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Old 07-10-2012, 02:30 PM   #1 (permalink)
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BSFC and Subar Impreza 2.5i - Confused!!

ok i have a 2.5L impreza sedan and im trying to find the rpm most efficient to accelerate to from dead stop and to cruise at on the highway. i have been reading all these stuff about brake specific fuel consumption and i have been utterly confused! the only subaru map that i have found is this:



i have difficulty understanding this but is this suggesting that BSFC is lowest at 3500rpm (190BSFC)? could someone help? thanks.

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Old 07-11-2012, 01:22 PM   #2 (permalink)
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While the BSFC is lowest at about 3500 RPM, it is almost as low from 1500 to 4000 RPM at or near wide open throttle.

If you drove 60 MPH on the highway at 3500 RPM, the torque would be down around 3 to 6 Kgf-M with terrible BSFC and MPG.

This engine accelerates most efficiently at or near WOT. The shift points could be anywhere from 2000 to 4000 RPM depending on how fast you want to accelerate.
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Old 07-12-2012, 06:33 PM   #3 (permalink)
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Quote:
Originally Posted by JRMichler View Post
While the BSFC is lowest at about 3500 RPM, it is almost as low from 1500 to 4000 RPM at or near wide open throttle.

If you drove 60 MPH on the highway at 3500 RPM, the torque would be down around 3 to 6 Kgf-M with terrible BSFC and MPG.

This engine accelerates most efficiently at or near WOT. The shift points could be anywhere from 2000 to 4000 RPM depending on how fast you want to accelerate.
thanks for the post. at 65mph my engine is doin about 2500rpm and by 75mph its at 3000. i know that at 65mph cruising on flat surface my car requires around 30hp. i'm not sure how this torque thing works but i found this:



someone else added the KW and blue lines. my car uses about 22-25kw cruising at 65 flat surface. so it would put me at label H correct? and when im passing if i downshift 1 gear and add slightly more throttle my rpms jump about 1000. im guessing power will jump to 60kw and so it would be some in the 200gr consumption range right? is that the most efficient passing then?
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Old 07-12-2012, 09:13 PM   #4 (permalink)
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Are you sure about the 30 hp at 65 MPH? My truck is larger than your car and it needs about 32 hp at that speed on a cold day, based on a coastdown test.

Time for some math......

30 hp X 0.746 = 22.4 Kw. From the second graph, 22.4 Kw at 2500 RPM puts the engine BSFC right at 210 grams / PS / hr. You should be at slightly less than half throttle, say about 16 to 18 inches Hg vacuum.

If you could add one more gear to the transmission to drop the RPM to about 1800 at 65 MPH, the BSFC would improve to 195. Steady state gas mileage would improve by 210 / 195 = 8%.

You can pass in either gear. If you pass at or near WOT, the BSFC decreases to 190 grams/PS/hr in either gear.

If you stay in top gear, the power increases from 22 Kw to 50 Kw. That should be plenty for most passing. The most efficient passing is with the least increase in speed because you then spend less time at partial throttle (300 BSFC or worse) to slow back down.
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Last edited by JRMichler; 07-12-2012 at 09:19 PM..
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Old 07-13-2012, 12:45 PM   #5 (permalink)
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cool thanks a lot for the help. yes im sure about 29hp is required for cruising but it could be +/- 3hp.
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Old 07-14-2012, 02:20 AM   #6 (permalink)
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Quote:
30 hp X 0.746 = 22.4 Kw. From the second graph, 22.4 Kw at 2500 RPM puts the engine BSFC right at 210 grams / PS / hr. You should be at slightly less than half throttle, say about 16 to 18 inches Hg vacuum.

If you could add one more gear to the transmission to drop the RPM to about 1800 at 65 MPH, the BSFC would improve to 195. Steady state gas mileage would improve by 210 / 195 = 8%.
not sure but im no where near 1/2 throttle. my idle TPS is 14 on scangauge. at highway cruising on flat surface TPS is 20. i think wot is 78 for me.



Quote:
You can pass in either gear. If you pass at or near WOT, the BSFC decreases to 190 grams/PS/hr in either gear.
doesn't that depend on the gear because the gear determines RPM?

Quote:
If you stay in top gear, the power increases from 22 Kw to 50 Kw. That should be plenty for most passing. The most efficient passing is with the least increase in speed because you then spend less time at partial throttle (300 BSFC or worse) to slow back down.
im not sure i understand the last part. isnt least increase in speed bad? if i have more load (more throttle) hence faster increase in speed i spend NO time at partial throttle? im sorry im so confused this part! So im gathering that i should pass on final gear at wot vs dropped gears with high rpm and 3/4 throttle?
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Old 07-14-2012, 04:39 PM   #7 (permalink)
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You might be better off looking at MAP than TPS. MAP is Manifold Absolute Pressure. On my truck, idle is about 4 PSI MAP and WOT about 14 PSI MAP. In between, torque is roughly proportional to MAP.

At low RPM, MAP increases a lot with only a little throttle opening. You can be near maximum torque with the throttle open only a little bit.

Back when I was getting 27 MPG, I was usually passing in 3rd gear. Now, most of my passes are in 5th gear. Although, I did do a 4th gear pass about 3 or 4 weeks ago.

My goal is to pass at no more than about 5 MPH over the speed limit. This requires advance planning.

Calculating the best way to accelerate and cruise from a BSFC map is a good intellectual exercise. Most people are better off to focus on improving trip mileage. I developed my acceleration technique by looking at trip mileage at a fixed point about a half mile from where I start. In my truck, the best technique is to use 1st gear to get rolling, then immediately shift into 2nd. I'll shift to 2nd when I hit 1500 RPM in 1st, and into 3rd when I hit 2500 RPM in 2nd. My accelerations are at 13 to 14 PSI MAP. Accelerating like a granny did not work as well.

You have to find what works best in your vehicle. The BSFC chart gives useful information, but does not tell the whole story.
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Old 07-15-2012, 12:19 PM   #8 (permalink)
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thats true. but i only use bsfc charts for acceleration purposes. i have been downshifting 2-3 gears to pass but recently stopped doing that. i can pass really well in top gear noe no problem with car churning about 3000rpm at near wot. my car is maf equipped and not map. although sg2 still reports map!? i also have programmed for maf in g/s. can that be helpful?
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Old 07-15-2012, 04:49 PM   #9 (permalink)
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If you are at or near WOT at 3000 RPM, then your engine is right at it's best efficiency point. If you downshifted and used partial throttle TO GET THE SAME POWER, the efficiency would be less and you would use more gas.

Of course, overall MPG will be even better if you don't pass at all.......

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