Driving on vacuum/gas use.

[barryogen]

I was reading about when driving on vacuum, and using the engine to lower your speed when heading up to intersection, the car uses 2/5s of f*ck all gas.

So I was wondering if it is a proportional thing... as in, the higher the vacuum the less it uses, or is it just a once the car is in a vacuum state, it uses almost nothing.

[Bumpy]

I guess when you are decelerating, when the valve opens it is sucking air in, thats where you get your vacumme. TPS would be at nill or idle so wouldnt use much gas.

[Dirtbag]

I think in many cars there is a fuel cut when you take throttle off at revs until it returns to idle. Read it in some toyota manual for 4ages

edit: just found it on in a 16v manual i have, fuel return is 1400 W/O AFM and 1200 W/ AFM

[sergei]

Quiet irrelevant to fuel consumption, especially in fuel injected cars.
What it is important is to brake the least and don't go full throttle all the time , but don't go too soft, the most efficient way to accelerate would be about 3/4 of throttle, pressing the throttle gently/slowly also helps. If you accelerate do it so you will not need to brake (be that engine brake or normal) on next corner. Try to be very constant on throttle (no on-of style of driving) and use inertia as much as possible (and legal) ie: if you go down hill do not brake, use the gain momentum to go uphill, and when you go up hill don't full throttle it, just enough gas so it will continue to the apex - let it slow down a bit (unless you have a line of traffic behind you). When you driving economically treat brakes same way as you treat gas - use them only when you need it.

[crazed_silvertop]

Quiet irrelevant to fuel consumption, especially in fuel injected cars.
What it is important is to brake the least and don't go full throttle all the time , but don't go too soft, the most efficient way to accelerate would be about 3/4 of throttle, pressing the throttle gently/slowly also helps. If you accelerate do it so you will not need to brake (be that engine brake or normal) on next corner. Try to be very constant on throttle (no on-of style of driving) and use inertia as much as possible (and legal) ie: if you go down hill do not brake, use the gain momentum to go uphill, and when you go up hill don't full throttle it, just enough gas so it will continue to the apex - let it slow down a bit (unless you have a line of traffic behind you). When you driving economically treat brakes same way as you treat gas - use them only when you need it.

Actually... the boys at TopGear did a show on this lol... and apparently, if you accelerate real quick, and then coast as long as you can it's more fuel efficient (they tested by using this method of accelerating, and then the same track, but with constant acceleration)

[Mr Revhead]

try that on the road

[IH8TEC]

afr wise, using engine to slow down in my car and nothing else brings it just over 60:1 or something like that. OR i put it in nuetral and cruise up use brake will be about 14:1

[SUBARUCONVERT]

yes if you engine brake and work down thru the gears rather than leave it to last second and use the brakes alot it will save some gas, also if over about 1500 rpm with the throttle closed the ECU cuts off all fuel, so going down a big hill engine braking lightly saves fuel compaired to poping it in neutural and coasting down a hill

[SUBARUCONVERT]

yes if you engine brake and work down thru the gears rather than leave it to last second and use the brakes alot it will save some gas, also if over about 1500 rpm with the throttle closed the ECU cuts off all fuel, so going down a big hill engine braking lightly saves fuel compaired to poping it in neutural and coasting down a hill

[jondee86]

A 60:1 air/fuel ratio is not a combustible mixture, so fuel cut has
occured. This feature (when enabled) happens on an EFI vehicle when
the TPS senses the throttle is shut, there is high manifold vacuum, and
the engine revs are above a minimum threshold (as posted above).

Until the revs drop below the fuel restore threshold (or you open the
throttle), the engine is just an air pump driven by the cars momentum

A 14:1 air/fuel ratio indicates normal combustion at idle.

Cheers... jondee86

[IH8TEC]

A 14:1 air/fuel ratio indicates normal combustion at idle.

Cheers... jondee86

this i know, and the 60:1 i know because thats what it showed on the air fuel meter when road tuning. no fuel cut nothing, he said it was normal and good.

[Lloyd]

60:1 was probably as low as the sensor could go, or a minimum voltage still showing or something. I'd say fuel cut would have come in if you were getting that reading. Hook a scanner up and watch the injector ms while you're off throttle and its pretty obvious its off and as you slow through the revs you can see where it kicks back in (usually 1500-2000rpm)

[Akane]

on the PowerFC it shows 0% injector cycle when i deaccelerate and leave it in gear, and above about 1500rpm, so yes if you use engine braking on a ECU that supports such function (most EFI cars do anyway), it'll save fuel.

[barryogen]

right, so pretty much any engine braking above 1500RPM(as a general rule) will use nothing, or at least next to nothing...

That'll explain why I have been getting a lot better milage(10-15%) since making my exhaust quieter... I've actually been able to use the engine to brake rather than flicking it into neutral and coasting, and Dunedin is hilly, so there is a lot of coasting.