I wish to check my timing on the 66 California Charger with a 383 HP engine. This was the first year a smog device was required and only installed on CA cars I believe. So I have a vacuum module that I believe retards the spark/timing and a CAP carb which has a vacuum port routed to the module and then to the vacuum advance on the distributor. I thought the initial timing would be the same on the engine but the books say instead of 10-12 degrees BTDC these are timed 5 degrees after TDC. The current timing, whatever it is, does not cause any pinging but I have been running octane boost and lead additive. I'm looking for advice before I go deeper into this. At one point I thought about disconnecting/removing the module but was not sure if the CAP carb would be affected by doing that. Then thought well it's running okay and I like keeping cars stock if possible. In CA it's no longer required for pre 1975 cars to be smog tested every two years. I recall many years ago I could plug a vacuum line with a BB to block a vacuum port. But can't recall now why. Should I leave this alone or can I get a better running engine with more power w/o the device?
In the 1966 and 1967 Cleaner Air Package (CAP) installations, the three included components are as follows:
- The carburetor is calibrated to provide a leaner mixture at idle and during low speed operation.
- The distributor is revised to provide retarded timing at idle.
- A vacuum advance control valve is added to operate with the distributor to provide advanced timing during deceleration.
Timing at idle is retarded in a CAP system. This, with a higher idle speed and leaner carburetor mixture was done to reduce the exhaust emissions. Although timing is retarded at idle, CAP allows conventional spark advance during acceleration and cruising.
The CAP vacuum advance control valve provides more spark advance during deceleration to reduce the exhaust emissions. The vacuum advance control valve is connected with three vacuum hoses -- one to the carburetor, one to the intake manifold, and the third to the distributor vacuum advance unit. Carburetor venturi vacuum signal and intake vacuum act on the internal spring of the vacuum advance control valve to control the signal to the distributor vacuum advance unit.
At idle, venturi vacuum is low and manifold vacuum alone does not move the spring, so no vacuum is applied to the distributor, similar to a standard ported vacuum system.
During acceleration or cruise conditions, the increased air flow in the carburetor venturi creates a stronger vacuum signal, and coupled with intake manifold vacuum, opens the vacuum advance control valve to provide vacuum to the distributor vacuum advance unit, similar to a conventional system operation.
During deceleration, emission levels are the highest in a conventional system. When decelerating, carburetor vacuum is low due to the closed throttle, and the vacuum advance control valve is activated alone by high intake manifold vacuum to provide maximum vacuum to the distributor vacuum advance unit, reducing emissions.
Although the CAP vacuum advance control valve fits in the distributor vacuum advance line, control is different from that used in the Orifice Spark Advance Control (OSAC) of the mid 1970s. OSAC was a delay of the vacuum signal from the carburetor to the distributor depending on ambient temperature.
Tuning a CAP engine to CAP specifications required a retarded initial timing at idle and advance was added as previously described during operation.
The CAP vacuum control system can be bypassed by running a vacuum line directly from the carburetor ported vacuum fitting to the distributor vacuum advance unit, and blocking off the intake vacuum line.
If the vacuum advance control valve is bypassed, additional initial timing can be set and the engine treated and tuned as a non CAP engine with conventional vacuum advance. With the initial timing now advanced more, idle speed will be even higher and must be lowered and balanced with initial timing increase(s). Overall distributor centrifugal advance curve changes might be made as well.
The CAP leaner carburetor may require adjustment to richen the idle after bypassing the vacuum advance control valve, and resetting initial timing and idle speed, depending on conditions. With an otherwise stock engine and exhaust system, it may run okay as is. In general, only the idle and primary circuits are affected by the leaner calibration.
Removing an operational CAP setup may not increase overall performance of the engine by its absence alone, but will yield a simpler setup for more direct tuning control, which can yield better performance. Removal without adjustments as described to compensate for it will yield worse performance until the timing, idle speed, and possibly carburetor adjustment has been performed to make it an operational conventional setup.
Although the specifications and settings are in the service manual if retaining the CAP system, the following images show the CAP settings for 1966 383 four barrel engines with manual and automatic transmission from the Chrysler CAP 1966-67 Engine Performance Evaluation with CAP document, TMC-103, for reference.
