Well, I just finished with a fairly long experiment. I have been moving the cam timing around on my car to see what effect it has on mileage. Keep in mind that I have shaved the head .040″ so the car cannot tolerate as much advance as when it was stock. I have tried from -2 to +11, and for each of them I ran the maximum ignition timing that I could run without knock. I gave up trying to do 2 tanks each due to the weather, and my having to drive less to work means that it takes me 2-3 weeks to run a tank of gas out now so it would be almost a year to do this test the long way. I did short tanks of at least 100 miles and filled the tank to the top with the car sitting at an angle to keep the air bubbles out.
The numbers are probably not 100% perfect, but they are pretty close and fit a curve reasonably well, so they still give useful information.
The curve is not perfect; I am pretty sure the 5 degree advance number is a bit lower than what it should have been. I would say advancing a stock XFI engine 5 degrees is ideal, and advancing one with the head shaved 3-4 degrees is probably ideal. I will redo this experiment later on going in one degree increments from 2-5 degrees advanced to see if I can fine tune my timing.
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Filed under: Metro