66cuda-s
Well-Known Member
Can anyone tell me what this gizmo does? It is on my 66 Barracuda 273. This car has a Cleaner Air Package (CAP) "option" that was required on California cars.
What's this CAP Vacuum Gizmo??
- Thread starter 66cuda-s
- Start date
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Suppose to retard your timing until the car warms up. i bought a car that had that POS. I ****-canned it.
slantsixdan
=..=
Nope, it is not a timing retard and it is not temperature sensitive. This what you're showing us is a distributor vacuum advance control valve used on certain CAP applications from 1963 (prototype CAP cars) through 1969. It 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 plastic cap, you'll see there's a spring underneath and a slotted screw running thru the center of the spring to adjust the tension.
What this valve does is advance the timing under closed-throttle, high-vacuum situations (i.e., deceleration or coasting with your foot off the accelerator). The way it works is simple: as long as the vacuum signal from the manifold vacuum port 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; this strong manifold vacuum overcomes the valve's spring tension and manifold vacuum is sent to the distributor vacuum advance. This advances the timing. Why? 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, but because the throttle plate is closed, you haven't got 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. It also reduces engine oil contamination. It does not take any bite out of performance or driveability, 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 action at idle (by manifold vacuum overcoming 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 tight it'll be as if the valve isn't there at all.
Read more here (Popular Mechanics November '63 discussion and test of the prototype CAP) and here (1967 Chrysler Master Technician Service Conference booklet on the system with detailed explanation of the distributor control valve —*keep reading onward; starting on page 10 are the check-and-adjust procedures for the valve).
Note that you can turn this piece of hardware to your advantage by advancing the base timing from the retarded CAP spec. I don't recall what the CAP and non-CAP basic advance specs are for a 273 in '66, but it'll be something along the lines of 5° BTDC for non-CAP and 5° ATDC for CAP.
What this valve does is advance the timing under closed-throttle, high-vacuum situations (i.e., deceleration or coasting with your foot off the accelerator). The way it works is simple: as long as the vacuum signal from the manifold vacuum port 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; this strong manifold vacuum overcomes the valve's spring tension and manifold vacuum is sent to the distributor vacuum advance. This advances the timing. Why? 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, but because the throttle plate is closed, you haven't got 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. It also reduces engine oil contamination. It does not take any bite out of performance or driveability, 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 action at idle (by manifold vacuum overcoming 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 tight it'll be as if the valve isn't there at all.
Read more here (Popular Mechanics November '63 discussion and test of the prototype CAP) and here (1967 Chrysler Master Technician Service Conference booklet on the system with detailed explanation of the distributor control valve —*keep reading onward; starting on page 10 are the check-and-adjust procedures for the valve).
Note that you can turn this piece of hardware to your advantage by advancing the base timing from the retarded CAP spec. I don't recall what the CAP and non-CAP basic advance specs are for a 273 in '66, but it'll be something along the lines of 5° BTDC for non-CAP and 5° ATDC for CAP.
CUDAGUY
aka Prince Valiant
so it works like vacuum secondaries on a holley, but to advance timing under low load/vacuum? thats neat!
66cuda-s
Well-Known Member
SlantSixDan,
Thanks for your (as usual) well-stated explanation. I will read the articles you referenced and perform the adjustment. I have noticed the ignition timing differences (CAP, Non-CAP) and wondered about it.
Thanks again,
66Cuda-S
Thanks for your (as usual) well-stated explanation. I will read the articles you referenced and perform the adjustment. I have noticed the ignition timing differences (CAP, Non-CAP) and wondered about it.
Thanks again,
66Cuda-S
31pickemup
Well-Known Member
Would this part be on a 67 383 California car as well?
slantsixdan
=..=
Factory parts catalogue says yes, the '67 383 with single 4bbl for California had the valve.
In fact, the '67 FPC says there were different valves for different engines. Some of the difference would be in mounting bracket configuration, but that's not all of it:
In fact, the '67 FPC says there were different valves for different engines. Some of the difference would be in mounting bracket configuration, but that's not all of it:
bruceo2011
Still waiting for Y2K
Also helps to eliminate backfiring when you take your foot off the gas.
CUDAGUY
aka Prince Valiant
who the hell wants that!?!?!?!:sign5:
31pickemup
Well-Known Member
man finding one of those for my 383 will be next to impossible
cudamark
Well-Known Member
I've a few different ones but I don't see any part numbers on them so identifying the exact application might not be easy. I have both green cap ones and black cap ones but they look the same underneath and the caps swap so that won't be a for-sure way to tell. The bracket for the valve has a couple of different mounting bolt diameters so that may help. The bracket is removeable from the valve but not easily....it's crimped... so it's unlikely they would be swapped. There is probably a way to measure the valve's action under different loads but I don't have a test jig for that. Just in Dan's chart, it shows 6 different part numbers.....anyone know how to tell them apart?
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