As I was summoned, maybe I can explain.
The burn rate has nothing to do with the octane. Your dealer is wrong. Any car, using any octane fuel, will carbon up if driven 'gingerly', but we're talkin' lil ol lady style here, not normal street driving at legal speeds. This is because the fuel is not completely burned. Incomplete combustion prevents carbon from combining with oxygen to form COČ and leaving the engine through the exhaust. Instead, it remains to foul the valves, plugs and oil. And there's LOTS of carbon - 80% of every gallon of fuel is carbon - about 5 lbs./gal. !
Octane is simply a rating assigned a fuel mix. It is the point where the fuel will spontaneously combust due to being compressed, compared to a 'known' combination of 2 of the components of gasoline - Octane and Heptane.
Low octane fuel will spontaneously combust under less pressure, whereas higher octane fuel can withstand greater pressure before it too spontaneously combusts.
Lower octane fuel usually (but not always) actually contains more energy because some of the actual gas is replaced by octane boosting chemicals in the higher octane fuel mixes. And, lower octane fuel (because of it's greater % of actual gasoline) also usually contains more carbon.
It's a popular misconception that higher octane fuel is somehow '
better' than lower octane fuel. This probably comes from the fact that higher octane fuel has mostly been labled as 'Super', 'Premium' or 'High Test' which are just marketing phrases. It's not 'better' at all, just DIFFERENT - intended for different applications - if you don't need it, it's not at all better.
The Boxster engine is a high compression engine with a compression ratio of 11:1. This is very high, especially compared to a Honda Civic with a CR of 9.8:1, or most production cars which have a cr in the 7.1:1 - 8.5:1 range. Jaguar once made a head which created a 14:1 cr, but this was later reduced to 12:1 for production models due to reliability issues. 12:1 is about the highest you'll see in a production engine. After 14:1, there is no fuel with an octane rating high enough to prevent detonation and you're getting into the realm of diesel engines which actually use compression, not a spark plug, to ignite the fuel. These are usually in the 20:1- 22:1 range.
A little side note: Compression ratio is just a comparison of the volume of the cylinder at the bottom of the piston stroke vs the volume of the cylinder when the piston is at the top of it's stroke. As a simple example, let's say a cylinder has a volume of 1000cc at the bottom of the piston stroke and a volume of 100cc at the top of it's stroke. This means the gasses (air/fuel) have been compressed tenfold and the cylinder is said to have a 10:1 compression ratio.
Well, when you compress a gas, you also heat it. At some point, this gas can get hot enough to ignite and burn uncontrollably, as in before the spark plug tells it to burn. So, in order to control this, the fuel must resist combusting until compressed to the smallest volume possible but not burn until it gets the added energy of the spark. This is done to release and capture the maximum amount of energy from the given volume of fuel and air. It's also why higher compression engines need higher octane fuel - to reduce igniting uncontrollably under pressure.
A high compression ratio is desirable because it lets a motor extract more chemical energy from a given mass of air and fuel due to its higher thermal efficiency and turn it into mechanical energy. High cr's place the available oxygen and fuel molecules into a reduced space and add the 'adiabatic' heat of compression. This creates better mixing and atomization of the fuel droplets. This is good because the fuel is just ready to ignite as the piston approaches it's topmost position where it allows increased power at the moment of ignition and the extraction of more useful work from that power by the expanding of the hot gasses. In the small amount of time allotted (about .002 - .004 sec.), you want the fuel to burn completely so all it's chemical energy is released. If not, some 'energy' ,in the form of unburned fuel is wasted, being expelled from the cylinder during the exhaust stroke.
