Can't improve mileage

Discussion in 'Fuel Economy' started by pickler, Jan 30, 2014.

  1. pickler

    pickler Well-Known Member

    I have been doing pulse and glide with my Subaru Impreza and I have been using a vacuum and AFR gauge to help me accelerate efficiently. I have tried acceleration (pulse) at different loads with leanest possible fuel mixture with locked torque converter at 1800-2500rpm depending on my speed. I pulse for about 5-8 seconds at (~5in-hg vacuum & 14.5:1 AFR) and glide for about 7-10 seconds. Doing this constantly down a freeway led to almost exact same results as just using cruise control on a flat surface. If I glide anymore my speed becomes too slow. Not sure why but my car engine brakes substantially more than I feel it's necessary during fuel cutoff. Anyway I haven't been able to improve my mileage much by doing stuff such as driving with load, driving without brakes, or engine shutoff during stop. Maybe 5% total savings in gas but nothing worth the effort. This is strange because when I drive efficiently with my dad's Acura RDX I can substantially improve MPG just by watching boost levels and following tactics mentioned above (I'm talking in excess of 15%). In fact the RDX can beat my Impreza's mileage sometimes despite being 1000lbs heavier and with a substantially larger frontal area. Maybe NA engines don't have much wiggle room?
     
    Last edited: Jan 30, 2014
  2. seftonm

    seftonm Veteran Staff Member

    Do you glide in neutral? "Gliding" with engine braking won't really do much to help MPG except in situations where you'd have to be using the brakes anyway.
     
  3. PaleMelanesian

    PaleMelanesian Beat the System Staff Member

    I'd guess not. If you're not using Neutral for the glide, don't bother with P&G.

    I don't know about Subaru, but in my Honda it's not problem to shift N to D at speed. The Odyssey has nearly 120,000 miles on it and I do it multiple times per mile whenever I drive it. (family car, so my miles are the minority)
     
  4. pickler

    pickler Well-Known Member

    actually dropping to neutral does increase glide time, but not by much. maybe 1 full second at most. Plus in neutral MPG gauge decreases as though the car is applying throttle and my RPM starts climbing to 1300rpm. But gliding in drive gives me infinite miles per gallon despite the engine braking.
     
  5. seftonm

    seftonm Veteran Staff Member

    Generally speaking, a longer period of 200mpg will help your average more than a shorter period of infinity mpg. Perhaps your car is just an oddity with its engine programming, but with most NA engines you can see big gains with pulse & glide. It's also odd that dropping to neutral does very little for your glide time, that means there must be lots of drag elsewhere. How fast are you going and what are your tire pressures at? Or maybe the AWD system itself is a huge drag.
     
  6. ksstathead

    ksstathead Moderator

    Best I find in my AWD CRV with 4 speed AT is to slow to 50 mph or less to reduce wind load and use DWL/DWB. Speed alone gives a >50% mpg gain.

    I do think the AWD introduces some drag (on mine), but not huge. I still don't feel negative g's when gliding in neutral.

    Would only glide in gear if need to burn speed without fuel flow as in approaching a light too fast (to keep moving).
     
  7. pickler

    pickler Well-Known Member

    I used neutral coasting today to get to work. It made no difference to my trip mileage vs just leaving it in drive and let it enter DFCO. Yes there was noticably less engine braking but the fuel consumption was about 2.5L/100km while doing this vs 0L/100km if in Drive. This actually allowed me to glide for about 8 seconds from 60-48mph vs. 5-6s. Speeds i'm driving at is ~55mph which is actually below the legal limit. In winter time I can feel the change in drag, scangauge device displays 50% engine load vs 40% during summer when cruising. I get crappier mileage driving slower in winter at 50mph than I get driving at 70mph in summer. I have replaced some factory parts with lightweight parts (flywheel, pulley, alloy wheels, tires with high psi). The car is actually pretty comparable to the prius in terms of weight and frontal area but drag coefficient is poor at 0.32.. The AWD is usually at a 60/40 split, that could be the biggest drag on the drivetrain.
     
    Last edited: Jan 31, 2014
  8. ksstathead

    ksstathead Moderator

    DFCO is a heat engine, not a part of pulse & glide. Gliding is the absence of engine/tranny drag.

    If gliding in neutral does not help, then abandon P&G.
     
  9. seftonm

    seftonm Veteran Staff Member

    2.5L/100km at 90 km/h in neutral is HUGE. Combined with your short glide times, it seems that your vehicle just won't work well for P&G.
     
  10. PaleMelanesian

    PaleMelanesian Beat the System Staff Member

    I can't metric, at least when it comes to mileage. Let me convert, for my own brain.

    That's... about 95 mpg. At 55 mph. Which means it's idling at over 0.5 gph. That's huge. My car idles at 0.22 gph. My 3.5 V6 Odyssey is about 0.55 gph.

    Does the idle settle down after a certain period of time? It may be programmed to run high idle for a certain time when you shift to N. That period may be longer than a P&G cycle, meaning you don't have much to gain from it.
     
  11. pickler

    pickler Well-Known Member

    The idle settles to 1.3L / hour only when I come to complete stop. At speed rpm never dips below 1300rpm whether in neutral or drive. It looks as tho the ecu is applying throttle to rev match when shifting back to drive?
     

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