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The Best Diesel Engines: Top 10 Of All-Time

The Most Revered Oil-Burners Ever Assembled

From the farm to the highway, construction equipment to gen-sets, and emergency vehicles to the trucks we drive every day, diesels have built—and continue to build—America. Even beyond our borders, in every mechanized blue-collar segment of the world, you’ll find a diesel in charge of the lion’s share of the workload. And although it goes without saying that all engine manufacturers strive to turn out the best product possible for every application, some power plants are simply better than others. Be it due to their durability, mechanical simplicity, or horsepower potential, these oil-burners are a cut above the rest—and we have 10 of the best diesels ever concocted to tell you about in the pages that follow.

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Many of our choices hearken back to the good old days, before the emissions crunch had completely taken hold and acronyms like EGR, DPF, and SCR had yet to complicate and ultimately cause so many of the reliability issues we’re familiar with today. Names like Cummins, International, Caterpillar, Detroit, John Deere, GM, and Mack all made the list, and the engines we’ve selected represent some of the finest examples of compression-ignition on the planet. Even with most of them now out of production, they remain celebrated and treasured. Without further ado, and in no particular order, we give you “the list”.

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International DT466

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Whether you know it or not, you’ve spent time around International’s DT466. School buses, box trucks, farm tractors, small bulldozers, and dozens of other applications have been powered by this legendary I-6. The largest of IH’s 400 series engines, the DT466 was the big brother to the 414 and 436 ci tractor engines, which all shared the same block. Its deep-skirt cast-iron crankcase housed an induction-hardened, forged-steel crankshaft with seven main bearings, utilized ductile-iron wet cylinder liners, and featured a 4.30-inch bore with a torque-happy 5.35-inch stroke.
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Thanks to a robust rotating assembly and six head bolts per cylinder being part of its mechanical makeup, the DT466 can withstand a lot of abuse in stock form. According to Hypermax Engineering, a big name in the world of tractor pulling, the iconic IH engine will handle 1,200 hp before the factory head gasket becomes a concern—and up to that point you can add all the fuel and air you want without worrying about hard-part upgrades of any kind.
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Aside from its anvil-like bottom end, simple 2-valve cylinder head, mechanical injection, and in-frame rebuildable design making it a favorite in the medium duty truck world, the DT466 found a natural home in tractor pulling also. Today, decades after discovering the limits of the factory components, replica blocks, billet cranks, stronger rods, and solid heads—combined with Sigma injection pumps, triple-feed injectors, and multiple turbochargers—DT466-based power plants can withstand 300-psi of boost and turn out more than 4,000 hp.

6BT Cummins

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Not only did the B-series 5.9L Cummins, the 6BT, revolutionize the pickup truck segment, it made it possible for a magazine like this to exist. Without it, the diesel industry wouldn’t have exploded the way it did and Diesel World might not have even been created. That’s how instrumental the 5.9L Cummins was in the expansion of the diesel marketplace. The 12-valve Cummins brought an I-6 design, direct-injection technology, and turbocharging to the ¾-ton and larger truck segment when it arrived in the summer of ’88, and Ford and GM have arguably been playing catch-up with the Chrysler/Cummins alliance ever since.
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One look at the 6BT’s connecting rods and you start to see why this overbuilt inline-six was capable of lasting a million miles, or surviving four-digit horsepower. Each forged-steel I-beam rod gets its own rod bearing journal (each one separated by a main bearing journal) and sports a 1.57-inch diameter floating wrist pin on the small end. When treated to shot peening, micro-polishing, and ARP rod bolts, these rods can live at 1,400-hp and more than 2,000 lb-ft of torque. The forged-steel and induction-hardened crankshaft is secured via 14mm main cap bolts, while the cylinder head employs six bolts per cylinder in its attachment to the cast-iron block.
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Though the 6.7L Cummins block, with the added strength that comes with its Siamese cylinder bores, is the favored crankcase for high-horsepower at the present time, the 5.9L block is still a viable option all the way up to 1,400-1,500 hp. Granted, the 5.9L’s threshold for horsepower is lower than that in street applications, but we’ve seen more than our fair share of them in the 1,100 to 1,200hp range.
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Simple yet proven, the 6BT Cummins’ crossflow, 12-valve cylinder head was cast from gray iron. However, its induction-hardened valve seats and ductile iron intake and exhaust valve rockers (not the common stamped steel type) suggest that it was built for the long-haul. In the world of diesel motorsports, the 12-valve head (and its aftermarket variants) is the weapon of choice. When treated to extensive porting (via CNC and/or by hand) and oversize valves, it can flow more than 300 cfm per cylinder (vs. 140-150 cfm stock).

