Jphank

True, while it might not be the safest, there are ways to mitigate the danger, it doesn't have to be extreme speeds, say, starting at 50mph and taking it up to 75mph, then coasting down.

But I do agree with you, it's not the safest.

alelser

I'd hazard a guess that if caught doing it (dunno how you would be, but it's a hypothetical, anyway) you'd likely get nailed for reckless driving. Knowingly disabling the vacuum to your brakes while driving by shutting off the engine could easily be seen as reckless endangerment of others on the road.

-Al-

2point5awd

Unfortunately my expertise does not lend itself to this conversation, however, I have $0.02 to add...

Top Gear did a couple of great fuel segments (that I know of thanks to YouTube).

1st: They took an Audi A8 TDI and attempted to get 800 miles out of 1 tank of gas. He claims that coasting in-gear opposed to in neutral uses no gas, whereas in neutral the engine has to keep itself idling. ,

2nd:

Quicksilver

Here's a test for whether you've just been told what you want to hear and are happy that your pre-existing behavior is validated,

or,

whether there's some truth to the mags findings...

Test: Do these loads and non-boost high throttle openings occur within the 'meat' of the engines torque curve?

Answer: If they don't occur under the 'meat' of the torque curve, then it's BUNQUE!!!

Good mileage comes at the engines highest torque. Acceleration is horsepower, and boosted acceleration is poor gas mileage, especially outside of the torque curve. The bit of truth to what the mags are saying is that fuel-wise, the most efficient acceleration occurs where you're in the space between torque curve and where the horsepower curve rises above then dips below. But not under boost...you have to have a pretty different ECU program to be able to get into that area without going into boost.

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2004 FXT 4EAT - Platinum Silver Metallic

Quicksilver

2.5AWD-
Coasting in gear requires gas. No gas equals deceleration/engine braking. Most engines are tuned to accept large fuel volume variations...you tune one to idle efficiently and your Audi would win by coasting while idling...
-Quick

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2004 FXT 4EAT - Platinum Silver Metallic

The Rookie

Wow talking of total bunkum......where did you pluck that from....

On a steady cruise the best FE is at the lowest road speed without ending up going rich (due to EMS fueling strategy in whatever car your driving), so sme cars you have to speed up a little on grades to keep it from going rich, best FE at rpm of somewhere near best torque ..... where is the logic for that? That would be at about 7000rpm in a Honda S2K!

As for tuning to idle economically, very difficult, and most manufacturers only tune it to idle economically when stationary, most jack the idel whe moving, and on manual cars retard the ignition to add a torque reserve in case the clutch suddenly comes up!

Simon

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98 S-turbo, next mod decat D/P and hybrid turbo!

Quicksilver

Simon-
Since you seem to know so much about this, perhaps you could explain the difference between an engine's torque and its horsepower?

Honda S2000s are one of my favorite cars, but not because it's a torquey engine, but because it's the highest horsepower per litre non-aspirated production engine made (at least up until the last few years, may still be), but note that it makes no claims on highest torque.

I'll give you a clue, the little torque it does make is in the upper third of its incredible RPM range. And yes, where it's maximum torque is, is where its best gas mileage will be found...find the gear where the desired road speed (let's say 55 for grins) puts the engine in that range, and that's where you find the fuel consumption sweet spot.

You're right about tuning for idle, it's a stupid thing to do, I wasn't suggesting it. But as the price of gas goes up, our craving for horsepower and performance is going to shift to getting good gas mileage...

Funny thing, back about 50 or 60 years ago, when folks weren't as affluent as they are now, car manufacturers were installing overdrives on cars...it was quite common for the overdrive system to be free wheeling when you let off on the gas...essentially putting system in neutral, to save on gas...yes, they were carbureted back then, but not for maximum horsepower...they were tuning for torque.
-Quick

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2004 FXT 4EAT - Platinum Silver Metallic

funkymonkey1002

I actually found that the give it gas and coast worked really well, but I was testing in our prius. It tends to almost bog at low throttle and you get terrible gas mileage the whole time you're accelerating, however if you just go (say 2/3 throttle) and get up to speed I've been able to average about 2-5 mpg better. The only problem are other cars on the road, if you end up having to use the brakes, it kills any increase in mileage you had gained. Also the prius shuts off the gas engine when coasting or just running on the electric so that saves quite a bit of gas.

