1965 Vacuum Advance TSB
Open the attached pdf to read the TSB.
The current replacement equivalent of the recommended 201 15 is stamped B22 (NAPA VC1802), which should be used for manual transmissions. For Powerglide use the 12" B26 (NAPA VC1765) These NAPA/Echlin numbers can be cross referenced to any other name brand. They are all manufactured by Standard Motor Products.
Both VACs provide maximum 16 crank degrees at 15" and 12", respectively. The reason to use the more aggressive VAC with Powerglide is because manifold vacuum idling in Drive is less than a manual idling in Neutral.
Beginning in '66 a 12" VAC (355 16) was used on 300 HP engines through at least '68 in order to accommodate automatics, but it's more aggressive than necessary for manuals. Basic rule of thumb is to use the least aggressive VAC that meets the Two-Inch Rule.
Also beginning in '66 maximum centrifugal advance was increased from 26 @ 4100 to 30 @ 5100. Both are lazy and performance can be improved with lighter springs that brings it all in at 3000-3500, which is about ideal.
To achieve best power WOT advance of 36-40, initial should be set at 6-10 on 30 degree centrifugal distributors, but earlier 250/300 HP distributors will require more. Run it as high as the engine will tolerate without detonation.
The '66-'68 spark advance map is near ideal other than the centrifugal being lazy and the 12" VAC being more aggressive than necessary for manual transmissions. Earlier 250/300 HP engines will benefit from grinding out the cam assembly slot to increase total centrifugal to 30 in order to avoid having to run excessive initial timing to achieve the ideal total WOT advance range, which could cause low rev detonation.
Note that the TSB says the 201 15 was phased into production about March 1, but in the field to use the ...163. This is because the ...163 was used as a service part to replace the 201 15 even though the 201 15 was still used in production. Why is anybodies guess because the ...163 requires about 18" to fully deploy (15" for the 201 15), so it doesn't even pass the Two-Inch Rule on a base cam engine. The ...163 should have never even been produced, and the 201 15 should have been the service part for the 201 15 used in production.
This is an example of "What the f... were those guys thinking?" Of course the aftermarket keyed off GMPD parts catalogs, and their replacement is stamped B1, which I have always referred to as a "boat anchor". It's not suitable for any GM production engine.
Duke
The current replacement equivalent of the recommended 201 15 is stamped B22 (NAPA VC1802), which should be used for manual transmissions. For Powerglide use the 12" B26 (NAPA VC1765) These NAPA/Echlin numbers can be cross referenced to any other name brand. They are all manufactured by Standard Motor Products.
Both VACs provide maximum 16 crank degrees at 15" and 12", respectively. The reason to use the more aggressive VAC with Powerglide is because manifold vacuum idling in Drive is less than a manual idling in Neutral.
Beginning in '66 a 12" VAC (355 16) was used on 300 HP engines through at least '68 in order to accommodate automatics, but it's more aggressive than necessary for manuals. Basic rule of thumb is to use the least aggressive VAC that meets the Two-Inch Rule.
Also beginning in '66 maximum centrifugal advance was increased from 26 @ 4100 to 30 @ 5100. Both are lazy and performance can be improved with lighter springs that brings it all in at 3000-3500, which is about ideal.
To achieve best power WOT advance of 36-40, initial should be set at 6-10 on 30 degree centrifugal distributors, but earlier 250/300 HP distributors will require more. Run it as high as the engine will tolerate without detonation.
The '66-'68 spark advance map is near ideal other than the centrifugal being lazy and the 12" VAC being more aggressive than necessary for manual transmissions. Earlier 250/300 HP engines will benefit from grinding out the cam assembly slot to increase total centrifugal to 30 in order to avoid having to run excessive initial timing to achieve the ideal total WOT advance range, which could cause low rev detonation.
Note that the TSB says the 201 15 was phased into production about March 1, but in the field to use the ...163. This is because the ...163 was used as a service part to replace the 201 15 even though the 201 15 was still used in production. Why is anybodies guess because the ...163 requires about 18" to fully deploy (15" for the 201 15), so it doesn't even pass the Two-Inch Rule on a base cam engine. The ...163 should have never even been produced, and the 201 15 should have been the service part for the 201 15 used in production.
This is an example of "What the f... were those guys thinking?" Of course the aftermarket keyed off GMPD parts catalogs, and their replacement is stamped B1, which I have always referred to as a "boat anchor". It's not suitable for any GM production engine.
Duke