mix 89 and 91 octane fuel to get an octane higher than 91??

[supergreen125]

ok ok... first of all.... i think this is total bs (which is why it's in the off topic forum.) i heard it from a mustang buff years ago and thought he was nuts. i actually watched the guy fill up with half 91 octane and half 89 octane. keep in mind that means he has to pay the cashier and walk back to his car twice... so he must have really believed this to invest the extra time after every 300 miles of driving.
i happened to mention this to one of my coworkers last week... today he walks into my office telling me that one of his friends randomly brought this same exact topic into a conversation, swearing up and down that it was true.
logic tells me the result would be 90 octane. am i wrong??

 

[dougjgreen]

NO, he might actually end up with pretty close to 91 octane, (higher than 90, that is) but he won't end up HIGHER than 91.

The reason for this is that some of the additives that boost octane work almost as well in smaller concentrations. So blending 1/2 89 and 1/2 91 might get you to 90.7, say, because the additives, when diluted by half, will still boost octane of the overall mixture by almost as much as they did in full concentration.


BTW, this whole concept might be obsolete. You mentioned that you saw this several years ago. It WAS true when it applied to blending leaded and unleaded gasoline.

If you blended 87 unleaded with 91 leaded, you would end up with a mixture that was very close to 91, and of course, had lead in it, at 50% the normal concentration of leaded gas.

I have no idea if this is still true to some extent where all gasoline is unleaded.

 

[nsxtasy]

Another point is this, and this may be what he's confusing this with. Mid-grade gas is generally overpriced, compared with regular and premium. You can do better for your money by mixing regular and premium.

For example, let's say regular (87 octane) is $1.70/gallon, mid-grade (89 octane) is $1.80/gallon, and premium (93 octane) is $1.90/gallon. These are typical prices in the Chicago suburbs right now.

And for the purposes of this example, let's ignore the factor Dougman brings up, regarding the effect of smaller concentrations of additives in boosting octane; let's assume that you get the average octane of what you pump into the car, pro-rated by the number of gallons.

a. You could pump 15 gallons of mid-grade and pay $27.00 for a tank full of 89 octane gas.

b. You could pump 7.5 gallons of regular and 7.5 gallons of premium, and for the same $27.00, you would have a tank full of 90 octane gas.

c. You could pump 10 gallons of regular and 5 gallons of premium, and pay $26.50 for a tank full of 89 octane gas.

Clearly, either (b) or (c) will give you a better deal than (a) - either more octane for the same money, or the same octane for less money - by pumping regular and premium, instead of mid-grade.

Perhaps that's what he's thinking of?

 

[AKUDOU]

Another point is this, and this may be what he's confusing this with. Mid-grade gas is generally overpriced, compared with regular and premium. You can do better for your money by mixing regular and premium.

For example, let's say regular (87 octane) is $1.70/gallon, mid-grade (89 octane) is $1.80/gallon, and premium (93 octane) is $1.90/gallon. These are typical prices in the Chicago suburbs right now.

And for the purposes of this example, let's ignore the factor Dougman brings up, regarding the effect of smaller concentrations of additives in boosting octane; let's assume that you get the average octane of what you pump into the car, pro-rated by the number of gallons.

a. You could pump 15 gallons of mid-grade and pay $27.00 for a tank full of 89 octane gas.

b. You could pump 7.5 gallons of regular and 7.5 gallons of premium, and for the same $27.00, you would have a tank full of 90 octane gas.

c. You could pump 10 gallons of regular and 5 gallons of premium, and pay $26.50 for a tank full of 89 octane gas.

Clearly, either (b) or (c) will give you a better deal than (a) - either more octane for the same money, or the same octane for less money - by pumping regular and premium, instead of mid-grade.

Perhaps that's what he's thinking of?

KEN: The blend ratio for mid grade 89 octane is 65% 87 octane Regular and 35% 93 octane Premium. The best saving on gas at this time is the Citi Bank Shell Card with a 10% rebate on every gallon of gas for 90 days after card is activated then 5% off after the initial 90 day period. I buy premium at 1.59.9 on super Tuesday 6 cents off then get another 10% off using credit card total 21.5 cents off which brings the cost of premium down to 1.38.4. I have watched similar antics as mentioned above, mater of fact my company has saved videos of such for our manager meetings for a laugh while having lunch. How much does a person value time spent pumping gas? Personally I wouldn't waste the effort and time to blend the gas.