I have discussed this topic with many in the past 2 weeks, have my own theories/rationales and EVIDENCE....but would really like it if somebody could explain the exact science behind this concept...

BACKGROUND:
I tuned with 101 octane to decrease the chances of suffering the damaging affects of detonation during my tune (if, say my A/F went lean and I allowed the motor to run for a millisecond with this bad mixture)

My plan was to ALWAYS run about 75%91 octane (the highest they have here at the pump) and 25% 101 octane....I figured this would yield at least a net octane rating of 94 or so.

So, we got things dialed in, except at about 5500rpm and up, the richest we could get the A/F was 11.7:1 (I wanted the A/F to be 11:1 across the board at WOT) ...at this point the injectors were seeing 90+% of their duty cycle, we figured we would have to turn up the fuel pressure a bit more and recheck the entire map, we had things to do, so our tuning session ended there...
We decided to pull one (1) degree of timing <----you like that out of the timing map from 5500rpm on up...I had a chance to run this a few times at WOT and it did not lower the A/F much (maybe .1point)

So...what made the biggest difference:
Following the tune, I went and filled up my tank with 91 octane (i had about 3-4 gallons of 91/101 mixture in my tank at the time). My A/F instantly went to about 11.3:1 at WOT!!! So, the lower octane fuel richened the A/F...this is the opposite of what I have heard from some...and consistant with what I have heard from others...so the question remains....

HOW DOES FUEL OCTANE AFFECT A/F RATIO?

any comments of brainstorming is very appreciated!

 

bump

 

http://www.sdsefi.com/techocta.htm

 

http://www.sdsefi.com/techocta.htm

Gas Octane shouldn't effect your a/f ratio but more so your resistance to knock.

 

Are you comparing what the dyno sniffer read at the time of the tune and what your A/F gauge read after the tune ? Is so...the sniffer will always read a little leaner than your A/F gauge because of the placement of the sensors

 

Read where Dyno sniffer(s) measure about .3 to .5 leaner than on board 02 sensor..i.e. if dyno sensor reads 12:0 on board A/F ratio could be anywhere from 11.7 to 11.5:..Anyone shed light on this subject

 

no, these comparisons were made on a bosch O2 sensor mounted in one of the test pipes while street tuning

 

cool informative article, but unfortunately it said nothing reguarding fuel octane and A/F ratio...

 

Were you tuning on the dyno and then logging on the street or just street tuning the entire time?

Is the fuel oxygenated? in either instance?

 

Octane won't effect AFR at all. You're still adding the same amount of fuel and air and that ratio is in essence what the AFR is. It's a ratio of quantities, not quality.

The only thing higher octane fuel will do for you is burn slower and resist detonation so you can run a more lean AFR with a higher octane and not detonate.

There's many different reasons why your AFR at WOT would change, temperature, baro pressure, humidity, and even engine operating temp.

 

Thanks for the imput,
It consitantly changed as soon as I filled with lower octane fuel...I see what you are saying, but evidence is showing otherwise....

any other ideas?

 

TRICK 101 (not sure about details of the fuel)...these comparisons were ALL made on street tuning in identicle situations-load, etc (as identicle as we could get)

 

As mentioned above, octane will not raise nor lower your AFR. Octane serves only to prevent gasoline from spontaneously igniting under compression without an iginition source ie. spark.. The higher the octane the more compression it can sustain before igniting.

Now, what will change AFR is if you are using gas that's less dense than regular pump gas. Like AvGas for instance.

Forgot to mention, in regards to fuel density and weight that MAY effect AFR, all gasoline is not created equal. What you get at Cheveron may not be what you get at BP or Texaco..

 

I am the first to admit that I don't have it all figured out, but I clearly understand the primary need/desire to use a higher octane fuel--it doesnt necessarily make any more power, it allows one to run more timing/ more boost with a greater margin for error towards experiencing the damaging effects of detonation...
higher octane= higher resistance to detonation

I am leaning more towards my ecu advancing a few degrees of timing, causing the A/F to lean out a bit (even though we checked the timing via an obd2 scanner--we must have missed the advance or something--we saw 26 degrees advance at WOT--I expected if it was advancing to see 28/29 in the same spots with higher octane fule--oh well)

 

I would trust the scanner. But one thing to note is that higher octane fuel is also a way to slightly lower timing. This is becasue higher octane fuels burn slower and in a more controlled manner.

 

sure , that makes ense, but this would create a richer A/F mixture (this is where the consensus was at)

but, yah, the scanner didn't show any more timing

Thanks again Dave and Mike!!!

 

I see what your saying and it does make sense. But, could it also be possible that the ECU is adjusting the injectors pulse width as well? If it doesn't see detonation and advances the timing, it may also try to shorten the "open time" of the injectors to conserve fuel.. Just a theory...

 

Take a look at this article.

http://www.mustangandfords.com/techarticles/30643/

Based on some of the technical info it seems a couple of things couple be happening.

1. The 100 octane is oxygenated for legal street use
2. The specific gravity of the 91 is lower then the specific gravity of the 100

I highly, highly doubt that the OBDII scanner is giving false info. That would be my last assumption becasue there is no reason for it and becasue this was initially designed as a diagnostic tool. If it gives false readings then there would be no purpose in it being a diagnostic tool.