Impreza - When to shift? BSFC & A/F

[ever_green]

Originally Posted by Daox

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

I go to college

[ever_green]

Originally Posted by MetroMPG

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.

thanks i have also come to the conclusion that my best option would be public transport in order to save cash. but being as i live in the suburbs that's not possible. unless i take the go train and park at the incredibly packed parking lot (if i find a spot) and pay more for a round trip than i pay for gas. so unfortunately i have to keep driving. i have started avoiding traffic by changing my times and it seems to be the best improvement ever so far. my overdrive engages at 30mph. not only is 2.5L too big for me in the city but it is also a full time 4wd drive-train.

[MetroMPG]

Aha.

Well, it's a wagon (OK, hatchback). What about sticking a bike in the back, finding somewhere to park along your route and now you have a 'hybrid'. Save fuel, get some exercise & enjoy the endorphins. Maybe not a pleasant winter option though.

[ever_green]

sounds good but it's a sedan lol.

[MetroMPG]

Ha!

I don't see many Impreza sedans, now that I think of it.

OK, so take the front wheel off and stick it in the trunk.

[ever_green]

Originally Posted by MetroMPG

Ha!

I don't see many Impreza sedans, now that I think of it.

OK, so take the front wheel off and stick it in the trunk.

yes a sedan with a rear wing, not sure how much drag that causes rofl. or i can just get a bike rack lol, anyhow not sure about biking along highway 7 in december lol.

[kennybobby]

Looking at the dyno data the torque peaks around 4400 rpm. This is the most efficient power production point of that engine, highest volumetric efficiency of the airflow in the intake, lowest specific fuel consumption, etc. If you had the BSFC chart it would show this for you graphically.

The dyno test was measured at WOT, which would have WOT-enrichment in the A/F ratio data, so don't be misled into thinking that 2300 is your most efficient operating point for acceleration purposes.

You may not have a need for acceleration in town, but when you do on the highway, put your foot into the throttle just short of WOT to let that engine breathe efficiently, then plane out and glide in top gear at the lower power level.

Shift up around the torque peak for the lowest fuel consumption to get you up to speed, plus it has the highest smile-on-your-face factor. Extraordinary claims require extraordinary data--try it and see if you notice a difference.

[Daox]

That isn't necessarily true. There are other factors besides peak torque to consider when accelerating efficiently. There is obviously fuel enrichment as you've stated, but there is also ignition timing that effects efficiency. I would tend to say that ignition timing is not optimized near peak torque because it is peak torque and that is the point where preignition/ping will be a problem. The most efficient point is likely at a lower rpm than peak torque.

[ever_green]

bsfc for older 2.2 subaru suggests 3500 most efficient near wot
http://ecomodder.com/wiki/images/thu...-ej22-bsfc.png

this is ofc different engine. new 2.5s have i-avls which uses dedicated high lift cams for higher engine speeds (similar to vtec).

[user removed]

Pick your shift point, probably close to 2500-3000 RPM depending on the engine. When you use higher gears and load the engine up properly for economy you can have 80% load at lower throttle positions. Beyond that throttle position adding throttle will have little effect on you rate of acceleration.

regards
Mech