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04FXT/4EAT Snorkus Free, WRX suspension, rota/falken/racingbrake/hawk

Geezer

This makes no sense at all! Keeping the throttle fully open and the turbo sucking air is a recipe for emptying your tank and your wallet faster.

As far as my own fairly extensive experience of driving (about 55 years) is concerned, driving as though you have a raw egg on both the accelerator and brake pedals is the easiest way to save on gas and money. Period!

Griff

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Griff (The Geezer)
Drives a 2009 Forester XS Ltd.

guroove

I have to agree with Geezer. Engineering and physics of engines are all good in the laboratory and classroom, but in the real world, you can't floor the pedal everywhere and not have to use your brakes more often. I've been driving like a grandma lately, keeping my engine rpm really low, and driving like there is an egg under my gas pedal, and I was able to get 280 miles from a tank where I used to be lucky to get 200 miles. I also stopped using my AC and stopped idling to warm up the engine, but still a 40% improvement just from changing driving habits is enormous.

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2005 Obsidian Black Pearl XT w/4eat

Quicksilver

guroove-
Not sure how low, really low is, but I can assure you that if you're below the meat of your torque curve, you can do better that you already are doing, with just a little more rpms. Like funkymonkey's Prius, when you're below your torque curve, your engine bogs. Your low throttle settings work just fine when in the 'meat' of your torque curve. "Eggshelling" is exactly right, once you're 'torqued' up.
-Quick

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2004 FXT 4EAT - Platinum Silver Metallic

Quicksilver

Jphank-
I'm glad you were sensible enough to exclude turbo- and super-charged engines from your at best speculative statement! I would be very interested to hear why WOT is recommended over say 1/2WOT or 1/4WOT...

And to accelerate with WOT to say 80, coast (read shut off engine, and engine accessories such as powered steering, powered brakes, etc) down to the desired (speed) limit, without specifying initial starting rpm, puts engine torque numbers out of the equation.

I've not had the benefit of your engineering specific schooling, so pardon me if I sound skeptical...perhaps you might more fully explain how wide open throttle is more fuel efficient than say half open throttle?

-Quick

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2004 FXT 4EAT - Platinum Silver Metallic

guroove

It seems the point I was trying to make was missed slightly. In my real world driving conditions, accelerating slowly allows me to keep distances behind other drivers large enough that I hardly ever need to use the brakes. What this ends up meaning in daily usage is that I often get up to 2000-2500 rpm before my automatic transmission shifts into the next gear. It may be more efficient for my engine to get up to 3400 rpm more often, but then I would be going too fast and need to use the brakes more when I get to the next traffic light, or to avoid hitting the car in front of me. On the highway, I tend to accelerate hard until I get to cruising speed, and I have been getting good results doing that as well, but for me, city driving is the norm.

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2005 Obsidian Black Pearl XT w/4eat

The Rookie

Sorry that is complete twaddle...are you trying to suggest that at 55mph the S2K would be more economical in thisrd when it revving at about 5K than it would be in fifth at about 2000rpm ish, cos it sure as heck aint! Not sure where you get that from but it is just plain wrong!

Simon

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98 S-turbo, next mod decat D/P and hybrid turbo!

El Sid

It is wrong.

Yes, the engine may be at it's most efficient power/fuel point, but to say that the engine as air pump burns less fuel at 4000 rpm when at 2000 it is using roughly half as much air and therefore fuel is just wrong.
The term roughly is due to the volumetric efficiency reaches a maximum at some point - say for a N/A DOHC 92% @ 4000 and only 88% @2000, but if there is less air our modern FI will portion to fuel accordingly.

Better percentage at torque peak - less fuel at lower rpm.

El Sid