Best Acceleration Load for a 2.5L non-turbo Subaru
 

I used to be following the official BSFC chart for a 2.2L sohc subaru engine which suggested best efficiency load of 80-90% below 3000rpm.

http://img198.imageshack.us/img198/1...opartorque.png

The Test


vehicle: 2009 subaru impreza 2.5i sedan auto

Experiment: Perform an acceleration run from dead stop to 70km/h and observe overall MPG at the finish checkpoint. Overall distance traveled will be 400 meters. All runs are to be in closed loop mode with A/F ratio of >14. Start and finish checkpoints are at exactly the same spots for all runs. Weather and temperature are the same (rainy and 2'c or 35'f). cruise control will be used to hold speed once at 70km/h. Car is fully warmed up before Run A. On a separate experiment find the best engine load for short distance acceleration (ie. heavy traffic).

observations :
Run....Load.......Shifts.........Overall Consumption........
A........60%.......1800rpm.......12.1L/100km
B........80%.......2300rpm.......12.3L/100km
C......+90%......3000rpm.......13.3L/100km

conclusion:
acceleration at 60% engine load provided the most efficient acceleration. Even still run B with 80% engine load provided almost the same efficiency. The gap in efficiency widened when engine load exceeded 80% and the torque converter became fully unlocked. The TCU shiftpoints also increased dramatically past 80% load but this might Not be the reason for the 10% increase in consumption. More experiments with fixed shift points and longer distances are needed to confirm this. for now i will be accelerating at ~70% engine load instead of 80-90%.

For heavy traffic / short distance acceleration i found that minimum engine load is best. so apply as little throttle as possible when stuck in slow moving traffic with lots of stop and go. if you are to stop in a short distance (ie. red light in 100 feet apply no throttle at all and let the car roll). any considerable engine load increases consumption for short distance acceleration.

Have you done any coast-down tests to determine your Crr? At this speed rolling resistance is a big factor in the power curve.

In imperial units it looks like you are doing a 1/4 mile acceleration run from 0 to 45 mph--did you only shift once during the run? What is the time to reach 70 kmh for each of the runs ABC--from that you can calculate your power and relative fuel consumption compared to the blue lines on the BSFC chart.

Also you might want to make some runs at 30 to 50% load to see if the consumption is higher there and that the best economy peaks at 60% and not lower.

hey at 50% load car never reaches 48mph by thr 400 meters end. thats why i didnt try it. i was in 4th gear by the end of all runs. i wanted to keep this a bit simple to minimize chance of errors. i will do 3 runs again today but with 35mph speeds.

from my tests again 60-75% load seems best for short distance acceleration aka. city driving. i trust my scangauge 2 fully, it is always right to +/- 0.1%. anything below 60% you will not accelerate properly for a typical 4 cylinder, so you spend less time cruising/coasting. higher than 80% load and my consumption just skyrocketed. A/F stayed stoich at 14.4-14.5, but it resulted in tremendous increase in airflow that called in for more fuel injection -> hence poor mileage at +80% load. im blaming the high lift cams that engage under heavy load. i think modern cars control pumping losses much better than they used to so i would go easy on the throttle.

the only case where i have seen low-moderate loads hurt mileage are on a 2.3 turbo RDX. it just seems that spending time in vacuum for acceleration kills trip mileage. so on the turbo car i accelerate with 1/3 boost (according to built-in boost gauge, i think thats 4-5psi).

If you do anymore runs be sure to record the acceleration time in seconds--without that we cannot calculate the power level of operation. e.g. 0 to 70 kmh in ?10 seconds, 15?, ??? seconds. The quicker the acceleration time then the higher the power level.

ok i will record the time to 70kmh next time. but if power is of interest, here is a dyno from when car was stock at WOT. AFR stays lean until 3500rpm.

http://img838.imageshack.us/img838/8346/stock2007.jpg

Thanks for posting the dyno chart!

i wonder if the WOT sensor wasn't working properly--looks like it was running closed loop up to 3800 rpm?

Also it looks to me that if you wanted to get minimum fuel consumption during acceleration then it seems that you would want to accelerate at WOT up to the second torque peak at 4500 to shift and then keep the revs up between the first torque peak at 2800 and the second peak until you reached your cruising speed (and shifting up into highest gear). That is the lowest fuel consumption region (highest volumetric efficiency) by the BSFC chart and it gets you to your cruise speed the quickest, so you spend less time accelerating and more time at cruise/glide. Or maybe just run it up past the first torque peak to ~3400 to shift? "Nearly WOT" in closed loop, or real WOT in open loop may make a difference also. Anybody have any test data on acceleration techniques?

yes my impreza stays in closed loop until 4000rpm according to my scangauge 2. hmm not sure, if i shift at 4500rpm the mixture is enriched. yes i think the 3400rpm suggestion sounds better. that's what i have been thinking, since the car runs lean at wot it may not be such a bad idea to drive leadfoot. but this doesn't seem to work for city driving...for now im shifting at 2500rpm. but if i was doing pulse and glide on a highway then it seems 3400rpm is best at 90%. after all oversquare engines shouldn't mind a bit of revving.

Remember that while accelerating at higher load is more efficient, you also increase the speed fluctuations by doing this and thus you increase the ultimate power requirements of the car. That's why I am starting to try to pulse at 60-70% load, that way I am doing less pulse/glide cycles, and my speed is more constant (pisses other drivers off less).

here is todays tests i did with avg fuel consumption at the end of 400m run, load % and acceleration times(0-50kmh)

12lhk at 80%, 12s
11.9lhk at 90%, 9s
11.5lhk at 60%, 17s
15.8lhk at wot 3-4s

once again the miserably slow 60% load gave best consumption number. but it seems 90% engine load is the healthiest BSFC number. i did not monitor engine rpm. b
maybe shifted at 3000rpm first and 2800 second gear.