There is surprisingly little information on these things out there. Designs along the same principles were made by Hadley, Delco, and Saginaw in the '60s and '70s. Hadley units were used in a lot of applications including dealer-fitted GM optional air horns: Delco Power Pac units featured in the GM "Pleasur-Lift" air shock system, a**** other things: Then there was the Saginaw Instant-Aire tyre pump, offered on Pontiacs, which was a much less serious proposition: Now, I suspect — because I have no direct experience one way or the other — that this is a case of one of my pet peeves, judging a technical concept by an incidental characteristic of the one iteration of the concept you happen to be familiar with. For every twenty blowhards on various online forums proclaiming loudly that the things never worked a damn and all of them have been junked in favour of 12V electric compressors, there is barely a single RV owner who has been getting 145psi from their Hadley over 40 years of frequent use, because they've got a mild, high-vacuum engine and a generous air receiver. It seems to me that at least the Delco and Hadley units are simple and straightforward, their main limitation of slow pressure build resulting from reliance on variable manifold vacuum and direct, on-demand supply to air shocks or whatever. Run it from an engine-driven vacuum pump and have it feed an air tank, and it might be much more effective. I don't know: does anyone have experience with these things? It'd even work with a new-type lubricated vane vacuum pump, like the kind often found on the backs of alternators these days, which discharges oiled exhaust air into the crankcase. There is no need to try to capture clean exhaust air from a diaphragm vacuum pump for low-pressure uses when there is another ready source of air pressure. All this from my perverse desire that no systems run on electricity if they could conceivably run on anything else.
Ned, It's been almost sixty years ago, but I had a car with one of these that powered an air horn on my 53 Plymouth. I don't know how it might have compared to a Delco or Hadley, but it was too puny to pump up a tire with the adapter and hose that came with it. Great pressure but no volume. It may well have simply been old or even defective. Not sure of the brand or source, probably JC Whitney, but I recall pulling it off for that reason and I also got some cooler sounding electric horns.
That's what I gathered. The compressor part is tiny. To get, say, 125psi from 16" of mercury would require a boost ratio of 16:1, meaning (given single action) a vacuum diaphragm diameter four times the compressor piston diameter. A unit small enough for the kinds of installation the manufacturers envisaged — on the pics it looks about 6"-8" in diameter? — would necessarily have a small compressor piston. Being a simple free-piston arrangement the stroke would be the same on both sides, and all that determines the speed at which it cycles is airflow, pressure, and inertia. For the same reason it'd probably handle a 100% duty cycle fine? I can't see compression or friction heat being an issue, or could it? So it outputs compressed air very slowly. Therefore getting a useful reserve of compressed air out of it means having it run almost constantly in the background, rather than on demand. The frustration is that there doesn't seem to be enough information out there about output pressure vs. vacuum vs. time to form an idea of what we've got to work with. Nobody knows; nobody cares, because electric motors are being dangled before our eyes like a hypnotist's pocket watch.
It's a clever idea to use a power source (vacuum) that's already there. Maybe a printed model could test a new design.
One of the first things I wondered was why the Hadley and Delco units both have their vacuum valves alongside what is presumably the air compressor cylinder, when it should be simpler to have them inline on a common shaft. It was only after @mustangsix got me to thinking about adapting bits of brake boosters and hand-held 12V tyre compressors that it occurred to me that the thing's stroke is likely to be so short that arranging decent-sized vacuum porting would be difficult. I'd expect there to be some kind of lever arrangement in there to amplify the motion of the diaphragm/piston ***embly to something with a useful stroke. The reason for this is that a straightforward 4/2-way valve probably won't work. We'd want to switch over from one direction of flow to the other only at the end of each stroke, and that means a very short-stroke valve with a lot of lash relative to the diaphragm/piston. For, say, 90% of the diaphragm/piston's stroke it's just using up lash, and only pushes the valve from one position to the other at the very end. That causes the diaphragm/piston to move in the opposite direction, but the valve stays where it is until the diaphragm/piston is 90% of the way along its travel back. All of this is really very simple, but it's extremely specific, where is the rub.
Probably not helpful, but in ~'68-'70ish, Cadillac used a tubular vac-powered air compressor to keep the rear air shocks aired-up. Iirc, it put out ~200 psi, but *very little* volume. From memory, it was ~4" dia x ~ 18-20" long. One vac-inlet port, one pressure outlet pot. Non-adjustable, iirc. Also kinda heavy... . Got a couple of them sitting somewhere, unused... Marcus...
As far as I could ascertain, Cadillac offered a system like that from '65 to '76, before switching to electric. This pic is from a heavily paywalled video, but the still shows something much like the Delco unit in post #1: Apparently this was integrated with an air tank of some kind. I'm seeing what more I can find.
Looks close. I'll see iffen I can find mine. I think that's the one. I never opened mine to see what was inside. Also, iirc, the factory shop manual had a small section on repairing these. Haven't seen that manual for awhile, don't know where it is now. Marcus...
This is one of those technologies that only had a brief window to mature, at least in cars. It wasn’t viable in the vacuum wiper era and was allowed to die on the vine once alternators were the norm.
Ironically modern cars use pump-generated vacuum for lots of automatic systems, but not for pumping air.
The Jalopy Journal
