Ignition timing with 95RON vs 98RON fuel

I have been doing some data logging recently and collected some data running 95 RON fuel vs 98 RON fuel to compare what was happening to ignition timing under load. The two charts below show the results. When running 95RON fuel, timing gets retarded 3-4 degrees under load (costing power).

The timing was pulled back less than I would have thought. I'm assuming your testing was near sea level? It would be interesting to see quarter mile acceleration times and speeds possibly using a Tip for consistency without thrashing the clutch. I'd imagine the octane rating would be less critical where I live at 5k feet. Thanks for posting

Can you see the knock sensor at the same time to see if that's what getting the computer to retard things? I have a hard time making out an rpm related to the timing being shown. It would help to know whats going on with the rpms also. Seeing the start values are different for the values I have to wonder if the O2 sensor or temp sensors are derriving some reason to cut back the timing. If it were a knock sensing thing I'd think after higher revs the cutting back would start. Was the engine wound up a little right before the lower octane readings started? Blah blah blah.

Not only is power lost, but fuel economy made worse and exhaust temperature increases when the engine controller has to retard ignition timing. And you're testing at higher loads where the engine controller may go open loop and then can add more fuel to produce a less likely to detonate mixture. Most street car engines speed the bulk of there time at part throttle (30% or less) and under these conditions cylinder filling is high and combustion pressures reach a max. The Ecu can retard the timing even more under these circumstances costing fuel economy and subjecting the combustion surfaces and other surfaces in contact with the exhaust gases to even more heat. Not good. Sincerely, Macster.

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An engine's octane rating decreases with elevation. I do not recall the ratio. However, a problem is that in some areas of the country we are limited to just 91 octane, when 93 is really what is wanted. Thus those of us who live in these areas we are already operating our engines at an octane disadvantage. What can be worse is at higher elevations in some areas gas stations only offer 90 octane. This may still be too low an octane even at the higher elevations. Then to make matters worse, an engine's octane requirement can *increase* over time due to combustion chamber deposits, injector deposits or wear, engine wear, etc. Sincerely, Macster.

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Higher RPM isn't necessarily the area where the knock is most pronounced in my experience, it's where the load is greatest that the knock occurs and that doesn't necessarily correlate to the highest RPMs. Start values for the timing are pretty dependent on the conditions immediately prior to the run starting and also (possibly) to some extent by adaptation of the ECU to the octane of the fuel. The testing was intentionally done at WOT in order to provide some consistency between the tests and provide a basis for comparison with what's happening when the car is on the dyno. The RPM trace is there on both charts by the way. The conditions were the same for both runs, car warmed up etc. Elevation was near to sea level.

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91 is the highest available here and it seems fine for all street legal applications. I had a hot rod Buell that I built the motor on and ran just over 11:1 CR. I ran 87 octane fuel for the 12 years I owned the bike. I did tear the heads off at 5k miles to inspect things and the pistons showed no signs of detonation. This was a carbed motor with no knock sensors. At 5k' with less air a point to point 1/2 of CR is lost to altitude. I always run the lowest octane that keeps a motor happy. Higher octane is slower burning and makes less horsepower if it's not needed. Why pay more for less performance?

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I'm kinda wondering about the deal there too. OZ951 My eyes and brain are dull but I do see now how the left side shows numbers and I figure the color coding for the bottom listed item. Now I'm still lost on torque and hp numbers. Possibly if your hp and torque aren't too different between octanes and you do want to run high octane then the sparkplugs might be the thing to mess with. I've always operated on the figuring advancing the spark is going to give me higher heats since we are popping things off more so during compression. But then this engine is in the computerized electronics game of things. While you are possibly wanting a more advanced ignition watch the head temps. Sure more power is nice but after run of the mill driving is out the window and sustained high performance is being demanded on the track the timing can come back to bite real hard in valves, heads and such. I was just remembering yesterday some old days of running lots of advancement. Valves don't like to melt Long story short, how much difference in power/torque? Nice 996 website. I tried finding your torque and hp from the pages but eh I'm lost.

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Pac - The car is tuned from the factory for 98 Octane fuel (UK), so the timing you see there on the 98 Octane data log is what Porsche chose, it's not 'extra' advance. The other data log (95RON) is showing what you lose in timing terms when you put in a lower octane fuel than Porsche tuned for. I've seen a few debates in the past about whether running 98 vs 95 makes any difference. The graphs show that it does so, hence they are a useful data point. Getting an ECU re-mapped is a different story of course, that's typically when timing is being advanced further than stock. The dyno chart on my web site is with the car in stock condition and running 95 RON and it produced 236.4Hp under those conditions. It hasn't been dyno'd yet with 98RON fuel - it's on the to do list.

Good luck getting a balance of things. I've even wondered if altering types of plugs would just change numbers. Like if a cold plug is more recessed in its hole you might get a more advanced timing from the computer to make up for the extra time before the cylinder fires. But not much would change except for the timing. Just a thought. If you have a durametric, I'm pretty sure there is a knock sensor reading. Knocking and the O2 sensor in combination with the MAF is what I figure might change timing to explain the difference. Like you know the torque and hp numbers are whats important. Maybe there are read out you could find for other simular porsche stroke and bore of engine with the performance tuning. Anyway good show on the tinkering with the car. I guess taking the car over to some McLaren shop is out of the options