3406 CAT

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There is no denying this engine’s presence in the Class 8 world, as thousands of over-the-road trucks still rely on it to propel them all over America in a trouble-free manner. It’s the legendary 3406 from Caterpillar and it’s a big reason why legions of truck owners swear by CAT power. Available in A, B, C, and eventually the coveted E model, the 3406 featured a bore of 5.40-inches, a stroke of 6.50-inches, and displaced 893 cubic inches (14.6L). Depending on the model, horsepower ratings ranged from 375-hp to 465-hp, with the highest factory torque output checking in at 1,850 lb-ft.
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As mentioned, the most revered 3406 carries an “E” at the end. While the 3406A and 3406B were fully mechanical engines (and the 3406C was a mix of both mechanical and electronics), the 3406E marked the point in time when CAT had mastered and then released a fully electronic unit injection system (circa 1993). Being full-on, electronically controlled meant that the engine control module (ECM) could be recalibrated, and when the right aftermarket reflash was uploaded some impressive horsepower gains were on the table. This hot-rod nature of the 3406E helped develop what became a cult-like following, where 550 and 600hp files were commonplace.
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In mega-horsepower, competition settings such as the 20,000-pound semi category in sled pulling, many drivers choose to campaign a 3406. This was the case for both Jerry and Jeremy Walker, a championship father and son duo that ran ragged edge versions in their semi’s, one a Peterbilt named “Two of a Kind” and the other an International coined “Down ‘N Out.” Both engines displaced more than 1,000 ci, each one being fed air via a massive single turbo with an inlet larger than 5-inches.

7.3L Power Stroke

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It’s not going to win any horsepower accolades or turn over happily in frigid conditions, but the 7.3L Power Stroke will never let you down. The largest diesel engine ever offered in the pickup segment, the Navistar-produced, 444 ci V-8 featured a 4.11-inch bore, a 4.18-inch stroke, and packed 210 horsepower and 425 lb-ft of torque when it debuted in mid-1994. Avant-garde for its day, the 7.3L Power Stroke featured hydraulically actuated electronically controlled unit injector technology (HEUI), an injection system developed by Caterpillar and leased to International to meet the coming emission standards. Direct injected and specifically designed around the use of a turbocharger, it was also Ford’s first serious answer to 5.9L Cummins-powered Dodge Rams.
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A forged-steel crankshaft, forged-steel connecting rods (although powdered-metal units were introduced after 2000), and direct-injection, cast-aluminum pistons with plasma-coated top piston rings speak to the 7.3L’s robust simplicity. The cast-iron heads featured two valves per cylinder but six head bolts per cylinder (with sharing). The valvetrain operated according to a single camshaft in the conventional V-8 location within the block, and hydraulic lifters meant no periodic adjustment of the valves was necessary.
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Low engine speed, big iron components, and (let’s face it) a lack of power is a great recipe for making any diesel last—and the 7.3L had all three of those characteristics working in its favor. After all, the most powerful version of the 7.3L produced kicked out just 275 horsepower and 525 lb-ft of torque (available beginning in ’01, but only in conjunction with the six-speed manual), a far cry from the 1,000 lb-ft diesels sitting on dealer lots at the present time.
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To mechanical injection fans, common-rail owners, diesel outsiders, or those whose first exposure to hydraulically actuated fueling came in the form of the 6.0L Power Stroke, the HEUI injection system is sometimes frowned upon. However, the system’s injectors, high-pressure oil pump, and injection pressure regulator were very reliable in the 7.3L application. Properly maintained injectors are virtually guaranteed to last 200,000 miles before requiring an overhaul, and many sets last well beyond that. Even today, 19 years after production of the 7.3L ceased, repair shops are still pulling original injectors (units equipped with black O-rings) out of engines.

