pump and is not huge on power. But it did what GM wanted it to do in a wonderful way. They were looking for a fuel effecient light duty diesel to replace the nightmarish Oldsmobile 350 gasoline engine converted to diesel. They built the 6.2 and some of those guys that bought them were talking mid 20's for mileage running down the highway in a 1/2 ton Suburban, that is pretty amazing when you think about it. Those guys were much happier with their 6.2 than were the guys that bought them to pull race car or horse trailers and they gave out at less that 75,000 miles.

The light truck diesel craze that we enjoy today started with Dodge and the 1989 Cummins powered pickup. They would pull a heavy load, run on the governor all day and some of those engines were/are still running and approaching or passed a million miles. The diesel horespower wars started and it did not take GM long to figure out their 6.2 and 6.5 engines were not going to take the competition on. They improved the block design on the 6.5 and governed them at a lower rpm, but even with that, they were failing from cracking in the main bearing webbing, thus we now have the Duramax, which is not my favorite diesel due to injection system nightmares, but nobody can deny the power a Duramax is capable of producing.

I took the liberty of copying and pasting out of the TM in the sticky at the top of the CUCV forum. Glen




Model . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 6.2 L
Type . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . Four-cycle, liquid-cooled, naturally aspirated​

STANDARD METRIC​

Dimensions:
Length . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 35 in. 89 cm
Width . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 28 in. 71 cm
Height . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 28 in. 71 cm
New weight, dry . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 650 lbs 295 kg
Cylinders:
Number . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . Eight
Arrangement . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 90° V
Firing order . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 1-8-7-2-6-5-4-3 (clockwise)
Bore . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 3.98 in. 10.1 cm
Stroke (nominal) . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 3.82 in. 9.7 cm
Displacement . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 392 cu-in. 6.2 L
Compression ratio . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 21.5:1
Maximum Torque (gross) . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 260 lb-ft @ 2000 rpm 353 N​

​

​

​

•m @
2000 rpm
Governed Speed:
Full load . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 3600 rpm
No load . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 4000 rpm
Idle speed . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 650±25 rpm
Lubrication System:
Type . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . Pressure feed
Operating pressure (normal) . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 40-50 psi 276-345 kPa
Operating pressure (at idle) . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 10 psi 69 kPa
System capacity (including filter) . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 8 qts 7.6 L
Operating temperature (normal) . . . . . . . . . . . . . . . . . . . . . . . . . . . . 180°-260°F 82°-126°C
Oil pump . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . Gear-driven
Cooling system:
Type . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . Liquid with fan and radiator​

Operating temperature (normal) . . . . . . . . . . . . . . . . . . . . . . . . . . . . 190°-230°F 88°-110°C

 

[Alucard-HD]

I agree. I wish i had a Duramax in my HD lol

 

[wallew]

Well, I own two CUCV's with the lowly 6.2 engine.

Neither of my vehicles see more than 2000 miles a year. I'm retired. I don't get out much. I drive which ever truck the task at hand requires.

And if my 'whaling' on the 6.2 engine causes it to fail at 50 - 60k miles, that's OK. I have a spare in the can. Plus spare tranny, spare transfer case...

Everybody whines, moans and complains about these engines. And yes, the CIVVY engines fell apart quite rapidly. Mine puked it's rear main seal at 130K miles. But it had a Banks turbo on it and was greatly stressed DAILY long before I bought it.

I basically donated it to a charity for the tax write off, because back in early 1990 the military wasn't selling 6.2 engines as surplus. Nor CUCV's for that matter. They were all still in service. And a rebuild on the civvy engine was way more than the Sub was worth. A shame too, because it was a nice truck other wise.

Can't say why folks think they can't find 3.53 gears (I asked about 3.54's and was corrected to 3.53's). They ARE available. All you gotta do is buy them and install them.

Then you are going to get good mpg and higher top end speed at lower rpm. Problems solved. Running 75 mph puts you at about 3000 rpm, which the engine will run at. Running 65 mph puts you at 2600 rpm. Not that I would want to run ANY 25+ year old vehicle at 75 mph without some SERIOUS upgrades on the brakes and other systems to make sure they could handle the stress.

Because lets be honest. The CUCV's were envisioned to run convoy speeds of the day. And as has been said over and over again, that speed was basically 45 mph. THAT'S what these vehicles were designed for, geared for and where they run the best and get the best mpg...

