I need to replace the vacuum advance on a stock points distributor on a '71 Cutlass 455. Rock Auto shows two AC Delco part numbers: D1393C (with a straight vacuum fitting), and D1394C (with a 90 degree vacuum fitting). Are the vacuum fittings the only difference here, or am I missing something else?

i went to acdelcos site and searched those numbers.

unfortunately no info is given on either one, only applications, both were recommended for 68-72 442 BBO.

maybe one is a manual trans vs auto trans one>

info like the degrees advance and vacuum required would be nice.

I've been looking into it myself. Unfortunately most new vacuum advances are made to handle many applications. I'm not sure what your "71 application needs but there are differences in the advance.

Search “Lars grimsrud” contact him directly as he says his writings not sent by him and pulled off the internet are mangled by others. I worked through my ignition setup with his help.

Also has a complete breakdown on vacuum advance. I can forward his info by email if anyone is interested.

Great tip on the timing articles by Lars Grimrud. After taking a closer look at the specs for both advance units, the vacuum advance with the 90 degree bend brings in advance at 8 - 10" Hg, which works well with a mild cam according to his articles.

Your vacuum advance specs requirement should be based on how much initial advance vs total mechanical and the amount of vacuum available. If your build and everything is set to stock then a stock advance will work. If you've setup your distributor for performance using the suggestions from the multitude of articles available, then it won't.

Where did you find the spec info?

The article is a little dated, but we're talking about older cars anyway. The paragraph 'Starts at Hg' was helpful.

http://www.metroli.org/pdf/WHITE%20P...0Operation.pdf