DFCO seems unnecessary to me.

[omgwtfbbq!]

Well alot of the new cars have this "fancy" feature built into their ECUs, as we all know it cuts ful under no throttle coasting conditions. This seems like it would be great for fuel mileage, but when I think about it, it seems like almost every car already has DFCO. My ECU adds fuel based on load, throttle position, and manifold pressure, but with the throttle closed I pull 26 inches of vacuum in the manifold at idle and under deceleration, so I know there can't be much fuel injected, if any at all. This would make adding an entire set of code for DFCO to an ECU seem like a waste of time. So is there a real difference between DFCO and what every other ecu does?

[Loren]

It's merely the difference between providing enough fuel for the engine to idle and providing NO fuel to the engine. Probably not a huge difference... but a difference.

Think of it this way:

You're coming to a light that's read, but you know it's going to turn green in 30 seconds. You have options.

Least economical: stay on the gas until you get there, brake at the last second, stop... and then blast away from the light from a complete stop in a blaze of glory.

More economical: Analyze the situation, clutch in, downshift or go to neutral and coast/decel toward the light maintaining as much momentum as you can and trying to arrive at the light after it's turned green without ever having to stop. Uses minimal fuel while coasting.

Old school hypermiler trick: Clutch in and shut the engine off as soon as you see the red light! Coast as above, and then shift to an appropriate gear and let the clutch out to restart the engine when you're ready to accelerate. They call it a "forced auto start" because you're forcing the car to start itself after you've shut it off. It's a little risky because it leaves you without power to get up and go if you need to, and without power steering, without brake boost, and <gasp> without air conditioning! But, use no fuel while coasting.

DFCO: Lift. Downshift to keep the revs above about 1500. Use no fuel.

If you're driving in the city and use it correctly, it can make a 10% difference. 10% is only 2 mpg when you're talking about the "average" American vehicle that gets 20 mpg. When you're talking about the kind of car that hypermilers typically use, 10% can be 4 mpg or more! So, while it doesn't sound like much... it's a big part of how hypermiling works.

[omgwtfbbq!]

Hmmm. I can't imagine putting your transmission through the punishment of repeated force starts.

But I would assume that regular ECUs are adding enough fuel to idle during deceleration? Mine doesn't seem to be doing that, if I coast in gear down to 800rpms, the car suddenly jerks forward as the idle begins to pull the car forward. It doesnt seem very smooth if theres already fuel being fed to the engine.

[Loren]

If your car IS cutting fuel under full deceleration in-gear... guess what? It has DFCO!

I really don't know how old the technology is or who started using it first or anything. I know it's pretty common these days and has just become popular with gas being as pricey as it is. I'd never heard of it before I bought my Yaris and started skimming the Yaris forum where the hypermilers hang out.

"FAS" if done with some degree of mechanical empathy shouldn't be any more stressful to the clutch or transmission than doing a normal shift. I'm just not a big fan of gliding around with the engine off.

[omgwtfbbq!]

Well if thats ture then it has been around for a while. I have a Japanese motor made between 98 and 01, with a Japanese ECU.

If its been around that long, then I don't think its anything for automakers to brag about now a days.

[FrodoGT]

Dude..my p28 has it. so its been around that long. Its call over-run cutoff in there. And also I tuned a haltech system that had fuel cut parameters.

Its actually not just for decel to red lights etc.. But when your cruising down the highway at 2500-4000rpm (honda:P) and you let off it will cut all fuel. and thats saving you milage right there, without even trying.. Just moving in highway traffic will save fuel as opposed to running while you lift off. Also helps with engine braking (ever so slightly)

Also if you are running down to 800 rpm in gear..your fuel has already been turned back on since at least 1500 rpm, and your mounts just suck.

[omgwtfbbq!]

No, my fast idle is set at 1000, regular idle is 700. My mounts are racing spec Innovative mounts, so I don't think they're to blame.

[Loren]

Right on, Frodo. I don't think it's "new", especially by the standards that all you "youngsters" on TR use. ;) But, it surely wasn't around in the 80's or probably the early 90's. Late 90's sounds about right, but that's just a guess.

And, yes, you can absolutely get benefit from using DCFO anytime you can. As I mentioned in another thread, anytime I'm cresting a "hill", I try to gain a few mph as I crest the hill, and then lift to get a "free" coast on the other side back down to my target speed. (someday I'm going to get a speeding ticket doing that... it'll be fun trying to explain that I wasn't really speeding, just trying to improve my fuel economy!)

[FrodoGT]

No, my fast idle is set at 1000, regular idle is 700. My mounts are racing spec Innovative mounts, so I don't think they're to blame.

Well then its your axles or some form of drive train slop that causes your car to surge, because fuel has most certainly been turned back on.

[omgwtfbbq!]

I don't know, it seems normal for the car to feel the way it does when I go from deceleration to a steady speed. Its just a bit of a jerk when it does. If you go to the next FAST autocross I'll show you what I mean. :lol:

[TIM TIM TIM]

Hmmm. I can't imagine putting your transmission through the punishment of repeated force starts.

But I would assume that regular ECUs are adding enough fuel to idle during deceleration? Mine doesn't seem to be doing that, if I coast in gear down to 800rpms, the car suddenly jerks forward as the idle begins to pull the car forward. It doesnt seem very smooth if theres already fuel being fed to the engine.

Half of the "hypermilers" techniques seem fucking retarded in that sense. Save a buck on gas but cost yourself 10 in vehicle repairs.. id rather spend the 1 dollar at the place where I just have to squeez the handle :dunno:

[chi town brat]

Old school hypermiler trick: Clutch in and shut the engine off as soon as you see the red light! Coast as above, and then shift to an appropriate gear and let the clutch out to restart the engine when you're ready to accelerate. They call it a "forced auto start" because you're forcing the car to start itself after you've shut it off. It's a little risky because it leaves you without power to get up and go if you need to, and without power steering, without brake boost, and <gasp> without air conditioning! But, use no fuel while coasting.

How hard is this on the engine and the clutch? Honestly I wouldn't do this just because of the drivers in Tampa and safety issues I would feel insecure with.

Interesting. I coast to red lights now and try to take off as gently as possible from lights but damn I love that low end torque. Well at 31mpg in my EG hatch I can afford to love it sometimes. :)

[Loren]

How hard is this on the engine and the clutch? Honestly I wouldn't do this just because of the drivers in Tampa and safety issues I would feel insecure with.

Interesting. I coast to red lights now and try to take off as gently as possible from lights but damn I love that low end torque. Well at 31mpg in my EG hatch I can afford to love it sometimes. :)

I think done smoothly, a "forced autostart" should cause about as much wear as a normal shift. You're using the clutch just as you would in a shift... obviously the transmission is doing what the transmission does... and you're restarting the engine without putting additional wear on the starter. I can't think of any way that it could harm the engine. Done right, I don't think it's a big deal. With most cars having the DFCO feature these days, I don't think it's necessary, though... and I'd be hesitant to do it, anway... I like to have the power available when I need it. Sometimes you just need to be able to get the hell out of the way!