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CAP/CAS Distributor Vacuum Advance Control Valve

CAPIn the mid-1960s, laws and regulations (first in California, then across the U.S. and in other countries) began to require automakers to clean up their cars’ exhaust emissions. After many years of building engines, ignition, fuel, and exhaust systems more or less the same way, the makers suddenly had to figure out new ways of doing it.

The cars still had to run as well as buyers had come to expect, but now they had to do it with much sweeter breath and without costing much more than buyers expected to pay. This made big problems. Not only were the technology, technique, and knowledge of engine emissions and combustion dynamics primitive, but for years the standard fix for any drivability faults was to fatten up the fuel mixture.

Balky cold starts? Add more fuel; enrich the choke. Stumble on acceleration? Add more fuel, use a bigger accelerator pump shot. Surge at cruise? Add more fuel, put in a bigger jet. Hesitation on flooring the gas to pass another car? Add more fuel, loosen up the power valve.

This was easy, cheap, and effective, but the resultant exhaust was filthy (and fuel mileage was stinkin’ to match). Once emission regulations came in, that method was out of the question.

As they bumped and ground along the learning curve, under constant tight deadlines and budget constraints, automakers tried all kinds of tweaks, twiddles, gadgets, and widgets to clean up the exhaust. Chrysler’s guiding philosophy was to avoid the afterburners and other big, expensive hang-on devices being talked about, and to improve combustion so less unburnt or partly burnt fuel would wind up in the exhaust.

Chrysler’s researchers found that ignition timing has an enormous effect on exhaust emissions and drivability, particularly when the extra fuel fat is trimmed away. The problem was that under conditions calling for a lot of spark advance to give a complete burn, there wasn’t much advance available. The engine wasn’t typically spinning fast enough for much mechanical advance, and there was no vacuum in the ported line (the line going to the carburetor) to the vacuum advance. But if they hooked up the distributor vacuum line to the intake manifold instead of the carburetor port, it caused problems with emissions and drivability under other conditions.

Losing all those extra fuel Band-Aids to cover up drivability flaws was an obvious first step to clean exhaust – but then how could they maintain drivability? The solution, at the time, was a distributor vacuum advance control valve, which could send either ported or manifold vacuum to the distributor depending on conditions. This valve handily overcame the shortcomings of other makers’ exhaust cleanup efforts, and made practical Chrysler’s first systematic emission control system, the Clean Air Package (CAP).

CAP was first launched on a trial basis in California in 1963. That trial was a grand success; read about it in this Popular Mechanics article from November of 1963. Starting in 1966, CAP became standard equipment on all Chrysler Corporation cars sold in the state of California, and Chrysler even offered “UCCAP” (Used Car Clean Air Packages) at dealer parts counters for retrofit to any 1955 or newer Chrysler vehicle.

CAP valve connections

The distributor vacuum control valve was used on some CAP-equipped engines from 1963 (the prototype CAP cars) through 1969, when CAP was renamed CAS (Cleaner Air System) and became standard equipment on all U.S.-market cars.

The advance control valve has three vacuum nipples: a large one to manifold vacuum, a small one to the vacuum advance port on the carburetor, and a small one to the distributor vacuum advance. If you pop off the black or green plastic cap, you’ll see there’s a spring underneath and a slotted screw running through the center of the spring to adjust the tension.

This valve is a pneumatic logic gate – a very simple mechanical computer. It advances the timing under closed-throttle, high-vacuum situations: deceleration or coasting with your foot off the accelerator. As long as the manifold vacuum is equal or less than that from the spark advance port, the distributor vacuum advance receives an ordinary ported vacuum signal as though the valve weren’t there at all.

When the throttle is closed, the spark advance port is above the closed throttle plate, so it produces no vacuum signal. But manifold vacuum is very strong under the closed throttle plate above idle speed, so it overcomes the valve’s spring tension, and manifold vacuum is sent to the distributor vacuum advance, advancing the timing.

CAP diagram

Why does this reduce exhaust emissions? Because with closed-throttle coasting or deceleration, you’ve got a relatively rich, slow-burning mixture headed into the cylinders; you’re hauling fuel through the idle circuit of the carb with the ultra-strong manifold vacuum created by the high engine speed and closed throttle, but because the throttle plate is closed, there isn't much air coming in.

Advancing the spark under those conditions gives more time for the mixture to burn, thus reducing emissions. It also increases fuel economy and reduces engine oil contamination. It does not take any bite out of performance or drivability, nor does it have any other detrimental effect.

The valve must be adjusted properly; if the spring tension is too loose you’ll get vacuum advance at idle (as manifold vacuum overcomes the too-loose spring). That will louse up your ability to set the idle mixture and speed for a smooth, stable, clean idle. If the spring tension is set too tightly, it’ll be as if the valve isn’t there at all. This 1967 Chrysler Master Technician Service Conference booklet has an explanation of the distributor control valve and, starting on page 10, test and adjustment procedures for the valve.

The CAP cars’ base ignition timing settings were very retarded compared to the non-CAP cars. A typical non-CAP setting was 2½° to 7½° before top dead center; CAP cars were set at top dead center to 5° after top dead center. The new control valve’s vacuum modulation fixed the sluggish drivability and poor fuel economy that would normally result from such late ignition timing settings.

Compared to a non-CAP car, the emissions of a CAP-equipped car were much cleaner, fuel economy was better, and drivability was unaffected. This was a much better result than other makers could come up; so, for several years, only Chrysler Corporation vehicles with CAP met the clean-exhaust requirements for purchase by the City and County of Los Angeles, the epicenter of the smog problem. A few years late(r), Ford and AMC began using similar valves.

Now, we have much more advanced technology, even for our old cars. Today’s gasoline burns much cleaner, today’s spark plugs and ignition systems do a much better job, and today’s aftermarket carburetors are much more precise than those of 1963. In that context, the 1963-69 distributor vacuum valve can be tuned to provide performance advantages by using it with non-retarded basic ignition timing settings. Hey, free gas mileage…who wants some?

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