I heard an exhaust note that sounded old, unusual, and unfamiliar before I came upon the source: this ol’ GMC pickup, running at a fast idle as the owner tinkered and fixed. I was out on a walk round our temporary neighbourhood just after we moved from Toronto to Vancouver in September 2011; we’d used something that came before Airbnb to buy a month’s overpriced lodging in a basement that smelt of litterbox, and so there was great motivation to spend as much unlodged time as could be.
As you can see, this truck is painted in the ideal and legally-required turquoise-aqua colour. I dig the chrome front bumper and grille—much classier than white paint—and the grille guard is totally tubular. I’m not so keen on the chain link licence plate frame, but my ballot must’ve got lost in the mail or something.
I don’t know its model year, so I’m calling it a ’64-’66 even though the front turn signals are the larger ones first used in ’63. But this truck rolls its (four) eyes at nerds oogling the turn signals and hollers Hey, you! Don’t watch that, watch this:
It has a V6, you see—the kind you couldn’t get in a Chev truck, only in a GMC—and this big chrome badge is here to make sure everyone knows it, even when they can’t hear the exhaust note.
By the time I walked back past the truck on the way back to Casa del Catbox, the owner had opened the hood. So:
There it is, lookin’ pretty tidy. The crankcase breather is ducted to the air cleaner, which suggests this truck was first sold in California where such was required starting in ’64—hence my guess at ’64-’66 despite the ’63-’66 turn signals. The (oil bath!) air cleaner is perched atop a Bendix Stromberg WW 2-barrel carburetor, the kind used right from ’60-’66, so no further narrow-in on the year there. I’m not familiar with these trucks; are there production date clues I’m not seeing?
It’s certainly not a ’63, which still had the dog leg windshield. I’m not sure there’s any obvious visual external cues on these GMCs between ’64 and ’66.
Dad’s 63 1/2 that he customed……
1 Ton springs Changed from 3/4 ton
Changed 3 on tree too 4 speed 1st being a low granny gear.
Additional 20 gal tank gas….
Custom rear bumper adjustable
Built his own dulles…….
I found this shot showing a Chevy 292 six with a similar crankcase breather tube in the 1964 brochure. The 230 six and the V8s didn’t have it. The 292 has the oil bath air cleaner.
I also found a ’63 brochure image showing the 292 with PCV. And a ’64 GMC ad that bragged about standard crankcase ventilation on the new V6 engine. It appears that this was both a standard feature on the V6 and other GM engine prior to it being mandated.
From Wikipedia:
By 1964, most new cars sold in the U.S. were so equipped by voluntary industry action so as not to have to make multiple state-specific versions of vehicles. PCV quickly became standard equipment on all vehicles worldwide because of its benefits not only in emissions reduction but also in engine internal cleanliness and oil lifespan.[1][7]
I’m not talking about positive crankcase ventilation, though; I’m talking about closed crankcase ventilation. The difference came up in re your own truck awhile back.
So why are you assuming that this GMC was first sold in CA because of the crankcase breather connected to the air cleaner. If it’s not a closed crankcase ventilation, then it wouldn’t be something CA specific, right? Or am I missing something, as is all-too often the case?
Closed crankcase vent—the crankcase breather ducted to the air cleaner instead of open to the outside world—was required in California starting on the ’64 models; it didn’t spread to the rest of the continent until ’68 (there might have been some models sold outside California with closed crankcase vent between ’64 and ’68; I can’t say for sure, but what I’ve seen over the years suggests not many). This truck in the pics has closed crankcase vent, which would have come on a California truck and probably wouldn’t have come on a non-California truck.
Separately from the ducted or open breather, PCV (crankcase fumes and vapours routed to the intake manifold) appeared in California for ’61, New York for ’62, and was roughly universal in North America by ’64, except Ford dropped it on non-California cars sometime between ’64 and ’65 because their PCV valves were causing problems; see here.
You’re confusing me.
The only system that is identified in numerous articles I just Goggled all describe one system, on which the PCV valve (or a substitute device) is the essential ingredient.
The system uses manifold vacuum to draw out blowby vapors from the crankcase, and there is an intake, either a breather on another part of the engine in “Open PCV systems” or from the air filter in “Closed PCV systems”, to supply the fresh air that is taken into the crankcase, to replace that being sucked out.