Cummins 855 Big Cam

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Designed to adhere to the new emission standards set forth in the Clean Air Act of 1976, Cummins released the 855 Big Cam. It soon proved to arguably be the most fuel efficient, most durable, and most powerful engine available in a Class 8 truck—thereby owning the road for most of the late 1970s and early 1980s. The 855 ci (14.0L) Big Cam I (four versions would be available in all) possessed a 5.50-inch bore, a 5.98-inch stroke, and most versions were available with power ratings that ranged from 250 to 400 hp. However, some 855 Big Cam engines were rated as high as 605 hp in gen-set form.
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The first three renditions of the 855 Big Cam (i.e. Big Cam I, II, and III) utilized Cummins’ PT (pressure-time) fuel system, which allowed for mechanically variable timing. Electronics entered the fold in 1985 aboard the Big Cam IV, but the Big Cam IV was soon replaced with the N14. Improvements throughout the Big Cam’s production run included the demand-flow cooling system and pulse exhaust manifold that debuted on the Big Cam II engine in 1979, a pressed-steel oil pan and Holset HT3B turbocharger added for the Big Cam III in 1983, and a revised cylinder head and pulse exhaust manifold for the Big Cam IV in 1985.

John Deere 50 Series 6-619

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It would be criminal to overlook the 619 ci 50 Series John Deere engine after having listed the International DT466 as one of the best diesel power plants ever. After all, exotic tractor pulling versions of the green I-6 have been duking it out (and often winning) against DT466-based mills for years now. Down on the farm, you can typically find the 619 swapped in place of a 5010, 5020, or 6030’s original 531 ci engine. Out on the track, the 619-based ‘Deeres tend to dominate in classes where their big cubic inches are allowed to run free.
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From the factory, the 619 ci (10.0L) 50 series John Deere features a 5.125-inch bore, a 5.00-inch stroke, cylinder liners, and six head bolts per cylinder—a perfect foundation for both longevity and/or huge horsepower. One of its most favorable attributes is the fact that its oil pump is crank-driven rather than cam-driven. Excessive cam wear was common on the 30 and 40 series engines that preceded the 50 series, and many owners upgraded to the 50 series when major mechanical failures or problems were encountered for this very reason.
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In the wild world of Pro Stock tractors, you don’t exactly see factory-based 619 John Deeres under the side shields. The block may be stock or a factory replica, but within it lies a billet crank, rods, pistons, a girdle, and a 1-inch thick deck plate holding it all together up top. In addition, most 619’s are brought up to the class maximum of 680 cubes to compete at the maximum displacement allowed. From there, a billet Sigma pump and billet cylinder head (some capable of flowing as much as 500 cfm) is combined with a turbocharger sporting a 5.5-inch or larger inducer. Rumor has it that the current crop of Pro Stock John Deere engines can spin 6,000 rpm and belt out as much as 4,200 hp on the dyno. Talk about Big League stuff!

Mack E7

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Any diesel produced in the 1980s but that survived a major emissions crunch before being retired is worthy of mention. Mack’s E7 is one such engine. The E7 line, which consisted of 16 different versions of the same I-6, was released in 1988 and underwent several changes (even becoming the E7 E-Tech) during its 20-plus year production run. To many, the 728 ci (12.0L) E7 epitomizes the manufacturer’s bulldog logo to a “T.” Its horsepower ratings ranged from 250 to 460 hp but, in true Mack-like fashion, earth-rotating torque was generated right off idle. The torquiest version of the E7 (the 460hp model) turned out 1,660 lb-ft of torque at 1,200 rpm. Also typical of a Mack mill, the 12.0L E7 produced torque numbers that rivaled what larger engines from competing manufacturers were producing at the time.
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The first E7’s were fully mechanical, but electronics would infiltrate the works beginning in the early 1990s. Mack’s initial (and partial) electronic control over its fuel system was coined V-MAC (vehicle management and control), which added a rack actuator, rack position sensor, and a timing reference sensor to the Bosch inline pump. Mack’s Econovance system also brought variable mechanical timing into the picture to help custom-tailor fuel delivery to the end user’s needs. In 1998, the E7 received Mack’s electronic unit pump (EUP) system, a pump that was cam-driven. By 2003, EGR was making its way onto the revamped 12 liter engine, and the million-mile engine slowly lost its longtime luster.

