AMC V8 Timing, $42 ASDU, 7747?

[shimniok]

I want to talk through setting up the Main SA timing table for a stock AMC 360 V8. I'm working on timing tables by looking at stock timing and starting with Bill USN-1's table from Binder Planet and tweaking based on driveability. I live in denver at ~5280'. The 1986 AMC V8 calls for 16* BTDC static timing at this altitude. I have the duraspark distributor phased and with vac and mech advance disabled.

I want to methodically step through a discussion of timing AMC V8 motors. While I have two questions at the bottom, first let's start with the stock timing curves for vacuum and mechanical advance systems as a baseline for understanding. The timing curves out of the TSM are:

Photo Apr 30, 11 07 26 AM.jpg

The diagrams show the allowable range of advance (min and max) for mechanical and vacuum mechanisms.

Let's look at the max allowable mechanical curve.

• Mechanical starts out around 0 until about 800 rpm.
• It climbs from there to ~8 degrees by 1000 rpm.
• From 1000 rpm to 4400 rpm, it climbs linearly from 8 degrees to almost 20 degrees.

Vacuum advance adds to mechanical of course. The vacuum advance mechanism is actuated by ported vacuum at normal operating temp from the factory. Let's look at vacuum starting with high vacuum, and use the minimum because it's a less complex curve.

• Above 13 in-Hg, the vac advance adds ~22 degrees.
• From 13 in-Hg down to 8 in-Hg, advance decreases linearly to 14 degrees of added spark timing.
• Then from 8 in-Hg down to 4 in-Hg, advance drops linearly to ~0 degrees.

With 16 degrees static timing, the '86 AMC 360 V8 idles at 16 + 22 + 0 = 38 degrees. I'd have to check with a dial back timing light to verify. How does the thing start with that much advance? I have read somewhere that the Motorcraft duraspark retards timing during start up. I don't remember by how much. But just enough, because I regularly got kickback if I tried to advance much beyond 15* static.

Maximum timing in this configuration at high manifold vac and high rpm is somewhere between 14+22+16=52 and 20+26+16=62 which is absurd. I don't know what to do with that. Low altitude static timing is 9* meaning 47-57* ... uhhh...?

Question: Can anyone explain how the TSM curves, adding up to maximum 47-57* low altitude timing (or 52/62 altitude timing), 4400rpm makes any sense? What assumptions are wrong, 4400rpm? Cruising with > 13 in-Hg at that rpm?

Question: is there a mathematical way to correlate TBI reported kPa to in-Hg? Or should I just get out the old vacuum gauge and write down what in-Hg corresponds to reported kPA?

[EagleMark]

Second question is easy. Make a value in your ADX file like this:





First question????? 1985-86 were the worst years for American vehicles to pass emissions as EFI was not really ready, so Detroit had to do it all with thermastatic controlled vacuum lines, dampers, one way valves and well the engine looked like someone left a roll of vacuum lines and parts on the motor by mistake. I'll take an 1985 Toyotas 22R and 22RE factory diagram for instance. 22RE was injected and had hardly a vacuum line on it, 22R was a sea of spaghetti, valves, switches, dampers etc...

I'm not sure any engeneer who did this knew exactly where timing always was? But it was where they needed it to pass emissions.

The only mistake in your thinking is Vacuum advance in Ported, so no vacuum at idle, port is above throttle blade. So idle 16 base + 0 mechanical + 0 vacuum. At cruise 4400 RPM you would have all your mechanical but I doubt more then 6 inches of vacuum, then decel it would increase vacuum advance until throttle blade closed past ported vacuum.

Then take into consideration all this spark advance was also with EGR! EGR is a leaner mixture and requires more spark advance.

WOT you get all your mechanical advance but because vacuum is 0 you get no vacuum advance... decel from there and spark increases till throttle blade closes ported vacuum.

[shimniok]

Second question is easy. Make a value in your ADX file like this:

Thanks, will give that a whirl.

The only mistake in your thinking is Vacuum advance in Ported, so no vacuum at idle, port is above throttle blade. So idle 16 base + 0 mechanical + 0 vacuum.

Ooops, of course.

At cruise 4400 RPM you would have all your mechanical but I doubt more then 6 inches of vacuum, then decel it would increase vacuum advance until throttle blade closed past ported vacuum.

Ah, that makes sense. So realistically the engine won't ever see > 13" vac at 4400.

Also, my motor doesn't ever rev past 4000, if even that high. My 85 never did either.

WOT you get all your mechanical advance but because vacuum is 0 you get no vacuum advance... decel from there and spark increases till throttle blade closes ported vacuum.

Ok, right. Without vacuum, at WOT, max timing would be ~ 20° + 16° = 36° which sounds sane.

[EagleMark]

That's really hard to explain, I'm glad you caught it!