I can see two sides to this. Before you read on, I must say that I didn't intend to type quite so much!!
1. As stated, I am in the camp that says higher octane fuel
will deliver more power on a modern car, providing that it can adjusting timing in real time.
2. Whilst it may seem I’m contradicting myself, I also think much of what drivers report is placebo effect and
not an actual improvement in power/performance.
So why do I contradict myself?
The additional power will only really be noticeable - even in ideal lab conditions - close to the point of maximum power as the ‘extra’ power produced by using a high octane fuel will be relative to the underlying power that the engine would produce at that engine speed using regular fuel.
In the real world, how often do we drive at even close to that? IIRC a normally aspirated CLK55 has been mentioned, and whilst I don’t have the data for that car at hand, I do for the W210 E55 which used the same engine...
To maintain the maximum legal roadspeed in the UK of 70mph, the E55 requires 24kW of the 213kW that a standard E55 should produce at the wheels. So it’s not normal driving.
When accelerating hard on the road - and judging gear shifts perfectly to keep in the power band - will mean that in reality the engine will only be operating at or near peak power for very short periods.
There are a number of critical factors though which will make the extra power performance more noticable:
A. Ability to adjust timing in realtime, hence drivers with cars featuring this capability may notice more than those without.
B. Proportion of time the engine is under heavy load, hence drivers towing may notice it more than those driving solo.
C. Underlying power output of the engine, and hence drivers of performance cars may notice it more than those driving a runabout.
D. Suceptability to knock, hence drivers of cars which have high compression high-revving NA engines or forced induction engines may notice more.
E. Fuel grade the car is tuned for, hence cars tuned for regular but with realtime timing adjustment capability may notice it more*.
F. Pattern of torque/power delivery, hence drivers with flatter output curves may notice it more as they will be driving that range more of the time
The list goes on, and the most important of all…
G. Drivers willingness to believe, hence drivers who do believe will be more likely to notice, or enhance the actual improvement through perception**.
* A car tuned to use super would need super super like BP 102 to see and improvement over super.
** The opposite is true, as a non-believer could wipe out all of those from the above!!
