Strictly speaking, DSMs do not require premium fuel.  However, the car was designed to accept higher-octane gasoline for a reason.

Octane is not a measure of the amount of energy in gasoline - rather, it is a measure of how well the fuel resists preignition, also known as detonation or 'knock'.  Detonation is very hard on the engine, and must be avoided.  The DSM cars incorporate a knock sensor specifically for the purpose of detecting knock.  Higher-octane gasoline resists knock better than lower octane gasoline.

During normal operation, the engine control unit (ECU) adjusts the engine timing as far forward as it will go.  This provides maximum power, but also increases the possibility of knock.  This is especially true for turbocharged or supercharged engines, such as the 2.0L turbo 4-cylinder found in the upper model DSMs.  Note that the ECU hears knock all the time - the problem has to be persistent to generate a reaction from the computer.

If the ECU detects excessive, continuous knock via the knock sensor, the engine timing is retarded until the knock goes away.  This results in a power loss, but saves the engine from damage.  So, if you fill up with 87 octane gas, your ECU will be forced to retard the timing quite a bit, and you will not get the rated power from the engine.  Not only that, but the ECU may also be forced to limit turbo boost levels to accommodate the lesser quality gasoline, which again means a significant power loss.  And yes, the ECU does have the power to do that - see "My boost is not strong enough.  Is there a fix?".

Once you switch back to premium, the ECU will eventually advance the timing back to original levels - after a while.

For full details on this and the role of the boost control solenoid (BCS) refer to the September 22nd edition of The Diagnostic Port, by Technomotive.

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The true way to answer this question is as follows: analyze gasoline samples from all the prospective gas stations in your area for a period of three to twelve months, then pick the consistently best gas.

In other words, this is an imposible question to answer. Gasoline quality and formulation vary enormously over the continent. Leaving aside race gas (100+ octane) there is no real way of telling which gas is the 'best' in your specific area. Just pick a high-octane brand and stick to it unless you have a problem.

For those unsatisfied with this answer, read the Gasoline FAQ (alternate link here), and seek out information from other local DSMers. Don't ask the Digest - somebody in California isn't going to be able to help you if you live in New York.

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DSMs will realize some performance benefits from ultra-high octane gasoline.  The extra octane will allow the ECU to advance the engine timing more.  Some people report significant gains with race gas, while others describe only minor improvement.  The prohibitive cost of these fuels generally restrict their use to drag racing applications only.

Note that this applies only to true racing gasoline, and not pump gas plus octane boosters. For more information, read the very detailed Gasoline FAQ (alternate link here), and this essay on high-performance gasolines.

For those who can't afford "straight" race gas, try using the gasoline blending chart provided by Osborn's Auto. Be certain to read the article that accompanies it.

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You can use leaded fuel, with the following caveats:
-  leaded fuel will quickly destroy the catalytic converter.
-  leaded fuel will damage and eventually destroy the oxygen sensor.

Leaded gasoline also leave white deposits on the O2 sensor and housing; some mechanics may recognize this as being from leaded gas, and deny your warranty for a new oxygen sensor.

Some people also believe that aviation gas is dirtier than normal pump gas.  Opinion on this is divided, with some people saying it is and others saying it isn't. Max Burke reports that aviation gas has none of the detergents usually present in automotive gas. This could be called a 'purer' fuel, but since it can't clean the engine as well, the engine might get dirtier inside as a result.

The most popular AVgas is 100LL (100 octane low-lead mix), which nevertheless still contains at least as much, and possibly more, lead as normal leaded automotive gasoline.  The "low-lead" rating is in comparison to other aviation fuels; this amount of lead WILL wreck the above mentioned auto components.  One Digest member suggested adding the Alcor TCP additive to the avgas to help prevent lead buildup in the engine.  Consistent use of leaded gas or AVgas will probably lead to significant clogging of spark plugs and oxygen sensors.

Except for the above details, it appears that for practical purposes AVgas can be considered the same as normal leaded automotive gasoline. According to the Gasoline FAQ (alternate link here), even 100LL has more octane than most auto gasolines, so you will get the same performance benefits as with unleaded race gas, as described above.  Also, some people can get aviation gasoline much cheaper than typical race gas prices, making the prospect more attractive. For more information, read the very detailed Gasoline FAQ (alternate link here), and this essay on high-performance gasolines.

For those who can't afford "straight" race gas, try using the gasoline blending chart provided by Osborn's Auto. Be certain to read the article that accompanies it.

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Many people use them.  No concrete drawbacks or benefits have surfaced despite the heated debate that surrounds these products, making them appear similar to highly publicized 'magic products' (some of which are listed here).  As with 'magic' products, virtually no experimental data is available to either confirm or deny the claims. For more information, read the very detailed Gasoline FAQ.

The only legitimate exception to the above paragraph may be gasoline additives that incorporate the tetraethyl lead additive, or TEL. TEL was the original 'lead' in leaded gasoline, and has since been banned from pump gas in North America. TEL was used because of its anti-knock properties, which have been well documented. TEL additives can still be purchased from Kemco Oil & Chemical; their effect on DSM engines is unknown, but presumeably TEL additives will have the same problems as leaded race gas.

Formulas for do-it-yourself octane boosters have been circulating various automotive lists for a while now, including the DSM list.  Read all about them in this post from 1996 - there are some interesting 'facts' on commercially produced octane boosters as well.  GS-Xtra claims original publication of the formulas, which are also listed on in the maintenance section of the Vintage Triumph Register home page and the Team.net Scions of Lucas (SOL) Technical section on fuel and octane boost formulae - they got it from Vettenet. Mario "Porsche Killer" has added his day-glo version of the homebrew formulae here, and there used to be a link in the (now amalgamated) F-Body organization page.

Although this information obviously gets around, there are no known records of anybody actually mixing and using their own octane boosters.

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Can I use gasoline tablets / fuel line magnets / other gasoline savers in my car?  Are they worth it?

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Don't bother - they are just more 'magic products'.

Don't believe me? Read what the FTC has to say on the subject.

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Maintained by Sean Costall. Changes and suggestions are welcomed!  If you have any information on the answers to any of these questions or wish additional questions, please mail me.

This page is an extension of Club DSM .