Calculating octane

[Maroon91RS]

Hey guys, havent visited this board in a while, but I need a bit of help and I know I've seen this topic here before, just couldnt find what I was looking for using search. I'm looking for the formula to calculate octane when you mix two different ratings. I mixed 3.5 gallons of 100 with about 4 gallons of 93 in my '97 GTP last week, and I was wondering how that affected the rating. I assumed I had to add the 2 together and divide by 2 since there was just about equal parts of both and came to a 96.5 octane rating, so I figured I was between 95 and 96. Am I correct, and what is the correct way to formulate octane when you have unequal parts? Thanks for anyone that can help!

 

[Ormand]

What you did will get you close, but there isn't any way to be sure. The additives that raise the octane don't give you a straight line increase. So, the refiner had 90 octane base stock, and added 5% of some additive, it might raise the octane to 92. But doubling the amount, to 10%, might only give 93, not 94 as you would expect. But, if your "blend" had better "base" gasoline in the high octane part, then the additive in the other part might give MORE boost than you would expect. So it's not an exact math deal, unless you know exactly what the additives are. In the case you ask about, mixing X gallons of one gas with Y gallons of another, you can estimate the octane of the mix by using this..
X gallons of fuel A with octane A, and Y gallons of fuel B with octane B. Mixture of the two will have X + Y gallons. The octane will be about (X times A plus Y times B) divided by X plus Y.
For your example, this gives (3.5X100+4X93)/7.5=96.3
But this is approximate. If you add leaded gas to unleaded gas, both with the same octane, the combination will have a higher octane, for example!

 

[SinistrV6]

Sorry, Ormand but additives do not add anything to the Octane numbers. The gasoline is blended to the correct grade (RVP) and octane BEFORE any additives are put into it.

 

[V6POWER]

3.5 x 100 = 350
4 x 93 = 372

350+372 = 722

722/7.5 = 96.27

 

[Ormand]

SinistrV6, no need to be sorry. You are MISTAKEN! If the fuel is leaded, it is blended to some octane- let's say 90- and then tetraethyl lead is ADDED to the blend, raising the octane of the finished fuel to 93. Maybe it's just a question of semantics, but to me, at least, the tetraethyl lead is an additive, not part of the gasoline. In the case of alcohol/gasohol, maybe you call the alcohol part of the fuel, but in my explanation, it would be an "additive" if it is used to raise the octane.

 

[Jerryl]

Try this link.

http://www.gnttype.org/techarea/misc/Octanemix.html

 

[SinistrV6]

Sorry,

Didn't realize you were referring to Lead as an additive. When I blend AVGAS, I consider is a component just like LITISO. Only the proprietary "additives" such as Techron was I referring to as such.

 

[Maroon91RS]

Originally posted by Jerryl
Try this link.

http://www.gnttype.org/techarea/misc/Octanemix.html

Kick ass, thats exactly what I was looking for!! I remember seeing that chart a year or two ago, but couldnt find it again. Thanks!