RC200

There's a big timing drop (12° BTDC) in the 4000-4500rpm area.
I already observed such thing on some logs.
apparently, it occurs when you add too much timing (+18° with lemmi, then you go over 25° BTDC at torque peak).
I suspect a kind of safety action from the ECU.

Nobody is surprised (or scared) about that ?

Flyboy

You’ve encountered an interesting result of running very high octane gas which is often misunderstood. To start, picture this, a bell curve with an x (horizontal-time ) and y (vertical-pressure) axis. The bell curve is representative of the pressure inside the cylinder as the fuel air mixture burns over time. Imagine that when the curve is centered on the y axis it represents the optimum crank arm angle to get the most leverage on the crank shaft and the peak of the bell curve is where peak pressure occurs. You want the peak pressure to occur at the best crank angle to put the most leverage on the crank shaft.

A/F ratio affects burn rate, the fastest burn (most rapid release of energy), occurs around 11.5-13 and you’d like to get the energy out quickly, when the motor geometry is close to ideal, to gain the most advantage from leverage. A faster burn rate means a narrower, higher bell curve. A slower burn rate leads to a flatter bell curve, and energy being released over a longer time. This slower burn means the crank arm is outside to optimal leverage range for much of the burn, and less energy is used for forward propulsion.

Ignition timing is what shifts that bell curve from left to right along the x axis.

As you add fuel you are slowing the burn, but timing is staying the same, so the bell curve is flattening and moving to the right. So the peak pressure moves away from optimum. CF’s decrease because the ECU doesn’t need to retard timing any, you are doing that with extra fuel.
You then start adding timing back, which begins to shift the bell curve back to the left, but the height of the curve doesn’t return to its former level, you’re making less pressure inside the cylinder. With less pressure there’s less chance for detonation so the ECU never tries to retard the timing advance, and you can run very high timing values.

In your case the effect is compounded because you are running fuel that is very resistant to detonation to begin with. An additional downside to running so much timing is that combustion (pressure rise in cylinder) begins while the piston is still compressing the air-fuel mixture, effectively countering the movement of the crankshaft.