In the late 1940s, Ford Motor Company began using a distributor that had no mechanical advance mechanism at all, relying solely on engine vacuum to provide advance based on both load and engine speed. They continued using it through the mid-1960s on their six-cylinder cars, and although it seems counterintuitive, the “Loadomatic” is actually a simple concept.
When I was 14, my parents got me a reproduction factory service manual of my own for my 1965 Mustang, and I recall being stymied by the Loadomatic (it’s sometimes spelled “Load-O-Matic” or “Load O Matic,” but I’ll use the spelling from the 1965 factory service manual). After all, when the engine’s manifold vacuum dropped to nearly zero under heavy throttle applications, how would the distributor advance?
I knew it must have worked, because Ford used it in V8 cars until 1957, and in all of their six-cylinder cars until roughly 1968. The 240-cubic-inch “Big Six” pictured above, which was a new engine in 1965, powers a 1965 Galaxie 500 Convertible. It came standard with a Loadomatic.
This is how a conventional distributor operates. The centrifugal advance mechanism is based on engine RPM: As the distributor shaft spins faster, the advance weights are thrown outward, which advances the points cam relative to the breaker plate. The advance springs not only return the cam to base advance, but they also control the rate at which the weights advance the points cam. Most cars used this method of advance in correlation with a vacuum advance canister, which senses the engine’s manifold (or ported) vacuum and advances the points plate itself against spring tension, which controls its rate of advance. The main difference between the two is that manifold vacuum advance is available at idle, because the vacuum port is located beneath the throttle plate in the carburetor, while ported vacuum advance is only active when the throttle is cracked open, because its port is located above the throttle plate. Both offer the same amount of advance once the throttle plate is opened, but manifold vacuum advance can help to keep an engine cooler at idle. Both work in combination to provide more ignition advance at cruise, when throttle applications are light, manifold vacuum is high, mixtures are fairly lean, and the engine is under light load.
Some engines prefer one, some the other, but debates rage on the internet, as they usually do (just ask someone what motor oil they use). For what it’s worth, Ford tended to use ported vacuum advance in the 1960s.
The Loadomatic, on the other hand, has no mechanical advance mechanism; instead, it has a canister with a diaphragm like a typical “dual-advance” distributor, but no advance weights. It does have advance springs as a dual-advance distributor has, and they too control the rate of advance, just in a different way.
For the Loadomatic to work, it needs to have a corresponding carburetor, which is why owners of pre-1957 Y-Blocks often upgrade both the distributor and carburetor at the same time. This Autolite 1100 is typical of the carburetor Ford used on its six-cylinders. The main difference can be seen in its “Spark Control Valve” and its “Distributor Vacuum Port.” The carburetor’s vacuum passages are expressly designed to work with the Loadomatic distributor.
The Spark Control Valve resembles the power valve on a Holley or Autolite carburetor, and it is basically the brain of the operation, balancing manifold vacuum with venturi vacuum, which is how the Loadomatic provides advance at all engine speeds and loads.
Based on this diagram from the service manual, it looks more complicated than it actually is, but the important part to remember is that the Loadomatic is fed engine vacuum from two main sources, a manifold vacuum port beneath the throttle plate, and a small port with a restriction located in the carburetor’s venturi. Under light throttle applications, manifold vacuum holds the Spark Control Valve open against spring tension and feeds the distributor vacuum canister. As the throttle is depressed and vacuum drops, however, the Spark Control Valve spring overcomes manifold vacuum, closing the valve, and then vacuum from the venturi is fed to the distributor. While there is a system of orifice restrictions (as shown in the above diagram) to maintain a balance between the two depending on engine speed and load (and to keep manifold vacuum from “escaping” through the venturi port), the Spark Control Valve is the “general” of the operation.
As you can see, because of the restrictors in the system, the level of vacuum actually making it to the distributor canister is quite low, no more than just under 5″ of vacuum. For comparison, full manifold vacuum in a 240-cubic-inch six cylinder as installed in a 1965 Ford would be somewhere in the range of 20″ at idle, and maybe even a bit higher at light cruise. Advance totals are similar to any engine with mechanical advance: the manual-transmission-equipped cars were tuned to (up to) 13 1/4 degrees of advance at the distributor (which equals 26 1/2 degrees at the crankshaft). When combined with the standard 6 to 8 degrees of initial timing dialed into Fords of the era, that equates to roughly 32-34 degrees of total timing.
