Coveted Inline-6 Diesel Engines

• December 8, 2020
• Story By
  Mike McGlothlin

We spend a lot of time here at Driving Line celebrating the Cummins found in ’89 to present Dodge Rams, but in at least one corner of the world that I-6 diesel is considered child’s play. After all, long before engines like the 6BT revolutionized the diesel pickup market, much larger and arguably more durable engines were already powering America’s Class 8 and medium duty trucks. Below, we’re spotlighting three of the most coveted versions: Caterpillar’s 3406, International’s DT466 and Cummins’ KTA-600. Each of them is now gone from production, but—thanks to the mark they left on their respective segments within the truck world—they will never be forgotten. Here’s why:

3406 CAT

CAT’s 3406 came in A, B, C and E models (ex: 3406A, 3406B, etc.) and then eventually morphed into the C15. Thanks to a 5.40-inch bore and 6.50-inch stroke, the 3406 displaced 14.6L (893 ci). Its block featured cylinder liners for ease of in-frame serviceability and the cast-iron 24-valve cylinder head incorporated an overhead cam. Cam-driven unit injection fueled the 3406, mechanically in the beginning and electronically later on. Depending on the model, maximum power output ranged from 375 hp to 465 hp, while the highest torque figure offered was 1,850 lb-ft. The engine’s governed speed varied from 1,800 to 2,100 rpm.

Most Coveted: 3406E

The 3406E is perhaps the most desirable version of the lineage. In large part, this is because CAT all but mastered its electronically-controlled unit injection system with this engine. It is known for being just as reliable as the mechanical 3406’s that came before it, but it possessed the refined mannerisms that come with being fully electronically controlled. The 3406E was also rife with horsepower potential. Its ECM was easily hackable, thereby leading to loads of hot-rodded or hot-running trucks tearing up the highways. With both reliability and performance on its side, countless Class 8 trucks were ordered with 3406E’s under their hoods from the mid 1990s well into the 2000s.

Off-Highway, Insane Power

Jerry and Jeremy Walker, a father and son pair of truck pullers, campaigned extreme, punched-out versions of the 3406 CAT in their 20,000-pound Pro Stock semi’s for many years. Carrying the driver side front tire the length of the pulling surface was par for the course for either the Walker’s “Double Down” Peterbilt (shown) or their “Down N Out” International. Each exotic rendition of the 3406 sported a giant single turbocharger with an inlet larger than 5-inches.

DT466 International

While sleeved blocks (cylinder liners) aren’t exactly cutting-edge in the Class 8 market, the wet sleeve design of International’s medium-duty DT466 was groundbreaking. This technology simplifies in-frame rebuilds and individual cylinder repair—and with the DT466 originally developed for light bulldozers and agriculture equipment it made all kinds of sense for prospective buyers. For decades, the DT466 was the engine of choice for school buses and box trucks, too. It’s an engine that International was able to make evolve in order to keep up with the times, having been in production for more than 45 years.

Half A Century Of Service

Perhaps the only blemish on the DT466 name is that its final rendition, coined the MaxxForce DT, exited with some reliability problems—nearly all of which were due to being fitted with modern emissions-control equipment. But in reality, what engine design concocted in the late 1960s could stand a chance in the post-2007 emissions era? It’s amazing the engine survived as long as it did…all the way until 2016 before Navistar ceased production.

Extreme Applications Are A Plenty

Over-engineered in essentially every way, DT466-based engines have proven capable of making 1,200 hp before its head gasket becomes an issue (simply pour on the fuel and add in more air via bigger or multiple turbochargers). And, when a wire-ring gasket or fire-rings have been added it’s game-on once again. All-out competition versions of the DT466 have been employed in Super Stock pulling tractors for decades, applications where more than 4,000 hp and 300 psi of boost is on the table. Above, a two-stage, triple-turbo and 540 ci version of the DT466 is shown, parked in Hypermax Engineering’s Spitzer chassis dragster. The one-of-a-kind, 3,600-pound rail has been 6.69 at 216 mph in the quarter-mile—which means somewhere around 500 hp was still left on the table!

KTA-600 Cummins

The KTA Cummins is an 1,150 ci all-iron behemoth—and the 600hp version of this power plant that was offered with limited availability in Class 8 trucks is the stuff of legend. Rumored to be capable of belting out 1,000hp without breaking a sweat, it’s no wonder this 19.0L I-6 used to be the go-to engine for most of the semi’s on the sled-pulling circuit. Boasting a 6.25-inch bore and stroke, the KTA is one of the biggest on-highway engines ever produced, but was also used in industrial applications, large farm tractors and even marine applications (fishing boats had them starting in the mid-1970s). Today, you can still find it in use in gen-sets around the world. In case you were wondering, the “K” stands for the engine series, the “T” stands for turbo and the “A” means it’s aftercooled. The KTTA, a Holy Grail version of this legendary engine, was twin-turbo’d.

The Only 600HP Semi You Could Buy 40 Years Ago         

When you could order a semi with the KTA-600 back in the late 1970s and 1980s, its 600 hp was a huge number. Remember, this was an age when even 500 hp was a big deal. The KTA-600 could be had in Peterbilt, Freightliner, International and Kenworth trucks (and probably more makes than that) and it turned out 1,650 lb-ft of torque at 1,600 rpm. Rumor has it that the torque rating was kept low (ish) due to no transmission manufacturer being able to guarantee durability behind any more grunt than that. This builds on the rumor that the only transmissions warrantied behind the KTA-600 were the eight and 13 speed units from Eaton Fuller.