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6.2 Diesel Highway RPM

16K views 30 replies 15 participants last post by  6.2Blazer 
#1 · (Edited)
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.
 
#23 ·
The TH400 is inefficient and badly geared. All it has in it's favor is that it was built for larger engines. The 700R-4 doesn't come with this advantage, but the inherent strength is there, in all the '88-93 versions, if you'll just address a few trouble items during the rebuild.
A TH400 can be race-built to survive up to 1000 ft-lbs in a 3000# car, while the 700R-4 is limited to around 600, if you want to retain street-friendliness, but how many of you have a real 600 ft-lbs anyway? Very close to none.
No matter how much you invest in a 700R-4, if you don't abuse it, then the OD will eventually make up the cost.
Now, if you spend the money on fuel instead of the OD, you end up with nothing to show for it. But once the OD finally does pay for itself, then you have a 700R-4 to show for it.
Everyone likes to join in the chorus singing the praises of the TH400's closer ratios, but in a half million miles of street-driving 700R-4s, never once have I wished for closer ratios.
I have tried a few 200-4Rs, which have in-between gearing, but I never found any advantage.
There used to be a kit to let a 700R-4 go 1-1OD-2-3-OD, making it effectively a close-ratio 5 speed, and it seemed great, but those who tried it reported it not worthwhile after all.
Hate the 700R-4 if you wish, it just shows you're stuck in an obsolete frame of thinking.
700r4 suck. Why Put in a crappy tranny that are know to be week? You can build a 700 with all your superstar parts an it will still be a pos. until they change their case they will continue to be week.
 
#3 ·
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.
 
#4 ·
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.
 
#5 ·
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.
 
#6 ·
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.
 
#16 ·
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.
 
#18 ·
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.
 
#21 ·
I would love to have another daily driver K5 with a 6.2 and 700r4. Properly geared they don't do bad around town, and with the 700r4 you can run lower axles gears to keep some power around town but still cruise down the freeway. With a mild lift and 33's my '90 Silverado K5 was pretty nice to drive. Won't deny it didn't have a lot of power but even after daily driving a 300 hp 6.0L truck for the last 9 years I wouldn't mind driving another 6.2 truck.

I've personally had many people tell me how crappy the 6.2 is after telling them what the K5 has. The funny thing is that NONE of them daily drove one for any period of time and the examples were always 150,000 mile beat up farm trucks that had never even had the fuel filter changed. That doesn't include the few ragged out ones with 3.08 gears and 35" tires.
 
#22 ·
The TH400 is inefficient and badly geared. All it has in it's favor is that it was built for larger engines. The 700R-4 doesn't come with this advantage, but the inherent strength is there, in all the '88-93 versions, if you'll just address a few trouble items during the rebuild.
A TH400 can be race-built to survive up to 1000 ft-lbs in a 3000# car, while the 700R-4 is limited to around 600, if you want to retain street-friendliness, but how many of you have a real 600 ft-lbs anyway? Very close to none.
No matter how much you invest in a 700R-4, if you don't abuse it, then the OD will eventually make up the cost.
Now, if you spend the money on fuel instead of the OD, you end up with nothing to show for it. But once the OD finally does pay for itself, then you have a 700R-4 to show for it.
Everyone likes to join in the chorus singing the praises of the TH400's closer ratios, but in a half million miles of street-driving 700R-4s, never once have I wished for closer ratios.
I have tried a few 200-4Rs, which have in-between gearing, but I never found any advantage.
There used to be a kit to let a 700R-4 go 1-1OD-2-3-OD, making it effectively a close-ratio 5 speed, and it seemed great, but those who tried it reported it not worthwhile after all.
Hate the 700R-4 if you wish, it just shows you're stuck in an obsolete frame of thinking.
 
#27 ·
It is a NP205 case, so WAY stronger than the NP241C

The truck is a M1028, and it does have the front locker.

I didn't even realized it was the 205 until I crawled under and looked at it. The shifter should have given it away.

Looks like I scored big time with this one.

And yes, my truck does have a 700r4, and it is a cracked 700r4.
 
#29 ·
My '84 C20 Suburban came from the factory with a 6.2L/TH400 and 31" tires. 65mph is about all I dared.

I enjoyed it much more after I swapped in the SM465, 3.21 axle, and 33" tires. :D I could get about 80mph out of it. :laughing:
 
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