[bds1904]

My 91 suburban needs a new tranny, xfer case and rear end, and the old 350 with 270,000 miles burns through some oil. I have the opportunity to purchase a good running m1028. I run 37's on the suburban. To keep the cost of the swap down, i would like to take the entire drive-train out of the m1028 and put it in the suburban. Axles, tranny, motor and everything. From what I understand the m1028 axles are 4.56, which means the 6.2 will be turning about 3000RPM on the highway at 70.

Is 3000RPM too high for the 6.2? I only drive about 15-20hrs on the highway a month. I don't tow with the truck either.

 

[northwest offroad]

My 91 suburban needs a new tranny, xfer case and rear end, and the old 350 with 270,000 miles burns through some oil. I have the opportunity to purchase a good running m1028. I run 37's on the suburban. To keep the cost of the swap down, i would like to take the entire drive-train out of the m1028 and put it in the suburban. Axles, tranny, motor and everything. From what I understand the m1028 axles are 4.56, which means the 6.2 will be turning about 3000RPM on the highway at 70.

Is 3000RPM too high for the 6.2? I only drive about 15-20hrs on the highway a month. I don't tow with the truck either.

Diesels do not like RPM, a more realistic RPM would be in the 1500-1800 range at 70. I don't know what trans you are using but you need an overdrive trans with those gears. A 700R or 4l80E would be a better choice and figure out your final gear with the tire size you will be using.

 

[geo in a K5]

I don't think its much different then the stock trucks with dinky little 235 tires and 4:56 gears. Yes it is kinda high rpm for the diesel so you have 3 choices, drive slower on the highway, re gear your axles or get an overdrive tranny.

 

[6.2Blazer]

For minimal highway driving it would probably be fine. The stock M1028 had 31" tires so running 37's drops the rpm quite a bit. On my K5 I ran tires that measured to about a 36 with 4.10 gears and never ran OD because it would just hunt between OD and 3rd at the smallest hill or wind, and never seemed to have any problem running 70 mph down the highway. If you had a true 37 and 4.56 gears it wouldn't be much of a difference.

This isn't an old school 1,800 rpm redline Cummins we are talking about either. From experience the 6.2 and typical 350 will behave about the same at that rpm...if you could run that rpm in a 350 than doing it in a 6.2 is okay also.

 

[DieselS10]

I ran a 6.5/4L80e in mine for a few years with 38's and 4.10's and I really think it was a little on the high geared side for the weight of the rig. Empty it was fine, running right around 1800rpm at 65mph, but with a trailer it down-shifted often.

I think with the overdrive 4.56's would have been better, but without I think the 4.10's would be better. With your limited highway time I'd just drive a little slower when I had to run down the highway.

 

[oldenginebuilder]

My 91 suburban needs a new tranny, xfer case and rear end, and the old 350 with 270,000 miles burns through some oil. I have the opportunity to purchase a good running m1028. I run 37's on the suburban. To keep the cost of the swap down, i would like to take the entire drive-train out of the m1028 and put it in the suburban. Axles, tranny, motor and everything. From what I understand the m1028 axles are 4.56, which means the 6.2 will be turning about 3000RPM on the highway at 70.

Is 3000RPM too high for the 6.2? I only drive about 15-20hrs on the highway a month. I don't tow with the truck either.

My friend Nate has an '85 Chev 1-ton 4x4 ex-army truck. It has the 6.2, TH400, and 4.56:1, he's on 32" tires. That thing is really annoying, and isn't good for much over 75 MPH, plus he reports about 17 MPG as his best.
My own '83 C-10 6.2 had a failing 700R-4 with no Od anymore, IDK about the lockup, had 3.42:1 on 19" tires, and did 24 MPG at a steady 55 MPH. Now I have that exact 6.2 in my '79 C-20 with 4.10:1 on 32", and I have a fully fixed 700R-4. I have seen 28 MPG doing 1800 RPM at 60 MPH.
The 6.2 loves plenty of first gear to get moving, but obviously gets more MPG with moderate gearing.
I haven't experienced a 6.2 with less cruise RPM than 1800 at 65, but I've read that 1600 at 65 is possible without too much lugging.
Bottom line, it's an engine that can be used with most any gearing, but it'll never be satisfying to a speed freak, no matter the gearing.

 

[mvanhelden]

Shouldn't your suburban have an overdrive in it now?

 

[geo in a K5]

Shouldn't your suburban have an overdrive in it now?

Read the original post again.

 

[mvanhelden]

Read the original post again.

Ok I did:flipoff2:
Then why would he convert to a shittier tranny? Why not rebuild the 're? What a waste of time completly stripping all the electronic speedo/harness, pretty much everything electricly related, guages, flush the fuel tank. Just to have a gutless louder Pos? If you have any smog req. Tons of paperwork if at all possible to even smog? I say just find another tbi 350. If you want something better, sell the CUCV and buy a 454.

