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Since I have installed the 78 403 into my Vista Cruiser I have been trying to find the correct initial timing specs for it. Google says 5*, my old Motors manual says 20* at 1100 rpm. Its a 78 403 out of a 78-79 Trans Am, it runs great and has plenty of power, a lot more power than the original 330 High Compression 4 barrel engine. I "think" the engine is mostly stock but does have an Edelbrock Performer intake on it and the timing gears and chain have been replaced.
So where should the initial timing be set?
That depends upon what distributor you have on the engine. I have a GM HEI from the same engine family, a 77-79 403, and I think the 403 engine settings were 20º initial.
I originally used a Moroso HEI advance curve kit weights and center bar to change it to match my engine's points setup with 10º initial and 24º mechanical. I later switched back to the original 403 weights and center bar to get a higher initial, which idled smoother, had more vacuum at idle (needed that for the power brakes). It's presently set with 18º initial and the vacuum advance is limited to 10º with a home-made stop (original canister was 24º).
That depends upon what distributor you have on the engine. I have a GM HEI from the same engine family, a 77-79 403, and I think the 403 engine settings were 20º initial.
I originally used a Moroso HEI advance curve kit weights and center bar to change it to match my engine's points setup with 10º initial and 24º mechanical. I later switched back to the original 403 weights and center bar to get a higher initial, which idled smoother, had more vacuum at idle (needed that for the power brakes). It's presently set with 18º initial and the vacuum advance is limited to 10º with a home-made stop (original canister was 24º).
My off brand Chinese HEI has a curve similar to the points distributor. I would use a dial back timing light and take the total to about 36 or 38 with the vacuum advance unhooked and plugged revved 3000+ rpm. Let the base fall where it may. I plan using my off brand with the new Edelbrock heads, trying around 40 degrees, which you could probably get away with, especially with less vacuum advance.
Any specs in the books are only authoritative if you are running the stock distributor and the stock carb and the stock trans/rear/etc.
Once you start changing stuff, then, well, you have to find your own tune.
With a mechanical/vacuum advance distributor I start by getting ~36% total advance at high RPM. Unplug the vacuum advance, rev the engine up until the timing stops moving (usually 3,000 to 4,000 RPM), set the timing to 36, lock it down. Let the engine go back to idle and see where the timing is. Could be anywhere from 12 to 25. Just depends on how that distributor was set up. See if the engine idles/starts/stops OK. If so, then you're lucky! Now you can mess with weights and springs to your heart's content.
If it doesn't idle well then you can try manifold vacuum advance to see if that helps. I personally use manifold or ported - just whichever leads to the best idle for that engine combo.
You also get to play around with the vacuum can adjustment to get as much advance as you can without it pinging at part throttle load. Slightly accelerating up a hill on a highway is a great stress test.
If you start with idle then the problem is you don't know where total is. HEI distributors usually (but not always) have less travel than points distributors, so setting an idle at 12 degrees, which was common in the points days, might lead to a total advance of ~26 degrees which will make the car feel very underpowered.
5 degrees is nuts. I have no idea when any Olds engine would have that low of an idle advance.
AI answers on classic car questions are nearly always wrong. They are only good for entertainment.
