Four Wheel Finesse

Diesel Details, Part 2

by Jim Allen

Diesel powered, light 4x4 trucks have become one of the hot enthusiast realms lately. Most people only remember diesels in the U.S. market back to the ’80s, but they go back a lot farther than that. There are unconfirmed reports that GMC built a handful of pickups with their ToroFlow V6 diesel in the early ’60s, but the first confirmed American-built entry was the ’61-69 Jeep CJ with a British made 192ci Perkins four. Land Rover was also importing small numbers of Series 1, Series II, Series IIA and Series III diesels from ’58-74.

In the pickup world, Dodge was first by having offered a production diesel option in a their four-wheel drive Power Wagon pickup starting in 1978. No, it wasn’t a Cummins, but rather a 243 cid, 107 hp Chrysler-Mitsubishi 6DR50A six-cylinder.

Going beyond these obscure and almost forgotten diesels of yesteryear, we thought you’d like to have details on the diesels available today, old and new.

Notes:

* Not the manufacturer’s rating. These are general guidelines from industry sources based on the particular engine’s basic design

** From manufacturer’s literature.

1993-up GM 6.5L Turbo V8

photo courtesy General Motors

The GM 6.5L was an evolution of the 6.2L. With a slightly increased bore and stroke, it was larger by a few cubes but it had a stronger bottom end, better heads and, best of all, turbocharging. Like the 6.2, it was used in the half-tons on up, but the applications were somewhat more limited than the 6.2L had been. There were 155 hp NA engines available for certain applications, the Humvee for example, but they are uncommon in general use. The turbo engines cranked out an honest 190 hp and 385 lbs-ft in most applications. The very first year used a mechanical DB-2 pump like the 6.2L, but the troublesome weak link has been the first generation DS-4 injection system that followed. It combined a mechanical pump with electronic controls and was a bit cranky at first. The fixes are in place to make it work well nowadays. Being a somewhat stronger platform, the 6.5 is capable of up to 300 reliable hp and about 550 lbs-ft., but you have to spend some fairly serious money (a special injection pump) to get it past about 250 hp. It desperately needs intercooling with the boost needed to make 300 hp. This engine remains in production, with some limited applications in the commercial GM truck lines and with the AM General military Humvee and the civilian H-1 Hummer.

Typical Specifications

1982-93 GM NA 6.2L V8

photo courtesy General Motors

The GM 6.2L V8 diesel of 1982 was the first major entry into what would become the light truck “Diesel Wars.” Dimensionally, it was designed by Detroit Diesel Allison (then a subsidiary of GM) to be comparable with the GM big block V8. In fact, at about 675 pounds, it’s the lightest V8 diesel commonly available. It used the same mounting system, bellhousing and engine mounts as the other GM V8s, so it was easy to integrate into the existing truck lines. Because of its light weight and modest dimensions, it was used in half-ton trucks and SUVs (Blazer/Sub), as well as three quarter and one tons. The 6.2L was also used in the first generation military Humvee, which appeared just as this new engine debuted.

Designed for a GVW of no more than 10,000 pounds, the first units had very modest power outputs; 135 hp and 240 lbs-ft. Towards the end of production in ’93, the over 8,600 GVW non-emissions units cranked out a bit more (148 hp in certain applications). The 6.2L could deliver high 20s highway fuel economy in some of the lighter 4x4 applications, and 30 mpg is reported in some of the stripped down 4x2s. This wonderful fuel economy is one of the trademark characteristics of the 6.2L, and is reportedly due to its very efficient Ricardo V combustion chamber design. A rare dealer installed turbo option was available from ’89-on in the form of a Banks Sidewinder kit. One thing to remember about the 6.2L is that it’s not a heavy-duty diesel. Because it’s built on a lighter foundation than the industrial engine based Ford and Cummins Dodge, the maximum power output is more limited. If you consider 250 hp and 450 lbs-ft the maximum safe power level, your 6.2L will be a trusty and efficient engine on a long-term basis. That’s about all the reliable power available from the stock DB-2 injection pump anyway.

