Optimal Acceleration and Coast Speed for Scion tC

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

  1. jcp123

    jcp123 Caliente!

    To be honest, I find my Echo more peppy in normal driving conditions.
     
  2. Hyper-tC

    Hyper-tC Member

    Do exhaust mods have an affect on FE? Or does it hardly affect it?

    If you're only going to be able to glide for about 5 seconds, do you bother gliding over steady state?

    If there's an upcoming hill, will you typically stay in gear and feather it for the whole hill till you hit the crest and then glide?

    Would you go 50 on a 65 speed limit highway with 3 lanes on my side during rush hour, medium traffic where the right lane is usually going about 55-60 and mid lane 60-65?

    The tC has a lot of torque. 2.4L engine that puts out 160 HP and 160 lbft. It makes me feel like I should be able to shift lower, but I can't find a BSFC for the 2AZ-FE to really know what's best.

    Other than getting a Scangauge II or simply testing myself over time, how else can I figure out my optimal shift points? This engine is used in the '02-09 Camry, '04-'12 Rav4, '08-'12 xB as well as many other cars...you'd think it'd be relatively easy to find a BSFC curve for it =/

    On a slightly related note, is letting VVT-i kick in usually bad for FE?
     
  3. RoadWarrior222

    RoadWarrior222 rockit serjun

    Well the torque curve will give you some idea. Generally efficiency is good from about 80% peak torque.

    Here's one for the 2AZ-FE that we'd all wish had more size and better scale, but by a quick eyeball, I'd figure most efficient range should be from 1500ish to 3500ish.
    [​IMG]
     
  4. PaleMelanesian

    PaleMelanesian Beat the System Staff Member

    Exhaust mods don't usually do much at low rpm. Sometimes they cost you mileage or power down at the bottom even. There's no need to change anything in an attempt to improve mpg. If I'm running at 2000 rpm, with a 7000 rpm redline, I'm only using 28% of the exhaust's flow capacity. Even if it's a restrictive exhaust, I'm still nowhere near its limit when driving for economy.

    I pulse and glide always. I'm stubborn. ;) If you only have opportunity for 5 mph variation, there's not much to gain, except where terrain is a factor. Any minor downhill is an opportunity for coasting while maintaining speed.

    I pulse and glide up and down hills. On a gradual climb, the pulses are longer and the glides shorter. At some point it gets steep enough to be one long pulse up the hill. Anything flatter than that and I slip in some short glides. Downhill is the opposite - long glides and sometimes small pulses when needed.

    There is one downhill on my morning commute that will accelerate me past the 35 mph limit if I crest the hill above 10 mph. It's a 2+2 suburban street, so that's just what I do - crest the hill at 10.

    I'd probably do 50-60 p&g in the right lane. Match traffic when it gets heavy, glide down 10 mph when there's a lull.

    Don't worry about finding a bsfc map. If you can gain speed at X rpm, that's usable. I'd guess with your engine you can pull from as low as 1200 with no problem. I like to think of my shift points in terms of where I will land, rather than where I shift from. I have in mind a minimum speed for each gear that gives me that minimum rpm. When I'm above that speed, I shift.

    But really, just keep it with a reasonable range, say 1200/1500 to 2200/2500 rpm and you'll be fine. Work on gliding.

    VTEC, VVTI, TI-VCT, and others are usually bad for mpg, especially when they come at high rpm.
     
  5. Hyper-tC

    Hyper-tC Member

    Oh that's an interesting way to look at it. So you're saying general efficiency is good from 80% peak torque up to 100% peak torque?

    Though that's confusing as...peak torque is 163, 80% of it is 130, which is what the curve pretty much starts at?

    That makes sense. Doing an exhaust mod would be more for my aesthetic pleasure than anything, wanted to make sure it probably won't hurt.

    Do you give it extra throttle when pulsing up hill to shorten the pulse or do you use the same amount of throttle as you would when pulsing flat?

    Cool, I'll have to try this. I'm worried about getting rear ended, but, I think it'll be okay if I match traffic as you mentioned.

    That makes sense about thinking about shifting in terms of where your rpms land and that VVTI-I is bad for mpg. I'll definitely focus on gliding, I see myself getting better and better at it and it's rather fun too.
     
  6. PaleMelanesian

    PaleMelanesian Beat the System Staff Member

    pulsing on hills - same throttle as any other time.
     
  7. jcp123

    jcp123 Caliente!

    +1. The maddening hills are the long, gradual ones. The same gentle slope which gives me a bonus on the morning commute is a killer on the way home. Hence I usually get better FE into work than coming home.

