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Old 11-19-2017, 05:08 PM   #1 (permalink)
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Mazda3 2.0 Manual can't break 40mpg

I'm hitting a wall at 40mpg with my manual 2017 Mazda3 2L with 10k miles on odo. i have studied the bsfc maps for this vehicle and sweetspot is 2000 rpm at roughly 70% engine load on scangauge. I try to shift at the following engine speeds to maximize time spent at sweet spot (2000 RPM):

RPM falls TO 1800 after shifting at below speeds
2900rpm 1-2 Gear
2700rpm 2-3
2300rpm 3-4
2200rpm 4-5
2200rpm 5-6

this for both city and highway and i try to maximize coasting in neutral or gear depending on situation. On highway i cruise at 65mph or 2300 rpm with 45% engine load on level ground.


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Old 11-19-2017, 09:18 PM   #2 (permalink)
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I think your low gear shift points are too high. BSFC isn't everything.
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Old 11-19-2017, 09:44 PM   #3 (permalink)
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Quote:
Originally Posted by ever_green View Post
I try to shift at the following engine speeds to maximize time spent at sweet spot
I assume that means you're skip shifting to upper gears in sub/urban driving a lot?

EG: shifting from 2nd or 3rd to 6th after accelerating to cruising speed of ~60 km/h
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Old 11-19-2017, 09:45 PM   #4 (permalink)
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45% load at 65mph seems like a lot for a slippery moden car.
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Old 11-19-2017, 10:42 PM   #5 (permalink)
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Here is a video of way i shift, usually i skip 5th and go right to 6th but sometimes like here i don’t. 45% load might be because of atkinson cycle retarding VVT and ignition to open up the throttle.

here is a video of way i shift:



https://youtu.be/RJRs0LE5DK8
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Old 11-20-2017, 05:24 AM   #6 (permalink)
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How long are your trips?
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Old 11-20-2017, 06:12 AM   #7 (permalink)
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The thing that strikes me in the graph is the curves that follow a set amount of power.
While the area around 2500 revs is most efficient when you consider equal torque, it definitely is not when you look at equal power.

At lower revs you need more torque to get the same power and that dramatically shifts the balance towards lower revs.
The 7.5 kW line has an optimal BSFC (around 250) just below 1000 RPM.
The 15 kW line hits 235 around 1300 RPM.
The 30 kW line hits about 225 just above 2000 RPM.

That seems to suggest that when maintaining speed on the level (10-25 kW at moderate highway speeds) you should select as high a gear as possible and only shift down when you go over 80% throttle.

Maybe it is as simple as this:
If you go over 80% throttle shift down. If you go below 60% throttle shift up.
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Old 11-20-2017, 11:03 AM   #8 (permalink)
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The optimal shift points when accelerating are lower than what you get from the BSFC chart. That's because you are using fuel to spin up the engine, then throwing that kinetic energy away when shifting.

I use first gear just enough to get rolling, then immediately into second gear. If not in a hurry, I'll shift out of first as low as 1000 RPM. My technique is mash the gas pedal about halfway or a little more, then set the shift points to get the acceleration that I want. Typical shift points range from 1500 to 2500 RPM, and rarely as high as 3500 RPM pulling onto a highway uphill.
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Old 11-20-2017, 01:33 PM   #9 (permalink)
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I'm not sure your shifting is hurting you that much. Yeah, maybe you can get marginal gains by shifting a little lower, but tbh your biggest gains on the highway are going to be from slowing down, and your biggest gains in the city will be from conserving momentum, and shutting the engine off when you don't need it.

Slowing down from 65 to 55mph improves my fuel economy from 75-80mpg to 95-110mpg.
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Old 11-20-2017, 03:36 PM   #10 (permalink)
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Looking at the graph it appears to me that the SkyActiv engine is happier at about 30-35% load at 2500rpm, not 70%. The island of efficiency is at around 35kW at about 2200-2500 rpms.

Like the other guys said, but it bears repeating: slow down. Aero drag goes up exponentially with the cube of the velocity, so slowing down pays dividends immediately and in a big way: doubling your speed multiplies the aero drag eight times. Backing down from 65 to 60 will reduce your aero drag by over 20%, while increasing your travel time by only 8%. Leave five minutes earlier, you'll never miss it.

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