Detroit Diesel 60 Series

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Without the Series 60 engine line, Detroit Diesel may have continued to lose market share in the 1980s and eventually folded up. Thankfully, that didn’t happen. When other engine manufacturers were just beginning to recognize that electronic control was the wave of the future, Detroit developed the 677 ci 11.1L and 778 ci 12.7L from scratch and hit a homerun right off the bat. Each engine’s unit injection system was completely electronic and performed flawlessly, as did the rest of the engine. In fact, the initial overhaul interval recommendations of 500,000 miles were changed to 750,000 miles soon after these engines were released.
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It’s been documented that Detroit Diesel, owned at the time by GM, reached out to John Deere in the mid 1980s for help in reviving its dying engine program. A short time later, the 60 series appeared on the scene, which virtually solved every issue the previous 50 series power plants had before it. Later on in 2001, Detroit set its sights on the larger engine market with the release of its 858 ci 14.0L engine. This version, possessing a long 6.62-inch stroke (and a 5.24-inch bore), was capable of producing 1,650 lb-ft of torque at 1,200 rpm, along with 515 hp at 1,800 rpm.

KTA Cummins

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Though pictured in gen-set trim here, the KTA Cummins was the largest Class 8 engine ever found on America’s highways. A square engine, its 6.25-inch bore and stroke made for 1,150 cubic inches (19.0L) of displacement, and although its original intent was never to power over-the-road trucks, the KTA made its way into them nevertheless. Single turbo Class 8 versions sported a single turbo and could crank out 600 hp—a very big number back in the late 1970s and 1980s. Peak torque from the factory checked in at 1,650 lb-ft, but it’s rumored that the KTA had to be fueled conservatively at low rpm to limit torque (that 1,650 lb-ft wasn’t available until 1,600 rpm), thereby increasing the lifespan of whatever transmission was bolted up behind it (usually an eight or 13-speed Eaton Fuller).
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Like the 855 Big Cam engines of the era, the KTA used Cummins’ PT (pressure-time) fuel system. With little more than fueling and turbo upgrades, the KTA could achieve four-digit horsepower and still go to work every day of the week. As for high-horsepower, truck-pulling applications, the 1,150 ci Cummins dominated the ranks of hot-rod type semi classes for years, in large part thanks to its solid foundation and obvious displacement advantage over other Class 8 engines. Now for the best part…There was also a KTTA, the double T meaning twin-turbo. The KTTA was primarily reserved for gen-sets and industrial-type use.

6.6L Duramax    

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It’s the only engine with aluminum heads and one of just two V-8’s on our list, but it’s certainly no less deserving of being here. When the 6.6L Duramax (RPO code LB7) blasted onto the scene in the summer of 2000 for ’01 model year GM HD trucks, it boasted the highest horsepower and torque any diesel pickup ever had before—but that wasn’t all. Beyond its class-leading 300 hp and 520 lb-ft of torque, the Duramax brought common-rail injection, the quietest and cleanest diesel engine operation anyone had ever heard or seen, and the aforementioned aluminum cylinder heads to the pickup truck segment.
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The LB7 Duramax’s 4340 forged-steel crankshaft was heat-treated, forged-steel cracked cap rods were employed, and the cylinder walls’ combustion areas were induction-hardened (something GM’s rivals weren’t doing). Looking back, the 6.6L Duramax’s same basic architecture has survived more than 20 years of production now, and with very few design alterations. It still utilizes a deep-skirt, cast-iron block (where both the Power Stroke and even Cummins have since gone to CGI), and the same bore, stroke, and valvetrain arrangement. While it’s one of the more complex diesels on our list, there is no arguing with its ability to go the distance. We’ve seen several 500,000-mile candidates, as well as an LMM version with more than 750,000 on the ticker.

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