93 Octane??

That's interesting, but as you say is pretty old. My understanding is the vapor pressure and octane aren't necessarily correlated. For example, in the northeast we have summer and winter gasoline. The octane ratings are the same for both seasons, but the gasoline has a different RVP in winter to be more volatile and thus help starting at cold temps. Lead was one of the earliest additives to enhance octane rating and it doesn't change the RVP. Maybe the newer octane rating additives do have a significant affect on RVP, but since our octane ratings don't change from summer to winter, there apparently are ways to modify RVP independently of octane rating.

Matt

It is from the post lead era, but from a time when many Fords used speed density (no MAF) / bank fire injection systems. I think these were less tolerant of less than ideal gasoline.

Chevron has an interesting web site if you want to read up on gasoline:

Mobil also has some good information (although a little more pro premium than Chevron):

Ed

I have never heard a credible source warn against using higher octane fuel in a car designed to use a lower octane fuel. Octane is simply a measure of the volatility of the fuel. Higher octane = less volatility and slower/cooler burn with more energy per molecule of fuel. Higher octane is also more resistant to pre-ignition/detonation than "the cheap stuff." The net effect is that combustion chamber temperatures are lower with higher octane fuel. Another benefit is that more air/fuel can be mixed in the chamber (under higher compression) resulting in higher power output for a given combustion chamber volume. That's why most forced induction applications (turbocharged/supercharged) tend to spec high octane fuel.

As for the methods of raising octane, there really are only 2 that are widely used for unleaded fuel. You can add MTBE (methyl tertiary-butyl ether) or alcohol. In many states (like NY as of the end of the month) MTBE has been banned so chances are that your fuel is a mix of petroleum distillates and ethanol to achieve the desired octane.

Other than unecessarily lightening your wallet, there should not be any reason to avoid higher octane fuel if your car doesn't call for it.

Cheers,

C

That WAS a problem at one time, when alternatives to lead were first being experimented with to obtain higher octane ratings than 85-87 (R+M)/2. I would date this to probably the mid to late 1980s, just guessing.

The fact that early "high octane" (what a joke!) unleaded fuels had this problem has in fact lead to a wide-spread misconception that "octane" and "volatility" are related, and that "high octane" fuel is "less volatile" than low octane fuel. In general, that is NOT true at all. Octane rating is a measure of resistance to self-ignition, that is all. Volatility has no fundamental bearing on the matter. If you were to take two gasoline base stocks and raise the octane of one sample by using, say, tetraethyl lead or MMT and leave a second sample alone, the two samples would have the same volatility (propensity to vaporize at a given temperature and pressure). However, if gasoline makers use aromatic hydrocarbons (or non-hydrocarbons for that matter) to increase the octane rating instead of lead or manganese, then the volatility may be adversely affected, as it was in 80s era fuels. Today, most gasolines except the EPA-mandated special brews seem to be very well compensated for volatility and octane. Using a higher octane rating might be a waste of money, but with modern fuels it is harmless. And in fact *some* makers put other goodies like detergent and lubricant additives in their higher octane fuels, so there may be some advantage to the fuel system even if the octane rating is of no benefit.

No. Octane is simply a measure of a fuel's resistance to spontaneous, uncontrolled ignition under specific conditions. The measure of fuel volatility is called Reid Vapor Pressure (RVP).

No. RVP (i.e. volatility) and octane do not have a causal relationship with each other in either direction. There are plenty of low-volatility, high-octane fuels, and there are plenty of high-volatility, low-octane fuels.

No. High-octane gasoline burns at the same speed as low-octane gasoline, as long as preignition ("ping", "knock", the uncontrolled spontaneous ignition referred to above) is not encountered. And the burn is at the same temperature, not "cooler".

No.

This is not an "also", it is the defining characteristic of high-octane fuel.

This is only the case if preignition is encountered in a particular combustion chamber with lower octane fuel. If it is not, then this statement is false.

No. The octane rating of the fuel does not affect the amount of air/fuel mixture that can be introduced into the cylinder. This is a function of engine and induction system design.

No. There are a great many refining techniques and chemicals that are used to increase the octane rating of unleaded fuel. The addition of MTBE and "alcohol" (ethanol) are only two of them.

Chris, you *really* need to read the Gasoline FAQ.

DS

Apparently so.

Pretty good guess. It was from 1991.

Ed

No, this much more comprehensive one:

DS

My 2004 mini-van from Chrysler comes with information in its owner's book advising against the use of high octane fuel.

Dan correctly noted that there are many ways to increase octane and some of those ways introduce additives and cylinder conditions that are not within the design limits of a motor not designed for to run on such fuel.

Richard.

Whoops, those are not my words, nor an accurate paraphrase of them.

DS

I intended my attribution to your statement to be limited to the following: (Sorry if that was not clear). My additional statement is based upon my experience one year at Mobil's Research Lab and as the individual who helped establish the NYS Fuel Quality Assurance Program.

Richard.

"There are a great many refining techniques and chemicals that are used to increase the octane rating of unleaded fuel. The addition of MTBE and "alcohol" (ethanol) are only two of them."

DS

Yep, that works. Thanks.

DS

Steve, Does higher octane fuel burn slower (slower moving flame front) - less like an explosion than lower octane?

Bill Putney (to reply by e-mail, replace the last letter of the alphabet in my address with "x")

In addition, the volatility of the fuel is altered depending on season to ensure less volatility in the summer, and acceptable performance in the winter. Of course this assumes you are in a place with real seasons. :)

It's mainly harmful to the wallet. :) Gasoline with higher antiknock index numbers ('octane') can give you lower mileage if your engine isn't designed to take advantage of it, since the antiknock index increasers can contain less energy per unit than gasoline, which they displace.

I don't think this is correct. RVP is the measure of volatility and octane is an indirect measure of the speed of the combustion wavefront. Not the same thing at all.

In the northeast our volatility changes dramatically from winter gas to summer gas, yet the octane ratings remain the same.

I agree with your last statement here. I know of no harm from using higher octane fuel other than to your wallet. Maybe there is harm, I've just never seen it documented or supported by data.

Matt

Did it say why?

Matt

Do you have a reference for this? Hadn't heard this before, but I'm not saying it isn't true. However, I would like to see a reference to read about this.

Matt