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ever_green 09-25-2012 01:01 PM

Impreza - When to shift? BSFC & A/F
help me find my sweet spot rpm for shifting and cruising rpm. i have been shifting at 1800-2000rpm at about 95% engine load without any noticeable gain in fuel economy. i have been trying to improve the mileage on my impreza in the city but i cant get any better than 22mpg. i have found an old subaru bsfc map but i think it's irelevant since it's for the old 2.2L with out AVLS (subaru's VVT). here is a dyno chart of 2.5L (red is stock):

according to some dynos i have seen a/f is leanest at 2.3k rpm at wot. i have more charts if needed.

Daox 09-25-2012 02:01 PM

Well, you have a automatic, so you might be loosing a bunch of power to the torque converter. Also, are you sure you're in closed loop at 95% load? If not, that will hurt mileage.

ever_green 09-25-2012 02:53 PM


Originally Posted by Daox (Post 330076)
Well, you have a automatic, so you might be loosing a bunch of power to the torque converter. Also, are you sure you're in closed loop at 95% load? If not, that will hurt mileage.

yea closed loop even in 99% load. only open loop if rpm over 4k and wot. 99% load doesnt equal 100% throttle though. the lod is kind of sensitive and maybe even inaccurate.

MetroMPG 09-25-2012 03:22 PM

I like to tell people not to worry so much about acceleration technique* in city driving.

The big, serious MPG gains are to be found by concentrating on the brake pedal - minimizing its use.

That means: don't use any more power than necessary to coast into your next transition (stops, slowdowns, corners, etc.). Coast as much as possible without pissing off other drivers. Every time you use the brake, somewhere a puppy dies. (Oh, and your MPG goes down because you're turning fuel into brake dust & heat.)

Add "avoid idling" to the list and those two techniques will dwarf any possible MPG gains of theoretically perfect acceleration efficiency.

(* unless you're pulse & gliding).

For cruising RPM, the lowest speed in your tallest gear is the safest bet for best MPG. Beyond that, more MPH = less MPG.

MetroMPG 09-25-2012 03:29 PM

PS - I realize this (avoiding the left pedal like the plague) is going to be challenging in 100% city driving in Toronto. But them's the brakes! Breaks. Whatever. :)

Daox 09-25-2012 03:33 PM

Metro is completely right. Me and him both use different acceleration techniques, but we can also both hit 100% above EPA ratings on our cars.

ever_green 09-25-2012 04:25 PM

i cant much avoid idling because traffic always inches forward every 10seconds or so. i have been driving without brakes and with load. that hasnt made much difference. mostly 10.6l/100km vs 11l/100km. but i have had no problems beating the highway mpgs by 30% without even trying, by just using cruise control.

as for cruising i knew drag increases exponentialy with speed, i learned that couple years ago in highschool physics. but i have read people talking about subaru running lean or its best at 3000rpm. I haven't been able to confirm this and plus that means braking the law at 75mph. to me 2300rpm seems the best but that puts me only at 55mph; not too safe for highway. IGN is 42 at 2600rpm going just under 70mph.

ah here is the a/f chart for a 2.5L na

roosterk0031 09-25-2012 04:29 PM

Stratus has about same size motor, I take it to about 25-2700 rpm with as much throttle as I can without it downshifting. Too low of rpm with a auto and the torque convertor is slipping alot, we need to get them into drive and TC lockup as quck as pratical.

Agree with the above most gains come from the coast. My 15 yo girl has been driving the Stratus the last month and is averaging 33 with her 4 mile each way drive to school, she's a pretty good coaster, didn't talk at all about how to accelerate during her pratice time, but did emphasize when she could start coasting.

Daox 09-25-2012 04:36 PM

See if your employer will let you start earlier or later so you can avoid traffic. I've done this multiple times.

MetroMPG 09-25-2012 04:39 PM

If you're stuck in stop & crawl traffic, you probably already know your goal should be playing the accordion: avoid coming to a stop. Let the vehicle in front stop and go, while you idle along behind it (like you see the big trucks doing) minimizing braking. You'll still have horrid mileage, just not as horrid as the car in front of you.

If your drive is as bad as that, your better options may be along the lines of shifting your commute time to avoid the traffic peak (if possible), public transit (for all or part of the trip) or biking (depending on distance).

As for highway, regardless of that AF map, I'd bet you a timbit you'll still find your lowest consumption is just after top gear engages.

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