Optimal Acceleration and Coast Speed for Scion tC

Discussion in 'Fuel Economy' started by Hyper-tC, Jul 23, 2014.

  1. Hyper-tC

    Hyper-tC Member

    Currently I can get 30.5 mpg from my 2006 tC by simply driving 55 mph on the highway, attempting to cruise through red lights, and cutting the engine when stopped at red lights.

    I'm really wanting to achieve 40 mpg!

    I plan to get a ScanGauge II soon enough, but for now I do not have any direct way to see my fuel consumption.

    Typically I slowly and lightly accelerate shifting at 2500 RPM. I'm reading that it may be better to shift at 3000 RPM and floor the throttle while I accelerate.

    I can feel the engine running differently between 2500 RPM and 3000 RPM versus 2000 RPM and 2500 RPM. I'm not fully sure what this means or if it's significant.

    It is much more fun to accelerate hard and shift at 3k than to accelerate lightly and shift at 2.5k for sure. The tC is a very fun car to drive overall, 160 hp and 160 torque is an interesting combination. It uses the 2AZ-FE engine which is also used in the Camry, so I imagine hypermiling techniques for the Camry is similar to what is needed in the tC.

    What should my shift point be? 2.5k or 3k? Maybe 3.5k? Can I tell from looking at a dyno curve?

    Should I be going open throttle or feathered throttle to accelerate?

    At what speed is it best to coast at? 55mph? 45 mph?

    Is there a safe way to pulse and glide on the high way? Does this achieve much more FE than feathering the throttle? What speed would I want to accel up to and glide down to?

    My drive has many many hills of varying gradients, I typically keep constant feathered throttle when going through them, is this the correct behavior letting my speed dip during the climb and regain during the descent?
  2. EdwinTheMagnificent

    EdwinTheMagnificent Legend In His Mind

    Lots of good questions. I don't have recent manual trans experience , but I will suggest 2000 RPM is all you need , but a larger throttle opening , which means..........shift quickly !
  3. PaleMelanesian

    PaleMelanesian Beat the System Staff Member

    Lower rpm is almost always better. I shift at about 2000, dropping down to 1500 rpm. You also don't want to fully floor the throttle, but look for half or three-quarters.

    What rpm do you run in top gear on the highway? That changes the recommendations. If you're running nice and low rpm, cruising works. If you have high revs (like mine: 2850 @ 60 mph), you'll do well to go into neutral whenever you can. Light throttle at high rpm costs you fuel just to turn the engine over. With my high revs, I'm almost never at a steady speed, I'm always P&G'ing.

    The ideal speed range for maximum mileage is usually pretty low. My route allows me the flexibility to do whatever, so I'm typically running a 25-40 mph p&g cycle in 5th gear. That obviously won't work on the highway, so I go the lowest I can while still being courteous and safe. On a 55 mph highway that's usually 35-55 mph. On the interstate I'll do 45-60 or so. You can time your pulses to be faster when a cluster of traffic moves by, to be less intrusive.

    Hills - it depends. If you can maintain speed without shifting down, do. If you would have to downshift and bump the revs up, try bleeding off the speed as you climb and then regain it on the back side. For more shallow climbs I'll p&g, with a fairly long pulse and a short glide.
  4. RedylC94

    RedylC94 Well-Known Member

    2.5k is likely to work better for fuel consumption.

    Try accelerating while neither "flooring" nor "feathering" the accelerator, but in between instead, at roughly 75% of peak torque available at the current engine speed. You can't tell much about when to shift for best economy by looking at a simple dyno curve---other than the speed at which the engine produces its maximum torque. You want to stay well below that RPM, except in emergencies.
    Last edited: Jul 23, 2014
  5. RoadWarrior222

    RoadWarrior222 rockit serjun

    This is for a Prius but..


    .. your basic "bang for buck" graph is that kind of shape, meaning at the lowest point it's costing you the least fuel per HP produced.

    So depending on how close or how far apart your gears are, you should try to stay in the flattest area, that would mean I guess not shifting up until you can be AT 2000 rpm in next gear at least. so might want to be shifting around 3000.

