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91/98 Duel Map Setup, Fuel Efficiency


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I'm looking at buying an XR6T as a daily but with the price of fuel sky rocketing, I'm wondering whether it's at all possible to run a setup where you can simulate the conditions of an N/A Barra running on 91 octane fuel? Then by using a duel map setup could you program the ecu to run an extremely fuel efficient 91 tune on very low boost but then also have a 98 tune built in running stock power & stock boost? I know 91 octane fuel should never be the fuel of choice for a turbo set up but these are hard times.

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Yeh definitely possible.  You would have to retard the spark to find where the engine is happy on 91.

I would go the other way personally.  Have a tune on 98 that is super efficient/low boost and have a switch wired for the custom tune that you could flick for high power 98.

98RON has more energy than 91RON and theoretically should give you more mileage for the same amount of fuel.  You would need to work out if better mileage works out with the price difference.  however coupled with more spark low down(slightly more resistant to knock) for torque and lower boost(less air/less fuel burnt)  could be a cool experiment to find out real world results.


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  • 2 weeks later...

Roland thats interesting.    When I had an FG Turbo, I was quite surprised at how poorly it would run on 98.  There was an interesting Engine Roughness which just wasnt there on 95.    Did Ford just tune for 95? - or even 91 on the turbos? - Mine certainly ran well on all fuels.  I ended up going with 95, and the odd half a tank of 98, always BP


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Pretty sure Ford tuned for  95 and up to 10% ethanol as well, putting in better petrol allows for more power but there are still places in Australia where you cannot buy 98 at all. The autooctane setting most tuners turn it off because they tell their customers it is tuned for 98, putting 95/91 octane in their tuned vehicle would lead to disaster.

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21 minutes ago, fordsrule said:

Just having a brief read of this topic, do i understand correctly, it's saying that the turning on or off of the "switch" in octane allows the use of 98 or 95 with no further changes?


It isn't magic. There is a spark adder (or subtractor) table which adds spark across the board. By default it is either 0 or subtracts 4 degrees. When it is turned on I believe the car was tuned with 95 octane, so when it detects enough knock it assumes 91 octane and pulls 4 degrees from the whole map. This logic is only turned on for the NA vehicles.

You can set anything you want in this map though, and enable it on a turbo, not that I would recommend it but it is an option.


These numbers are not necessarily true or accurate.



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