One thing that is implied is that under cruising situations with high manifold vacuum, the system will provide even more advance than this, as the vacuum advance system in a dual-advance distributor would. These numbers represent the most the distributor will advance under full throttle, when detonation could become an issue. Unless I missed it, the Loadomatic’s distributor advance tables in the service manual do not list totals under light cruise.
For comparison, here are the advance rates of a 289-powered 1965 Galaxie with dual advance. At 2000 distributor RPM (4000 crankshaft RPM), the centrifugal system will provide up to 13 3/4 degrees of distributor advance (27 1/2 degrees at the crankshaft). At light loads with high manifold vacuum, the vacuum advance provides up to 9 3/4 distributor degrees of extra advance (19 1/2 degrees at the crankshaft). That means that cruising down the road at 60 miles per hour, the engine is running on a lean cruising mixture and the distributor is providing over 50 degrees of advance. Where some people get their numbers mixed up is in thinking that that is “total” advance. It isn’t. Only the initial plus the mechanical advance is “total” advance, because vacuum advance drops out when the engine’s manifold vacuum drops to zero (or near zero, depending on carburetor sizing).
The Loadomatic distributor is quite a simple piece. The posts that hold the “advance” springs in the distributor are basically adjustable “cams” that can change the spring tension in the distributor for a faster or slower advance rate, and all these systems work together to provide the correct advance for the engine to operate efficiently.
So what can go wrong? Well, I’ve had cars with faulty vacuum advance canisters, and in this case, that would mean NO distributor advance. In essence, you’d be a slow-moving chicane on the highway, and I would imagine something similar would happen with a faulty diaphragm in the Spark Control Valve. Also, because of the restrictions in the system, those who have experience with Loadomatics say that it is slow to advance the timing compared to a dual-advance distributor. Anecdotally, those who have converted their old Fords to dual-advance system don’t seem to regret it; still, Loadomatic is an interesting piece of archaic engineering.
Related Reading
How does a Ford Load O Matic Distributor Work? (This posting is an excellent, in-depth presentation of the Loadomatic, and in combination with the factory service manual, it was indispensable in putting this piece together.)
I remember these!
I think it may have lasted into the 70s sometime in some applications. There were lots of 6 cylinder Falcons, Mustangs and such around that used it. Pickups too. It was a pretty trouble free system as long as the breaker plate wasn’t stuck and the diaphragm worked. Same as any distributor.
I remember my high school shop teacher, who had been a Ford technician, explaining it at length. His theory was that Ford originally came up with it as a fuel saver and a means to allow for the use of low octane gasoline. He was an excellent instructor who was always willing to stay late and expand on a topic for those of us who were interested or planning on a career as a mechanic. We didn’t know how lucky we were at the time!
One of the dumbest things I hear often: carbs are easier to work on. Translation carbs are easy to fiddle with…
My Buell Blast has an interesting idle control valve that uses heated wax to alter it’s condition as the engine warms up. This controls both mix ratio & idle speed. It’s operating range is controlled by how far it’s screwed into the carb. And on a Blast forum I’ve seen hundreds of posts asking why it wont idle after adjusting that valve wrong
Honda lawnmowers use a heated wax pot to control the choke. Sometimes that wax pot fails and they wont idle after warming up
Heated wax? That’s a new one for me. Never too old to learn. Thanks for the insight.
45 years ago cruise missiles used wax to cool the electronics that controlled the tiny turbine engines. The latent heat of transformation sucked up the wattage long enough to do the job.
My Prelude VTi (H22A8) had that.
When the wax leaked out, it would sit there revving up & down like a boy racer at the traffic lights.
A new IACV was an easy install.
I was devouring Ford ads and such more than sixty years ago, but have no memory of the Load-O-Matic phrase in any review, technical bulletin, whatever. Thanks for being my teacher today with a nice, clear writeup—all of it news to me.
