Leaded fuel will poison the o2 sensors which means check engine lights. I would be surprised if you noticed anything from 100 AKI unleaded race gas. I threw some 100 octane in my push mower once. It smelled pretty cool but it was still a regular old mower.

Great info in here. arghx7 what do you do?

And I'm pretty sure I've seen you on another forum before

I'd rather not say.

He's obviously a very knowledgable landscaper.

Heh, jk...

Since there are lots of knowledgeable folks on this thread.. I've got a related question about 91 vs 93 octane. In CA only 91 is available except for a very few places. I've considered adding about 3 gallons of 100 octane to the 10 gallons of 91 in the tank. This would bring up the avg to 93 or so.

Any benefit to this?

Since it would be mixed going forward, I'd just periodically add some more 100 when I pass that gas station. If it's not done right I could see the octane rating going well above 93 (and also wasting money on too much 100) ..

I'm not as concerned with performance as avoiding any kind of knock and forcing the ECU to relearn the 91/'low octane' thats being used on a regular basis.

I wouldn't do it all. The difference you'll see in regular driving will be negligible but the difference you'll see in the thinning of your wallet will easily be noticed.

Just pump 91 for daily driving. Save the expensive stuff for a day a the track.

I knew there was a reason I wasn't getting any work!:paddle:

Thanks Dimman I needed to laugh today...

One aspect of modern vehicles that you will have to accept, all modern engines knock ,especially at low load high timing and AFR. What you have to be concerned with is knock that is consistent, and happening during high load, open loop operation. To a certain extent the the engine will correct the reason for the knock (either retarding ignition timing, increasing fueling, or a combination of the two). If the manufacturer did not want the vehicle to be run with 91 octane than it would not be sold in your state with a warranty. I live in New Mexico, and our gas is as bad as yours, interesting as we are so close to Texas... I degress. Anyways, your vehicle will not be at it's most efficient running 91 octane, only you can decide if the extra $5.00 to $8.00 is worth spending every time you fill up.

And also Rice, yes ALL OBD II ECU's have a octane parameter built into the factory ECU system, whether it is determine proper ignition, fueling, or different alpha N calculations (Honda) for the cylinder events. Yes different automobile manufacturers have different ways of dealing with knock, but ALL have a sub routine to change the octane count on the fuel once it is detected.

Jedibow: in asking if all OBDII ecu's do that, I wasn't referring to ignition management via knock control. That I knew. I was referring to auto-advancing to take advantage of higher octane than what the car was designed for. My 1996 Ford Econoline Van designed to run on 85-87 octane doesn't seem to "figure it out" when I've put in 93 octane and I have a sneaking suspicion that most OBD2 ECU's are not "adaptive" like the Camry 2GR. The discussion wasn't based around retarding once detecting knock, it was based around automatic detection of higher octane for specific design and subsequently automatically altering ignition timing.

All OBDII ecu's retard timing when knock is detected. Check
All OBDII ecu's on cars designed for 87 octane automatically detect and advance timing when higher octane is introduced than what the car was designed to run on? I don't think that's true.

But.. :iono:

I have this suspicion that it's like this, if the car is meant for 87 then it might not have a high octane map onboard, but if it's tuned for 93/91 then it can detect the higher octane after it's been drinking 87 for a while?

:iono:

Sorry, only certain ECU's are proactive to determine what ignition timing and fuel tables to run, namely Toyota, Subaru, and Volkswagen mostly. Most ECU's are reactive, and only retard ignition as they see knock, so the maximum timing it will run will be the value on the timing map (they cannot exceed these values) unless effected by a multiplier table such as barometric pressure, or tempurature adjustment tables.

I would also like to state that even though subaru ECU's are proactive, there is also a maximum value that the ECU would apply for timing, and no it would not take full advantage of using 100 octane fuel.

I'm sorry I hope that clears things up a bit, I've had a lot going on lately and have been in a bad mood. Again I'm sorry.

One thing to keep in mind is that an ECU really consists of 3 things:

1) Hardware (injector drivers, analog inputs, CAN communication, processor, RAM, flash ROM, etc)
2) Software (the way things are calculated, not the actual values in the tables)
3) Software calibration (the tuning of the maps and values inside)

Software is being developed all the time, but it's not always changing on every engine. If you've got an old engine on a vehicle that doesn't change much, it may have old software. That cold mean old knock control logic that is mostly reactive in nature. The manufacturer isn't going to update it unless they feel it's necessary to meet an emissions, engine output, or driveability/NVH (noise/vibration/harshness) goal--development time costs money.

Look at the WRX and STi for example. The main non-emissions calculations inside have barely changed in 10 years. Some stuff shifted around when they went to electronic throttle, AVCS (variable valve timing) and a 32bit ECU, but that's about it. The EJ series engines are 20+ years old.

When a mostly new engine goes into development, you can expect some of the newest software to be implemented. So it's less about the make (Toyota etc) and more about the individual needs of the vehicle under development.

Btw, much of this software is developed in collaboration with suppliers or purchased off-the-shelf. Sometimes most of the engine tune is outsourced to a company and you never hear about it. Here is a partial list of the main suppliers of ECU and ECU software/control logic:

Denso
Yakazi
Hitachi
Delphi
Siemens/Continental
Bosch
Magnetti Marelli
Renesas

Most German makes use Bosch or Siemens. Toyota uses Denso on almost all engines; Subaru uses Denso a lot but dabbles in other brands. Nissan uses Hitachi. Hyundai uses Siemens. GM uses Delphi and Bosch mostly; Chrysler uses Magnetti Marelli stuff. I don't know every supplier used on every project, but it's perhaps half true to say something like "Volkswagen ECUs respond better to low octane than others." VW/Porsche use various versions of Bosch software anyway.

Hope that clears some things up.

I wrote engine models using Simulink that ended up in Delphi controllers... arghx7 has/is working in industry :)

What he's trying to say is that if you increase the octane you will see more ignition advance which may lead to more power... IF there isn't ignition advance limits.

What he's not said, which I think is important is that with the increased octane you will not have your knock control strategy retarding the ignition advance... which means that the ECU won't reduce the power output... which conversely means that you'll be making more power.

For the poster that was saying that you won't be able to ignite higher octane fuel... that's a fallacy. You could run 100 octane in your BRZ/FRS and you're not going to have the thing misfire. This isn't like trying to start an engine running on E98 when it's 40 degrees outside.

If you study the combustion process in the cylinder a bit more you'll see that it's the temperature and pressure the end gas sees near the cylinder wall is what the addition octane is there for. Once the flame kernel reaches the end gas it will be ignited... no problem there. For fun think about the in cylinder temperature and pressure that the end gas sees... degree by degree. It'll start making more sense.

My turn. I put 100 octane in my 04 Solara v6, it has an 11.1:1 compression ratio which I'm pretty sure was damn high back then. I disconnected the battery for 30 min to ''reset'' the ecu and off I went. I drove her hard like I usually do and thought I may have noticed a slight increase and smoother running but it was most likely due to placebo or simply ambient conditions. However, the reduction of the weight of my wallet was significant. I read that the FR-S will infact make more power on 93. I do believe that above that there are little to no gains to be had with the stock ECU.