Today I was pumping gas at my favorite gas station and was watching the guy with the fuel tanker truck filling up the big tanks in the parking lot. I figured I'd walk over and ask him a few questions about octane. He seemed very knowledgeable and said that there are no mid-grades of gasoline; only 2: regular and premium. Anything else is just a mixture of these 2 grades and the pump simply mixes based on the correct percentage to yield the correct octane rating. This guy could have been an idiot but if so, he had me fooled. I'd figure if you have such a relatively dangerous job as driving a gasoline tanker truck you probably know your product.

So I mentioned that my owner's manual calls for 91 octane but I can't find this anywhere in my area-- only 87, 89, and 93. He said if I pumped 1/2 tank 93 octane, and then added 1/2 tank 89, I would have a full tank of 91. Too much trouble IMO, I'll probably just alternate between full tanks of 89 and 93. Or maybe just use one or the other depending on any performance or gas mileage gains. I drive up and down hills all the time here in Tennessee and I don't want to hear any knocking!!

 

http://auto.howstuffworks.com/question90.htm
http://www.repairfaq.org/filipg/AUTO/F_Gasoline6.html
-2 good resources for engine octane

btw..you wont hear the knock even with 87 octane.
you have a knock sensor sitting in between the cylinder banks that "listens" for knock and will retard timing accordingly.

 

I guess what I meant to say was that I don't want the knock sensor messing with my performance! I have been driving a 4 cylinder for soooooo long!

 

That's definitely not true, knocking is heard with 87. As a matter of fact, hard acceleration, uphill, I hear it with 93.

But I agree wholeheartedly with the USA Today article, basically saying premium is not needed. The spokesperson from Mercedes Benz said they sell vehicles in so many countries that they have to be able to operate them where only regular is available.

I buy premium because it is simply so darn cheap nowadays. I get it for about 8-10 cents more a gallon than regular on Tuesdays, I mean what is that, 5% more? No brainer. If premium ever gets expensive again, like it was when regular was 1.099, about 18% more, I may consider switching back to 87. Elasticity of demand in action.

 

The first post in this thread is correct. One of my friends owns a gas station. You buy low grade and high grade and all the mid-grade levels are mixed at the pump when you pump it. However, I dont know if trying to "mix" them in your tank works...???

 

Are you sure it's not the rattley interior?

I've NEVER heard knocking on my car with the knock sensor working even while watching with my own two eyes my timing drop from 23* down to 18* (knock retard) on my OBD-II scanner on a 95F day with 93 octane with moderate acceleration.

I DID hear knock in the same exact conditions but with the knock sensor bypassed (no timing retard in the event of knock). Timing stayed at 22-23* instead of getting pulled and I heard some mild knocking.

If all is working properly in your car, you should never hear any knocking.

 

David N. I really hope you didn't just find that out today, that would be sad.

 

so what?

 

I always thought if there were 3 grades available at a pump, there were 3 separate storage tanks.... call me stupid if you like... impolite as it may be.

 

Frank, you might check for an intake leak somewhere, like at the manifold gasket. Like Stevtec says, you shouldn't be hearing any knock unless something's wrong. For instance a lean afr at WOT.

Stevtec, what kind of scanner do you have? [Edit: Never mind, I just found in another thread that you have an Auterra.]

 

This is simple automotive knowledge that everyone should have. Octane is octane and mixing is perfectly ok. People have been doing it since gasoline came in different grades. It's VERY EASY to find you octane rating with a mixture.

For instance: say you want 10 gallons of gas.
5 gallons of 80 (making the numbers easy for myself) + 5 gallons of 100 = 10 gallons of 90. It's very basic 5th grade math.

It's just an average of the octanes and amounts. 80+100 = 180. Divided by the ratio of octanes, in this case 50/50. So 180/2 = 90.

Or an easier way: 80 * 5 = 400. 100 * 5 = 500. 400+500=900/10 = 90.

I used to use this every weekend when I'd mix 104 octane with 93 in my DSM.