Speed of octance change in ECU

I searched but couldn't quite find this. Where I track offers higher octane fuel (even 100), but I'm more interested in the 95. Does the computer change to accept the higher octane immediately if I had been running 91-93 in the previous tank? Am I even asking this right? I'm sure this is out there already, but I may as well put it here for completeness: how much higher octane is necessary to notice a difference? Thanks John

I'm just guessing with the answers below, but the guesses are based on my experience with other brands of cars. The ECU won't recognize the change immediately, but probably within the first 50 miles or so. Unfortunately, the answer is "it depends". If your car is running properly, and the fuel you've been using is working just fine, there probably won't be any performance difference with higher octane fuel. In fact, there might be a very slight reduction in performance, because higher-octane fuel burns slower than lower-octane fuel. On the other hand, if your car is detonating badly with your normal fuel, increasing the octane even a few points can make a huge difference. The way most ECUs work is to pull (reduce) timing when detonation is detected. If there is no detonation, there is no timing reduction and the engine runs at it max timing. The issue with running higher-than-needed octane fuel is that once the ECU reaches max timing, that's as far as it can go. So, any octane over the minimum needed for no detonation is essentially "wasted". Also, you need to remember that the octane rating of any fuel you add will get mixed in with the octane rating of any fuel left in the tank and piping. It's a simple formula to calculate (it's not quite exact, but close enough for anything we'll ever do with these cars): ((# gallons in tank * octane rating of that fuel) + (# gallons added * octane rating of new fuel)) / (# gallons in tank + # gallons added) = new combined octane rating. As an example, if you add 6 gallons of 100 octane gas to the 4 gallons of 93 left in your tank, you'll end up with 97.2 octane fuel (assuming the original ratings were accurate).

BillC is correct, blending above the point the ECU can adjust for is not helping anything. In Europe, they use 98-99 octane fuel, equivalent to our 94-95 fuel, so that has been the informal standard that many people use. Since CA, NV, AZ and other western states usually have 91 octane premium and the fuel door sticker is asking for 93, I blend with 100 to get to approx. 94. I am lucky to be close to one of the few stations that has 100 octane pumps, it costs $4.99/gal.!

Good stuff here, ndporsche - it will depend on what octane you are running in the car right now - but if you're runnng less than 93, then you can mix higher octane gas with it and get a higher effective octane rating. I do this all the time (since in NV and CA, as mentioned, only 91 is generally available) before a track day or an autocross competition. AFAIK, the ECU will recognize the new mixture as soon as it is detected - which means fuel already in the lines will have to be used before the new mixture is available. I've generally felt a good difference in about 5-10 miles. Since the ECU is optimized for 93 octane (as measured here in the states) mixology that gets you any higher than 93 is a waste of money. brad

Thanks everybody - I'm getting 91 only here, I'll either show up almost empty (I know, its not too good to be empty) or I'll blend. So, unless the vehicle is modified for it, that high stuff isn't all that its cracked up to be. First Cayman track weekend next weekend, I already know the car is WAAAAAY faster than I need for now anyway. John

True - The only way to modify the car to take advantage of higher octane fuel is to increase the compression ratio - turbo cars with variable boost, on the other hand, handle the increase quite nicely - my 2002 996 turbo loved higher octane gas - I used to fill up with 96 or 100 and the difference was amazing - it came up onto the boost faster and with much more boost when it did. brad

And a reference ... If your car doesn't knock or ping on lower octane gasoline, paying for higher octane is a waste of your money. http://www.quiktrip.com/gasoline/myth.asp

Another consideration (I think, others please correct me if I am wrong) is air temperature. I believe that in hot vs cool climates, there will be a greater risk of detonation from the lower O2 content of the warmer air. So, higher octane might be more important on a hot summer track day.

I agree with other respondants here, no advantage going over 93 octane, as that's what ours are optimized for. Also, remember higher octane fuel burns SLOWER, so unless your timing and ignition is set so you can use abit more time to burn completely and efficiently, you may not burn it all. On track days, I dump 4 gallons of 100 octane Sunoco into my tank and top up to full with 91 octane (AZ....) and I notice a difference in 5-10 miles -- likely this corresponds to the volume of the fuel lines. Greg

Yes, this is true - Porsche ECUs are programmed to begin to retard timing when the ambient temperature reaches 68 degrees, F. The warmer it gets, the more it does. Increasing octane above 93 though, will not compensate for higher temperatures. 93 is still what the system is optimized for. brad

Thanks for the confirmation, beez. Yes, I would not have thought any need to go above 93. Just that in cool weather, it might be OK to stick with 91. I think it also depends on what RPM range you expect to lap in. If you like to run at high revs the higher octane is more important. If you use more of a momentum strategy and use more midrange RPM, the benefit of the race fuel is less. So far, at cool temps, I have found little problem with 91 in CA. More limited by not being ready to turn off PSM. Actually, more limited by the driver

Octane and timing Actually, in all cars I know about, the ECU doesn't care what the octane is. It is listening to the knock sensors. As long as there is no knock, it puts in the maximum amount of timing that is programmed in the ECU. When it hears knock, it starts retarding. So it doesn't care what the temperature is, whether it's raining, etc. It just ensures that the maximum timing is present that doesn't cause knock. Now the engineers who created the fuel/timing mappings may have considered all kinds of things but that is frozen in the data. It's possible Porsche is different but I doubt it. I also can't find a reference to an ECU temperature sensor in the shop manual (but given the random organization of the shop manual, I could have missed it).

Well, temperature, altitude and to a much lesser extent humidity do have an effect on the power output of an engine, however, I have no idea to what extent a Porsche measures or compensates for these factors. It would be interesting to know how sophisticated the engine management really is. As to temperature and octane requirements, I am not familiar with the details of that, but there is a relationship between altitude and octane, the higher you go the less the octane requirement. As I recall the octane requirement drops one point for approx. every 2500 ft. of altitude. In the aircraft industry all engine horsepower ratings are stated using the ISA Standard Day, which is sea level altitude at 45 degrees north latitude, dry air, temp. 59 F, barometer 29.92 hg. I don't know if the vehicle manufactuers follow this standard.

HP There is an SAE standard for measuring horsepower. In fact it changed a couple of years ago which resulted in some manufacturers having to reduce their advertised horsepower.

The link http://wahiduddin.net/calc/calc_hp.htm mentioned in this http://www.planet-9.com/showthread.php?t=847&highlight=altitude has some related info.