91 Octane

[istabemos]

Right now i just have my flashpaq on the 87 octane settings cuz that was what was in the tank but im going to fill up on 91 octane and change it to the performance settings next refill would it be best to get tank right at empty or can there be some 87 still in tank? tnx

[cerbomark]

just put 93 this time and then whatever 89or91 your tune calls for next time.

[VMax2007]

NO!!!! You can NOT mix low octane with high octane, because if you do, you will have...

 

 

Medium octane

[omegafiler]

Just get it as low as you safely can, fill up with good quality premium, and tune for premium. Those tunes aren't aggressive enough to cause any serious concern and should work just fine. But if you did get detonation for some crazy reason, you can easily go back to the regular octane tune until your next fill-up.

[TheColter]

If you plan on filling up with 93 octane fuel with some 87 octane in your tank, you'll want to wait till there's about 1/4 tank of 87 left. The resulting mix will yield you around 91 octane.

 

1/4 - 87 Octane + 3/4 93 Octane = 91.5 Octane

[bryanhunt]

Octane Rating as viewed on the pump is not a quantitative measurement of any quantity inside the fuel. Its a number assigned to a certain mixture and how it performs on a give scale.

 

Mixing 87 and 93 in a mixture will not yield a reliable result because the numbers are not related to the fuel mixture parameters. Simply drain the tank as far as you're willing to go and fill up then run it down again and fill up with your desired fuel.

[cerbomark]

I guess I learned something new today, but can you explain what octane you have if mix 1/2 tank of 87 with 93???? He only using a 91 tune, i didn t understand him to beleive he needed 93 ocatne..

[bryanhunt]

You will have a tank with 50% 87 and 50% 93. It really depends on what the refinery used to alter the octane rating to begin with. They could very well be different between the two fuels.

 

Octane is a rating of how well a fuel mixture performs under a certain set of test conditions. On top of that there are two different scales to deturmine that, the pump rating is an average of those two scales.

[asilverblazer]

Likely won't matter any who, I ran my truck on 87 octane with a 91 octane tune, no detonation.

[bryanhunt]

Likely won't matter any who, I ran my truck on 87 octane with a 91 octane tune, no detonation.

 

Pretty crappy gas mileage though? Since there is a knock sensor on these engines the PCM would retard the time to eliminate the knock.

[asilverblazer]

Just like it retarded it with out a tune. Regardless of tune or no tune running 87 octane will result in the same timing, it just starts at a higher threshold and retards timing till no knock is detected.

[TheColter]

Hate to tell you, but most gas stations only have two tanks in the ground. one Regular (87) and one Premium (91+). They get thier mid-grade fuel by mixing the two. Most stations that offer only 87 and 89 (casey's around here) do so by adding ethanol to the 87 to yield a higher octane rating, but worse mileage due to the ethanol.

 

You can be fairly accurate mixing fuels... I mix 110 octane Race Fuel with 93 Pump gas (50/50 mix) to yeild around a 102 octane, which is perfect for my set-up. Any lower and i'd start getting detonation, and trust me i'd know as the quad has no knock sensors or computers to automatically retard the timing.

[cerbomark]

OK, now we have two ideas on mixing octane. Anyone care to offer more proof on their ideas??? I am now confused.

[bryanhunt]

OK, now we have two ideas on mixing octane. Anyone care to offer more proof on their ideas??? I am now confused.

 

I stopped arguing the point because I realized that those not willing to put in the time to think about it.

 

First you have to understand what Octane rating is all about anyway. It is a scale from 0 to 100 is seriously nothing more than "how badly a fuel knocks". If it sounds subjective, that's because it is. While fuel companies have done a decent job standarizing the test there is still room for some slop. They test fuel blends by running through this standard engine test (there is an idle test and a WOT test, thats how you get RON and MON numbers... averaged together you get your average knock index.... which is what is posted on the pump). Each blend of fuel is a complex mix of many molecules that perform differently (cumbustion is not a simple process). It's like plumbing a mapp tank with a propane tank and running the mix out the torch as saying it'll burn at the average temperature of the two when there are many other processes to consider.

 

Mixing unleaded fuels is a "good enough" situation. Yes mixing two fuels often seems to perform like a mid grade fuel but you can't absolutely say the mix is that octane rating without running it through a the full test. One example people have known about for a really long time is that if you mix a slight bit of leaded fuel with the same octane unleaded fuel you get a higher octane fuel (budget racer's trick from the carburated days).

 

There are always impurities and adititives in fuels. Mixing them from station to station does alter the octane rating and its something you live with in normal situations because most things we drive can live with the slop. The important thing to remeber is that octane is not a measure of a ratio of molecules in a mixuture. Octane is a qualitive assignment of fuel mixture's performance in a standardized I.C.E. at idle and WOT.

[cerbomark]

Interesting. Thanks for putting the time in for the explaination. I m shocked as I ve done this blending thing for years starting with the leaded and unleaded fuel to get a higher octane with lead when they started to do away with the priemum leaded fuel. If I understand you correctly I wonder how the engine reacts to the two different ocatanes mixed in one tank? I guess it would attempt to keep adjusting the timing? Can t be good for performance.