Originally Posted by SSEimatt93
what is KR? Id love to put 87 or 89 in mine too, it costed me 55 bucks to fill it on Saturday with 91...
KR is knock retard. When the computer detects predetonation in the cylinders (by using knock sensors) it will pull timing from the car in order to get it under control. predetonation happens when a hot spot in the cylinder ignites the fuel charge before the spark from the spark plug does. When the two parts of the fuel charge ignited from 2 different sides (1 from the hotspot, 1 from the spark) collide this causes knock. Too much of this can destroy a piston. Higher octane is harder to burn so the its harder for a hotspot to set off the charge the reducing knock. Being harder to burn however means less explosion from the charge and thus less hp. However if your not getting knock due to higher octane then the computer can advance the timing of the car resulting in more hp so its kind of a trade-off. Make sense?
The advantage of high octane is that the computer can always advance timing when conditions are right, so your car can only perform better. Using low octane fuel with no knock is ok until something happens to offset the balance (you develop a hotspot, either through booststacking, or heatsoak) and the only thing the computer can do is pull timing to try to cool it down, this significatnly reduces your hp and can damage the engine. I for one will always use 94 or better. I could possibly get away with 91 but with my mods if something were to happen and the computer couldnt compensate then by-by engine.
This is why running lean in a supercharged car is so dangerous. More air and less fuel means its easier to combust so a hotspot can set it off a lot more easily. A custom PCM can add fuel to help make the fuel charge more resistant to predetonation but this also reduces hp. Getting the air/fuel mix just right so your not getting knock but not making the charge too fuel heavy is a fine line and a great place to try and tune the car.
Leaner = more horsepower but also closer to the edge where if something goes wrong it could be catastrophic for the engine.