High compression engine fuel requirements
 

So I know that higher octane helps prevent detonation (knock).

I know that we need 91 minimum.

What I don't know is why we need 91 octane? Is it so that we can run more advanced timing? More RPM?

Just asking because Mazda's Skyactive motor (which is 12:1 compression) can run on regular (87) gas.... but it makes less power and has lower RPM.

Before any tells me not to complain about the fuel requirements, i'm NOT. I just am curious about the factors that contribute to the 91+ requirement.

Higher octane is only to prevent early detonation AFAIK. You answered your own question. Advanced timing and or tuned for higher power output.

Skyactiv is more like nonreactiv (gas pedal).

I think you'll find the Mazda is 13:1 at least the 2L skyactive version here is.
I was surprised it ran on the low octane too. It just shows the advances in combustion chamber design and fuel delivery to achieve that.
My sister has one. The 6 speed auto that is rated at about 6.4L/100km compared to the 7.8L/100km for our manual 86's
In reality they get closer to 9L/100km while my manual GTS86 has averaged 6.9L/100km over the first 2000km. However my 98[RON] octane costs 10% more than their 91 octane.

Put simply, there is a lot that affect the det threshold of an engine. Bore size, stroke, crown and combustion chamber shapes, swirl and tumble characteristics, valve timing (thus lobe profile as well), obviously ignition timing, charge air temp, fuel temp, piston temp, quench height, quench pad, squish, air fuel ratio...separately for GDI is now injector timing, flow rate, angle, nozzle design (hole count, size, spray pattern, spray angle; as they effect atomization), injection pressure (again atomization),etc...

So where do you want to start with the comparison? You can also look at the new GDI LT1 and see that it's only 11.5: (IIRC) and suggests but doesn't require premium fuel.


To put it really simply; every in an engine has some effect on everything in an engine.