what is ignition timing...

[johnf2003200]

...1978 trans am? has a 400 I have been told it was 16btdc
It is a auto I dont belive it to be the 6.6 just a garden varity 400

 

[smokey]

call your local GMC dealer for the correct timing

 

[Goatman]

There should have been a emission sticker on the core support that has that information on it , if that is gone I would suggest setting it around 15-20* btdc as a start and test driving it to see if you hear any detonation (ping) if you don't and it runs OK that should be close ..
with an older engine the timeing wont be critical as they only had 7 to 8- 1/2 to 1 compresion and are Not really bothered by timeing being off a small amount ...
for the best mileage use a little less timeing for better performance use as much timing as you can without hearing any (pinging)
it's trial and error but the factory settings are seldom optimal for todays fuels ..