The PCV valve an essential element in both systems, as it has to close at idle, since vacuum is very great then, otherwise the fuel-air mixture in the intake manifold would be grossly leaned out. And then it opens as vacuum is reduced at higher throttle settings. And at WOT, when there is no vacuum, there is another hose connected to the air filter (or just before the MAF in FI engines) to use the partial vacuum there even at WOT to provide the necessary suction to purge the crankcase. But that only comes into play at or near WOT.
I’m regurgitating this to you because it represents all I have ever known and understood about the PCV system and could find online to reinforce it by their description of this obvious and simple system. Hence I am baffled by your descriptions of apparently two different systems. If you’re saying that “Open PCV systems” and “Closed PCV systems” are two substantially substantially different systems, I’d disagree. The difference is technically pretty minor, but undoubtedly the closed system has advantages in reducing vapors and emissions.
All the articles say that GM essentially “invented” the PCV system, and that it was introduced in 1961 in CA and 1962 nationwide.
So here’s your chance now to rebut or correct what I have distilled from memory and reinforced by what I could find.
You’ve got all the pieces in front of you, just you haven’t snapped one piece into the puzzle. It’s this one: Some systems (“closed PCV system”) draws its fresh air from the air filter as an alternative to a breather on the engine.
Starting from the start:
There must be airflow through the crankcase, so air must enter the crankcase and leave the crankcase.
The crankcase air outlet for many years was a road draft tube which ducted the air + crankcase fumes down under the car where air passing over the end of the tube created a draft if the car was going fast enough.
Those crankcase fumes were heavy in unburnt hydrocarbons, which are a precursor to photochemical smog. By law in California starting with 1961 cars and in New York starting with 1962 cars, the road draft tube was replaced by positive crankcase ventilation (PCV): the crankcase air outlet was instead a vacuum hose—via a metering valve or orifice to avoid a massive vacuum leak at idle—to the intake manifold so the crankcase gases would be burned in the engine. PCV improved things not just for air quality, but also for the car and its owner, and by 1964 most makers just put PCV on all their vehicles rather than having one configuration for CA + NY and another 48-state configuration.
The crankcase air inlet is the part you’re tripping over. For many years, the crankcase air inlet—called the breather—was open to the atmosphere. There was typically a metal or fibre mesh inside the hollow oil cap, to exclude spiders and pebbles entering the crankcase along with the air drawn in by the draft created by the road draft tube (or by manifold vacuum with PCV). But the volume of crankcase gas would exceed the flowrate into the intake manifold because of excessive blowby on a worn engine and/or weak manifold vacuum under high loads/heavy throttle. Then the crankcase fumes would flow out the breather and into the atmosphere, just like with a road draft tube.
So starting in 1964 in California, the crankcase air inlet was ducted to the air cleaner instead of being open to the atmosphere. That way, the excess crankcase gas still flowed out through the breather, but now to the air cleaner and was drawn into the intake tract anyhow, instead of out to the atmosphere.
Part of the confusion is that Chrysler, among others, called the first kind of PCV system (with the atmospheric/unducted breather) “closed crankcase ventilation”. Which was sorta true compared to the previous road draft tube, but the crankcase ventilation wasn’t really closed until the ducted breather came along.
As to GM inventing PCV: maybe or not; there are many articles claiming GM invented the catalytic converter, too, and that’s wrong. PCV systems of one configuration or another were used long before 1961 in engine applications where there was no road to create a draft across a tube. GM’s AC division were early to commercialise PCV metering valves, and they sold them to at least one other automaker (Chrysler); probably others, too, which might’ve given rise to the notion that PCV was invented by GM.
How’d I do? (Did you read the “came up in re your own truck” link at the start of this subthread…?)
That’s all exactly as I understood. I wasn’t tripping over anything except
But here’s the part that is still a bit of a question: as I said, under WOT there is no vacuum, yet that’s when blow-by is the greatest. My understanding is that all PCV systems also have a second tube that is used in WOT situations, to utilize the modest vacuum available there even in WOT situations, to create the necessary suction to vent the crankcase.
Where is the hose for that function?
Your understanding on that point is not correct; the second tube exists only on the closed systems with the breather ducted to the air cleaner. On the systems with atmospheric breather there is only one hose, from the crankcase to the intake manifold.
The open systems that do not have the hose used an oil filler cap or downward routed draft tube for the venting. There were some engines that used the closed system in advance of any legal requirement in order to stop oily residue that would be on engine from the breather style oil filler caps or blown back on underside of vehicles from draft tubes. In the 40s and 50s there were accessory catch cans sold that slid up on draft tubes and had baffles to stop oil escaping from vent holes at top of can.
