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Ethanol & Flex Fuel Tuning: Swapping Fuel

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Swapping Fuel

04.06

00:00 - In the last module I mentioned the issues that can be caused by residual fuel left in the tank, diluting your new fuel when you swap backwards and forwards between ethanol and gasoline.
00:11 It's not uncommon to leave five litres or more of a fuel in the tank.
00:15 And this can be difficult or impossible to drain, and particularly if you're only topping up with perhaps 20 litres of your new fuel this residual fuel can have a significant effect on the overall composition of your fuel.
00:29 For example if we had five litres of gasoline left in the tank and we poured in 20 litres of E85, the actual ethanol content would be reduced to approximately 68%, which can have a serious impact on your tuning.
00:45 The more concerning issue though, is when the tank is empty again, and a further 20 litres of E85 is tipped in, we'll end up with a much different fuel composition and again the tune will become inaccurate.
00:58 Without the benefit of a fuel composition sensor to adjust the tune, we want to be particularly careful of how we go about changing fuel, and I have a couple of tips here.
01:09 First of all if you're building a fuel system in a race car with dual fuels in mind, it's worth adding some drain plugs to your tank and your surge tank so that you can properly purge the fuel system when swapping fuels.
01:22 This is fine for a purpose built fuel system, but less suitable for a factory stock fuel system.
01:29 In a stock fuel system we have two options.
01:32 If the fuel tank does have drain bungs, then this is a nice easy solution to removing the fuel from the tank.
01:39 On modern cars drain bungs are now pretty uncommon though.
01:42 So the other option we have is to disconnect the fuel return line and run it into a container before powering up the fuel pump.
01:51 The fuel pump will likely only operate if the engine is running so we can power the fuel pump using the laptop or a scan tool to put the fuel pump output into a test mode.
02:02 If you are planning to do this frequently, it can be worth wiring in a bypass switch to control the fuel pump relay directly so you don't need your laptop or scan tool.
02:13 Draining the fuel in this way will almost certainly still leave some fuel in the tank, but there's little we can do about this.
02:20 The technique I use to help reduce this contamination is to start with the new fuel by pouring in only two to three litres before draining the tank again.
02:31 This still won't completely eliminate the contamination, but it does help.
02:36 And it also purges fuel through the fuel lines and fuel rail.
02:40 Of course the downside of this is that you are going to waste some fuel.
02:45 Once we have as much of the fuel out of the tank as possible, I also suggest completely filling the tank with your new fuel.
02:54 The reason for this is that it will reduce the affect of any residual fuel contamination remaining in the tank.
03:01 As we discussed earlier, if we have five litres of gasoline remaining in the tank and we add 20 litres of E85, our actual ethanol content is reduced to about 68%.
03:13 If the tank holds 60 litres though, and we completely fill it with E85, we would now have five litres of gasoline remaining, and then 55 litres of E85, so the overall ethanol content in our tank is around 78% which is much closer to the mark.
03:32 Understanding the effect of this fuel dilution, as well as the potential effect on your tune, is a big part of managing ethanol fuel properly.
03:41 If your fuel system doesn't lend itself very well to being completely drained then it's smart to consider this in your tuning, and leave a wider margin of safety, with a slightly richer mixture than necessary, and a more conservative approach to your ignition timing and boost.