Hello.
my AFR used to be in 11.4-6's in upper RPM. recently, i put 100 octane and at the same time, disabled lean spool up.
now my AFR drops down to mid 10's in upper rpm.

can 100octance change AFR? i am suspecting it's all b/c lean spool up disable but just wondering if 100octance can also cange AFR.

thanks.

 

Disabling lean spool will do it.

 

higher octane fuel is going to burn more slowly which can make you rich too

 

hmm...so i shouldn't modify my fuel map based on high octane now. i will be using 91 for the most of the time. only pumped 100 octane for boost controll set up, sometimes i get overboost/knock.

 

correct, if you run 100 octane fuel and lean out the AFR to say 12.0:1 and then later fill up with 91 octane and don't add fuel, you're probably going to burn a hole in the piston

 

100 octane fuel is also heavier vs. 93 octane, and C16 is heavier than 100 octane. Until you get into alcohol based fuels, usualy the rule of thumb is the higher the octane, more additives, the more it will weigh. If your out of injector on 93, you might have enough on 100. Any time you change fuel types you should re-tune a/f ratios.

 

I was under the impression that octane value didn't affect the fuel burn at all, only the resistance to knock. I don't have any real knowledge of this on my own, I've only read it in a few articles.

This is one place I've seen it ... Wikipedia's "Octane Rating" entry.

I guess this means that 100 octane could have other properties that will change the burn that aren't directly related to its octane rating.

 

here's a for instance... I was tuned for pump gas ~ 11.5:1 on my stealth... I put in 5 gallons of c16 when I got to the track and the WB02 sensor died and I ran 125mph traps at 25psi pig rich.

I got home, installed a fresh WB02 sensor and found out that the C16 had me at 10.5:1. I ran the C16 out and filled up with pump gas again, the AFR went back to 11.5:1 and I ran 125mph traps again but this time with less octane and 22psi since I was a the proper AFR for the octane.

 

C16 is leaded though. I assume that leaded fuel will burn differently than unleaded. Is the 100 in question leaded?

 

Was the ignition timing untouched when the C16 went in? 125 is quick for a Stealth

 

The high octane fuel vs. pump fuel debate has been going on for ages it seems.
They "burn" at the same rate. Regardless of the octane level.
The difference is how it reacts with heat and the way in which the flame front is propagated.
Lower octane fuel such as (everyones) **** 93 octane will seem to have a tendency to burn thru because of its volatility rate vs. the ignition advance used. You are actually just measuring, thru the 02 sensor, the portion that was burned. Technically you are running MUCH richer on the lower octane fuel than what the wideband is reporting. You don't realize it until you are dealing with an octane level that can take the heat and burn completely thru the cycle. One of the reasons you will run into rich misfires when running race fuel with insufficient ignition advance.

So to answer the above question.. you NEED to check AFR's and make adjustments. But its not the burn rate that effects anything.

 

I monitored knock and ramped the timing up..never saw any knock but I went so far with the timing that the car actually dropped to 122-123mph traps so I pulled it back down a litte bit and got back to 125's

thanks, it should go mid 130's at full tilt (~ 35psi max) when I get back out there.

 

Hey Chris,

So you busted your O2 sensor because of the leaded C16 right? - not because you were running rich!

 

leaning it out to 12.0 to 12.2 on C16 didn't extend the life significantly... leaded fuels kill WB02's

 

This is correct. Octane rating does not, in itself change the AFR. It is the SG and other components in the Higher octane fuels that change it. What I tell customers is if the car is optimized for a fuel, any change, higher octane fuel included, should be recalibrated for best performance.