| Since the oil companies raised gas prices I have seen | | | | There are many factors that affect octanes |
| more and more articles about the benefits of using | | | | requirements, and altitude is one of them. However, |
| high-octanes gasoline versus the question of whether | | | | the affect of altitude on octanes requirements is |
| it is a waste of money. Your primary considerations | | | | very slight on modern engines. |
| in choosing fuel octanes levels are the manufacturers | | | | Modern engine management systems can |
| recommendation and the compression ratio of the | | | | accommodate this adjustment, and in some recent |
| engine. As a very general rule, an engine with | | | | studies, the octane number requirement was reduced |
| compression ratio of 9.0:1 or over will benefit from | | | | by 0.2-0.5 (R =M)/2 per 300m increase in altitude. The |
| high octanes gasoline, and the higher the compression | | | | larger reduction on older engines was due to: |
| the more the engine will benefit from successively | | | | " reduced air density provides lower combustion |
| higher-octanes levels. The benefit is both in power | | | | temperature and pressure. |
| production and lack of detonation. | | | | " fuel is metered according to air volume, |
| Modern engine-management systems use knock | | | | consequently as density decreases the stoichiometry |
| sensors to detect engine detonation (aka pinging, | | | | moves to rich, with a lower octanes number |
| knocking), which then transmit the information to the | | | | requirement. |
| electronic control unit, (ECU) which in turn retards | | | | " manifold vacuum controlled spark advance, and |
| ignition timing to compensate for octanes deficiency. | | | | reduced manifold vacuum results in less spark |
| The trade-off, unfortunately, is a reduction in power | | | | advance |
| output. One way to offset this power loss is by | | | | Another factor is, U.S. oil companies tend to use |
| installing performance exhausts, high-grade cold air | | | | better detergent additive packages in higher-octanes |
| intakes and quality air filters. Performance exhaust | | | | fuels even though detergent is blind to octanes level. |
| systems give about 1 to 2% increase in fuel | | | | Many engines have a long suffered from loading up |
| economy as they reduce impurities in the cylinder, | | | | with carbon on the tops of the pistons, which is one |
| thus achieving a purer gas and oxygen mixture. Purer | | | | of many factors that can increase octane |
| burn results in performance boost and fuel efficiency. | | | | requirements, sometimes beyond the operational |
| Cold air intakes and performance air filters on the | | | | ability of the knock-sensing system to compensate, |
| other hand, supply the engine with more quality air | | | | resulting in detonation. So, from the stand-point of |
| which improves engine power and in turn enhances | | | | your engine, you should be running a high-quality |
| the vehicles fuel economy. With an automatic | | | | gasoline additives periodically regardless of the octane |
| transmission, low-octanes fuel can also result in a | | | | fuel you are using along with upgrading the quality of |
| distinct and very noticeable hesitation when | | | | air that is coming into your engine through your air |
| accelerating off idle, this is when the knock sensors | | | | intake system as well as leaving the engine through |
| sense the detonation and the ECU retards timing | | | | your exhaust system. |
| abruptly to prevent engine damage. | | | | |