Fuel System Analysis, 328i, MT, E92 by ProfessorCook

4 minute read

Fuel System Analysis, 328i, MT, E92

Compliments of ProfessorCook @ www.bimmerfest.com

I've taken advantage of a rainy day to compile more data about my January build coupe.

First, mileage:

I attribute the general, modest, upward trend through June to the mild increases in average temperature during that period.

From mid-August to date, I've been trying for higher mileage by having fun during accelerations, but then using the biggest gear possible to keep the revs below 2k. I refer to this as "driving in Prius mode." Not as much fun while driving, but I enjoy the challenge of seeing if I can break 30 mpg. (Not quite yet.)

Next, how accurate is the OBC fuel consumption gage?

I really can't explain why it's been getting better, but as you can see, my OBC mileage is stabilizing at only about 1% to 2% optimistic. Pretty accurate, really. I'm impressed.

Next, fuel prices.

With only one exception, a tankfull of Sunoco premium 93 octane, I have only filled with Shell premium gasoline which, around here, is always 93 octane. I wonder how long we'll have $3/gallon gasoline.

The last time I posted, somebody asked how linear our fuel gages were. I started keeping track. Here are the results:

For those math and science types, look at that R value! But you don't need a college degree to see how linear these results are.

Two important take away points here:

1. When my fuel gage reads zero, I can put 15.64 gallons in. That doesn't mean the tank is dry though... there still might be some left even when the gage reads zero.

2. When you fill up, there's about 1 gallon of gas "extra" beyond the fuel gage reading of 100%. So I should be able to drive about 20 to 30 miles before the gas gage begins to show a value below 100% full.

Finally, how does the BMW's OBC Miles to Go value measure up?

I got my own value by taking 15.64 as my total capacity, subtracting out how much I filled to get the estimated fuel remaining in the tank. I then multiplied that by my calculated fuel consumption to estimate how many miles I would have been able to go before my gage read zero.

As you can see, the BMW is consistently optimistic and from the equation of the linear fit, the BMW is optimistic by about 24%. That's important. It tells me not to put too much faith in the OBC's estimate. If the OBC tells me I have 100 miles to go, I now know I only have about 80. Now, it's possible that the OBC is taking into consideration some amount of fuel that still exists in the system when the fuel gage reads zero, but if that were true, the linear best fit shouldn't have an intercept at the origin.

Overall conclusions:

1. You can drastically improve your mileage by keeping your revs low. (Not that you want to always drive that way... gotta have some fun too right?)

2. The OBC consumption value is pretty darned accurate.

3. The fuel gage is beautifully linear but there's an extra gallon hiding beyond "full."

4. The miles to go value from the OBC appears to be optimistic by more than 20%. Careful!

5. Who the hell knows where fuel prices are going to be a month from now?