Typical Specifications

2001-up GM/Isuzu DI Turbo Duramax V8

photo courtesy General Motors

The Duramax appeared just in time to put GM back into competitive standing in the diesel wars before it was completely overwhelmed. It wasn’t that the old 6.5L was a bad engine, it was just vintage technology that had the added burden of being power limited by it’s internal design. Especially when compared to the other more modern diesels on the market. The Duramax, co-developed by GM and Isuzu, was a very modern iteration of the V8 diesel, with a few twists. It was direct injected with a high pressure, electronic common rail injection system and 32 valves. The block is a burly cast iron piece, but the heads are aluminum. That raised more than a few eyebrows. There are many advantages to an aluminum head, including heat transfer and savings in weight. Decades of aluminum heads on Isuzu diesels has proven the benefits and durability of the design, so the eyebrows have dropped. At under 900 pounds of weight, the Duramax has the best engine weight-to-horsepower ratio of the modern “big-three” diesels. The Duramax started out at 300 hp/520 lbs-ft and has had one power upgrade since. Modified, it can produce truly incredible outputs.

Typical Specifications

Note that EGTs are monitored and limited by the computer.

1993-94 Ford 7.3L IDI Turbo V8

photo courtesy Ford

The ’93-94 7.3L IDI turbo is one of Ford dieseldom’s forgotten and unsung heroes. It appeared for less than two years and coexisted briefly with the first generation Powerstroke in 1994. It was factory rated for a modest 190 hp and 388 lbs-ft , but sources indicate it was deliberately “tuned down” so as not to out-power the Powerstroke, which was close on the horizon at the time it appeared. With very few mods (pump adjustments, getting rid of the crushed turbo outlet pipe and wastegate adjustments), this engine can make a very easy 225 hp and 440 lbs-ft for almost no money. Internal differences between the turbo and non-turbo 7.3 consist mainly of stouter anodized pistons with larger wrist pins, improved piston rings and Iconel exhaust valves.

Typical Specifications

1989-93 DI Turbo Cummins 12-Valve Inline Six, Generation One

photo courtesy Cummins

The first generation Dodge Cummins of 1989 probably did more to fire up the “diesel wars” than anything. It cut deeply into Ford and GM diesel truck sales and turned an “also ran” manufacturer into a contender. With only 160 hp and 400 lbs-ft, it was modestly powered by today’s standards but ahead of the competition at the time. At that power level the Cummins engine was hardly breaking a sweat. It was built for medium/industrial/agricultural use after all, having appeared first in 1984 as a joint development of Cummins and Case tractor.

The Cummins was an inline six of 5.9L capacity. It used a Bosch rotary injection pump and was turbocharged but without an intercooler. Engines built after the end of 1990 did use an intercooler for emissions purposes. It’s very easy to crank this engine up to about 225 hp and over 600 lbs-ft at the rear wheels. The injection pump is the limiting factor and the earliest version can really use an intercooler and a turbo housing upgrade. This version was built to the end of 1993.

Typical Specifications

2003-up 6.0L Powerstroke V8, Generation 3

photo courtesy Ford

The 6.0L Powerstroke is the new kid on the Ford diesel block. The bore is almost 3/8-inch reduced (3.74 vs 4.11 inches) and it has a fractionally shorter 4.13-inch stroke. You can see many design hints from the old Powerstroke engine, but with only a handful of carryover parts, you have to call it a new engine rather than an evolution. The block is heavily revised and strengthened.

The biggest news mechanically is the 32 valve head configuration, four valves per cylinder with the injector centered in the cylinder, which greatly enhances breathing ability and combustion efficiency. It uses the same basic type of HEUI injection system, but at a much higher fuel pressure (26,000 vs 21,000 psi). It also features a new variable turbo, called EVRT (Electronic Variable Response Turbocharging). Boost rate is controlled by a unisyn ring, which is a movable trim ring around the compressor blades that controls boost. This is a much more elegant way of adjusting boost to suit engine speed than a wastegated turbo, and offers lower EGTs as well. Essentially, the engine can optimize boost at any rpm range without the restriction of a wastegate. The 6.0L also features “Pilot Injection,” a.k.a. Split Shot injectors, which inject the fuel in a couple of stages during the idle compression stroke rather than all at once. This is said to decrease idle noise and emission. These injectors were introduced at the end of the 7.3L Powerstroke era but weren’t used in all engines.

The end result of all the new tech is a stock 325 hp/560 lbs-ft output. Thus far, upgrades have yielded nearly 400 hp and 700 lbs-ft at the rear wheels, with more to come. Note that EGTs are monitored and limited by the computer.

Typical Specifications

Note that EGTs are monitored and limited by the computer.