This is a screen shot from a 1963 Canada market Chevy truck brochure. The 292 six clearly has a hose from the air filter to the valve cover.
It sure does. Doesn’t surprise me that a ducted breather would be found on heavy-duty vehicles prior to California’s ’64 mandate; it meant better cleaning of the air entering the crankcase. It does torpedo my use of that configuration to date this truck, though!
Which is precisely what started me on this (largely redundant) discourse: your assuming this had to be a CA truck. I’ve seen these hoses to air cleaners on older trucks before, outside CA and in brochures.
And that lead to my confusing interpretation of your descriptions of the two systems.
Some engines had the hose to air cleaner well before legal requirements. Crankcase venting through oil filler caps and draft tubes created oily messes on engine and undercarriage. The air cleaner vent hose took care of this.
the second tube exists only on the closed systems with the breather ducted to the air cleaner
But wouldn’t that require two hoses to the air cleaner? One that “sucks”, and the other that allows air into the crankcase. Inlet and outlet. One hose can’t be used for both, obviously.
The closed systems have two hoses: one from the crankcase via a metering valve to the intake manifold (the metering valve means gas can flow in only one direction: crankcase to intake), and the other from the breather to the air cleaner (gas can flow in either direction).
and the other from the breather to the air cleaner (gas can flow in either direction).
How does that work? Under WOT, that hose cant be bi-directional at the same time. If under WOT it’s getting its intake fresh air from the air cleaner, where’s the dirty fumes going?
There is no airflow into the crankcase under WOT, because the volume of blowby is great and the manifold vacuum is nonexistent. All flow under that condition is out of the crankcase, through the two hoses. When gas flows from the crankcase out the breather to the air cleaner, it is pushed from behind (pressure), not pulled from ahead (vacuum).
Air cleaners create enough air path restriction that the area inside element or oil bath has faint vacuum and will draw from crankcase if the vacuum on PCV valve goes to zero or experiences minor pressure pulses due to pressure waves reflected as intake valves close. On gasoline engines built today this is still done. Some turbo engines use a venuri “vacuum pump” that uses intake feed (pressure from turbo) and blows through a venturi fitting tonvacuum crankcase and discharge it to air cleaner pipe in front of turbo inlet. The turbo inlet will have faint vacuum as well. BMW engines using intake valve lift as the throttle and run with plenum throttle partly open but just enough to have just under atmospheric (faint vacuum) to assure crankcase venting.
The 1966 GMC had more modern squared-off lettering for the grille emblem, so that’s another year shorn off the range. AFAIK there is no difference between ’64 and ’65 models…and GMC didn’t even have “formal” VIN-encoded model years until the 1970s, so the distinctions are completely moot.
Nice truck! I’m not so familiar with the V6 sound on start up but wow do I recall that distinctive dere dere dere dere dere shudder purr sound of these trucks starting up. In our neck of the woods there would have been 1 283 for every 693 6 cylinders. Fun times.
GMC is a 60 degree V-6, which has a different exhaust note than the much more common 90 degree configuration.
Nice find. That would have been a pretty deluxe unit in the ’60s. I think you may be right about it’s California roots, as I’m not sure the V-6 was ever sold here in Canada in pickups.
There were a few mediums around with it, but Canadian market light GMC’s used the same powertrains as Chevrolet. You could probably special order the V-6 if your pockets were deep enough (there was a substantial tariff on US built vehicles up until the Auto Pact took effect ca. 1967).
Another clue as to it not being a Canadian native is that it still exists at all. These trucks were epic rusters in our climate. I see the telltale start of cancer in the lower front fender, I hope this one was saved and is still around.
Yes I don’t think the GMC pickups were sold in Canada with the V6. My Dad told me this when I was young and you never saw them up here at all. Lots around when we went down to Washington State though.
This brought back memories! I was a mechanic for a Cadillac/Oldsmobile dealer. They had a GMC pickup with a hoist in back and a 4 speed transmission, used as the dealer’s tow truck. We had a “very old” customer who was in the US Army Cavalry in WW1. He liked to bring his 1958 Lincoln Continental to our service department. Guess he didn’t like the Lincoln service dept.
Anyway, one day his car died on the road and he called us for a tow. I got the assignment to take the GMC out and get him. I hooked his Continental up and was amazed at the torque the truck had at a little more than idle. What a beast!
