From everything I've read, unless you are tuned to take advantage of race gas, it is a waste of money. When I tuned my car last week I was running 100 Octane already and I had a full tank. It was running like a champ ...extremely noticeable power increase over the previous tune.
When I ran low on fuel I went back to 91 (the highest we have at most gas stations in California) and after many full throttle runs through the gears at multiple temperature conditions there is definitely a feeling of less power than I had with the 100 in the tank. Does this make sense? The tune is for 91 so why would the 100 make such a noticeable difference in power? It isn't just my imagination...the PSM isn't coming on nearly as much as it did with the better fuel.

 

because the ecu will add timing on its own when it senses the octane... a oem car will do that too to a point.... your target timing may be a little higher too... Im sure they added timing.... its fine... enjoy it...

 

Crap - knowing that I will be tempted to blow a lot more money on better fuel since it is available fairly close to my office.

 

This is exactly what I see. I do not have a race file, just a 91 octane file. When I mix in 100 or 105 octane, I do not feel any difference in the performance of the car. I don't think my ECU is adapting up. Now if my tune was designed for 93 and I was running 91 and added some 100, then I would probably notice a difference.

 

Interesting thread. I have a 91 octane tune on my X50 vehicle and I can definately tell a difference between 91 and 100 fuel when the outside temperature is over 90 degrees. I believe that in higher temperatures the IAT is high enough that at 1.1 bar tune forces the computer to start pulling timing. This get real noticible over 100 degrees and/or stop and go traffic and heat sink.

This is exactly why I am contemplating changing intercoolers to the late model 997.2 units so that timing does not get pulled.

 

Thanks for the clarification. I thought that was the case but when I put 91 in it felt down on power. The weather did get warmer a the same time so maybe that has something to do with it. At least now I won't be tempted to waste money on 100.
Do you have a tune I could add (for additional $) that would take advantage of 100 or meth injection and automatically retard timing when I only put in 91 or would I have to buy multiple tunes and download a different one depending on which fuel I use? Is all this just a waste of time with K16 turbos?

 

tony when the ecu senses knock does it only pull timing or does it also pull boost .

 

No way. Look at the 60-130 times list and look at how fast some of the stock K16 turbo cars went with tune and other minor mods. You can squeeze a lot out of a stock setup.

Unfortunately, I don't think you can have a 100 octane file that just adjusts down with timing with these cars. Tuners know better this answer, but if you have a 100 oct tune and you put in 91 or 93, your ecu will advance timing and boost like it normally does for 100. At some point in the run, it will start pulling back timing, boost or both to prevent knock. At this point your ecu is just putting out fires and does it in the fastest way possible and it usually takes away way more power than you would have had if you had a tune designed for that fuel. That is my understanding of how our ecu works. Better to have a tune designed for your type of fuel.