Octane q's

[bruspeed]

How does octane ratings work? I am trying to decide if buying 5 gallons of say 100 octane fuel and mixing it with 5 gallons of 91 octane fuel is going to give me a good jump in octane rating.

I don't know if there is a ratio and I'll end up with something like 95 or 96 octane fuel, or if there is any definitive way of really knowing.

I would like to run some decent fuel in the Camaro for autox weekends, without spending $30-40 a day on gas.

Any help would be appreciated.

Chris.

[cody]

Here's a cool little spreadsheet if you have Excel. You can use it to figure out how much race gas to mix with 91 octane to get what you want. Just leave the amount of Torco at 0 unless of course you're using Torco.

[sperry]

I'm going to guess that 10 gal of 91 + 5 gal 100 = about 93. Now, off to check the spreadsheet to see how my intuition stacks up!

[sperry]

Originally Posted by sperry

I'm going to guess that 10 gal of 91 + 5 gal 100 = about 93. Now, off to check the spreadsheet to see how my intuition stacks up!

The answer's 94. Which is what I was running at the race track last season when I was running w/o water injection.

[NevadaSTi]

According to that chart, I was running 88.8 octane this weekend. LOL.

[wrxkidid]

wow.

higher octane = higher knock protection right?

so running autox or on the track with a mixture that would give you 94-95 octane wouldnt hurt it would be more protection for the motor?

is there aq point to where the octane rating is too high if you arent tuned for it?

[Dean]

Originally Posted by wrxkidid

wow.

higher octane = higher knock protection right?

so running autox or on the track with a mixture that would give you 94-95 octane wouldnt hurt it would be more protection for the motor?

is there aq point to where the octane rating is too high if you arent tuned for it?

Octane is only good when tuned appropriately for it. Otherwise, it reduces power because it affects the flame front speed and other combustion characteristics.

[wrxkidid]

i see.
so much for my brilliant saftey idea.

[Kevin M]

A small bump in octane is never a bad idea when going to the track. If you normally run 91 octane, bumping to 92-93 octane has no downside other than the $10 or so you spend boosting it. Assuming you have a turbocharged or relatively high compression motor.

[cody]

Yep, a couple extra octane points may keep the ECU from having to pull timing due to IC heat soak or an imperfection in the tune.

[Dean]

At our altitude, it is not likely to do much of anything except rob power IMHO. And as long as your tune has not disabled them, there are reasonable safeguards built into the ECU to protect you from detonation.

[Kevin M]

Originally Posted by Dean

At our altitude, it is not likely to do much of anything except rob power IMHO. And as long as your tune has not disabled them, there are reasonable safeguards built into the ECU to protect you from detonation.

Dean, the ecu will prevent your motor from suffering damage if detonation actually happens. A slight octane boost, while less necessary at altitude, will prevent detonation in the first place (unless you have serious issues elsewhere).

[Dean]

Originally Posted by BAN SUVS

Dean, the ecu will prevent your motor from suffering damage if detonation actually happens. A slight octane boost, while less necessary at altitude, will prevent detonation in the first place (unless you have serious issues elsewhere).

So, are you agreeing with me or disagreeing?

[MikeK]

1 - 2 gallons of 100 per tank is a good idea at a track day.

[Kevin M]

Originally Posted by Dean

So, are you agreeing with me or disagreeing?

Yes.

Running at RFR in the summer on 91 shouldn't cause damage to a relatively stock turbo Subaru because of detonation. However, detonation can cause the ecu to slow you down to protect the engine from det. Higher octane will likely prevent some det in the first place. So 92-3 octane>91 octane at the track in summer when tuned for 91.

[wrxkidid]

sounds good.

[sperry]

Mike Warfield has instructed me (on many occasions) to run 4-5 gal of race 100 per tank of 91 in my car.

It's not just for the "safety" boost in octane over 91, it's also because pump 91 can actually be 90, 98, 87, who the hell knows... quality control ain't nearly as good for the 91 at the pump as it is for race 100 out of a can. How many people have heard the phrase "I must have just gotten a bad tank of gas"? Tossing 1/4 to 1/3 of a tank of 100 in there may bump the octane "uselessly", but it may also turn that tank of 90 you accidentally got into 93 and save the day.

A few gallons of 100 is good insurance, even if it costs you 3 or 4 hp.

[murphy]

if your car requires 91 oct. (turbo, suprecharged, high compression) that is the min rating to avoid det. and knock. 93 would be better choice. no seeing as we don't have that..to get the best results of mixing fuel go off a 1/4 tank of 91 mix in aprox 1/2 a quarter tank of 100 you will raise your oct to about 95. thats using the fuel tank cap. of my evo. 14 gallons, 3.5 gal per 1/4 tank.
The + of upping you oct. is easy yes you get more knock and det. protection it will also allow you computer to pull less timing under boost (if you have a turbo car) allowing your engine to make more power.