For the last several days I've been reading all I can about vacuum vs mechanical advance and how they interact and about ported vs manifold vacuum. I've learned that I know alot less than what I thought I knew. Most of the info that I've come across is discussing V's, but I'm pretty sure that the same principles will apply to inlines though.
Based on what you guys have said and what I've been reading I'm going to do the following this weekend : 1st - limit my vacuum advance enough so that I can set the base timing to 4 - 6 degrees BTDC (near factory) and still maintain 32-34 at 3K; 2nd - move the vacuum advance to the manifold and readjust the carb; 3rd - install a line so that I can run my vac gauge into the cab.
Then drive it and fiddle with the mechanical advance springs (I have 4 sets that came with a kit I bought) and moving the total advance at 3K up some and down some and drive it after each change.
My carb is 390 Holley, mild cam, zero decked, cast iron headers, Offy intake.
Now it's time for questions!
Tony P - do you consider a 390 to be a 'small' carb and should I expect vacuum at max throttle (4200 - 4600 rpm's)?
Panic - are you saying that I may have to increase the idle spark higher than 4 - 6 BTDC? If yes, then I'll also need to limit the vacuum advance further to keep my total timing near 32 - 34, correct? I'm not aware of what your talking about when you say 'disc edge too far into the transition slot' (excuse my lack of knowledge/ignorance - I'm learning as I go!) You said vacuum advance at max throttle is 'dangerous', dangerous to what?
Both of you - do you agree with total advacne at 3K near 32-34 degrees? Is it possible to have both good performance and fuel economy or is one the sacrifice for the other? Is it worth my time to get an adjustable vacuum cannister?
I'm open to all suggestions and thanks!
