(first posted 6/30/2012) It’s beginning to sound like a broken record, but the years 1960-1961 were undoubtedly GM’s most creative and adventuresome ever: rear-engine Corvair; aluminum V8, the Tempest with half-a-V8 four and swing axles, the Buick 90° (three-quarters-V8) V6, Greenbrier van and Rampside Corvair. And there was another: the GMC truck V6 engine, which appeared in 1960. And unlike the Buick V6, this was a proper one indeed: 60 degree cylinder angle for even firing pulses, and a compact block assuring a short and stiff crankshaft. It was quite the bombshell; never mind its V12 “twin-six” version (post here).
Why a V6? Good question. To be different, for its own sake? Possibly, but not likely. And why not a V8? Hmm. Let’s look at the historical context, and perhaps a good answer will formulate.
GMC was famous for their truck in-line sixes; well, actually, that’s all they ever built, as gas engines. It was a venerable line, starting in 1939, when GMC stopped using the Pontiac six. The primary family included displacements that varied from 228 inch³ to the legendary 302.
270 inch GMC sixes powered the legendary “Deuce and a half” Jimmy 6×6 military trucks and other equipment in WW2, of which some 800k were built. Take that! We drove one and posted that here, with video.
These rugged ohv engines also were popular with the hot rod set in the late forties and early fifties, because of their big cubes (270 and 302), overhead valves and ultra-tough construction.
There was also a “large-block” GMC inline six, with 426 and 503 cubic inches, for the really big rigs and buses.
Given that a six cylinder intrinsically has a torque advantage over a comparable-sized V8 (all other things being equal), staying with that proven number of cylinders makes gobs of torque sense, for true truck engines. There’s a reason why literally every semi on the road has a six under its big hood.
And it was the era for GM to feel modern, so why not go all out and design the very model of a modern six? The 60 degree cylinder angle is of course the preferred one, since it affords an even firing interval of 120 degrees if a split journal crankshaft is used, as had been the case since Lancia’s V6 since 1950. In cross-section, it looks somewhat similar to the little Chevy 60º V6, eh? Well, the block is a bit deeper and beefier.
The GMC V6 was designed for serious truck duty, and came in a range of displacements from 305 inches³ to 478 inches³. Oddly, there was even a V8 version with 637 cubic inches, with twin balance shafts to compensate for its uneven-firing and vibrations, due to its odd 60° block. They were all relatively low-rpm engines, with some versions producing their maximum power below 3000 rpm. The M “Magnum” series that appeared in the mid-sixties had bigger ports and valves, and less constricted exhaust headers, and are the most powerful and desirable of the family.
I drove a GMC V6-powered medium truck, for an interstate sign erector outfit, and its characteristics were distinctive, in relation to the other trucks in the fleet. Its low-end torque put the typical Chevy small-block to shame, and was palpably better than the Ford FE-powered ones. It bit hard right from idle, and its characteristic sound was of course unique. We’re used to the slightly-tense grumble from the Chevy 60° V6, and the GMC V6 shared some of that, but through a megaphone. At full chat through some shorty pipes, it was memorable; more so than its actual power output, which was hum-drum.
It was America’s love for big V8s in their pickups that killed the GMC V6 in that role. As GMC started offering more Chevy V8 power, the V6 was relegated to entry-level duty, and eventually disappeared. In truck applications, it soldiered along until 1978, eventually also replaced by the growing appetite for diesels, or the big-block Chevy V8, which was undoubtedly cheaper to build.
The GMC V6 was also built in a diesel version, the Toro-Flow, as a cheaper alternative to the “Million-miler” Detroit Diesel 6-71. Probably the less said, the better. It had a spotty reputation, and I doubt anyone ever racked up a million miles on one. Many were eventually replaced with gas V6s or something else.
Some folks did put them in their GMC pickup trucks, like this one installed in 1967 in a then-new GMC pickup. They did better in less-demanding applications.
The gas 305 was the standard engine for pickups, and most likely is what thrums under the hood of this fine old veteran. Rated at some 135 or 140 hp, this was designed to be a blend of six-cylinder economy and V8 power, although some early complaints had those qualities switched around.
Which may help explain the mystery of the Scotch plaid valve covers some of these engines had. This picture is from the web site 6066gmcguy.org, which offers two alternative theories as to why a number of these motors sported them. What did I say about GM feeling adventuresome?
This particular truck first caught my eye when we moved here in 1993, sitting a couple of blocks down the street from us. It’s right up my alley, as you undoubtedly know by now. And it’s obviously got a granny-low fours speed, with a stick a yard long. Plenty of elbow torque to make sure it gets into the next gear.
This one sports a “Custom Cab”, the Brougham version of upgrade in the still-spartan early sixties. Mainly chrome, on the grille, bumpers and some trim. And that luxurious color-keyed arm-rest, on the interior. It was the way to distinguish a hard-core work truck from a soft-core work truck.
What really makes this truck exceptional is its original aluminum canopy. It and the truck have weathered the decades so gently.
That also explains the non-rotted out boards in the bed. Have they ever felt sunshine or rain? Maybe not.
