[trube78]

A gallon of gas whether high test or regular has the same amount of energy. The engine of your Boxster will convert the chemical energy of the gas to mechanical energy to move the car. Porsche designs its engines under the assumption you will use 93 or 91 Octane and sets ignition timing to ensure there is no knocking and the car gets the max power and fuel economy for the expected Octane. When you use lower Octane fuel the car will have a tendency to knock so the knock sensor will sense the beginning of engine knock and tell the ECU to adjust (retard) timing to protect the engine. Eventually if you consistently run low test the ECU will automatically assume there is something unusual and either automatically retard timing or spit out an error code (according to some Porsche Gurus the ECU will adjust after about 25 to 30 hours but I have never seen it in writing.) That assumes all of the protective features work proberly. If not, for example if you go full throttle and there is no early indication of knock you could knock before timing is retarded and harm your engine.

Since the car will frequently have retarded timing while running 87 Octane, it will not be operating at peak efficiency and thus produce less power and worst Gas Mileage. Cars that are designed for regular will see no improvement by using high test. However, many chip tuners get their best results by programming for 93 Octane vice the 93 or 91 octane spec called premium in various states and recommended by Porsche.

ATB.
Tom