They can last quite a while with proper maintenance and a good supply of spare parts (engines, trannies, transfer cases, axles, etc) and they were available on the cheap over the past ten years or so. Though the supply is beginning to turn from a flood to a trickle, parts are still out there. Not as cheap, but they can still be found.

It just depends on where you want your money to go. Car payment or maintenance bills. I've opted for maintenance instead of car payments. I might change my mind one of these days, but for now, I'll stick with my two CUCV's. And a fair pile of parts to keep them running.

 

[Alucard-HD]

 

[ken]

I have 3.40's in my 82 3/4ton. With a 6.2 and a banks turbo,4speed manual, NP205. But i have the 10bolt front axle. The tallest gear for your dana 60 is 3.54. So the tallest combo you can run it 3.73's. I drive my M1028 betwwwn 45 and 50. I've already slung apart a balencer or both of these trucks and had to replace engines. I try to keep RPM's down now. If im in the M1028 i just leave early and keep it slow. It gets 17MPG at 50. But the civi truck gets 26MPG at 65MPH. I sudjest swapping to 3.73's that way you can keep your drive train strength. The R4 is not even close to the strength of a TH400.

 

[bbqfan5909]

Have you thought about larger tires?

 

[91W350]

It is hard to beat a 4.56 gear down by running a taller tire. A 50" tire with no sidewall flex will give you 71 mph at 2200 rpm. Not sure what kind of body-chassis-lift-fender trimming that takes to fit.

The early Dodge 4x4 Cummins trucks had 3.08 gear ratios in the Dana 60. Other than that special application for the early Cummins, 3.54 is the tallest I know of.

I am sure the military bought them to run convoy speeds of 45 and still be able to pull a decent load in all kinds of terrain.

They are what they are and I really enjoy mine. Glen

 

[SPECIALTYLC]

There was quite a few K30s built with the 456 gears and they were meant to run all day long at legal speeds of 55 and 60. I drive my 1008 at 60mph and if it wont survive this then I will throw it away and buy something different.

 

[Wolf.Dose]

After propper brake in the engine is good for a very long live time, 300,000 to 500,000 km or 200,000 to 300,000 miles. You have to care for regular oil change and the propper spec. of the oil. Than it runs well also on long distances between 2700 and 3200 rpm if the noise does not disturb you. In my M715 the engine is like an automatic transmission, from 16 mph to max speed no shifting required, like an old steam engine. Very nice driving, even with a loaded trailer.
In Germany max speed for trucks over 3,5 tons GVW is limited to 80 kph (50 mph), which means about 3000 rpm with an acceptable fuel consumption, without trailer about 18 l per 100 km (13 mpg), with trailer 20 l - 22 l/ 100 km( 11,7 - 10,6 mpg). Most slopes on the autobahn do not exist any more, so the fuel consumtion is ok. Maximum possible speed is clearly over 60 mph. But that costs a lot of fuel, which we call Expresszuschlag (Express exra fee).
Wolf

 

[91W350]

I hope that you guys are right and I just live in the land of poor 6.2s. I was just at the GMC dealer today with my M1008 getting the front toe set, it has been pulling to the right a little. Anyway, the shop manager and the business owner both want my M1008. The shop manager said he used to work for our local Chevrolet dealer prior to making the move to the GMC dealer. He said he used to do a lot of head gaskets, seals, fuel leaking issues, fuel pumps, and block replacements on the 6.2 engines. He also told me to figure out what speed 2400 was and to try to stay under that. I am in no way knocking this engine, I like them, just trying to share what our local guys tell me. I had dropped it off yesterday and went back by to get some tires and wheels out of my M1009, tossed them in the bed of the M1008. He was all jacked up about the M1008, not quite so excited about the M1009. He said the M1009 sounded good and asked me to look for a nice M1008 for him. When I picked it up today I asked him if he took it for a test drive. He said "No, I was afraid we would not come back." With a big grin he gave me the keys and I was off.

 

[akonitony]

With my Favorite M1009, then one I drive the most anyway, I went with Synthetic oil, and added a very large transmission cooler, then stay with the 4.56s and TH400, and after a 6" lift, put on some 36" tires. This is the poor man's fix. The trans cooler will keep the temps within range for the engine, and the synthetic oil never forms sludge, so you get no wear. I have logged well over 100k miles between my two properties in Alabama and Florida, which are about 447 miles apart, with no breakdowns.
If you want a nice 700R4 that will take everything that 6.2 will give, and also let you tow in overdrive, I can build you one for about $800.