By Todd A Schafer

Since the oil companies raised gas prices I have seen more and more articles about the benefits of using high-octanes gasoline versus the question of whether it is a waste of money. Your primary considerations in choosing fuel octanes levels are the manufacturers recommendation and the compression ratio of the engine. As a very general rule, an engine with compression ratio of 9.0:1 or over will benefit from high octanes gasoline, and the higher the compression the more the engine will benefit from successively higher-octanes levels. The benefit is both in power production and lack of detonation.

 

Modern engine-management systems use knock sensors to detect engine detonation (aka pinging, knocking), which then transmit the information to the electronic control unit, (ECU) which in turn retards ignition timing to compensate for octanes deficiency. The trade-off, unfortunately, is a reduction in power output. One way to offset this power loss is by installing performance exhausts, high-grade cold air intakes and quality air filters. Performance exhaust systems give about 1 to 2% increase in fuel economy as they reduce impurities in the cylinder, thus achieving a purer gas and oxygen mixture. Purer burn results in performance boost and fuel efficiency. Cold air intakes and performance air filters on the other hand, supply the engine with more quality air which improves engine power and in turn enhances the vehicles fuel economy. With an automatic transmission, low-octanes fuel can also result in a distinct and very noticeable hesitation when accelerating off idle, this is when the knock sensors sense the detonation and the ECU retards timing abruptly to prevent engine damage.

 

There are many factors that affect octanes requirements, and altitude is one of them. However, the affect of altitude on octanes requirements is very slight on modern engines.

 

Modern engine management systems can accommodate this adjustment, and in some recent studies, the octane number requirement was reduced by 0.2-0.5 (R =M)/2 per 300m increase in altitude. The larger reduction on older engines was due to:

 

"reduced air density provides lower combustion temperature and pressure.

 

"fuel is metered according to air volume, consequently as density decreases the stoichiometry moves to rich, with a lower octanes number requirement.

 

"manifold vacuum controlled spark advance, and reduced manifold vacuum results in less spark advance

 

Another factor is, U.S. oil companies tend to use better detergent additive packages in higher-octanes fuels even though detergent is blind to octanes level. Many engines have a long suffered from loading up with carbon on the tops of the pistons, which is one of many factors that can increase octane requirements, sometimes beyond the operational ability of the knock-sensing system to compensate, resulting in detonation. So, from the stand-point of your engine, you should be running a high-quality gasoline additives periodically regardless of the octane fuel you are using along with upgrading the quality of air that is coming into your engine through your air intake system as well as leaving the engine through your exhaust system.