OCTANE RATING

[Dick #44978]

Does anyone know if you mix 2 different octane fuels does the octane become the average of the 2 fuels? 93oct+100oct=96.5 octane rating.

[Rob A.]

Re: OCTANE RATING

Dick,

Heard a rumor(years ago) that if you mix high and low octane together, you actually create one higher than the one used in the mix...have no idea if it's true.

[John H.]

Re: OCTANE RATING

Most gas stations that sell three grades (regular, mid-grade, and premium) only have two underground tanks (regular and premium); the mid-grade is created in the pumping system by blending those two.

[Mark #28455]

mixing leaded with unleaded

The rumor about mixing gas and getting a higher octane came from mixing leaded with 93 octane unleaded.

Don't know if it's an urban ledgend, but it goes like this - 93 octane unleaded is more highly refined than the base used to make the racing leaded gas, so if you add a couple gallons of leaded to the 93, the added lead (4 grams per gallon of cam2) raises the octane of the 93 unleaded by a bunch.

Mark

[Dale S.]

Re: OCTANE RATING

This is no rumor, however it is a very small percentage. Dr Dean Hill and I proved this, years ago at N.M. State Univ. We mixed (and I dont remember the exact octanes) but for demenstation purposes 100 octane and 90 octane. It came out to about 95.45. Today if they are mixing the top grade with the lowest grade your best buy dollar wise for octane is Medium grade. We had a single cylinder engine with a variable compression ratio, and used fuels like Chevron white pump etc. The reason I used 100 octane in the example was I remember the Chevron white pump was 100 octane. Dale

[Duke W.]

Re: OCTANE RATING

If you mix two non-leaded fuels, the approximate octane of the blend will be the weighted average of the two.

If either is significantly leaded it gets a little more complicated due to the decreasing marginal effectiness of TEL, but the blend will probably be a little higher than the computed weighted average.

Duke

[Duke W.]

Re: mixing leaded with unleaded

Urban legend. It's about the decreasing marginal effectiveness of lead as a knock suppressant.

Say you mix a gallon of iso-octane (100 R0N) that has 4 grams/gal. TEL, which is (by definition) 104 octane with a gallon 90 RON unleaded base. This is equivalent to the 90 octane with 2 grams/gal. and the isooctance with 2 grams/gal, which is 102 RON.

Adding the 2 grams of TEL to a gallon of 90 will yield about 94, so the average is (94+102)/2 = 98 RON versus the simple average of 97.

Duke

[Doug Flaten]

Re: OCTANE RATING

Last year a station near me used to carry 94 octane that had a price differential of about 27 cents per gallon over 87 octane regular. Just prior to the hurricanes they switched brands and they stopped carrying the 94 octane. But they started carrying 100 octane priced at least $1.25 more than the 87 Octane. So if I were to blend equal amounts of regular and 100 octane, it will only be 93.5 octane and would carry a 62 cent price differential over the regular gas. I did not realize how good I had it till they did away with the 94 octane. I drove by there last night just to check, and it appears they now stopped carrying the 100 octane, so 91 octane is the highest they have now.

[Joseph R.]

Re: Octane Rating

Reading this begs the question, if you have access to a higher octane gasoline, why would you want mix and end up with an overall lower octane? My guesstimate is for an overall lower price per gallon. And what about proportions, say, 19 gal of base + 1 gal of highest octane = the middle octane rating?? Or is this formula extimate base on 50/50 mix???

[Doug Flaten]

Re: Octane Rating

Yes in my case, my Pontiac runs better on a higher grade fuel like 94 octane. Currently with 91 octane, it is pinging again and I'll have to retard the timing some more. That makes the car's response is a little sluggish. Plus, the retarded timing compounds my problem with a hot running engine (due to an old radiator). I did not need to fork over the money for 100 octane if it ran good on 94 octane.

[Ray C.]

Re: Octane Rating

Could some one please explain how the higher octane is achieve? The person that rebuilt my engine told me that additives are used to increase the octane and that some additives increased performance and were better for the engines than others.

Thanks Ray #36314

[Duke W.]

Re: Octane Rating

Gasoline is a blend of hundreds of hydrocarbon species and each has an octane rating. What you get in a given blend is basically and average of everything in the blend. Back in the days of TEL a few grams per gallon could increase the octane rating several points, but no more.

Modern gasoline blends walk a tightrope between vapor pressure, limits on certain hydrocarbon families such as aromatics, and octane, but increasing the octane of a given blend with TEL is not longer available except for Avgas which only has about 0.5 gram/gallon or "off-road" blends like race gas.

Commmercial octane enhancers are mostly a waste of money, and are mostly alcohol. Most of the common alcohols have over 100 octane, but many gasolines already have enthanol and adding more can increase fuel system corrosion, and a pint of alcohol in 20 gallon of gas isn't going to have much effect on octane.

If your local pump gas doens't have enough octane to keep the engine out of detonation, the only really effective choice is to blend in some race gas or avgas. As little at 10 percent may do the job, and usually no more than 25 percent unless you have a L-88.

Duke