I read this in a 305 build up article.

"We considered a computer chip for the Camaro, but we decided to wait until we had more accomplished before making the investment. In discussion with Steve Cole of The Turbo Shop (TTS), he said that the computer-controlled advance curve is very limited in this combination—roughly around 22 degrees of spark timing. While more timing could be added with a chip, the least expensive route was to simply add more initial timing to the engine. This is a good idea anyway since the factory initial timing setting is 0-degrees BTDC! Tim checked the Camaro and that’s where the timing was set, so we had plenty of room for improvement."

My Question, What is the difference between the computer-controlled advance and the "Initial" timing? I've used a timing light to time my cars sp if I do that with my carbed 305 what happens to the computer advance? Anything? Since the carb is computer controlled also, will I have to adjust it if I advance the timing or will the computer compensate?

 

The computer does not "compensate". The computer "controls".

Consider a traditional pre-computer distributor with spring & weight (centrifugal) advance and vacuum advance. You install this, and set the "initial" timing at low RPM, below the RPM at which the weights and springs become active; and you disconnect the vacuum advance, so that it is also deactivated. You then twiddle the distributor as appropriate.

Now consider the computerized distributor. It contains no springs & weights, and no vacuum can. Yet, the engine still requires that the actual ignition timing be varied according to various operating conditions. An engine is still an engine, regardless of the control system.

What happens is, as RPMs increase, the computer adds more timing, exactly like the springs and weights used to do; and it adds timing when the throttle position and vacuum indicate that it is needed (lean high-speed operation, i.e. cruising), just like the vacuum can used to do. To adjust the static setting, the human still has to disable the computer's adjustments, i.e. "tell" the computer somehow that the human is preparing to set the distributor to its "baseline" point; then twiddle the distributor body around to the desired "starting point", from which the computer then applies an "offset" to once it's "told" that the human is through; just like the spring & weight system, and the vacuum can, used to do back in the Spanish-American War days.

The computer hasn't the foggiest clue what the human twiddled the distributor to. All it can do, is look at the pulse it receives, and look up in the tables programmed into the "chip", how long it's supposed to wait before it puts out the pulse to the coil for the next cylinder.

With the computer carbed cars, the way you "tell" the computer you're adjusting the static timing, is by disconnecting the 4-wire Weatherpak connector that hangs out of the bottom of the dist. You unplug that, start the motor, set the static timing with the light, turn the engine off, plug it back in, and leave it alone after that. Now, where you set it to, is up to you; the factory has a "spec", just like the old Civil War system, but you have the power to make changes.

Ideally, the engine is going to want the spark to occur somewhere areound 34-36 degrees BTDC at high speeds (300 RPM or greater) and full load. If you want to measure that, disconnect the vacuum line to the MAP sensor, speed the engine RPM up to something beyond 3000 RPM, and check it with the light. You could, in theory, twiddle the dist housing under those conditions to achieve the 34-36 degree setting, and just lock it down and forget it. I can tell you though, with the L69 computer, you'll find it won't be too far different from the "spec" static setting.

 

Curious:
What will the effect be if he adds 4° to the mechanical baseline without reprogramming the ECM baseline?
Does that increase the total timing to 38°?

Why is 0° used on these engines versus 6° for my TPI?

 

Yes. And every other timing setting as well.

I have no clue why they did that. I wasn't there at the time. It's more or less arbitrary; one of those things where they can pick a number, any number (within a reasonable range at least), and write the programming in the chip from there. Probably the numbers were chosen by 2 different committees composed of different people that met several years apart. (TPI introduced in 85, TBI introduced in 88)

There's no accounting for the reasoning behind such things, or lack of it, after the fact.