Octane numbers are something we see every day and probably take for granted. We all know this is a rating means to measure a fuels resistance to pre-ignition, but how does that apply in a practical sense. Let's first assume you have a car that runs well on 91 octane. What could you gain by altering the fuel you fill up with. By going down to 87 octane you will achieve better mileage, but could risk detonation depending on your cars engine. By going up to 93 octane you might allow your car to advance the timing gaining horsepower. Due to lower BTUs per gallon you will give up a few miles per gallon. So right off you can see that it is a balancing act.
Looking closer at this, you notice that the extra horsepower comes from the ability of the car to advance timing and not the fuel itself. Higher octane fuel has less BTUs, but still nets power due to the timing advance and higher boost it can achieve. In modern vehicles with knock sensors the timing is constantly varied to achieve the best balance between performance and economy. Because this adjustment takes time, simply switch to higher octane at the track is not good enough. The fuel needs to be run in advance to allow the car to compensate for it. So using 91 all he time and then going to the track and filling up with 95 is not going to help your track times. You will have a faster car on the ride home though.
The other end of the spectrum is less octane. The down side here is the risk or pre-ignition and high EGTS. Both of these can lead to melted or bend pistons. Valves and the head is also placed in risk. If you have a modern vehicle it will detect this and retard timing to prevent damage. This timing modification is easy to detect with an OBDII scanner and it an easy to diagnose too fuel with too little octane.
Because of the above reasons, a lot of people turn to the boosters as a way of having both good fuel commonly and good performance at the track. The problem with them is most do not work. If you do find one what works, adding it to your tank a day before you go to the track can be beneficial. Despite the benefit, use boosters sparingly as most rely on MMT as a means of boosting the octane. Excessive MMT can cause problems with sensors, injectors, or even the exhaust.
By now you are probably confused again by what all of this means. To sum it up, if your car has no changes to compression, raised RPM limiter, or lots of boost, you should probably use whatever fuel the dealer recommends. If you have a race built motor, stick to a race fuel that meets the need of your engine. If you have a lightly tuned engine and enjoy the occasional track day, throw in a bottle of octane booster the day before and call it a day. Do not rely on boosters all of the time.
A review of several octane boosters can be found here: http://volvospeed.com/Reviews/octane_boosters.html.
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