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So I was researching as part of a recent project what my timing should be set to - lots and lots of comments on this that varied greatly in some cases so I was curious when looking at Emissions stickers (valve covers) what it might clarify.
In example: (all 1978 NON-CATALYST)
Label D8TE-9C485-ADA which is a 351M with manual transmission has 6 BTDC, 650 RPM with 2 Rich
Label D8TE-9C485-VA which is a 351M with automatic transmission has 14 BTDC, 650 RPM with 3 Rich
1) 8 degrees with a different transmission, why?
Label D8TE-9C485-ACA which is a 400 with automatic transmission has 12 BTDC, 650 RPM with 3 Rich
It's all about emissions. they had different curves for manual and auto transmissions .
When air pumps started in the 60's they were only on manual transmissions at first. manual transmissions have always been harder to meet emissions with and it's one of the reasons so few are offered today.
Just struck me that the timing could swing by up to 8 degrees on the same motor - I was "tuning" mine earlier this year and moving more than 2 degrees would take me from a pinging engine to one that would run great but hard start. Ended up at 11BTDC
It might be worth it to check your curve with a dial back. you might need to make some changes, it can make a huge difference if you get it right.
I probably shouldn't have said it's all about emissions, but 90% emissions. most distributors were curved a little different for manuals even before emissions so some of it is for performance and longevity.
