Re: 2000 7.4 MPI fuel pressure question
I ask because I am concerned about E15 being approved and how much it will lean out the air to fuel ratio in my engine. Gasoline requires a 14.7:1 air to fuel ratio. Ethanol contains oxygen and requires less air so it has a 9:1 air to fuel ratio. E15 will be running 9% too lean if the injection system is setup for plain gasoline. That could be troublesome.
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You bring up a very interesting point, and one which I've never considered before, but before we get into that, I would be very surprised if Mercruiser uses a variable rate fuel pump and a fuel pressure input to the engine control unit. I have no direct experience with the Mercruiser system, but I suspect that it is a typical speed density control that uses inputs from engine RPM and manifold pressure as the prime inputs to calculate the injector pulse widths depending on engine speed and load.
As for the air fuel ratios, stoichiometric ratio ( the ratio where all fuel and all oxygen in the mixture is consumed ) is indeed 14.7:1 for gasoline and 9:1 for ethanol. This is assuming a standard specific gravity, so the ratios will vary a bit, but not much for practical purposes. Stoich ratio is not what is required to burn the fuel, it is just the ratio where least emissions are produced. What is important from a performance stand point is what ratios produce the best power, and what ratios produce the best economy? For best power on gasoline it is generally agreed that between 13 and 12.5:1 works best, any richer will produce less power and the excess fuel is just used for cooling. Best economy is as lean as the engine will run without a "lean stumble". That is usually between about 16 and 17:1. I have engines that will run quite happily lightly loaded at 17.5:1.
Now as far as ethanol is concerned, I have no idea where the best power ratio would be. Stoich is at 9:1, but the emissions from burning pure ethanol are completely different than what happens with gasoline. I would think that best economy would be determined the same way, by how the engine runs, but I really don't know.
So, getting back to your point that E15 would result in a 9% leaner mixture. First of all, I'm not sure how you came up with that figure. According to my math, and admittedly, math is not my strong suit, if you have a mixture of 85% gasoline, and 15% ethanol, at stoich, you end up with a fuel air ratio of 13.84:1. What does that mean in terms of energy available to burn or emissions? I have no idea.
So, How will this affect how your engine will run? I am also assuming that Mercruiser does not use an O2 sensor to provide feedback for a closed loop system. This means that the fuel maps were prepared by data from lots of dyno time using standard gasoline under typical conditions with a bit of safety factor thrown in. Now you will be burning a mixture that has not been used in preparing these maps. Will this mixture have leaning effect, or a richening effect? How much of a safety factor did they build into the maps, and will it make any real difference at all?
Not an easy question to answer from the layman's point of view.