(And thanks to Phil for the info about “heated wax’—–again, 100% new info for me today.)
Pretty much never read anything about them, either, in any material outside of Ford. Knew about through my 1965 and 1967 Ford Sop Manuals. Then, again, I never had a Ford six.
You’re welcome, George!
The thing that’s fascinating about this is that when it was developed, vacuum advance was less than 20 years old. Until the early ’30s, automatic spark advance was usually centrifugal only. Vacuum advance was first adopted around 1932, and it took a couple years before it became universal.
I’m very familiar with these, as my ’66 F100 has one. Never had any issues with the distributor except for breaking the little drive pin, which obviously cause its forward motion to end abruptly and unexpectedly. And annoyingly.
I just took this shot of a spare I have in the garage:
That ’65 Galaxie 500 convertible with the 240 six is worthy of a post!
I call dibs on writing it up! 🙂
It’s all yours!
That’s a good picture because you can see how the spring posts could be rotated to change the advance rate; I imagine there was a special tool that Ford offered to latch onto the top of those half-moon-shaped posts.
Speaking of adjustments, I have never put a timing light or any other proper device on it to check its timing. I’m the crudest shade tree mechanic there is; I just advanced it until it pinged under load at low speed, then backed it off a hair. 🙂
Runs great!
An excellent overview of a often misunderstood system. I also appreciate that you brought up the fact that on the more conventional dual advance (vacuum and mechanical) systems the timing can, under certain conditions, run at over 50 degrees advanced. This is something that is rarely, if ever, mentioned!
I remember the carburetors that had that valve, we were taught to call it the ‘power valve’ and when it inevitably failed the symptom was crystal clear : hot start never touch the accelerator, if you did the engine would flood and you’d have the devil of a time getting it started again .
I bet there’s a N.O.S. K-D Tool hanging somewhere to adjust those spring posts .
I guard my 50 + year old K-D Tools with my life, if you don’t know the tool # they’re getting hard to find .
-Nate
I don’t just remember but spent enough time sorting out the right PV on my Ford 2100 carb lasz year! One of the more recent issues is that the PV gaskets coming with the recon kits nowadys fall apart when they sniff Ethanol – and then you have fuel directly being poured into the intake manifold with everything you described plus constant rich running. Also, the point in which the PV cuts into operation is something people totally neglect, you can’t just use any PV, it has to fit carb size and vacuum on light throttle…
As for the weird double vacuum advance carb, everyone I know on my FB groups and other Ford forums ditch them asap. They may work but require a different type of knowledge to set right and if you’re looking for better performance, aftermarket support in unfortunately non-existent.
The Spark Control Valve for the Loadomatic doesn’t flow fuel, just vacuum.
You’re right on power valves though; power valves are tailored for each engine combination. Most rebuild kits come with a 6.5″ power valve, which means that fuel enrichment begins when manifold vacuum drops to 6.5″. Many cars with stock camshafts, however, do better with an 8.5″ or even a 10.5″ power valve, both of which are a little tougher to come by.
Yep. Mine calcualted to about 5.5 – I get my stuff from Mike’s Carburetor Parts (see link below) who has the bigger sizes and most of everything else for those and other Ford carbs. He also has the patience to provide advice to those of us who are no experts!
Regards the PV gasket issue, I forgot to add I don’t use the gasket in the kit but obtain Viton rings which can live with ethanol.
https://www.carburetor-parts.com/?srsltid=AfmBOoonr2LZQ6Ji59GRzI4YeHpnNq7Y96Wzk4Hdax8PAzoi5xusg71I
Except for a few 1950’s exceptions, air-cooled VW’s used vacuum-only advance. The exceptions were centrifugal only, such as the famous .009 distributor that most everyone thinks is an automatic performance upgrade, which it is not, but necessary when using aftermarket carbs, as with the Ford.
Emission controls, especially NOx, made vacuum advance tricky. VW added a dual vacuum advance-retard. Cheap retrofit NOx controls (required in California) simply capped the vacuum lines on American advance cars, with some danger of overheating, and a gas mileage penalty.