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6.2L GM Diesel Conversion Information for M-715

Contributed By: David P. Dusenbury

Editor's Note: This was posted pretty much as you see it on the FSJ List. Thanks, David! -Doc

Date: Sun, 16 Feb 1997 20:25:56 -0500
From: "David P. Dusenbury" <>
Subject: SUMMARY: 6.2L GM Diesels (LONG!)

Thanks for everyone's help regarding swapping a 6.2L Diesel into an M715. I originally cross-posted the question to both the Military vehicle List and the Full-Size Jeep List.

Sorry about the length if you're really not interested, but there seems to be a large amount of interest in this information.

My original questions are in all caps.

The following folks contributed information to this effort. I mixed and matched messages, so I was not able to really give credit on individual sections.

  • Hal Johnson
  • Joe Schaefer
  • Carter Ledbetter
  • Lee Phillips
  • Gale Barrows
  • David C. Decker
  • Marcello Gustavo Torres
  • Matt Oliver
  • rick
  • john


Speaking as someone who has honcho'ed a military rebuild line for these, just because they're in a container doesn't mean much. These COULD be rebuilt, or could be sick ones that had been pulled and were declared surplus...

Because of parts shortages, some units have ordered entire motors just to get injection parts (pumps and injectors), pull what they need, and send back the rest as the core.... others would swap entire engines despite only needing to replace the parts mentioned above...

On the other hand, these could be motors needing rebuild that were pulled, and declared surplus...

Finally, they could have been rebuilt. Look for stencils on the outside of the shipping contained indicating status of the contents; also look for the shipping papers inside the container (in a plastic package wired to the motor), and a engraved data plate on the engine indicating when/where/who rebuilt this pig. This plate should be CLEAN if the engine has been recently rebuilt!

A rebuilt engine is tested pretty extensively; we had to run them for about 4 to 6 hours on test-stands; they had to produce a certain minimum amount of power, and not exceed certain temperatures (coolant and exhaust)...

Obviously, you'll have to examine these pretty carefully to know what you're getting!

If they are in the cans, perhaps they are complete with the accessories, be sure and check before bidding. Even if they are used, they could still be in good shape and have a lot of life left in them, look for any indications of oil leaks, cracked blocks, etc. Anything which might give you a clue. If you are not a mechanic (diesel) find the local know-it-all (perhaps the diesel mechanic at the Chevy Dealership) and pay him $20 to take a look. This is really cheap insurance and you will learn a lot of stuff about the model and what it will fit. Buy the guy lunch.

If and when I go to put a diesel in mine, I will try to buy a wrecked Chevy Pickup with one already in it. That way I will have all the parts like the exhaust, the wiring, radiator and all the nickel and dime items which add up very quickly in any conversion.


Don't know about the bell housing pattern; however, it could probably be fit up somehow, may be you would have to fabricate.

As stated, being a GM product, it probably came from a CUCV or a Hummer and it should fit. Another trick is to find the flex plate from a automatic trans and take it with you, however, and this is why I have always loved GM products, there is a lot of interchangeability of engine/trans. Depending on whether or not you want to keep the stock 715 trans and transfer, you may or may not need an adaptor. The company, Advance Adaptors, can match darn near everything.

Should be standard Chevy, Available with Auto and Manual from factory

Since these are Gov't versions all bets are off, but Yes it should be a Chevy.

If I had to bet on this, I would say that these are take-out 'core' engines which have already had a life in either a military Blazer or Pickup (CUCV) There is also a VERY good chance they came from a Hummer, which would mean they would bolt up to a Chevy 400 automatic.


Just about any of your questions could be well answered at your local Chevy dealership as this is the standard chevy diesel pickup engine used in the years around 1986.

I don't know this for a fact but before doing this, get some advice on the axle ratios. I think you'd have to go down to 4.xx:1's for the dsl to be effective.

The big problems would be the RPM ranges required. In general, fuel injected diesels DON'T LIKE TO RUN over about 2400 RPM. It isn't that the bottom end won't take it; it's that the fuel shot ERODES THE PISTON TOPS at the higher RPMs, because of the higher fuel pressures required to deliver the required volume of fuel at the higher RPMs. This is particularly a problem in diesels run at higher elevations like Mexico City and environs, where you have an essentially normally aspirated engine and fuel injection [Horsepower is proportional to the product of Torque and RPM]. This implied you have to run the engine at a higher RPM with lower available manifold pressure to get a constant horsepower. The implication of this, assuming that the horsepower ranges that you are comparing are similar, you'll have to have a drive train that allows the diesel to operate in the range of 1800-2200 RPM, for a typical engine; i.e., this swap could get expensive

No matter what engine you put in a 715, I would guess you will want to change the axle ratios. I have 4.88 and put in a Dana Locker when it was apart in the rear unit. This allows me a higher top speed and with the low transfer gear no problem with climbing hills or pulling.

It would probably be more comfortable at 45 to 50 MPH. You could use taller tires such as the Super Swamper 11.00-38 x 16 or re-gear. The M715 uses Dana Axles so gears in the 4.88 range should be available.

You'd be at the high to max end of the 6.2L.

