E60714

I've heard some people say that getting higher octane gas is pointless unless your car is "tuned for it." What does that mean? Aren't there clear performance gains from using 93 over the 91 CA residents use? Why won't 94 (if available) be better? I asked someone about using race gas in particular and they told me it would do nothing unless my car was tunes for it. I didn't think you needed a mod to use race gas or higher octane fuel in general.

I figured all cars benefit from higher octane fuel, not just turbocharged ones.

jfk92

Octane rating is an indication of the burn rate and detonation point of the fuel mixture. Period. Octane rating DOES affect the motor/power performance but here's how. A 'standard' engine only "requiring" 87 Octane will experience minimal if any extra power/performance by using 93 Octane. However a 'tuned' (meaning an engine that was designed for 91 or higher octane) requires the higher octane - higher detonation pressure/temperature and slower or faster burn rate WOULD run sh*tty on 87 octane b/c you would experience pre-detonation of the gas in the engine cylinder. When that happens you hear the 'engine knock'..this is actually the fuel flaming up before the piston reaches top dead center (or the point in the stroke cycle for which it was designed) and the mechanical force of the crank/rods still force the piston upwards despite the fractional seconds of predetonation/burning of the fuel...that 'knocking' is the piston against the walls and really any other part of the mechanical engine drive that has any play and is effected by the forces.

Basically, the timing of the the plug spark in conjunction with the stroke (compression of the piston) location as well as the octane rating (think detonation pressure/temperature and burn rate) are used to optimize the power output of the engine. You obviously want to have no forces working against you while you are compressing the fuel/air in the piston by having the mixture explode 'early' (hate it when that happens...honey, I swear it's never happened to me before... ) and use up some of the power on the up stroke of the piston. You want the fuel to be totally compressed, then spark it when the piston is just at that point in the cycle to stroke down and deliver ALL power from that cylinder explosion to your crank shaft, transmission and axles.

The burn rate, also a function of Octane, will determine how fast or how slow the fuel air mixture burns in the cylinder...which also effects the rate of delivery of the power to the piston head and then eventually to the wheels.

To take class a step further, you may hear drag race strip people chatting (or the buddy down the streen that has a distributor cap on his 69 camaro or another buddy that has the software hooked to his Honda to electronically remap the ignition for his street race) where they'll mention "Advancing" the timing or "Retarding" the timing....this means that they are changing the point at which the spark happens to ignite the fule, either "Advancing" it to happen prior to the TDC (top dead center - point at which the piston is at its highest in the cylinder) or "Retarding" it to happen after the TDC point of the piston. This is done to optimize current race or environmental conditions to maximize, again, the power output of the motor...for the conditions you are racing in....OR also to account for where you need your most power delivered...early in the race or down the strip..as in older cars...there was no 'on the fly adjustment, of timing by a computer in the car...you chose where you thought the power would help you most in the race...

Was this more than you'd ever hoped to hear about?! Hope it is helpful and clarifying.......

E92-Lighting

just dont use ARCO gas

BET530

^^^haha, simply put and so true!

pjinca

.............or Valero, or Thrifty, or any other cheap gas. Also, keep in mind your manual says Premium Fuel required.

infinityjk

this sounds daft but on my gas cap it states i can us btween 84 to 107 octane fuel so i presume the engine has different maps for different fuels......

jfk92

yes, the engine ignition mapping can adjust to different operating conditions and fuel ratings. can only assume there are also limits to where you will quickly damage it as well as slowly damage it....is the gas cap original to the car?! as most BMW manuals will specifically state use only 91 Octane and above....