Highest useful octane rating?
11-15-2002, 09:48 PM
#1
Racer
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Highest useful octane rating?
My understanding is that if I use less than 91 octane rating fuel, the ECU will adjust spark plug
ignition timings, basically 'detuning' my car.
Fair enough. But what's the upper limit? I've heard anything above 92 octane rating is wasted since the stock ECU keeps the timing the same for
all octane ratings above that value. Is this true? I've read on the 964 board that up to 94 octane rating can help.
I'm not thinking of regular use, but just for track use to fill up with the higher rating, if it makes a difference.
Btw, this is for a stock 964.
ignition timings, basically 'detuning' my car.
Fair enough. But what's the upper limit? I've heard anything above 92 octane rating is wasted since the stock ECU keeps the timing the same for
all octane ratings above that value. Is this true? I've read on the 964 board that up to 94 octane rating can help.
I'm not thinking of regular use, but just for track use to fill up with the higher rating, if it makes a difference.
Btw, this is for a stock 964.
11-16-2002, 12:30 PM
#2
Rennlist Member
Interesting question. I have reviewed all the factory Information Technik books on the 964 regarding octane requirement. For all Turbo and non-Turbo cars except the 964 Carrera RS the fuel requirement is listed as RON/MON=95/85. For the Carrera RS it is listed as RON/MON=98/88. RON means "Research Octane Number" and MON means "Motor Octane Number".
This implies and average fuel grade of only 90 is required for all 964's except the RS which requires a 93 octane average.
The RS would be the only car "detuning" to run 91 octane.
This implies and average fuel grade of only 90 is required for all 964's except the RS which requires a 93 octane average.
The RS would be the only car "detuning" to run 91 octane.
11-16-2002, 02:45 PM
#3
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Hmmm, on the fuel filler for my car, it says 'minimum octane rating 91 (R+M/2)' or something to that effect. Which is good, because I can't get 92 octane here in California. I must say, though, that the car seems to run better when I fill it up in Oregon, where 92 is available (and significantly cheaper than 91 is here 
) - although that could just be my imagination.
) - although that could just be my imagination.
11-16-2002, 03:57 PM
#4
Rennlist Member
Good points.
The problem is that the number that is the most important is the Motor Octane Number(MON). A low MON can lead to knock despite a 90 RON+MON/2 average. We really don't know the RON and MON numbers used to achieve the average mix by the oil company so Porsche prints 91 on the filler door to be safe. In their technical literature Porsche are careful to specify the numbers used to achieve the average at 98 MON + 88 RON.
My car has an RS chip and pinged a little when the throttle was tipped in too agressively at low revs on the previously available 92 here in California. The DME dials back the timing and you can feel and hear the difference. With ~20% 100 octane mixed with 92 it runs great. I have always mixed 100 with the 91 that became all we can get here in California when our 92 dissappeared (good weather bad gas). I don't have the heart to try the 91 knowing 92 was already an issue.
<img src="graemlins/yltype.gif" border="0" alt="[typing]" />
The problem is that the number that is the most important is the Motor Octane Number(MON). A low MON can lead to knock despite a 90 RON+MON/2 average. We really don't know the RON and MON numbers used to achieve the average mix by the oil company so Porsche prints 91 on the filler door to be safe. In their technical literature Porsche are careful to specify the numbers used to achieve the average at 98 MON + 88 RON.
My car has an RS chip and pinged a little when the throttle was tipped in too agressively at low revs on the previously available 92 here in California. The DME dials back the timing and you can feel and hear the difference. With ~20% 100 octane mixed with 92 it runs great. I have always mixed 100 with the 91 that became all we can get here in California when our 92 dissappeared (good weather bad gas). I don't have the heart to try the 91 knowing 92 was already an issue.
<img src="graemlins/yltype.gif" border="0" alt="[typing]" />
