# Regular or Premium Unleaded?



## F1Crazy (Dec 11, 2002)

Epi330 said:


> In USA the highest you can get at regular pump is 93 (and in California 91 is maximum). However, our 91/93 is called "premium" - probably, in your country this number is calculated differently? I do recall different formulae used in US vs Europe, but don't remeber the details.


Basically we have 2 octane ratings: Motor Octane Number known as MON and Research Octane Number known as RON. Methods of testing are different and RON method is not indicative of the performance of the fuel under all conditions but it produces higher numbers and is often used for marketing purposes. RON rating is used in England and most of the Europe and was used in US years ago. 
US adopted AKI (Anti Knock Index) which is the average of the RON and MON (R+M/2).

The closer the RON is to the MON, the more stable the fuel is and it is best if MON is not lower than 10 points below RON. This is very important in engines with compression ratios over 10:1.


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## RKT BMR (Sep 7, 2002)

Nick325xiT 5spd said:


> Assuming that TRICK is pure toluene, 1.5 gallons mixed into a 15 gallon fillup would get you to 93.9 octane.


Price out 1.5 gallons of TRICK (or any of the other octane boosting additives), and get back to me :bustingup :rofl:  (been there, done that :wow


Nick325 said:


> Now, the real question is whether or not it's bad for your engine to put that stuff in your gas...?


Not from what I found researching it. Toluene is a common component of all gasoline. Indeed, varying the proportion is the primary means of adjusting octane in unleaded gasoline.

In the old days, octane rating was modified primarily through lead additives.


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## RKT BMR (Sep 7, 2002)

racerdave said:


> Or the cat...


Should have no effect on the cat, unless you dip him in it.

Nor a catalytic converter either, so long as the engine is running a proper stoichiometric combustion. Unless there is something wrong with your engine management system, it will be.


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## RKT BMR (Sep 7, 2002)

Soupcan325i said:


> Just to buck the trend, has anyone actually seen a peer-reviewed academic study on the benefits of premium vs. regular gas (adjusted by make/model of car to account for different engines, compression ratios, etc.)?


Not in the form that you're asking about here. However, I've researched the issue quite extensively as a consequence of adding a blower to the RKT, and then having some knocking under heavy load in the hot summer.

My investigation in to this confirmed something I've heard anecdotally many, many times: There is absolutely no benefit to using higher octane gasoline than is necessary to prevent knocking (a.k.a. detonation a.k.a. pre-ignition), other than lightening your wallet for the gasoline company.

Octane is simply a measure of the activation energy of the fuel. IOW, how much energy it takes to initiate combustion. Once it starts, the energy release of the combustion process is far in excess of that necessary to activate further combustion, so the whole batch in the cylinder burns up.

Higher octane fuels require a greater input of energy to get started. Another way of looking at it is a higher temperature initiating event. In a gasoline motor, the heat to fire off combustion comes from an electrical spark, which is WAY hotter than the activation energy of even the highest octane fuel.

However, air heats up when it is compressed in the cylinder on the compression stroke. Under certain conditions, it may be hot enough to ignite low octane fuel immediately when it is injected -- in advance of the spark.

So, all that said, there is really nothing "premium" about higher octane gas. It's not "better" in any meaningful way from the standpoint of energy density, emissions performance, etc. A gallon of 91 has the same energy content as a gallon of 87.

It is only better if your motor is experiencing detonation as a result of the lower octane gas. This is damaging to a motor. To compensate, virtually all modern motors have knock sensors and can compensate by adjusting timing parameters to stop the pre-ignition, but at the expense of power and emissions performance. So, under these circumstances, higher octane gas will give you better performance, but as a side-effect of engine management activity, not because of something inherently better about the gas.


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## RKT BMR (Sep 7, 2002)

Found one of the links from my research on this a year ago: Octane.

I'll keep looking around to find some more. It's been a long time, and I didn't save the links away like I should have.


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## ctbmw (Aug 30, 2003)

Good one blueguy-aiming in the same direction myself.
-M


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