1983-88 Ford 6.9L and 1988-92 7.3L NA V8

photo courtesy Ford

Ford’s first entrant into the diesel wars came in 1983 with the 6.9L V8. Developed by Navistar/International, it cranked out 170 hp and 315 lbs-ft and out powered the GMs by a good margin but, in the 4x4 lines, was available only in the F250HD and F-350. It’s a very stout engine that’s capable of about 275 hp with the stock injection pump (a DB-2) and add-on turbocharging. The engine could probably handle over 300 hp with a high capacity pump.

The 6.9L lasted until 1988, when it was replaced by the similar 7.3L. Externally they looked almost identical, but inside, the 7.3L had a larger bore for the increased displacement, larger head bolts and (supposedly) improved valve stem seals. Overall, the early 7.3L isn’t considered quite as reliable as the 6.9L, though the 180 hp and 345 lbs-ft were improvements owners liked. Some spec sheets show the last year of 7.3L NA, 1992, with 185 hp and 360 lbs-ft.

The 7.3L NA probably has about the same power potential as the 6.9L because it’s limited by essentially the same injection pump. Again, it needs an add-on turbo kit to bring it up to good power levels. Bear in mind that, like many older diesels, the 6.9 is more or less designed to use small amounts of oil. My own has, even when it had only a few thousand miles. The new valve stem seals on the 7.3L were designed to solve that but had the unexpected bad side effect of causing early valve guide failure.

Typical Specifications

1994-98 Powerstroke DI Turbo V8, The First Generation

photo courtesy Ford

The first generation 7.3L Powerstroke of 1994 was very much an evolution of the IDI engines. The lower ends are similar and share the same bore and stroke, though the Powerstroke lower end is very much beefed up. The heads are radically different, given the differences between direct and indirect injection. The DI Powerstroke has a combustion chamber in the piston and a lower compression ratio (17.5 vs 21.5). The Powerstroke heads also contain the high-pressure hydraulic passages and fuel passages for the hydraulic HEUI injection. The Gen 1 Powerstrokes were conservatively rated for 210 hp and 425 lbs-ft (some sources show 450 lbs-ft, but this is probably the heavy-duty variant), but are capable of much more. The electronics are a bit clunky compared to the later engines, but 300 hp and 600 plus lbs-ft is very possible with mods from the ’94-97 engines, but should include an intercooler, which the factory trucks did not have. They were also used non-wastegated turbos.

Typical Specifications

1998.5-2003 Powerstroke DI Turbo V8, Generation 2

photo courtesy Ford

The 98.5-2003 7.3L Powerstrokes made a bunch of technological leaps. They were intercooled, got new electronics and a new electronically controlled wastegated turbo. Internally, they were given a stronger crankshaft, redesigned pistons and rings, and revisions to the block. Power and torque was way up, 235 hp and 500 lbs-ft. and was later bumped to 250 hp/505 lbs-ft for automatics and 275 hp/520 with the 6-speed manual. It coexisted in 2003 with the new 6.0L Powerstroke and finally the direct progeny of the original Ford/Navistar was retired. This engine has a big potential for power, with well over 300 hp and 680 lbs-ft possible at the rear wheels.

Typical Specifications

Note that EGTs are monitored and limited by the computer.

1994-98 DI Turbo Cummins 12-Valve Inline Six, Generation Two

photo courtesy Daimler Chrysler

The second generation Cummins used a Bosch inline injection pump that’s generally considered the best of the mechanical pumps in the light truck lines. It develops the highest pressure and has the most hop-up potential. The Cummins in this era were intercooled and output varied and was limited by tranny type, with manuals having considerably more juice. The ’94-95 manual trans engines cranked out 175 hp/420 lbs-ft, with automatics keeping the old 160 hp/400 lbs-ft rating. There were many internal engine upgrades in this era, including pistons and rings as well as the camshaft and lifters. The turbo housing was changed to increase boost pressure. Most Cummins in this era also used a catalytic converter.

Typical Specifications

1998.5-2002 DI Turbo Cummins 24 Valve Inline Six, Generation Three

photo courtesy Daimler Chrysler

The Gen-3 Cummins that appeared in ’98 was a 24 valve unit, featuring four-valves per cylinder and a centered injector. Both of these design elements offered power increases (with potential for more), better emissions and increased fuel economy. The block was also upgraded in many ways to increase durability, reduce leaks and reduce noise. It used a Bosch electronically controlled rotary pump that pumped out 20,000 psi. Power outputs increased greatly, though the automatic transmissions remained the limiting factor. An HO engine appeared in ’01 that offered over 500 lbs-ft., but it came only with the 6-speed manual that year. The rpm limits of the engine took a jump with the 24-valve engines. Instead of being governed for a mere 2500, they could crank up to almost 3000 rpms. These engines are very easily modified with lots of potential for upgrades.