 

[geo in a K5]

Ok I did:flipoff2:
Then why would he convert to a shittier tranny? Why not rebuild the 're? What a waste of time completly stripping all the electronic speedo/harness, pretty much everything electricly related, guages, flush the fuel tank. Just to have a gutless louder Pos? If you have any smog req. Tons of paperwork if at all possible to even smog? I say just find another tbi 350. If you want something better, sell the CUCV and buy a 454.

I was trying not to be a douche !! :flipoff2:

But he's in for work either way, you could buy a crate 383 from summit , get the trans rebuilt, find a decent used t case and swap in a 9.5 sf 14b rear axle and your sub would be good to go for years to come and have a healthy motor too. It's just gonna cost you around 4k.

Or swap in the diesel stuff and hope it isn't on its last legs either....

 

[ChevyBoy2010]

1800 rpm cruising will net you the best fuel economy and still land you in your power band.

 

[Loftis61]

1800 rpm cruising will net you the best fuel economy and still land you in your power band.

This. Spinning 3k on pretty much any stock diesel for an extended period of time will equal :nuke:.

 

[g-wizz]

This. Spinning 3k on pretty much any stock diesel for an extended period of time will equal :nuke:.

redline anything long enough will probably= broken... lucky for you the 6.2 diesel defuels at 3600 rpm soooooo anything under that should be good... not with out saying that if something else is wrong you wont have problems but i really doubt anything will happen if your under the factory redline. it wouldnt hurt to pop the valve covers and at least check to see if your plastic rocker guides/retainers are still good, or there. other than that the valve train on the 6.2 looks pretty solid especially for the rpm range they work with.

 

[oldenginebuilder]

1800 rpm cruising will net you the best fuel economy and still land you in your power band.

But assuming your aero drag plus rolling drag is as modest as mine, 1800 at 65 ( 1650 at 60 ) will net more MPG at 60 than gearing for 1800 at 60.

 

[bds1904]

Ok I did:flipoff2:
Then why would he convert to a shittier tranny? Why not rebuild the 're? What a waste of time completly stripping all the electronic speedo/harness, pretty much everything electricly related, guages, flush the fuel tank. Just to have a gutless louder Pos? If you have any smog req. Tons of paperwork if at all possible to even smog? I say just find another tbi 350. If you want something better, sell the CUCV and buy a 454.

How is the TH400 a "shittier" tranny than the TH700R4? Every single 700R4 that I have ever owned took a shit in under 80,000 miles, even on 100% stock vehicles. Never had a problem with a TH400.

I already have a 454 powered suburban, and I don't like the 6MPG. If I get 15MPG out of the diesel, I'll be happy.

I picked up the CUCV for $1300 because the body is torn to crap, but it runs and drives great.

The electronic speedometer, the computer that makes it work and the reluctor ring in the NP241C don't work right now, so I am already running an aftermarket speedo with a custom made pickup. Running the xfer case from the CUCV will allow me to run a normal electronic speedometer pickup, and is a stronger xfer case.

Needs a fuel tank anyways.

Tranny is cracked on the bellhousing.

Xfer case is cracked down the side where all the 241C's crack

The TBI 350 has trouble running on the highway under 2300RPM on 37's with the 4.10 gears I have now. Hell, one of my other suburbans (2wd) that is stock and in very good condition can't get out of it's own way on the highway.

The CUCV has the 60 front and a 14 bolt rear, both with lockers already, so why not run them anyways.

Did I mention I got the truck for $1300?

So all in all, I'll have about $2000 in the swap and end up with a strong drivetrain. Not gonna be a fast truck, but strong.

But assuming your aero drag plus rolling drag is as modest as mine, 1800 at 65 ( 1650 at 60 ) will net more MPG at 60 than gearing for 1800 at 60.

1991 suburban rolling brick lifted 4" on 37's isn't "low drag" in any way, shape, form or dream.

So all in all, if I drive 65MPH for now and look for an overdrive tranny or overdrive unit for the TH400, it looks like it'll be just fine.

 

[g-wizz]

throw it in, it wont be unpossible to get 20mpg, you might have to change a thing or to. and you definatly eventually want an overdrive with the 6.2 if you dont or cant have one now.

 

[mvanhelden]

If that suburban's so fucked up, why bother? So your telling us every possible thing is toast? Wow

 

[ken]

That CUCV will have an NP208 transfer case, which I believe is a bit weaker than your broken 241 was! Unless you get super lucky and find a CUCV with a 205.

 

[Dstang97]

I'm running 37' on my m1008 and can do 65mph. I really would not run it any higher. It will run 62-65 all day long. I still have the th400 and would say i get around 16 mpg with my currant setup

 

[OutlawRider]

Have you checked the front 60 for a locker? From what I've read, lockers in the front are rare.