The manufacturer’s plate is still there, and I had to do a double take when I first read it. I assumed there was another name ahead of MFG. INC, as in NIEDERMEYER MFG. INC. What was it, and why was it missing? Then I realized it was “GEM MFG”. Duh! A long-time maker of high quality pickup toppers. And from Portland too, which confirms this to be a “native”.
Always wished my truck had a nice big bumper like this. They were optional; trucks back then came without rear bumpers unless you wanted one. And this is the one to want.
Our walk-around is at an end. This truck went up for sale shortly after I shot it, and naturally, I was tempted. But what was I going to do with another old truck? A young guy bought it, and I saw it in another part of town for a while, and then it disappeared, for good. Hopefully, it’s being kept in a way so it will be around for another fifty years. It deserves it.
This body style GM truck seems really thin on the ground now. In fact I see more Advance Design trucks running around, some them still obviously working, than later ‘50’s and ‘60’s pickups, until the ‘67 and newer GM pickups. By contrast, there are still quite a few early- and mid-sixties Fords on the roads here. Due to sales numbers? Durability? Collectibility taking them off the roads and into museums or garages? I don’t know.
In middle school when I stated taking the bus to school, it was a GMC with the V6. I wonder which V6 full sized school buses had? I remember it had a 4 speed manual transmission, only once did the driver start off in first when we were stopped on a hill. Usually second was used from a stop. I think the driver would sometimes (or maybe all the time) would double clutch between first and second.
This was in the late 60s. One bus route used one of the GMC ‘old style’ rounded transit busses with the automatic that sounded like a 2 speed.
“Suspect is hatless, repeat, hatless!”
Some confusion is found in the description of this engine.
“60 degree cylinder angle for even firing pulses, and a compact block assuring a short and stiff crankshaft.”
Even firing is the result of coordination of crankshaft and block design. In this case, a 60 degree V6, most engine designers use flying webs to connect the two rod journals. Flying web crankshafts are longer than overlapping journal cranks and both are weaker than common rod journal cranks like that found in a V8, V12 etc. A longer crank usually means a longer block.
“The 60 degree cylinder angle is of course the preferred one, since it’s the only V6 configuration that gives even firing without split crank journals, something that hadn’t been invented yet, as the early Buick V6 painfully attests to.”
If you look at the cross sectional drawing of the engine titled “GMC’s V6 Engine,-TODAY’S MODERN TRUCK POWER” you can see the flying webs of the crank in between the rod journals in the illustration to the right. These are not overlapping journals of a 90 degree V because the journals would be too large to make it a good engineering choice. A 120 degree V such as the latest McClaren and Ferrari V6’s have common rod journals just like a 90 degree V8. The preference for 60 degree V6’s now mostly has to do with transverse front wheel drive applications. They may be better balanced than a 90 degree V6 as well.
The 90 degree Buick V6, derived from the 215 cubic inch aluminum V8, had odd fire due to budget issues, as in let’s not spend the money on a new crank design now, not because the flying web crank design had not been invented.
“Oddly, there was even a V8 version with 637 cubic inches, with twin balance shafts to compensate for its uneven-firing and vibrations, due to its odd 60° block.”
Yamaha designed a 60 degree V8 with Ford to fit inside the narrow engine bay of the 1996 SHO Taurus. This is similar to the 90 degree V10 Ford Triton and Chrysler V10 found in trucks and the Viper. The front of the engine is expensive to design and make. Spreading the costs over more units will reduce unit cost. This is almost certainly the reason for the separate heads of the Twin Six, an engine that would not see large, by GM standards, production volumes. Using the design and castings for both the Twin Six and the V6 would save money.
Thank you Oita Ikki, JZ78817, Bill and Ate Up With Motor for your clarity.
Thanks for pointing out that obvious error in the text. This was originally written back in 2009 at the other site, and I was a lot dumber then!
I’ve amended the text.
Hey Jerry, I’ve been a Master Tech for 40+ years and the first 305 V6 I heard of, I saw yesterday. Surprised the hell out of me. Anyway, the owner is a respectable young guy, in Escondido Ca. Wants to sell it. Granny 4 speed and all. Looks to be mostly original.
Back in middle school in the late 60s I rode on a GMC school bus. That’s what got me to become a manual transmission fanatic. I never realized it was actually a 4 speed on the floor until one day we were stopped on the hill leaving the school. The driver started in 1st, a very short gear with a distinct whine. He usually started in 2nd, and as noted elsewhere here typically ‘floored it’ with each gear up until 4th,
The bus had the V6 logo – and I wondered what displacement it was. Per Dave’s post in December 2015 it must have been the 351. I could identify the GMC V6 buses by the sound, in high school there were a couple of International buses that also had a distinct sound, I think those were V8s, In middle school one route had an ‘Old look’ GMC bus with the V6 diesel and what sounded like a 2 speed automatic transmission.
I assume being a truck engine, the 305 and 351 V6s may not have had balancing to reduce vibration like the 231 V6 used in cars of that era. Then again, I test drove a 1965 or so Buick Special in 1973 with the V6 and 3 speed manual on the column. At idle, definitely a noticeable vibration, Not so much with my friend’s ’74 Olds Starfire.