Drivetrain from early m998 to newer 6.5 turbo and 4l80e trans with new t-case

INRISEMPERFI

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Hello all, i am new here so go easy on me, lol. I have a 1991 m998 with a 6.2 naturally aspirated backed by a 3l80 (aka turbo 400h) with the early model transfer case. I hydro locked it having to much fun... And well now is a good time to do the complete updated power train swap with the 6.5 turbo, 4l80e and updated transfer case. I know am general did the swaps to all the old m998's, i know you have to seperate the body inorder to install the new wiring harness. My question and reason for this post is apart from the obviously complete power train, what are all the harnesses am used to do the conversion? I would like to keep the military specifications and do it the way am general has done it in order to maintain the water fording abilities ect. Any pic of the 6.5 turbo engine front accessory brackets and a complete pic of the complete engine would be awesome as i would need those to reference those to build up the gov planet 6.5 engine i am going to be receiving soon. If all is successful i would not mind doing a complete write up with pics for and easy future reference thread. If that thread already exists, forgive me as i am new here and i could not find the search this forum bar anywhere. Thanks again guys for your inputs
 

LiftinTheVeil

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Out of curiosity, what kind of cruise speed increase will you expect to see with this upgrade? I’ve been wanted to do something similar for a while now.


Sent from my iPhone using Tapatalk
 

Coug

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6.2 and 3 speed reportedly cruises at 50-55 comfortably, with the 6.5NA and 4 speed people report 70-80. I'd imagine since the transmission is the same the 6.5 turbo will be 70-80 as well. It's more about the transmission than the engine, going from 3 speed to 4 speed makes the cruising speed difference, engine just determines how fast you get up to cruising speed.
 

TOBASH

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6.2 and 3 speed reportedly cruises at 50-55 comfortably, with the 6.5NA and 4 speed people report 70-80. I'd imagine since the transmission is the same the 6.5 turbo will be 70-80 as well. It's more about the transmission than the engine, going from 3 speed to 4 speed makes the cruising speed difference, engine just determines how fast you get up to cruising speed.
I agree with one major exception. 6.5 and 6.2 are similar in normally aspirated vehicles, but a turbocharger on a 6.5 will make vehicle able to drive more effectively at higher elevation. That means acceleration and top speed at higher elevations with a turbo will be somewhat better.

That being said, Juan Prado has a vehicle with 6.2 and TH400 that does 70 at sea level.

Because I wondered why Juan's vehicle could do that, I brought my rig to my mechanic, and after correcting an issue with the transfer case, my M1025 A1 with TH400 and stock rims/tires now does 70 without over-revving. I have 2.58 gears in the diffs, and I think that is a major reason for my higher speed. A rubber upgrade with taller tires will also increase top end speed, but you will need to change your rims or add 1/2 inch to the walls of your rims as there are only limited tires for 16.5 inch rims on the HMMWV. There are better selections at 17 and 17.5 inch rims.

Ultimately I think a 4 speed is a great choice, but a TH400 with 2.58 gears in the pumpkins seems like it will be OK for 65% of highways in the USA.

Best,

T
 
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Coug

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according to specs, max RPMs are supposed to be 3600 under load (4k no load, but what's the point?)

Peak torque is supposed to be 2000-2100rpms, so if you're running it much above that you're wasting fuel.

As for "over revving" to me it's anything that your cooling system can't easily keep up with. Some people have run the 6.2L trucks up above 60 for periods of time without doing anything to lower the RPMS, and while the engine will do it, some have also destroyed their engines in the process.
 

BLK HMMWV

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Hello all, i am new here so go easy on me, lol. I have a 1991 m998 with a 6.2 naturally aspirated backed by a 3l80 (aka turbo 400h) with the early model transfer case. I hydro locked it having to much fun... And well now is a good time to do the complete updated power train swap with the 6.5 turbo, 4l80e and updated transfer case. I know am general did the swaps to all the old m998's, i know you have to seperate the body inorder to install the new wiring harness. My question and reason for this post is apart from the obviously complete power train, what are all the harnesses am used to do the conversion? I would like to keep the military specifications and do it the way am general has done it in order to maintain the water fording abilities ect. Any pic of the 6.5 turbo engine front accessory brackets and a complete pic of the complete engine would be awesome as i would need those to reference those to build up the gov planet 6.5 engine i am going to be receiving soon. If all is successful i would not mind doing a complete write up with pics for and easy future reference thread. If that thread already exists, forgive me as i am new here and i could not find the search this forum bar anywhere. Thanks again guys for your inputs
The guy that replied to you first would be worth a call to in my humble opinion.
If he will give you some info it's way worth what you will pay for it. And don't let that price fool you. Sounds like it might just cost you a phone call.
He works on the things all the time.

If you plan on keeping it in a stock configuration with stock military parts he is your guy.

BH
 

TOBASH

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BLK is correct about RTW/Retired War Horses.

If you want to place a 4 speed or do other things, he is a FMV restoration specialist.

You should talk with him
 

Milcommoguy

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Mr. T, What tweaking to the transfer case can get you to 70 MPH and stay below the governor. You coasting down a steep hill. lol

55 and stay out of my way, CAMO
 

TOBASH

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Mr. T, What tweaking to the transfer case can get you to 70 MPH and stay below the governor. You coasting down a steep hill. lol

55 and stay out of my way, CAMO
Juan Prado has the same setup with claims of the same speeds.

There is a remanufacturer in Cali that makes the same claims for the same speeds.

Alls I know is my mechanic is a Diesel mechanic and a marine Diesel mechanic and a rally car builder on top of all of that, so I go with what he says. He was using GPS speedometer and not internal speedo to record speed.

I'm waiting to drive her myself after all the repairs, but I won't question my guy's veracity.

I absolutely understand your concerns and statements. I was surprised by these numbers too. It is just that Juan Prado also uses the same numbers to describe his rig, so it seems ok.

Either way, I want to go with a 700R4 swap. I was just surprised with his GPS speedometer reading of 70MPH.
 
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