ACN91 (Arizona, California, & Nevada 91 Octane)

[silverbullet]

In my research for an ECU tuner for my car, I was reading up on the available off the shelf (OTS) tunes/maps. Interestingly, this is the first time I came across ACN91 (Arizona, California, & Nevada 91 Octane). Apparently, ACN91 is a lower quality fuel (for power) and thus the tunes/maps for ACN91 fuels are the least aggressive, with 91 and 93 octane maps being more aggressive respectively.

MY question is this: What makes ACN91 worse than other 91 octane fuels? What is it about ACN91 fuels that requires a slightly less aggressive tune/map than compared to a 91 octane fuel from other states?

I did ask the ECU tuner manufacturer's tech support department and didn't get a satisfactory answer.

 

[dparm]

I don't get that either and I think it's baloney. 91 octane is 91 octane. Octane is not arbitrary.

I drove a car requiring premium in AZ for 4 years (undergrad) and it ran just fine on this so-called "low quality 91".

 

[dparm]

Just did some Googling and found this, which is the only thing that's even remotely useful:


They're all 91 octane. ACN Fuels are higher in detergents. Technically they're still supposed to burn at the same rate/heat as all 91 octane fuels, otherwise they couldn't receive the federally regulated 91 octane rating... But many have experienced earlier knock with ACN fuels, and so cobb detunes things a little bit for the ACN area. Personally, I lived in southern california and had my 91 octane stage 2 map loaded, and had no ping (knock), but others have. Granted, I always loaded up with our military base fuel which was rumored to be different than ACN fuel since it was on a federal base and didn't have to conform to state standards... but I never bought into that theory... logistics wouldn't make sense to me.



Sounds like a lot of [censored] to me.

 

[A_Harman]

No, not all 91 octane fuels are the same. The octane number posted on the gas pump, is an average of measurements taken in two different tests in a standard test engine. You've seen the (R+M)/2 script on fuel pumps, right? R stands for Research octane, and M stands for Motor octane. The Motor octane number is always the lower of the two because it is taken at increased intake manifold temperature and advanced spark timing. In the real world, Motor octane number means more. If you have two different fuels that have the same (R+M)/2 value, but one fuel has a higher Motor octane number, it will have more real-world knock resistance in your engine. The difference between Research and Motor octane numbers is called Sensitivity, and can be 6-10 points lower in Motor octane.

 

[NateDN10]

Originally Posted By: A_Harman
No, not all 91 octane fuels are the same. The octane number posted on the gas pump, is an average of measurements taken in two different tests in a standard test engine. You've seen the (R+M)/2 script on fuel pumps, right? R stands for Research octane, and M stands for Motor octane. The Motor octane number is always the lower of the two because it is taken at increased intake manifold temperature and advanced spark timing. In the real world, Motor octane number means more. If you have two different fuels that have the same (R+M)/2 value, but one fuel has a higher Motor octane number, it will have more real-world knock resistance in your engine. The difference between Research and Motor octane numbers is called Sensitivity, and can be 6-10 points lower in Motor octane.

But this does not explain why ACN 91 octane is different from 91 octane elsewhere, because all of the US uses the (R+M)/2 method.

Edit: Re-read a couple times, and I see now that it could explain it.

 

[silverbullet]

FWIW, this ECU tuner manufacturer is a well respected company with proven products. For a company to develop separate, slightly less agressive ACN91 tunes/maps (as compared to the 91 octane tunes, but of course more aggressive than factory tune), there must be a practical difference between ACN91 and 91 octane from other states despite both being "91 octane". I do believe there is a difference but my goal of this thread was to find out "why" or "what is it" about ACN91 fuels that makes it different.

 

[getnpsi]

91 octane with a 10 percent ethanol content will not be the same as "puregas.org" All gas in cali has ethanol.

you can combine 87 e-10 with some e-85 and hit an effective 96 octane mix in your tank which will kill most knock, but you will burn leaner at WOT which needs to be watched in boost.

Although pure ethanol has 30 percent less BTU than pure gasoline, 10 percent ethanol doesn't count for 30 percent loss in fuel economy or output as people all over the internet claim, its more like 2-3%--something that can be attributed to any deviation, and in a race, the driver even. I totally get it that being forced to use ethanol blends in carb engines is a bummer but oh well, nothing california can do about it unless we use racing fuel in our lawnmower.

 

[Scuderia]

Sorry for the old thread bump, but I think I figured out why ACN91 is of lower quality, but still gets the 91 rating. My guess is it has to do how the different test methods affect gasoline that uses ethanol as an octane booster.

To get research octane, all they do is raise compression in the test engine. That's it. So gasoline with ethanol probably performs really well in this test.

To get the motor octane, the RPM is bumped to 900 (from 600), ignition is advanced, and the key step is that they pre-heat the fuel.

Ethanol's claim to fame as an anti-knock agent is that it cools the combustation chamber. In the motor octane test they preheat the fuel, which might negate (to a degree), ethanol's ability to cool the engine, and suppress knock.

So non-ACN 91 might be 99 research, and 83 motor. 99+83/2=91 octane

ACN 91 might be 103 research, and 79 motor. 103+79/2=91 octane.

So basically, ACN Gas probably a lower motor octane rating and a higher research octane. ACN gasoline also has other additives to reduce emissions, that might widen the discrepancy between research and motor octane even more.

 

[Riptide]

To add. I have a boosted gen1 coyote car and when I lived in montana was able to run the current tune with exxon 91 fuel and had no issues. After moving to the reno/sparks nevada area I filled up with (expensive) shell 91 and saw knock retard in the data log. Pulling as much as 2 degrees during the pull. This was not an issue with the exxon from montana.

Burned 2/3 of the tank and then filled up with "cheap" kroger 91 + some boostane additive. Now the car is back to adding timing during the pull and not pulling it.