3900 6 speed MPG test

[josephupson]

I conducted an impromptu MPG test on the way home from GA to prepare to swap out the forged motor which I damaged after a series of preventable mistakes that I suspect has resulted in at least one broken piston ring or ring land to be covered in the build thread later in the next week or so. Despite the damage and resulting blowby and detonation causing oil contamination, the hwy mpg was higher than the initial pre damaged test result of 30 mpg hwy, with the 80 lb lighter 15" wheel and tire combo and stock ride height. That test was conducted in about 95% openloop using code59 which automatically suggested better potential due to the 13.4:1 AFR it ran in during that time.

The first leg of the trip to Atlanta after the first test and with the wheel swap, ~1" drop, plus a few changes to the tune still in openloop due to locking up the computer before I could change it produced 20 MPG hwy and a panic since the trip was just shy of 500 miles. Once the laptop was straightened out and closedloop activated the hwy MPG immediately jumped to 25 MPG at 14.7:1 AFR. The first and second tests were conducted using code59. I thought the 18" wheels were responsible for some of the 5 MPG loss along with the car being weighted down a bit.

It turns out that was partly correct but for a different reason. The wheels and tires sat for at least 6 months before install. When I finally brainstormed the MPG loss, and looked at the car, I found all tire pressures to be in the twenty psi range and since the car was not aligned after being lowered front and rear ~1 inch with coilovers in the rear, the alignment was way off and it still is now just not by as much. So that's what caused the negative result in the second test.

Todays results were using 8F Turbo Grand Prix code with lean cruise turned off at about 70 mph using cruise control and 14.7:1 AFR, 89 octane.

3900, stroked to 4.2L rounded, ~11.5:1+ compression ratio (certainly less due to damaged rings) turbocharged, first year F40 from G6, 225/40R18 front, 245/40R18 rear.

189.4 miles/5.685 gal=

33 MPG HWY, it's lousy around town though probably 21 mpg. I stopped at McDonalds in the middle of the test so that along with dropping .3 should account for any variance in the fuel top off at finish by erroring on the low side. Hope the stock motor does even better while its in place until this one is repaired.