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Old 07-20-2010, 10:18 PM   #12 (permalink)
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Originally Posted by Meph View Post
no EGR, JDM engine. I have a vac/boost gauge int he car and the values between my MAP sensor and boost gauge are comparable. The X axis is all in PSI, the conversion is somewhere around 2.03, I cant talk in -psi or in-Hg, but not many people use -psi.
Well, from what I'm seeing on your graph, your 7.74 PSI x 2.03 = 15.75 inches Hg seems about right for a steady cruise speed.
Without EGR and with your low compression the numbers I gave as an example should be pretty close. You said that you're going a bit slower than 60 MPH though, so unless your car has a lot of drag somewhere then you may be able to get away with a little more than what I have in my example for your cruise speed.

If you really want to tune to the edge and are worried you won't hear the knock, then there are aftermarket stand-alone kits that allow you to use a knock sensor and display the feedback for you. More sophisticated ones can also adjust the SA automatically, but they are more pricey.

As for a lean AFR: I have tuned a 1998 C5 Corvette that was very steeply geared at 3.15:1 with 29" diameter rear tires (the owner/driver weighed around 370 lbs) - it was a drag racer.

I set up an open-loop tune (no O2 feedback) with:
DECEL AFR of 16.5:1 or DFCO with less load (aggressive DFCO setting)
steady cruise AFR of 15.5:1
very light ACCEL AFR 14.5:1
moderate (but normal) ACCEL AFR 13.8
heavier (but not PE mode) ACCEL AFR 12.8:1

I never hypermiled it to test, but the car had a factory MPG gauge that averaged around 25 MPG with his normal driving.
The SA numbers for that engine wouldn't be comparable to yours so I won't even mention them. I just wanted to offer an example of AFR varying with engine load.
If you try to run 17.5:1 across the board, then you will definitely experience some lean misfire when you load the engine in your upper gears.
Very lean mixtures like that require excessive SA, and that will reach a point of being counterproductive in the search for efficiency. It will also be hard to keep the engine from surging when transitioning from cruise to light ACCEL.
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