What happens on in gear decceleration?
What happens on in gear decceleration? There's a big thread on another forum that started out discussing which method uses less gas. The situation is your slowing to a stop, or coming down a mountain. Is it more fuel efficient in a manual car to throw it in neutral, letting the engine idle and use the brakes, or is it more fuel effiecent to keep it in gear and let engine braking slow the car, or keep the car at a certain speed(if going down a mountain).
The discussion then turned to what happens when you snap the throttle shut causing the engine to deccelerate and begin engine braking.
Some people are saying the fuel injecotrs completely turn off, injecting no fuel until the engine gets down to idle rpm.
I believe there's still fuel being injected into the cylinders, just a very lean mixture. I'm guess approximately the amount of fuel it takes to idle the engine. I say this because as most of you know when you let off and allow our engines to deccelerate the car, if your in gear, it burbles and backfires. To me that means there's still fuel being injected into the cylinders. If there where no fuel the car would just make air pumping noises, meaning sound like it's not running, instead of the burbling rumbling and backfiring.
Help me out here, prove me wrong, prove me correct. I just need to know what the injecotrs are doing on in gear decceleration with the throttle completely closed?
The discussion then turned to what happens when you snap the throttle shut causing the engine to deccelerate and begin engine braking.
Some people are saying the fuel injecotrs completely turn off, injecting no fuel until the engine gets down to idle rpm.
I believe there's still fuel being injected into the cylinders, just a very lean mixture. I'm guess approximately the amount of fuel it takes to idle the engine. I say this because as most of you know when you let off and allow our engines to deccelerate the car, if your in gear, it burbles and backfires. To me that means there's still fuel being injected into the cylinders. If there where no fuel the car would just make air pumping noises, meaning sound like it's not running, instead of the burbling rumbling and backfiring.
Help me out here, prove me wrong, prove me correct. I just need to know what the injecotrs are doing on in gear decceleration with the throttle completely closed?
If you're me, on a late night highway run in H2 with a Radix running 10psi, the engine ***** itself on decel after snapping the TB closed. Two pistons gone at 10,000 miles... Sucks to be me I guess... LOL
It depends on how the DFCO is set to operate in the calibration.
DFCO shuts the injectors off and uses no fuel.
If you're popping while decellerating, then DFCO has not activated and you are still getting fuel... probably way too much fuel.
DFCO shuts the injectors off and uses no fuel.
If you're popping while decellerating, then DFCO has not activated and you are still getting fuel... probably way too much fuel.
TECH Fanatic
Joined: Jun 2002
Posts: 1,979
Likes: 3
From: Upstate NY
Originally Posted by FAST LS1
There's a big thread on another forum that started out discussing which method uses less gas. The situation is your slowing to a stop, or coming down a mountain. Is it more fuel efficient in a manual car to throw it in neutral, letting the engine idle and use the brakes, or is it more fuel effiecent to keep it in gear and let engine braking slow the car, or keep the car at a certain speed(if going down a mountain).
The discussion then turned to what happens when you snap the throttle shut causing the engine to deccelerate and begin engine braking.
Some people are saying the fuel injecotrs completely turn off, injecting no fuel until the engine gets down to idle rpm.
I believe there's still fuel being injected into the cylinders, just a very lean mixture. I'm guess approximately the amount of fuel it takes to idle the engine. I say this because as most of you know when you let off and allow our engines to deccelerate the car, if your in gear, it burbles and backfires. To me that means there's still fuel being injected into the cylinders. If there where no fuel the car would just make air pumping noises, meaning sound like it's not running, instead of the burbling rumbling and backfiring.
Help me out here, prove me wrong, prove me correct. I just need to know what the injecotrs are doing on in gear decceleration with the throttle completely closed?
The discussion then turned to what happens when you snap the throttle shut causing the engine to deccelerate and begin engine braking.
Some people are saying the fuel injecotrs completely turn off, injecting no fuel until the engine gets down to idle rpm.
