An octane booster is a product which is designed to raise the octane rating of fuel. Many octane boosters come in lavish packaging which makes extravagant claims about their effectiveness. Testing of octane boosters in controlled settings has suggested that these products are actually not terribly effective, with a few exceptions, and that if people want high octane fuel, they should simply pay for it. Since many octane boosters are quite expensive, it's often also more cost effective to buy high octane fuel than it is to use an octane booster, even a product that works.
The idea behind octane boosters is that by increasing octane in certain types of engines, people can increase power, and sometimes vehicle efficiency. This only holds true for high compression engines, however, such as those found in sports cars, and a number of factors can influence vehicle performance, which means that adding an octane booster to the fuel in an average car may not create a very demonstrable effect.
Most cars come with specific octane ratings from the manufacturer. The engine, timing system, and other systems in the car have been calibrated to work with gas in a specific octane range. Sports cars, performance cars, and race cars tend to be designed to run on higher octane fuels, because these fuels can generate more power. The higher the octane rating, the greater the resistance to preignition, which translates into an ability to increase the compression ratio in the engine, generating more power.
The computer systems in cars are usually designed to make minor adjustments to deal with changes in octane rating and other conditions. This means that high octane fuel needs to be regularly used to get good performance, and that an octane booster needs to be added well in advance of the time that it is needed. Someone trying to boost the octane of a car used for racing, for example, couldn't add the octane booster at the track, because the engine would not have time to adjust.
There are a number of concerns about octane boosters which consumers should be aware of. The claims on such products are not verified, and statements like “adds up to eight octane points” can be very misleading. Consumers might think that this claim turns 89 octane gas into 97 octane, but in fact the “points” are usually expressed in 10ths, so the octane booster is really capable of increasing the octane up to 89.8 octane, and often much less in practical conditions, which will not generate much of an improvement. Consumers who are interested in using such products should ask for recommendations from mechanics or enthusiasts who work with cars similar to those handled by the consumer.