: question about mixing of octane at the pump

05-31-05, 10:25 PM
When I go to the pump I mix half 89 octane with half 93 octane to make 91 octane. I heard from a friend that this is not good, he said that it would mess up the emissions sensor. Anyone have any comments on mixing of octane when filling up at the pump? Is this bad or o.k.?

05-31-05, 10:28 PM
Well, what you're doing seems like a waste of time, but it won't hurt anything. You can put 87 octane in it if you want to. It won't have as much power, but the knock sensor will prevent any damage and you'll save a few dollars. A few members here have been using nothing but 87 octane for years with nothing bad happening. It works fine. Why are you mixing grades anyway?

05-31-05, 10:40 PM
to make the recommended 91 octane in the owners manual. i also heard that you get better gas mileage with a higher octane, which will save you money in the long run from having the better gas mileage.

05-31-05, 10:55 PM
The owner's manual recommends 91 or higher, so you can use 91, 92, 93, or 105 if you want. The performance benefits are negligible and probably not even noticeable by going with a higher octane like 105 race fuel (especially not at $4.00+ per gallon!). If you really want to know, try it for yourself. Fill up from empty with 87 octane and reset your average mileage. Drive it normally until it's empty, then fill up with 93 and do the same thing. My guess is you'll see very similar mileage and better performance with the 93. If I was you, I'd save the time of mixing grades and just stick with one or the other. Mixing doesn't hurt anything, but it doesn't help anything either. The few cents' savings at the pump doesn't seem worth the hassle. You can put regular, mid, or premium in it and have very similar performance and mileage, but the regular will cost you less. No damage will result from any of those choices.

05-31-05, 11:04 PM
thanks mcowden, your information is very helpful. i will give the 87 a try. where i fill up they have their 87 octane a lot cheaper than their 89 and 93 because they buy the 87 in massive amounts. so that will help the old pocket book a lot. thanks again.

05-31-05, 11:22 PM
No problem. Let us know what your mileage and performance results are. It will make for an interesting debate. This is the time of year when stations are rolling out their summer reformulated gasoline in the EPA mandated areas (June 1 if I remember right), so gasoline-related questions are in season right now.

I want to clarify something also: the knock sensor prevents damage to your engine by telling the PCM to retard the timing when it detects the sound of detonation in the engine. Detonation can occur because of high compression and a number of other factors, but in the Northstars it's primarily the compression. 10 to 1 or more in most if not all of them, whereas most production road car engines are 9 to 1 or less. By retarding the timing, performance suffers a little, but no damage will result. Higher octane gasoline is recommended in the Northstars for maximum performance, which means that theoretically the knock sensor doesn't have to effect any retardation of the timing to account for detonation due to the lower octane rated gasoline.

Gasoline is not a single product. There is no such thing as "straight gas" or "pure gas" sold at gas stations. It's a blend of lots of different stuff, and performance can vary slightly from tank to tank, even using the same grade from the same station. It's a common misconception that "race fuel" will make your car go faster. That's really not the case because theoretically the timing should be advanced to its maximum value using 91 octane gasoline in a Northstar. Therefore, the increased octane won't cause the timing to be advanced any further and no performance gains are seen. Also, they have to use some nasty stuff in race fuel to boost it up to those octane numbers. That stuff can degrade fuel system plastic components over time.

More than you wanted to know, I'm sure, but thought I'd share a bit. I read too much about this stuff. :drinker

05-31-05, 11:25 PM
To confirm what Mcowden said, I have been using 87 for 3 years now and haven't noticed any difference in performance or milage, only in my wallet.

06-01-05, 12:36 PM
Some gas stations have two tanks, one of 89 and the other of 93. To get mid grade octane, it's a blend of the two.