Typical Specifications

2003-up DI Turbo Cummins 24-Valve Inline Six, Generation Four

photo courtesy Daimler Chrysler

The current generation Cummins is still a “chip off the old block,” but with many upgrades to enhance power and emissions. The newest generation has gone fully electronic, with common rail injection. Common rail systems use a high pressure fuel rail to which all the injectors are connected. Pressure is supplied by a single high-pressure pump. The injector opening is accomplished electrically, so that the duration and amount of injector opening can be precisely controlled. These engines also use pilot injection, a system where a small amount of fuel is injected early to start the combustion process. It makes the engine much quieter and cleaner-running at idle. The standard engine, both manual and automatic trans rigs, were rated for 250 hp/460 lbs-ft (except in California at 235 hp). The High Output option got you 305 hp and 555 lbs-ft. Beyond the calibration changes needed, this engine also got beefed up with oil-cooled pistons to handle the extra load. For 2004, the Cummins dodge rocked the diesel world by offering the Cummins 600 HO engine. Optional in 50 states, it cranks out 325 hp and a whopping 600 lbs-ft of torque. This bruiser features a factory 4-inch exhaust system, a larger oil-bath type turbo, a bigger intercooler, updated valves and seats, updated pistons, high temp exhaust manifold castings, improved head gaskets and the necessary programming updates. It’s the first factory diesel pickup to break the 600 lbs-ft barrier, though don’t expect it to reign there for long.

Typical Specifications

‘Wheeling a Diesel

My own ’83 Blazer diesel proved the viability of a diesel engine on the trail to me. Stock, it had the power of a small gas V8 with the economy of a small V6. When built up with a Banks turbo kit and other mods, it had the power of a big gas V8 and the economy of a medium sized V6. It never ran out of grunt on the trail, though it came close in sand when it was stock.

The four-wheeling basics don’t change just because there’s an oil burner under the hood, but there are a few extra nuances you can exploit with a diesel.

Frankly, a diesel is absolutely wonderful on the trail. It chugs along happily, never stalling or belching at angles, sipping fuel at a very low rate and requiring very little throttle. It always seems to have the grunt to pull itself over a ledge, even at low speeds.

It’s too bad that there aren’t more diesels in trucks other than the monster 3/4 and 1-tons. The new Jeep Liberty diesel due out next year is an exception. The early GM trucks with diesels may be one of the better choices for a ’wheeling diesel because they came in half-ton applications. Plus the engine is lighter than the others. The Ford diesels, from 6.9L to Powerstroke, are thousand pound engines that only came in 3/4 ton applications, or larger. Ditto the Cummins, which is even heavier. In general, stock 3/4 or 1-ton trucks aren’t the best choice for a trail rig due to extremely stiff suspensions and large size, but some of those things can be revised for better trail performance.

The extra weight of a diesel up front is both a help and a hindrance. It’s a help on a climb because it puts a little extra weight on the front and you end up with more grip from the front tires. The opposite is true on a descent, where that extra weight unloads the rear wheels more and makes the rig more prone to swapping ends.

Because of their heavy internal parts, diesels rev at a slower rate than gas engines and stall less easily. The slow revving aspects make it very easy to control wheelspin and throttle control in all situations is easier. You do have to watch applications of throttle with a diesel in low traction situations because a diesel can break the tires loose easier at low speeds when you use too much throttle. As long as you keep the intake clear, diesels are virtually impervious to water, so water crossings are nearly trouble-free.

One of the best aspects of a diesel on the trail is fuel economy. Running at low speeds, the diesel is a miserly beast. I’ve literally spent the entire day on a trail with my 6.2L GM and the fuel gauge barely moved. Granted, I’m easy on the pedal most times but trail buddies who have a similar driving style, but run gassers, had empty tanks in the same amount of time... and with smaller engines.

One negative aspect of a diesel on the trail concerns your buddies. You’ll often find yourself as tail-end-Charley because people generally don’t like the smell of diesel exhaust. I acquired the name “Stinky” with the diesel Blazer. I deliberately turned back my injection pump trying to lose that nickname, but alas all I lost was some power. The name stuck!