I believe there's still fuel being injected into the cylinders, just a very lean mixture. I'm guess approximately the amount of fuel it takes to idle the engine. I say this because as most of you know when you let off and allow our engines to deccelerate the car, if your in gear, it burbles and backfires. To me that means there's still fuel being injected into the cylinders. If there where no fuel the car would just make air pumping noises, meaning sound like it's not running, instead of the burbling rumbling and backfiring.
Help me out here, prove me wrong, prove me correct. I just need to know what the injecotrs are doing on in gear decceleration with the throttle completely closed?
My conclusion is that the GM engine may not completely shut off the fuel during closed throttle decel. I have tried different gears (A4) on long downhill grades, and instant mpg is slightly different in 3 vs 4, but not anywhere near the ratio of the gears.
Some GM automatics with their "fuzzy logic" will downshift to lower gear(s) if you are in a closed throttle decel (coming down a long grade), and the vehicle speed is still increasing. This gives more engine braking, which is a safer thing than coasting at idle with the trans disengaged.
FWIW, if you have a mpg computer, watch instant. mpg during your normal driving, especially before the cold engine gets to normal operating temp. 3-5 mpg. at light acceleration is common with a cold engine, while it will double or better when the engine is warm. Of course letting it warm up without moving gives zero mpg, so driving away moderately right after start is the way to go. Many owners manuals say just that.
DFCO comes in and cuts off fuel... depending on coolent temp, Baro, TPS, and speed...
if everything is set right, it does completely cut off fuel... however, if you hover the gas pedal on slightly, or its not warm enough to enable DFCO, ect.. then you're using fuel.
if you have a good scan tool, you can look at the injector pulsewidth to see if its cutoff completely.
that said, ive thought about it.. lol and logged and studied it.
with my car a "02 camaro" if its warm out and i dont touch the gas going down a steep grade, and i have it in 5th (not 6th) it uses NO fuel at all.
in 6th, it runs really lean (pegged lean on my wideband.. so its above 18)
if i touch the gas, it brings more gas in.
but as long as im actively using engine breaking, high enough RPMs, ect.. it does completely cut fuel.
your results will vary depending on the car and the tune.
if you have any GM EFI tuning software, you can tune the DFCO in your car to be more agressive, and work better for MPG... although it may not always be seemless kicking in and out like it is stock.
if everything is set right, it does completely cut off fuel... however, if you hover the gas pedal on slightly, or its not warm enough to enable DFCO, ect.. then you're using fuel.
if you have a good scan tool, you can look at the injector pulsewidth to see if its cutoff completely.
that said, ive thought about it.. lol and logged and studied it.
with my car a "02 camaro" if its warm out and i dont touch the gas going down a steep grade, and i have it in 5th (not 6th) it uses NO fuel at all.
in 6th, it runs really lean (pegged lean on my wideband.. so its above 18)
if i touch the gas, it brings more gas in.
but as long as im actively using engine breaking, high enough RPMs, ect.. it does completely cut fuel.
your results will vary depending on the car and the tune.
if you have any GM EFI tuning software, you can tune the DFCO in your car to be more agressive, and work better for MPG... although it may not always be seemless kicking in and out like it is stock.
I've wondered about this also. With my engine hot after driving 20+ miles on a hot day, I've noticed my AF meter nearly pegs out rich on deacceleration with no throttle. I need to look into DFCO, not exactly sure what it is, but I'm sure it will boost my mpg as I do more braking with the engine than I ever do with the brakes.
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Staging Lane
Joined: Mar 2006
Posts: 50
Likes: 0
Lumpy cam guys might have trouble getting to low enough MAP to enable DFCO in higher gears at lower engine speeds. Getting consistent combustion and accurate fueling is also very difficult at low low loads. I'd adjust the settings for DFCO depending on the cam to allow most cutoff, with acceptable return to idle, and acceptable drivetrain jerk when exiting cutoff.
braking, burble, car, closedthrottle, cut, deccelerate, deccelerates, decceleration, engine, fuel, fuelcut, gear, jerks, point, shut, slowing