I had started working at a Canadian GMC truck dealership in 1966, and the light and medium duty GMC’s had the same engines as the Chevrolets. Starting about 1967 the 351-401-478 Magnum and Toroflow engines were available in specific HD GMC models.
Closed crankcase PCV draws clean air from the the air filter on the clean side usually, some used a secondary crankcase filter inside the air filter housing. This means that if an engine overwhelms the capacity of the PCV system, the excess blowby still gets drawn into the intake tract and burned. I had a ’64 Studebaker with an open crankcase PCV, it drew fresh air in through the mesh filter in the oil filler cap(s). Same deal on my current ’62 Dodge D-200. Excessive blowby will escape out the breather/filler caps to the atmosphere. I love the big V6 as they really rumble with a blown out glasspack dual exhaust. 10 mpg no matter what in the one I had. Great truck but a bit weak on the braking side.
This reminds me of the pickup with camper that John Steinbeck drove when he drove around the country writing Travels with Charley except Steinbeck’s GMC was a 1960. It can be seen at the Steinbeck museum in Salinas.
The cranckcase ventillation system on the GMC V-6’s was a bit unusual. The PCV valves (there were usually 2 of them) were under the valve covers and screwed directly into the intake ports in each cylinder head. Fresh air came into the crankcase through a small air filter behind the left cylinder head. This air filter had a replaceable element with a vented screw-on cap if it was an ‘open’ system, or had a pipe and hose connecting it to the air cleaner if it was a ‘closed’ system. The filter wasn’t replaceable on the ‘closed’ system, air going through it went through the carburetor air filter first. The pipe could be removed and the filter cleaned with solvent if needed. The filter and pipe is clearly visable in the picture, round orange object between the left head and firewall. From what I understand, the ‘closed’ system was an option on these trucks unless sold in California where it was mandatory beginning in 1964.
Thank you for the details, Bob! I love that the majority of the discussion thread is about the minutiæ of crankcase ventilation systems. It’s almost as good as if it were about, like, the particulars of the lens facets and filament shields in the low beam headlamps on trucks made before versus after some date, or otherwise like that. 🙂
My pleasure. BTW, I enjoyed your piece on Chrysler’s Cleaner Air Package. I had not thought about those control valves in a good long time.
I have a set of reproduction Guide T-3 headlights on my ’67 GMC and was thinking about doing a low beam shift comparison with G.E.’s on a similar vintage Dodge truck. Or not!
Ah yes. I well remember the distinctive sound of a GMC V6. It was hard to mistake them for anything else at idle up to full chat, and they weren’t uncommon back when I was growing up. I know these engines were a fairly serious effort to create a commercial duty powerplant, and weren’t “value engineered” to the level of a regular old small-block Chevy. Reading period advertisements reveals GMC going to great lengths to extoll the virtues of its purpose built workhorse engine, especially in the earlier years… hence the hugemongous “V6” badging.
I’ve never driven a vehicle powered by the 60 degree GMC mill, but everyone I’ve spoken to who has swears by it and not at it. All stories corroborate, and give the impression that these were extremely durable engines that did more work than their horsepower figures would suggest, with the biggest downside being high fuel consumption. They’re definitely a reminder of days when General Motors actually had the resources to supply bespoke engines to each and every one of their divisions. Kind of crazy, eh?
I have been able to compare GMC V6s against Chev V8s in medium trucks (25,000-45,000 GVW) and there was a big difference. Much more dramatic than the numbers suggest, you always seemed to be slower and stronger.
Do people notice that the plugs are in the valley side of the heads, not the outside?
Slower and stronger…? I can’t tell if you’re saying the GMC V6 was better or worse than the Chev V8.
Sorry I wasn’t clearer. Stronger at lower RPMs. Torque vs. horsepower. In 1972 Motor’s a GMC V6 351 only has 154hp vs a Chevy big-block V8 366’s 200, but it was at 3600rpm instead of 4000. The 351 only has 295ft/lbs vs the 366’s 320 but it comes in at 1600rpm instead of 2800. Maximum torque at 1600rpm. That’s HD diesel speed. You wanted to be at a low rpm in the GMC and a high rpm in the Chevy. It’s quiter and more relaxed.
These trucks rarely had tachs, the driver would know the layout and maybe displacement, but not actual power at speed. So this is subjective, feel, but not actual performance.
