I have a 2017 scat pack which recommends using 91 octane just wondering if there's really any difference between using regular and premium?
With 91 octane gasoline the engine controller can run more ignition advance which starts the combustion process at the more optimum time and thus obtain more mechanical energy from the chemical energy of the burning gasoline/air mixture.
This is important. Emissions are lower and fuel economy is better. If you switch to 87 while the initial cost is lower you will find the engine burns more 87 than it would under the same operating conditions as with 91. The difference will have you buying more 87 and often enough to wipe out the initial lower price of the 87 octane gasoline.
There's more bad news. With a lower grade of octane the engine controller will have to dial back the timing to avoid detonation. This lowers the pressure in the combustion chamber which eliminates detonation but raises exhaust gas temperature. The mixture starts burning later in the power stroke and stops burning later. Thus subjects all components exposed to the exhaust gas to higher operating temperatures. This includes the exhaust valves, O2 sensors, converters. All very expensive items to replace if they go bad.
You think because if you drive the car "easy" you can avoid detonation and the ignition timing retard? Nope.
Detonation is not a WOT only problem entirely. At part throttle at relatively low RPMs and in a higher gear -- which is with the automatic transmission equipped cars is how the transmission shift map works -- cylinder filling is quite good. As a result combustion pressure can be quite high. So driving around town easy with 87 octane gasoline and thinking one is doing no harm is just the opposite of what is going on.
High compression is a very good thing. The power it takes to compress the air/fuel mixture is almost all regained by the initial push back of this against the piston. If the spark can be triggered at the right time the mixture burns and develops pressure earlier in the power stroke which pushes down on the piston at the most optimum time. Running the correct octane grade allows the engine controller derive maximum benefit from the high compression.
Best I can in some way detail how this works is if you ride a bicycle you just know when is the best time to apply pressure on the pedal to get the most out of your leg muscles. If you delay applying pressure -- which is what happens with low octane gas and the engine controller retarding ignition -- you get a very good idea of how inefficient this delay in applying pressure really is.