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. It’s a bit more challenging to significantly raise the output on a DD, but it’s been done.
There are special marine units of the 6V-92 that make 700hp, and I just viewed this video of an 8V92 that makes some 2000 hp but it took some serious engineering to do that, including fitting a complete common trail fuel injection system and of course two massive turbochargers.
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.
But for some, the 12V-71 lives on, even if it’s in a (big) rat rod.
Related CC reading:
The company i worked for in 1972 to 75 had four kw coe with the 12v71 for pulling double trailers on i90. This was before radial truck tires. I had to change the drive tires every 15k miles (about two months) the massive torque just burned them up. Excellent source of carcass for the recap shop.
Splendid overview and pictures, very welcome!
Here’s a duo of Detroit Diesels V12, joining forces (Flickr/Donald de Kloet):
Love that hill climb. I wonder if they worked out plan B ahead of time and if so what it was.
Old diesels are gold.
Awesome also the old Scania trucks V8 engine.
The 12v-71 was the king of the hill, but even in the 70s there were a few challengers. Cat had thier complicated, expensive and heavy 1693 at up to 425 hp, it even had dual overhead cams. Which meant it couldn’t be fitted with a Jake brake, only a brakesaver.
Cummins had thier problematic NTA series with up to 410 hp, and even Allis Chalmers fielded a 425 hp version of thier big agricultural 6 cylinder for a couple of years. They painted them purple and called it “Big Al”, but it was not succesful in over the road applications and were more of a rumour than a fact.
There were all kinds of little tricks to pull more power from what was available at the time, some worked and some produced little more than smoke. Here in mountainous BC everyone was after more power, even after engines like the Cummins K series and Detroit 92 series came along.
Just like hot rodding your car though, more power meant problems elsewhere. The strongest transmissions and clutches of the day were pushed to thier limits by these engines, so unless the operator understood how to use that power things would break.
The factory 12V-71 was probably the best way to get power with a minimum of problems, and there were a few around. A trucker I talked to many years ago said he could get an extra trip between Vancouver and Calgary every month, which just about paid for the extra fuel he burned with his!
I’m trying to understand the limitations on boosting a DD 2 stroke. I understand the exhaust valves have to stay open briefly to allow the incoming air to push the burned mixture out of the exhaust, I assume this limits the amount of boost available? Anyone have anymore information?
The blower on a naturally aspirated Detroit is not actually there to provide boost. It’s actually a scavenger blower intended to facilitate airflow out through the exhaust ports. Turbocharged Detroits produced boost like any other engine, and they had slightly different cam timing which I believe was intended to allow boost to build. I don’t recall how much boost was produced, but it was enough to require the compression ratio on turbocharged engines to be lowered from about 18.5 to about 17. Don’t quote me on the numbers, its been a long time since I built one.
On an 8V-71 the turbo was good for maybe 30-35 hp, and much more importantly about 200 ft lbs of torque. Once you got into the higher altitudes you could really feel the difference.
I worked in a mine years ago and the dump trucks (WABCOs, if memory serves) had 16V-71s.
Pretty impressive.
Great post and pics. I often heard truckers saying they had a “318” which I assumed was a CAT, but only learned recently that it was actually an 8V71.
My comment was specifically related to this line from Paul, “This is simply impossible with the DD 2-cycle diesels; there’s almost no way to significantly increase their power above their original ratings.”
Your comment seems to back that up. 30-35 horsepower isn’t much over the ~318 an 8v71 made. Theoretically, 7 psi should up horsepower a little less than 50%. Modern diesels push 40 psi boost pretty easily.
I’m guessing there’s some limitation in the ability to fill the cylinders with more pressure. Are the intake ports open to the crankcase when the piston is at the top of its stroke? That would certainly limit the ability to build boost.
*Edit, I found a cutaway, the piston blocks the port.
Maybe the injectors wouldn’t work with a higher cylinder pressure?
I don’t think the injectors were an issue, but after a certain point heat became the issue. In marine applications with raw water cooling even a 6-71 with a big turbo and big injectors could pull 400 hp or more and survive. Truck 6-71Ts, the few that were around, were limited to 275 hp. Same deal with the V series engines.