Ah, gotchya. Thanks for the clarification.
I remember those Gimmie V6’s. Vaguely, or at least on used vehicles. My recollection is they were a good engine, not a lot of power, but a basic truck engine. And after a couple of years in pickups they were relegated to 2 ton and heavier trucks. Never quite understood that, but in the mid 70s I did have occasion to drive one, hauling 500 gallons or so of hydrochloric acid at a chemical plant I briefly worked at. 2 speed axle (that didn’t work) and all, it was a real truck, not just a pickup. Short trips, it worked ok, no drama.
1. Daniel and Paul, thanks for that excellent back-and-forth discussion about the history and evolution of the PCV. Great stuff!
2. An oil-bath air cleaner still used in the mid-’60s? That’s a bet I would have lost.
3. “I was out on a walk round our temporary neighbourhood just after we moved from Toronto to Vancouver in September 2011; we’d used something that came before Airbnb to buy a month’s overpriced lodging in a basement that smelt of litterbox, and so there was great motivation to spend as much unlodged time as could be.”
Aieee! Daniel, you have triggered an ancient memory … it was October, 1979, and I had returned from a summer working out west, my life neatly planned, to find that I had fallen out of favour with my girlfriend, who had neglected to send me a “Dear John” letter. I had arranged, months before and sight unseen, a nice place to live – a two-room suite in an older character home, with a nice young couple as live-in landlords.
But oh, what’s this? There were three other rental suites – one in the basement, one also on the main floor, and one on the 2nd floor. The landlord couple had the 3rd floor, and about half the 2nd floor.
And it turned out my 2-room main-floor suite actually comprised two rooms (a kitchen and a bedroom) separated by a public hallway. The bathroom? Oh, that was shared, and in the basement. The guy in the downstairs suite had a dog he kept chained in the bathtub. Through no fault of its own, the dog did its business in the tub. On several occasions I left a roll of toilet paper in the bathroom, only to find it floating in the toilet the next time I visited the facilities. (Not long after moving in, I bought an alumni gym membership at the university, and used to drive down after work to shower. I also got quite good at holding my bladder.)
The fellow who rented the suite on the 2nd floor was an alcoholic who was often very angry at me because I had parked in my designated parking spot. One evening he was desperate, and I drove him to the liquor commission so he could buy a bottle of whisky. He was nicer to me after that.
What about the other suite on the main floor? Oh, that was the Cat Girls – two sisters, from whose suite wafted the pungent odor of neglected litter box/industrial-strength cat urine. (This was the memory Daniel’s statement dredged up.)
Did I mention that the gas oven never worked during my seven months there? The part was on order, and was “going to come in any day”. I did fry a lot of food on the operable range, though. This was the pre-microwave-oven era for most of us.
But on the upside, it was $100 a month (cheap even then).
To bolster the relevance of my ramblings, my car at the time was a ’68 Impala 2-door fastback, red with lots of tin worm, with the commonplace 307 and Powerglide. It turned over 147,000 miles the first day I moved in, and was at around 155,000 when I moved out. It was consuming copious amounts of Quaker State by then.
Oil bath air cleaners really merit their own CC article, because the way they work is fun. I’ve added it to the list! They lasted even past the ’60s; see here.
I use an oil bath cleaner that would have come on my 65 F-100 352 V8. When I got the truck it had a 1972 360 with typical Ford air cleaner. I obtained two correct Ford oil bath air cleaners. One had a nipple for the fresh air hose and the other did not. You can see in the picture which one I used and is now on the rebuilt 390 in the truck today.
My 63 had the stock 305E V6 with the rare plaid valve covers when I bought it to customize. I originally planned to keep it in the truck but it was just too big and parts were too hard to find. I still have the engine in may garage though.
Here is my GMC 305 V6 next to a Chevy 305 V8
Ready for a PCV laugh? My ’70 Opel Kadett with the dual carb 1.1 has no PCV valve. It has a short rubber pipe between the valve cover and clean side of the engine air filter. There is a small manifold vacuum line teed in midway. Thats it. I have no idea how that was allowed but truth is stranger than fiction.
Interesting configuration, but how does “allowed” enter into it? The purposes of the PCV valve can also be served by a fixed orifice and separate flame trap arrangement of one kind or another—Volvo did it this way, and I imagine others did, too; your Opel sounds like something along this line.
My ’65 Corvair uses a fixed orifice PCV system, as well.