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I was discussing E85 and scaling ignition timing with a friend last night over dinner and we got to a point where I was unsure of the correct answer and thought that I would see if anyone could help out. Int this specific example we are discussing a 2JZGTE that is factory compression running sub 30PSI of boost from a 6266 precision. When the car was originally tuned we were able to hit MBT without any detonation on 83% ethanol. The question that came up was how low of an ethanol percentage could be run and still keep the cars ignition timing the same without it knocking? If we filled up and the ethanol content was lower....say 70% are we still safe at the old timing levels? Now I am very aware that it varies from engine to engine and that know one over the internet can tell me exactly where this specific setup will start to run into issues.
But as a rule of thumb could you tune a properly setup 2J on an ethanol content of 70% and still run the same ignition timing as 85% ethanol with boost being below 30 PSI in both cases? If not where does that start to fall off approximately?
The very broad rule of thumb with Ethanol is that below 20% there is not much of a gain over Petrol, and that over 70% the gains tend to plateau into a shallow rise with only small gains to be had. This, as always, depends on the brand of fuel used and whether it is blended with regular or premium ULP, the water content of the ethanol and the engine itself.
I'd agree with BlackRex's comments - In my experience once your over about 70% ethanol there isn't much gain/difference so you're likely to find that you can maintain the same ignition timing between 70% and 100%. If I'm tuning an ECU that offers a blend table base doff ethanol content I typically retain the gasoline ignition and boost maps until I'm above about 20% ethanol and then slowly ramp them up towards the full ignition/boost by about 70%. I don't use a straight line interpolation for these points though preferring to be a little more cautious and slow ramping the boost/timing in. This results in a sort of exponential shape to the blend table.
Of course the amount of timing and boost an engine will take is very dependent on the specific engine so you still need to test and confirm that the tune is safe at several ethanol contents in between your expected minimum and maximum values.
Awesome! Thank you both for the great answers and your time.