I'm trying QC mode with E4 emptying whatever ammo i have on a single ai from different angles. Yet the best i can get is fuel leak/engine fail after very prolonged times. Yes i'm shooting close, and trying to aim engine/pilot but it doesnt bother ai damage model.

Too close is no good you need to be at the optimum convergence of the guns for a start.
Make sure you are hitting engine, fuel tank or pilot area (which does have armour).

Some people think it needs toughening

I'm sure some others can chip in with help on it.

Oh and Hurricanes were rugged, one of its advantages over the Spit.

Can't seem to find where to set convergence.

You can't via controls, but it can be done manually editing files but the point is to try to fire at whatever the setting is on.ChiefWH has a mod that changes them and the gunsights to match etc (see mods sub forum for detail).,

The .xdp config file of an aircraft has convergence set (top lines).