    In the rolling hills around here, I simply keep a constant throttle input bleeding speed up the hill and keeping in the throttle to gain the speed back. Best for longer trips. On the commute I learn the hills and pick a point where I can glide to a low speed at the crest and glide down the other side, both of the more major hills I have terminate at stop signs. One of the hills I could probably get a bit of improvement on, I crest a at 25 with a PSL of 40, and hit the bottom at about 40. Problem is cops hang out there now and then and although there's a second lane to pass...nobody ever does.
     
  8. Hyper-tC

    Hyper-tC Member

    I bumped my engine for the first time today going about 30, was way easier than I expected! Pretty cool haha

    It's been quite amazing how far I can glide sometimes, about half my local commute to the highway I can do in two pulses because of the hills where as before I was using gas much more often and braking several times. I also managed to brake only once in about 10 miles of highway, another first for me.

    Okay cool, that is what I suspected.

    So you like to P&G as much as possible from what I read. When do you find yourself simply using steady state?

    In a 50 mph zone with 3 lanes on my side, would you do p&g from 40-50 or 35-45? In 5th at 35 my RPMs will be about 1700 I think which is a bit on the lower side but enough for me to accel just fine with my torquey tC.

    yeah hills really change things up. I take a different route home than I do to go to work because of traffic, my route home has about 2-3 miles of nothing but huge sweeping hills with 50 mph speed limit and no stop signs or traffic lights. I've been able to gain a lot of speed going down the hills and that carries me easily through the next hill.
     
  9. RoadWarrior222

    RoadWarrior222 rockit serjun

    Yah that's not a great graph, might be around 1200 where you're exactly at 80%. Older motors had to be optimised in the midrange at the expense of top and bottom, with computer control, variable timing and other tech they can be optimised all the way across now.

    Also, it's not actual AT that output you get best FE, it's more like at that RPM at about 80-90% load. At 100% is where motor is breathing hardest and you start to have airflow inefficiencies show up, then also most ECUs are programmed to go "open loop" at 90-100% throttle opening, which might mean your mixture drops from a controlled 14.7:1 to a "safe guesstimate" 13.5:1 or something.


    Exhausts get confusing with all the nonsense often spouted about back pressure. A good exhaust for naturally aspirated engines works somewhat like a siphon, use too small a tube and it will just drip drip drip, use too big a tube and you might get a gush just after you suck hard on it, then flow will stall, use just the right size and it flows great. But exhausts are not just tubes, the muffler plays a part also. A badly flowing muffler will stall exhaust velocity, steal the gas momentum, and then your motor has to push on the gas to get it all the way to the end... also if the pipe is way too big, it's effectively slowly pushing at a large cylinder of slow moving gas. Turbocharged motors are kind of different, all the exhaust momentum goes to the turbo, it's already relatively "slow moving" so to get the most volume moved at a lower velocity, you go to a huge pipe. The siphon effect on naturally aspirated if momentum kept up, actually allows it to evacuate the chamber fully at end of exhaust stroke maybe actually sucking on the piston a little and removing the resistance the piston would have if it had to PUSH the exhaust out due to slow flow due to oversize pipe, too small a pipe, or restrictive muffler or tight nasty bends.

    However, it's rare that you'll find a really good flowing exhaust in a diameter that enhances fuel economy as it's main objective, unless you happen to have the situation where there's a spec racing series for a smaller motor model of that vehicle... (Where it was actually designed for top-end in that one, but enhances midrange nicely on the bigger motor) So, in order to get a good flowing exhaust, you have to buy a performance one, which might be relatively oversize for bottom end on a stock motor. This can be the case especially if it was offered by a company that has cams, tunes or other power enhancements, it was probably designed to complement them.

    Stock exhaust sizes have generally increased from "ridiculously small" as having the gases cool off before they hit the catalytic converter, or having too small a diameter converter to process high RPM output would have been emissions problems. So it may be the case that your stock size is theoretically pretty decent for that size/horsepower motor... I would guess they may have used 2 1/4" on it... IF you can get a mandrel bent exhaust and performance muffler in this size, take it, it would offer best mpg advantage.

    But, it may be the case that the performance parts are at least a size up, 2 1/2"... this is not too terrible and there is a chance you will see a slight MPG gain over the stock system still, depending on how good the stock system was in the first place. It will tend to be more efficient towards the high end of the the rev range though. Although if all you can get is 3"... then you may take an MPG hit, you may feel it "lose pep" on the bottom end, you probably have to have other modifications to the motor and be making somewhere over 200HP before this is necessary, and before it has any advantage whatsoever.