The 6.2 operates at peak torque at 1800 rpm and at peak power (best combination of torque and hp) at 2800rpm. The 6.2 is governed by its fuel pump to 3600-3800. However Jim said to keep rpm under 3000 since power drops off rapidly above that. Whats all this mean? The M715 runs 5.88 gears and a T98a transmission. Using a formula found on the 6.2 page I calculated the rpm vs speed using the transmission ratios of the T98a. I got the T98a ratios out of an Advance Adapters catalog. The 6.2 in first would top out at about 8 mph in 1st, 17mph in second and 28mph in 3rd. In 4th gear the top speed would be about 50mph with at 3061 rpm. If you were to change the differentials to a 4.56 gear set you would see a marked improvement with a top speed of 60-65mph. The peak power(2500rpm) would occur around 60mph. That in theory should yield about 20-23mpg.


Not the same mounts but easy to adapt, either fabricate locally or possible obtain from a company called MARS. It will fit with a little banging on the fire wall sheet metal for the valve cover to clear.

The 6.2 is based on the Chevy Big block i.e. the 396/427/454. It is a little heavier

It's 379 cu in. It's heftier, best bet is to find a full size blazer with one and look... I'd say it's pretty close, if not the same size.


The beauty of the diesel is you need only clean fuel and a starter motor to get it going. If you keep the manual trans, in an emergency, you can push or pull start it. What computer? Yes, you do need power to the glow plugs, but with simple wiring and common sense usage, it is not a problem.

No computer. However there is a controller for the glow plugs, you need it...

Another thing to keep in mind is that any military 6.2 would have 24 volt glow plugs, this is the only electrical portion that I know of which could be considered 'computer controlled'

As far as glow plug relays I corresponded with Chiltern Automotive in England they are doing 6.2 conversions into Land Rovers. They use 2 80amp timer relays to heat the glow plugs. The only problem is that it is a 12 volt setup. I would like to keep the truck 24volt. I looked at a Chiltons manual at the wiring diagram for a 2.2 diesel and it seems the glow plugs are wired off of the starter so they come on as soon as one turns the key.


I have a 95 Civilian Hummer and would not have anything else. They are bulletproof engines and will last forever if properly cared for. On the other hand, if you do break something they are expensive to repair. Here is an informative site to visit.

As to "wanting" a diesel, that depends a lot on what you want the vehicle to do and what your climate is. In cold weather, when temperatures are routinely below 20 degrees F , you have to change to using #1 fuel so it will flow better. There is a small loss of power when using #1 instead of the normal #2 but not too much. At temperatures getting much below 10F you should start adding a good anti-jelling additive with either fuel grade or filters will start clogging and when it is really cold, say -20F or colder, the fuel may just turn to a gelatinous mass and you can lose all fuel flow till the fuel warms up again.

The gas Chevy 350 engine is the universal choice for conversion. Couple that with an automatic or stick with the stock trans and any mechanic in the US can work on it and hot rod parts, stock overhaul parts and cheap engines make this a real good choice.

Personally, I would spend the first money on a stock 715 on changing the axle gears and getting a set of decent tires.

The diesels you refer to are used in the HMMWV, aka HUMVEE or yet HUMMER in the civilian marketplace. They are rugged, produce lots of torque at -extremely- low rpm -which, BTW, is a very distinctive diesel trait, are reasonably trouble-free and should be a very proper choice for the M-715, at least in terms of profile - big engine, "high" mileage, high torque, noisy, smelly; exactly what the doctor ordered!

There is a book called the Diesel Guide by Chilton's. Very good info, read it first. I like Diesels, but if you don't know about 'em, you're in deep doodoo...


MARS(Military Acquisition Restoration & Supply) offers a product called the 'Hercules Diesel Re power Kit'. The information indicates that the kit is designed to re power the 3/4 ton M37 & M43 trucks, the 1 ton M601, the 1 1/4 ton M880, M715, & M725.

The kit consists of a DT3.7 turbo-charged four cylinder diesel engine with 4.89 gears(front and rear). It is a drop-in replacement requiring no drive train or chassis modifications. The engine generates 108 horsepower at 2800 rpm's. There is also an optional system that generates 125 horsepower for the M37.

With the diesel engine and higher speed differential gears, the trucks are reportedly capable of 70mph (including uphill), with a fuel mileage of 18-24 mpg.

The kit includes every part needed for the conversion(down to the front and rear engine mounts, front axle lift kit, pressure plate, clutch disc & dust shield, fuel transfer pump, fuel filter/water separator with electric heater and dashboard indicator warning light, ether starting aid, fan and fan spacer, belts and hoses, after-cooler and piping(125 horse power unit), air filter, vacuum and air piping, 24V windshield wipers with arms and blades, all wires, terminals, and tie wraps for existing wire harness, muffler, exhaust pipe and tail pipe, all clamps and hangers, parts book, instructions, and wiring diagram.

For more information, contact Paul Caudell at:

  • MARS(Military Acquisition Restoration & Supply)
  • 4712 W. Yellowwood Road
  • Nashville IN 47448
  • (812)988-2330 Phone
  • (812)333-3082 Fax
Thanks to everyone who contributed to this information!

David P. Dusenbury
Sun, 16 Feb 1997

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