Timing a 454 bbc
My brother in law bought a 1976 chevy 1/2 ton short bed 4x4 truck a couple days ago and brought it over for me to look over before he handed the guy the money for it. To my suprise this truck had a 454 big block on it and a turbo 350. The engine has a dual plane wieand intake manifold on it and a quadrajet 4 barrell carburetor and it has some hedders on it along with a gm hei distributer. When the engine was idling it had a real big lope to the idle, i put my vaccum gauage on it and it was making about 14 to 14.5 hg of steady vaccum at idle. I put my vaccum gauage on it just to see what it was making and i thought at fisrt it might have had a vaccum leak because of how un-steady the idle was, but i think it may have a pretty good sized cam, but not 100 percent sure on that.
Anyways, i did a compression test on it for him and each cylinder came out at 140 to 145 psi and we live 4000 feet above sea level, so in my book it passed the compression test with an A+. I decided to put my timing light on it, i have a actron digital timing light, initial timing was at 10 degrees and the idle rpm was around 750 rpm. I did not check the total timing because at the point it was starting to get dark outside, but i did take the vaccum advance hose off the ported vaccum source they had it running to and put it on the full time manifold vaccum source on the carburetor and instantly the idle improved and the idle rpm raised to around 950 rpm, i dropped the idle down to 850 rpm and it idled alot better with more timing at idle and he told me it drove better too.
I know it still needs some adjustments on the timing because 10 degrees of initial timing seems to low to me for a 454, but i dont have much expierence with Gm's, I have a crate 360 magnum in my 91 half ton short bed dodge truck and it likes 16 degrees initial and 34 degrees total timing with another 12 degrees of timing added at idle from the vaccum advance. So that leads me to believe that 10 degrees initial for a 454 is probably too low. But i do not know how much mechanical advance is in a gm hei distributer. For example the distributer on my 360 magnum has an adjustable mechanical advance and i have it set at 18 degrees so i can run 16 degrees of initial timing while manintaining a total of 34 degrees. From what i have read and been told big block chevys like 36 to 38 degrees of total timing and anywhere from 16 to 20 degrees of initial timing depending on cam shaft size, but i planned on just increasing the inital on this 454 2 degrees at a time untill it kicks back against the starter and at that point back it down 2 degrees from there, thats the way i was taught to find the best initial timing setting on an engine that you do not know what the cam spec's are.
I have read that most Gm hei distributers have 20 degrees of mechanical advance in them, and if this is true and his inital timing is at 10 degrees then that means it is at around 30 degrees total timing, wich is not enough total timing. I just thought i'd ask here and see if anyone knew how much mechanical advance is in a Gm hei distrbuter, i dont know what year of distributer this is and i'm sure that they vary from year to year.