    Try googling BSFC and your engine model to find one that's exact for your motor.
  6. RedylC94

    RedylC94 Well-Known Member

    That plot doesn't show the effects of throttle position. Ones that show BSFC at every combination of speed and load are more useful. Usually best BSFC is at a moderate engine speed, at somewhat less than WOT throttle angle.
  7. No Static At All

    No Static At All Active Member

    Shift as early as possible w/o lugging the engine in the next gear. I would aim for 2000-2500 with 1/2 to 3/4 throttle. Have driven the Scion & it has much better low end torque / lower gearing than the Corolla, so as low as 2000 RPM should be doable under most conditions.
  8. RoadWarrior222

    RoadWarrior222 rockit serjun

    Yah, if I had my druthers, they'd have to give you a full tech package with every car sale with BSFC maps, torque curve on decent scale, etc etc.

    Although most of the time you're only spitballing load by perception/buttdyno, so tryna be sooooper accurate ain't really gonna happen without more instrumentation.

    Though my Escort does well with extra mass, think it must load motor easier, jump up the map.
  9. Hyper-tC

    Hyper-tC Member

    I appreciate all the replies, they are helpful!

    My rpms look about like this:

    55 mph - 2500 RPM
    60 mph - 2750 RPM
    65 mph - 3000 RPM

    My redline is 6500 RPM if that helps.

    I noticed you don't have a hybrid, so when you P&G do you shift to neutral and turn the engine off?

    Yeah it does have plenty of torque compared to the Corolla. That's the other car I drive much at all, big big difference in tuning and torque haha.

    Does producing enough torque keep the engine from lugging?

    I won't lug at 2000 RPM I know that for sure. It's under 1500 RPM and closer to 1200 RPM is where I think the engine starts lugging, I'm not 100% sure on what lugging really feels like. Is it anything more than what sounds like things being strained and the sound of the engine getting really deep?

    Cool, didn't know these kinds of curves existed. I'll see if I can find one for my 2az-fe, though so far I'm not having much luck. This thread is one of the top results haha.
    Last edited: Jul 25, 2014
  10. PaleMelanesian

    PaleMelanesian Beat the System Staff Member

    A deep or kind of boomy sound is fine. When it starts bucking, that's when you're lugging. Lugging isn't just a function of rpm, it also depends on load. Think about this: it idles at ~800 rpm. It's not lugging there. If you ease it into gear and just roll along not touching the pedal at the same 800 rpm, is that lugging? If you mash the gas pedal, it most definitely will be.

    My weaker engine, with almost the same gearing as yours, has usable power output from about 1500 rpm. It's not fast, but that's not my goal. So long as it can gain speed smoothly, it's fine. My experience is that higher rpm eats more fuel exponentially, so 3000 rpm is something like twice as thirsty as 2000. Low rpm is where the magic is.

    Oh, and gliding. Actually, that's where the big magic is. Neutral always, engine off most of the time. With a manual, I do a smooth and gentle clutch start in 5th gear, then shift to my needed gear for the pulse.
  11. Hyper-tC

    Hyper-tC Member

    Hmm so then I've never really lugged my engine I don't think.

    Are there any mpg comparisons of doing P&G in neutral vs off? It feels so dangerous doing P&G with the engine off during my work commutes which does have traffic. I know I can brake about 3 times fully before my brakes won't work anymore.

    Do you do P&G with the engine off when going at lower speeds too?

    If you have a hill coming up and you're already at the low speed of your glide, do you feather the throttle till you're over the hill and then pulse back up with 3/4ths throttle?

    I also have a couple hills on my drive that I usually am stopped for and then need to accelerate on, is it better to feather the accelerator up the hill here also and then give it 3/4th gas once I'm over the hill to get up to speed?
  12. EdwinTheMagnificent

    EdwinTheMagnificent Legend In His Mind

    The hard part of P&G , from my perspective , is knowing just when to pulse and when to glide , when you have traffic constraints. You need to look for ( or create) opportunities to do so.

    Listen to Andrew.
  13. PaleMelanesian

    PaleMelanesian Beat the System Staff Member

    Don't do anything you're not comfortable with. Stick with engine-on coasting until / unless you get comfortable and practiced with it.

    I do adjust my timing to work with traffic. As a cluster of traffic comes up behind me (and they're always clustered :confused:) I'll pulse so they can pass me gradually instead of a major bottleneck. And I have adjusted my route to work better - less 2-lane and more multilane roads. That way I'm not blocking traffic.