The big advantage that a turbo offered a Detroit wasn’t so much raw power, but rather the turbo allowed the engine to produce rated power at higher altitudes. A 318 over the Rockies was a long pull, the turbo really helped.
It’s all ancient history now of course, 600 hp is common as is 2000 ft lbs of torque. All you gotta do is write a cheque.
I used to enjoy tuning Detroits, theres an art to it and I still have all the different injector timing pins, governor gauges etc. Up until maybe 10 years ago there were still a few guys running them in gen sets or boats, but they are now pretty much extinct.
I specifically thought I had removed that line late last night. Why? Because I found a video of a guy who’s modified his 8V92 to make 2000 hp. (I’ve embedded the video into the post now). But it took some serious doing, including fabricating a complete common rail injection system electronically controlled. Yes, the DD unit injectors are not capable of dealing with both the higher pressures and fuel demands. And of course it has two enormous turbos.
I wrongly assumed that due to the short duration of when both the intake ports are open and the exhausts are closed it would be impossible to boost these to very high pressures, unlike 4-cycle diesels which can be boosted to very high levels. I would be curious to know just how much boost he can get in there.
The other way he gets so much power is to substantially increase the revs.
In any case, that line was incorrect and shouldn’t have been there, although the stock truck DDs generally didn’t have much of a boost in power from being turbocharged.
Well, I read through the (excellent) previous post on the history of the DD, and I thought I’d got the turbo-ing of them (and the limitations) straightened out in my head, but it seems not. The comment by Robert Bray below is too much complication for my non-engineer’s brain! I’ll try and find a cutaway I can almost understand…
Keep in mind that unlike a small 2 stroke gasoline engine, the DD 2 stroke intake ports are not open to the crankcase. They are open to the ‘air box’, a sort-of cast in the block intake manifold separate from the crankcase that the blower pressurizes. Neat thing about the air boxes is that they have removeable cover plates on the side of the block, and when you remove them you have a pretty good view of the rings and cylinder liner walls. Or a lot of motor oil if the engine is shot…..
71 Series Detroit’s have become my favorite big diesel engines. As Paul knows, it started 22 years ago working on a Coast Guard boat with a pair of 6-71’s, that howl is engrained in my memory and I crave to hear them still. I also spent a lot of years working on the 47′ Motor Lifeboat with a pair of 435hp 6V-92TA’s, those turbos scream with over 105 decibels on the back deck cruising at 1750RPM! 2 years ago I was fortunate to briefly drive another older Coast Guard boat with twin 12V-71TT’s, 55′ aluminum hull gets up and goes a solid 25 knots. Of course, they are Detroit’s which means that if they ain’t leaking, they ain’t running but I really love the sound and look of the 71’s.
Great article. I, for one, know very little about diesel engines whether in a 300D or trucks. Had to look around for some videos of this engine in rolling trucks. Found one mean looking truck for this bad ass engine.
Great article, Paul. You are very correct about the 8V-92T’s replacing the 12V-71 in most highway truck applications. The 92 was lighter and externally smaller and produced the same or more power with better fuel economy.
Thank you both for the replies. I suspect tiredoldmechanic’s answer is the main reason they didn’t come with more power from the factory; heat.
Exhaust gas temperatures are important to watch with a diesel, especially when boosted, as the pistons are prone to melting above a certain level, especially when subjected to those temperatures for longer durations (like pulling a mountain pass). Twice as many heat cycles could only make that happen more quickly.
So capable of making a lot of power, but not for as long or in as forgiving of a manner as a 4 stroke.
Great post, and I’ll confess I watched the vids.
That rat-rod at the end twists ‘n’ shouts even at idle: not surprising when you notice that the v12 weighs in at about 1.5 tonnes.
I dont know if anyone here knows the old Canadian west coast show “Beachcombers” but Nick’s boat in the show is obviously DD powered by the sounds of it
Had three 6V92’s on fire trucks on my fire dept. One was a 3000gal tanker with a 335hp 6V92 and the other two were an engine and staright stick ladder with 400hp 6-92’s. All you had to do was change injectors for the hp increase. Absolutely LOVED that sound from those engines!!