Too Easy To Do......

[Wadula]

And how would that save gas? With a manual transmission, if you leave the car in gear and take your foot off the gas, you will get better gas mileage than if you put it in neutral and coasted. Why? Because in neutral, the fuel is required to idle the engine, in gear the injectors can shut off completely while the movement of the car keeps the engine "running".

but do the injectors actually shut off? i doubt it. the friction would be too big without ignition compared to at least a minimum rate of fuel being injected.

why i think so: take a car at full stop. pop into neutral and rev it up to 5000 rpm. then

A) just switch the car off (which also stops injection )

B) just step of the gaz.

result at a) is that the engine goes off instantly and the needle can't even follow the drop of the revs. with b) the revs drop slowly to idle. of course, the movement and it's energy will keep the engine from stoping instantly, but it will still go significatly fast. and it doesn't. my idea at least and i never thought about so i just constructed it. what do others think or know?

 

[bahnstormer]

lol sorry guys but this thread is so funny for me b/c i hate automatics with a passion.
yeah everyone here is right, D -> N no need for button or brake pedal. jungle had a REALLY
interesting point about the ice patch...not sure if its true but i've no reason to doubt it, sound
logic.

but my advice is get a manual !
lol =]

 

[wileycoyote]

but do the injectors actually shut off? i doubt it. the friction would be too big without ignition compared to at least a minimum rate of fuel being injected.

why i think so: take a car at full stop. pop into neutral and rev it up to 5000 rpm. then

A) just switch the car off (which also stops injection )

B) just step of the gaz.

result at a) is that the engine goes off instantly and the needle can't even follow the drop of the revs. with b) the revs drop slowly to idle. of course, the movement and it's energy will keep the engine from stoping instantly, but it will still go significatly fast. and it doesn't. my idea at least and i never thought about so i just constructed it. what do others think or know?

I'll try this on the way home. At 60 MPH, take foot off gas, leave in 5th gear, reset computer, check average fuel economy as car slows to 30MPH. Return to 60 MPH, put car in neutral, take foot off gas, reset computer, check fuel economy as car slows to 30 MPH. (Or should I travel a fixed distance, say 1/4 mile?) We'll see which yields higher fuel economy.

 

[Wadula]

field tests....cool....make it 5 per set up to get an average i guess.

furthermore another thing came to my mind. basically, to switch off injection is switching of a cylinder right? well that is what mercedes introduced 1.5 or 2 years ago with their V8 and that was new and revolutionary. if bmw did that since years already, hey cool but we should tell mercedes, shouldn't we?

 

[frolf]

yeah, stepping off the gas in most situations in a manual tranny will shut off the fuel pump, this is known as compression braking.

 

[Wadula]

how does the fuel pump know how to jump in again before stalling? the breaking is known to me aswell, but i doubt that the fuel pump stops or in general no fuel gets into the combustion chamber how so ever.

 

[Wadula]

okay, i admit i am a lazy rat. i did the research and found the answer. it is as said, the pump stops working and jumps in when a certain idle is reached.

learned somwething new today which exceeded my imagination. thanks for that guys



now i am reaslly curious if that really saves fuel in a noticeable way!

 

[codex57]

And how would that save gas? With a manual transmission, if you leave the car in gear and take your foot off the gas, you will get better gas mileage than if you put it in neutral and coasted. Why? Because in neutral, the fuel is required to idle the engine, in gear the injectors can shut off completely while the movement of the car keeps the engine "running".

It's just a guess by me. The way I figured it was in neutral, it would be at idling which barely takes gas. if I just take my foot off the gas, it's still running at around 3500 rpm or whatever it's at and VERY slowly drops down. When I accelerate, I get worse gas mileage the higher the rpms go. So, my thinking was that lower rpm meant higher gas mileage. Nothing to back it up, just my wild guess. I dunno about autos shutting off the fuel injectors. I know some autos can lock up the torque converter (or whatever locks up) that saves gas, but I don't think my Civic did that.

 

[Pink_Floyd]

What years did BMW start adding all the safety features? Did they come with the OBDII cars?

I know in my '92 325i there is not really any safety features that I can think of. I can shift into any gear without depressing the break and without the key in the ignition. Also, the car doesn't bother me with stupid beeps if I don't lock the doors or don't wear my seat belt or something like that.

In my dads 2003 Santa Fe you can't do anything wrong. When you have to press the break to shift all the time and even in the "steptronic" mode the engine will shift automatically at like 4000RPMs. Also, the doors lock when you start to drive and the thing wont stop beeping if you don't wear your seat belt.

I'm so glad my car doesn't have any of these stupid features. My mom's 2002 Concord even has a interior trunk release with a picture of guy opening the trunk and jumping out. It even glows in the dark so you can see the release if someone kidnaps you and locks you in the trunk, lol.

I think we can attribute all this to stupid people who file lawsuits because of something that happened that could have easily been avoided by using common sense.

 

[Wadula]

pink_floyd, you're last sentence pretty much hits the nail on the head!!!!

do you have auto tranny or manual? just curious cause you said you can shift through all gears without pressing a button or pressing the brake. that really sounds like manual.

 

[xplode]

i've verified this in my MINI (i know, bring it on, i can handle it). don't know if this trick work in bmw's but i suspect it does - if i hold the trip reset button while in key position one, i get access to a small number of tests (shows up in the odometer window). test 19 will unlock a number of diagnostics, number 4.0 and 4.1 shows momentary fuel consumption in l/100km and l/hour. perhaps you can do the same.

anyway, while "engine braking", or "compression braking", the readings drop to 0, i.e. injectors are switched off. when your revs are low enough that momentum alone can't keep the engine "running" they kick in again (and you can kindof feel this as a very small jolt forward).

i don't see why an auto wouldn't work the same, but i can be wrong.


(oh, the gf drives an '04 325xi as of yesterday, so that validates my being here)

 

[Wadula]

xplode, welcome aboard! thanks for the report and that exactly confirms what i doubted.

oh, and nice taste...the girl i mean. nice car get her to be a member too and post pics both of you

 

[wileycoyote]

Test results were inconclusive. Under both conditions, the computer reads 99.9 MPG.

 

[Wadula]

hm...maybe the difference is that small that the computer just does not recognize it. whatever.

 

[wileycoyote]

Seems logical, or under both conditions the MPG exceeds 99.9, which appears to be the display maximum. I choose to conclude difference between the two methonds, manual or automatic trans, yields an immeasurable fuel savings.

 

[Wadula]

calculated on a lifetime it might be a gallon

what makes me wonder is that you actually do not feel the acceleration a little after stepping on the gaz. the pump has to restart and build up pressure etc....that it does not cause a retarded reaction...

 

[Pink_Floyd]

pink_floyd, you're last sentence pretty much hits the nail on the head!!!!

do you have auto tranny or manual? just curious cause you said you can shift through all gears without pressing a button or pressing the brake. that really sounds like manual.

I have an automatic ... though I wish it were a manual.

Sorry I wasn't specific enough when I said that. I mean that I can shift thru most of the gears without the button, obviously a few are designed so you can not. Also, I never have to press the break or have the key in the ignition. Is this normal or is there something broken on my car, lol.