The Legendary Detroit Diesel V12 (12V-71) - The Most Powerful Engine Built For Highway Trucks In Its Time

Although more powerful four-cycle diesel engines would eventually surpass the DD 12V-71 as the most powerful engine built for highway trucks, it was certainly true in its heyday. With 475 hp and a shrieking “Screaming Jimmy” roar from its twin exhaust stacks, it turned any truck that sported one into king of the road. This was in the late ’60s and early ’70s, during the great muscle car boom making the 12V-71 was the semi equivalent of an L-88 427 Corvette or a Hemi Charger.

The automobile sector wasn’t alone in having a horsepower race that started in the early-mid ’50s. Big trucks entered the post war era with maybe 150 or so horsepower, resulting in highway speeds of 45, maybe 50 mph. As both the economy and the highway system expanded rapidly, the demand for more power was almost insatiable. Time is money, never more so than in the trucking business. By the late ’60s, 250-300 hp was becoming the norm, allowing trucks to roll along at 60-65 mph on the new interstates.

But even that was not enough for some, mostly owner-operators who wanted to reduce trip times by flattening hills and roll along at 75, even 80 mph in the pre-55 mph era, at a time when diesel really was cheap. They wanted more power, and they found it in the Detroit Diesel 12V-71, as it was smaller and lighter than what a four-stroke engine of comparable hp would have been. And the lure of saying “I have a V12 under my hood” was about as powerful as the engine itself.

I’ve done a very comprehensive history of the DD engine in trucks, but just to give some perspective, they were first available in 1939, primarily in three and four cylinder versions. The 4-71, as installed in this 1939 GMC, had 110 hp, which was about the norm back then, and quite suitable given the conditions and expectations of the times.

The 165 hp 6-71 was available for the biggest trucks, like this West Coast Kenworth, as a more economical alternative to the very powerful but thirsty gas-powered Hall-Scott engines.

In 1960, the new DD 8V-71 engine was available on special order in GMC’s biggest trucks. It was rated at up to 290 hp, soon increased to 318 with larger injectors.

I assume that the 12V-71 went into production at or about the same time as the 8-V71. Undoubtedly its initial intended mission was for marine applications, where it became a popular choice for ever-larger yachts as well as working boats. There were also 16V and 24V variants, but they were essentially multiples of the 8V engine, with separate but cojoined blocks. The 12V-71 was the largest of the family with its own single block.

But as power demands increased for trucks, and truckers discovered the 12V-71, it soon found its way there, starting in the mid-late sixties, as there are a number of survivors from this era, quite often long-hood Peterbilts, Kenworths and Brockways on the East Coast.

According to my book “GMC Heavy Duty Trucks 1927-1987” the DD 12V-71 became available as a special order option on the Astro 95 COE (and almost identical Chevrolet Titan 90) in 1970, and as a regular production engine option in 1972. This same source quotes a gross rating of 475 hp for 1970 and 434 hp for 1972 with 1205 ft.lbs. of torque. The 1973 GMC Astro 95 brochure switched to net ratings, with 390 hp and 1078 ft.lbs. of torque.

The above-mentioned source book shows this special order 1964 DFWX7120 off-highway rig powered by a 12V-71. Undoubtedly there were others, including some on-highway trucks by owner-operators that had exceptional needs or just wanted an exceptional engine under the hood.

There were always those that craved more power, and the DD 12V-71 would have been the first new V12 to come along since this operator had a Hall-Scott V12 installed in his 1951 Kenworth. That beast produced either 450 or 600 hp, depending on the source, but its thirst for propane gas was prodigious. The DD 12V-71 would have been a fuel miser in comparison.

At the NWP Truck Show I attended recently summer, there were several 12V-71 powered trucks, the oldest one being this 1967 Peterbilt 351-ST.

It’s a typical example of the time the 12V-71 started finding its way into large trucks.

It was used by a heavy haulage firm, meaning oversized and over-weight loads, so it was presumably ordered this way from Peterbilt, which was undoubtedly willing and able to accommodate such special orders.

This 1973 Kenworth W 900 A Longhood was another 12V-71 powered truck in attendance.

Its 12V-71N (non turbo) has the later-style alloy valve covers; earlier versions had either painted or chromed steel valve covers in a flatter style.

Here’s another look from the other side. Visually it’s almost as impressive as that Hall-Scott V12, although it has considerably less displacement, at a mere 852 cubic inches (13.97 L). That’s less than most of the 4-cycle diesel six cylinders, but that was the big advantage of the DD 2-stroke format.

There was another on there, a 1970 Peterbilt 359 EXHD, in more original condition.

A turbocharged 12V-71T was also available, with a 20% increase in maximum torque and 15% more hp. It must be noted that due to their 2-cycle design, DD engines did not show the large increases in torque and hp as some of the 4-stroke diesels could and did.

It’s common for some to radically increase the boost in 4-stroke diesels to increase power to 2500-3000 hp. This is simply impossible with the DD 2-cycle diesels; there’s almost no way to significantly increase their power above their original ratings.

Here’s a 12v-71 powered Kenworth in Australia pulling a road train of vintage trucks. I’m not going to pretend I can actually tell the difference between these and an 8 or 6 cylinder DD, because with so many overlapping 2-stroke power cycles there really isn’t any, or if so it’s very subtle. But sweet, in any case.

If you just can’t get enough, here’s one of a sustained full-power maximum rev pull up a steep quarry road. You get the idea. No wonder they were dubbed “The Buzzin’ Dozen”.

The DD 12V-71’s reign as the king of the road was not very long. In 1973, Freightliner unleashed the Powerliner, and although its standard engine was the 12v-71, it drastically upped the ante with its optional 600 hp Cummins KTA, an 1150 cubic inch 4-stroke turbocharged six that belted out 1580 lb.ft. of torque. Game up!

In reality, it was game up for both of these super-power diesels in trucks, as the energy crisis and resultant higher fuel costs and the 55 mph national speed limit changed the rules almost overnight. Efficiency was the new watchword, and that was never in the 12V-71’s brief.

The other thing that killed it was the introduction of the DD V92 series of engines, with more displacement per cylinder (92 cubic inches instead of 71). This meant that the 6V92 was now as powerful as the 8V71 had been, with 270 to 335 hp, and the 8V92 even eclipsed the 12V-71, with 360-435 hp. And with less internal friction and other improvements, it was inherently more efficient. There was of course a 12V92, but as far as I know it was never offered in production highway trucks.