Gasoline Octane Numbers

Date: Wed, 3 Jan 96 17:25:38 PST
Subject: Re: 93 Octane
From: johnw@acuson.com (John Williams)

Dave Schurr writes:
[regarding carb farts in the winter]

>A friend who rides all year round told me that in colder weather he
>burns lower octane fuel to eliminate this problem.  I gave it a try
>and the problem disappeared.  Seems the 93 octane has a lower ignition
>temp and wasn't allowing the engine to warm completely(??).
Naw, nicht, not. I don't know where these misconceptions about gasoline grades comes from, but let me assure you, all unleaded gasolines burn at the same temperature, regardless of octane rating. The only exception to this would be gasahol.

Gasoline octane numbers are adjusted by two methods: adding anti-knock compounds such as tetraethyl lead (now illegal in many states) OR using higher refining temperatures and pressures to reform the hydro- carbon molecules into more knock resistant compounds. Obviously, unleaded gas uses the second method, which is why unleaded gas costs more than leaded.

Let it be known to all, that regardless of the octane, there is more or less the SAME amount of energy (about 115,000 btu/gal) in gasoline grades. Octane numbers are adjusted by tweeking the proportion of about a hundred different hydrocarbon compounds that form gasoline. This adjustment does not effect the combustion temperture, just the fuel's resistance to knock.

There IS a big difference in the blending of fuel depending on the climate and season. One of the big things they control is vapor pressure. This is to reduce hydrocarbon pollution from evaporation in the summer. Also, they add a lot of butane to the gas in winter to allow the engine to start.

Also, in many areas, they now add oxygenates to the gasoline to lean the mixture in the winter, which also reduces hydrocarbon pollution. These oxygenates (MTBE, Ethanol, and sometimes Methanol) will definately affect engine performance, most notably fuel economy. In *theory*, there is only 2-3% less energy in oxygenated fuel, but I sure as hell notice more than a 3% drop in fuel economy with oxy-gas. Here in the Bay Area of California, there has been a very noticable drop in smog in the winter, so I guess the oxy-gas deal is working.

Back to carb farts and octane, there has to be something else going on, as the combustion temp theory is boloney. Gas companies often add other compounds, such as deposit control and anti-icing agents to the high octane grades, mainly as a sales gimick to get you buy the high priced gas whether you need it or not. It is *possible* that this might be causing the farts in cold weather. I tend to use 87 octane MOST the time, and run a tank of 92 octane thru every fourth tank or so to clean out deposits.

It is also possible that in the winter, you might need to tweek the idle mixture screw a little to help cure the carb farts. Air temperature can also affect the burn *rate* in the combustion chamber, and thus require tweeking the ignition timing. This again has to do with gasoline volitility in the winter. Oh well, who the fuck knows? I've rambled too long on this thread, don't ya think? ;^)

Later,

John Willie
San Jose, Left Coast, USA

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