vac advance or not for FE Cobra

[olddog]

I just graphed out the timing tables from a tune called A9L2 I got from a dyno tuner. He basically modified the A9L for a 89-93 Mustang GT manual trany. He eliminated the WOT timing and sea level tables, and made one table for everything. It makes the older EEC4 A9L act more like to modern Ford EFI.

At light loads 10% - 30% cylinder fill (high manifold vacuum), the timing comes in very fast at low rpm. By 2500 rpm the timing is coming in at a much slower rate. It is fully in at 44 deg from 4000 rpm up at a 10% load. The graph looks much like a square root function, for the mathematically astute.

At a 90% load (Wide Open throttle) the timing comes in very slowly from 750 - 1250 rpm, then starts to increase more rapidly until about 2000 rpm. From 2000 up the rate of increase in timing is starting to decrease. At 3000 rpm the timing is at 26 and it slowly goes from 26 to 28 by 4000 rpm. I would describe this graph as looking somewhat like a stretched letter "S".

Now this is for a bone stock 5.0 engine, and they do not take a lot of timing. Ford is only putting about 28 in at WOT high rpm.

But the biggest point is that the timing maps are curves. Mechanical advance is a Linear straight line. I believe the Vacuum advance was Linear too.

This map was done for boosted engines as well. At 200% load, which would be 16-18 psi boost, it is real interesting. He pulls all the timing out (o) and then steps it up a bunch at about 1750 rpm. Obviously he is concerned about destroying the engine at low rpm.

It would be interesting to see what Ford is doing with their ecoboost engines.