    Engine-off is more useful at lower speeds. Say your car idles at .25 gallons per hour (low end of average). Idling at 70 mph, you're getting 280 mpg.
    60 mph = 240 mpg
    50 mph = 200 mpg
    40 mph = 160
    30 mph = 120
    20 mph = 80
    10 mph = 40 mpg

    It's almost not worth doing engine off at highway speeds, but VERY worth it at urban speeds.

    Braking - if you use the brakes once or twice, you still have some reserve left, but you're running low. That's a good time to bump the engine on again and replenish it, before you're in an emergency. Better to burn a drop of fuel and be safe.

    Stopped on a hill is just an ugly thing. Go ahead and give it moderate-to-high throttle. That's the best answer in most cases, especially with a manual. With an automatic it's harder, because it'll fight your efforts and jump the rpm up when you don't want.
  14. RoadWarrior222

    RoadWarrior222 rockit serjun

    Personally, if I were to get serious about engine off coasting on my older rides, due to a fuel crisis or something, I would do the following mods.

    Engine cutout button, normally closed button that cut the ignition long enough for motor to stop, without turning the key on and off. i.e. hit button to kill motor, no risk of locking steering, car returns to state where you can bump start instantly, or it's one click to key start not several.... could add starter button also.

    Bigger vacuum reservoir, for brake booster reasons, I'd do this with check valves and those aluminum drinking water bottles. (Also note that some cars use vacuum to switch heater flaps, so do not adjust while EOC otherwise you might have none left to assist)

    Electric pump for auto transmission.... mayyyybe. I think mine are safe to flat tow up to 5 miles at 40mph, so I'd maybe just avoid fast highway to EOC
  15. Hyper-tC

    Hyper-tC Member

    That makes sense. My work commute is mostly highway atm, do you typically prefer highways over local roads?

    Oh wow I didn't think about it like that at all. So it's really not worth doing almost at all on the highway.

    I've barely been P&Ging on non-highway roads, do you do a same type of pulse to about 5 mph over the speed limit and then glide down to 10 mph under?

    I've never bumped the engine before, is it as simple as put the car in 5th gear, release the clutch a bit so the engine starts and then disengage the clutch so it doesn't stall and put it in the needed gear?

    I haven't thought about doing mods for engine off coasting. How would you make such an engine cutout button?
  16. PaleMelanesian

    PaleMelanesian Beat the System Staff Member

    I prefer the roads with fewer stops, but i've chosen my commute route for multi-lane so traffic isn't a factor, and timeable lights - visible from far enough out I can adjust my speed and hit green.

    25-40 mph p&g is the best in my car. In most cases, that's completely below the limit even at the top end. If it's a 35 road, I'll do 20-35 or something. I aim to not exceed the limit. When traffic is a factor I'll do a smaller range, maybe limit to 10 below, or only 5 below if it's really heavy. At that point you're almost going steady speed so there's not much gain to be had.

    Bump-start, yes you've got the idea. Using the highest gear minimizes the shock when the clutch engages. 5th works down to 10 mph or so, below that I'll use 4th or 3rd. Never 1st or 2nd - just too much jerk.

    I'm perpetually thinking about an engine cut switch. If you can find a fuel injector relay or fuse, that's best. Cutting the ignition is ok, but will leave a few cycles of unburnt fuel in the engine.
  17. jcp123

    jcp123 Caliente!

    I used to shift my SVT Focus around 16-1800rpm. That would drop it to about 1200ish in the next gear. On gentle acceleration it would not lug, and that's a 7500rpm redlined performance engine.
  18. RoadWarrior222

    RoadWarrior222 rockit serjun

    Well it's somewhat the case that the more motor you've got, the lower you can shift, because it's making 20HP where the stock motor is only making 15. And performance models are much more refined in the last 2 decades than they would have been in the past, less "cammy" and with a broad power band for public consumption.

    Yes fuel cutout is a good plan. Might depend on the "wiring paradigm" for the car, some you'd just have to trip out the ASD relay, which everything is slaved off. But if you're not comfortable messing with your car's electrics, don't.
  19. jcp123

    jcp123 Caliente!

    It certainly wasn't torquey, under about 4k it felt like a "regular" 2.0l. At 4500 the VVT came in (got that Ford VTEC, yo) and the intake switched to the short-ram tract. Then and only then did the engine feel like any kind of performance machine. It was a fuel hog too.
  20. RoadWarrior222

    RoadWarrior222 rockit serjun

    And that's how they do it, two motors in one... the modern stuff is half a dozen motors in one or more with the continuously variable valve timing.

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