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| Autolite C5AZ-12370-A Advance | C5AZ-12370-A Rear View |
The 428 Super/Cobra Jet engine uses a distributor vacuum advance (Ford basic part number 12370) that adjusts ignition advance in response to changes in engine vacuum. Both single diaphragm (with one vacuum port) and dual diaphragm (with two vacuum ports) advances were used depending on the type of transmission installed in the car. Cars equipped with an automatic transmission used a single-diaphragm advance and cars equipped with a manual transmission used a dual-diaphragm advance.
Dual diaphragm vacuum advance units use carburetor vacuum on the outer, primary diaphragm to adjust ignition timing in response to engine speed. Manifold vacuum is used on the inner, secondary diaphragm to retard timing during periods of closed throttle idle to reduce emissions. The secondary diaphragm is also used to control ignition advance in response to coolant temperature; see the distributor vacuum control valve page for details. Single diaphragm vacuum advance units respond only to carburetor vacuum.
| Application and Notes | Service Part Number | Motorcraft Sales Number | Calibration Code | Marking |
|---|---|---|---|---|
| 1968-1970 with Automatic Transmission Single Diaphragm |
C5AZ-12370-A | DD-190 | 303 | N/A |
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| Autolite C5AZ-12370-A Advance | C5AZ-12370-A Rear View |
The advance used during the 1968 and 1969 model years on cars equipped with a manual transmission is a dual diaphragm unit that can be identified by its narrow silver band and "HP 6" mark.
| Application and Notes | Service Part Number | Motorcraft Sales Number | Calibration Code | Marking |
|---|---|---|---|---|
| 1968-1969 with Manual Transmission Dual Diaphragm |
C8AZ-12370-A | DD-631-A | 357 | HP 6 |
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| Autolite C8AZ-12370-A Advance | Rear View |
The engineering number found on the front of the dual diaphragm advances doesn't uniquely identify the advance, so it shouldn't be referenced for identification purposes. Apparently different castings with different numbers and "Autolite" markings were used.
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| C8AZ-12370-A Front View C8AF-12378-C Engineering Number |
D0ZZ-12370-B Front View C8AF-12378-D Engineering Number |
The advance used during the 1970 model year on cars equipped with a manual transmission is a dual diaphragm unit that can be identified by a wider (than the 68-69 band) gold (zinc dichromate) band and an "X 6" mark. Some information I've seen suggests that a different advance with a screw-in front sitting may have been used early in the 1970 model year, but my research thus far is inconclusive.
| Application and Notes | Service Part Number | Motorcraft Sales Number | Calibration Code | Marking |
|---|---|---|---|---|
| 1970 with Manual Transmission Dual Diaphragm |
D0ZZ-12370-B | DD-258 | 322 | X 6 |
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| Autolite D0ZZ-12370-B Advance | Rear View |
Here are close-up shots of the "HP 6" and "X 6" markings found on the dual diaphragm advances.
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| "HP 6" Stamping | "X 6" Stamping |
Both the single- and dual-diaphragm advances are calibrated and adjusted using a combination of washers and a vacuum chamber spring that can be found under a plug on the front of the unit. Different springs were used for different applications, with lighter springs used in advances targeted for high performance vehicles. Adding a washer will decrease advance and removing a washer will increase advance. Adjustment procedures can be found in your factory service manual.
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| Advance Calibration Parts |
Thanks to Tim O'Connor for the pictures used on this page.
© 2009 Scott A. Hollenbeck. All Rights Reserved.