    Stock systems may now be SIZED right for the motor, but the beancounters may have been at work, meaning it's made cheaply with crush bends that give it the effective area of a 1 3/4" exhaust in the bends. Also NVH factors play into things with the muffler selection, they may want reviewers to say it's "refined" on the highway rather than a little growly, so a muffler that's a little more restrictive may be used.

    So penny pinching and marketing factors may override what the engineers intended in the first place.
     
  10. Hyper-tC

    Hyper-tC Member

    Cool that is quite useful. I really need a Scangauge to better figure out what throttle I need during accel from stop.

    You mentioned turbos, do you think it's possible for a turbo to increase mpg? Does having a higher compression usually help mpg? I can turbo my car for less than $2k...would be nice to have an excuse to do so lol.

    The exhaust I've been eying for years has 2.5" piping and 4.5" muffler tip. Stock is 1.75" piping and I think 2" tip.
     
  11. EdwinTheMagnificent

    EdwinTheMagnificent Legend In His Mind

    Modifying or replacing your exhaust system is not likely to improve MPG.

    Adding a turbocharger is even less likely to improve it.

    This site is mostly about adjusting the nut behind the wheel.
     
  12. schuylkill

    schuylkill Well-Known Member

    This is something for me to work on. I have several 2-3 mile down hills where I always engine off glide, would you also? And with urban speeds as you word it, do you have minimum distances of glide before you engine off? My Elantra does not like to bump start below about 25 mph so I consider that as well.
     
  13. RedylC94

    RedylC94 Well-Known Member

    Why, on that car? That normally ought to be near the easiest speed at which to do it smoothly.
     
  14. schuylkill

    schuylkill Well-Known Member

    I don't know, there is a hesitation, a "dead space" with the electronic accelerator that often leaves a delay between letting off of the clutch and the tranny engaging when bump starting. I have adjusted the switch on the pedal and it is better than it was but it's not my cable controlled civic, that was smooth.
     
  15. EdwinTheMagnificent

    EdwinTheMagnificent Legend In His Mind

    I had really good luck with my cable-actuated clutch in my Civic Si. Of course , at that time, I wasn't bump-starting it ( unless I ran the battery down).

    I guess what I'm trying to say , in my typical off-topic fashion , is that I had MUCH better luck with operation and longevity of my cable-actuated clutch than the several hydraulic Civics we had afterwards. The Si's clutch ( and cable) lasted 233,000 trouble-free miles. It was still going strong when the rest of the car died from rust. The hydraulic cars ALL required maintenance.
     
  16. RedylC94

    RedylC94 Well-Known Member

    Yep. Zero clutch cable problems in about 764K miles so far on my two Japanese cars, and zero in roughly 450K on my brother's two VWs. My first encounter with hydraulic clutches was getting drafted to help repair a leaky cylinder (forget whether it was master or slave) on a much newer Saab.
     
  17. Hyper-tC

    Hyper-tC Member

    Sweeeet! My last tank I not only achieve my first 400+ mile tank, I almost hit 500 miles on the tank! 489 miles on 12.789 gals of gas! That's 38mpg, about 173% of the EPA for my tC.

    It's been really cool learning my car so well, gotten the hang of bump starting quite well. Mostly did pg nice-on 50-60mph on the highway, pg 35-45 fas off the highway, dwb, shifting before 2k if possible at 1.8k giving it 10-30% throt, when I hit 5th I give it about 80% throt, also upped my tires to the max sidewall pressure.

    I'm trying to optimize things further. I still need to get a ScanGauge, but speaking from experience, is there an optimal range for p&g, like 15mph? Still figuring out how to deal with hills properly, I was doing 5 mph p&gs to try and get through hills with minimal acceleration, does this sound about right? Do you ever extend your glide to try and utilize downhills more even if it means slowing down to say 30mph instead of 35mph where my normal pulse starts?
     
  18. PaleMelanesian

    PaleMelanesian Beat the System Staff Member

    Nice result!

    Yes, there is an optimal speed spread for p&g. There is some loss at each end of a pulse while engaging and disengaging gears. It's better to minimize that, so a wider spread is good. Too wide a spread and the top end is just too fast (rpm and aerodynamics). I've found the best results at about 15 mph. 25-40, 30-45, 45-60, etc. 5 mph is probably too small.

    Sounds just about perfect. I'd go ahead and give it more throttle in the lower gears as well. I've managed nearly as good results in 4th gear at lower speed as 5th gear at higher speeds, using similar rpm ranges. I hit 3rd gear as low as 20 mph, so there's not much time to worry about throttle during the quick 1-2-3 sequence. 4th and 5th I do focus on it.
     

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