[jpd80]

Re Barra I-6
The Barra 190 engine had a vane-type VCT phaser – produced by Aisin – on each camshaft
that provided continual variable adjustment within a 60 degree range (10 degrees advanced
or 50 degrees retarded from the initial pin lock position).

when observed from the front of the the engine, the crank and camshafts turn in the clockwise direction
So any advance of either cam timing or spark timing means the event has to happen earlier or in the
anti-clockwise direction.

Since it takes time for the spark plug to ignite the flame front and to to completely combust the mixture,
Firing before top dead centre is essential to time for maximum work from the combustion but this depends on,
Engine speed, mixture strength and whether full throttle, any boost pressure or high vacuum due to light throttle.

Under light throttle high vacuum conditions, the compression pressure is much lower than the nominated compression
In some instances it could be as low as 3 to 1 or 2 to 1 and combined with lean cruise mixture, more spark advance
is required to account for the slower flame travel and less dense mixture charge.

Forget all the stuff you learned about 1970s engines advancing and retarding cams,
The modern VCT engines advance and retard cams depending on load, not just RPM.
under light load, the 3- valve 5.4 V8 would retard up to 40 degree cam timing to permit
a form of internal exhaust gas recirculating to meet emission controls, I suspect the Barra does similar.
Those Aussie engineers were so smart and employed a lot of brilliant ideas in the I-6.
They even had Jaguar engineers assist with tuning the harmonics of the engine to make it smoother.