In AT going from N -> D
Burning Brakes
Joined: May 2006
Posts: 1,158
Likes: 1
Energy creates heat, therefore spent exhaust.
Are you saying in this downhill scenario, that there is no exhaust with the engine running?
You are still creating combustion. Minimally yes...but you still have combustion, therefore spent fuel and exhaust.
Are you saying in this downhill scenario, that there is no exhaust with the engine running?
You are still creating combustion. Minimally yes...but you still have combustion, therefore spent fuel and exhaust.
Last edited by S PAW 1; May 8, 2010 at 03:05 PM.
Team Owner
Joined: Mar 2006
Posts: 20,172
Likes: 1,818
From: Bakersfield
In fuel cut mode you have compression going on but no combustion. You'll raise the exhaust a few degrees above ambient by the compression itself but no appreciable heat will be present at the tailpipe. At this point all the engine is doing is moving air from the intake, compressing it, and then exhausting it. There is no combustion. The wheels (by momentum or gravity) are now powering the engine instead of the engine powering the wheels.
Team Owner
Joined: Mar 2006
Posts: 20,172
Likes: 1,818
From: Bakersfield
I was a complete car nerd back when I got into racing. I did all sorts of weird tests and information gathering that no normal person would care to know.
I would watch the oxygen sensor under various conditions. Under fuel cut 02 volts would go 0.
If you had a mountain long enough, the engine temperature would go below the thermostat setting. Going down the grapevine around here, I would sometimes put the GN in neutral or the heater would quit working when coolant dipped into the 140 degree range.
If you have open exhaust with no cats you can hear the exhaust pop a few times as it enters and exits fuel cut. When I first lifted off the gas, it would sound like it was running rough for a second or two and then get completely smooth as the fuel was shut off. Ironically, the exhaust is a little louder when the fuel is shut off than with it idling.
With the scan tool I used back then, it displayed if it was in cruise fuel, decel fuel cut, power enrichment, and wide open throttle fueling so I could monitor what happened right at the moment of fuel cut.
I also learned that if the coolant is running hot, downshifting and getting it into fuel cut will cool it off very quickly. Not only do you have the coolant trying to do it's job, you now have ambient air being drawn in without combustion to internally cool it.
I would watch the oxygen sensor under various conditions. Under fuel cut 02 volts would go 0.
If you had a mountain long enough, the engine temperature would go below the thermostat setting. Going down the grapevine around here, I would sometimes put the GN in neutral or the heater would quit working when coolant dipped into the 140 degree range.
If you have open exhaust with no cats you can hear the exhaust pop a few times as it enters and exits fuel cut. When I first lifted off the gas, it would sound like it was running rough for a second or two and then get completely smooth as the fuel was shut off. Ironically, the exhaust is a little louder when the fuel is shut off than with it idling.
With the scan tool I used back then, it displayed if it was in cruise fuel, decel fuel cut, power enrichment, and wide open throttle fueling so I could monitor what happened right at the moment of fuel cut.
I also learned that if the coolant is running hot, downshifting and getting it into fuel cut will cool it off very quickly. Not only do you have the coolant trying to do it's job, you now have ambient air being drawn in without combustion to internally cool it.
6th Gear
Joined: Jan 2013
Posts: 6
Likes: 0
From: Columbus, Ohio
"Hard on the brakes"?? Lol not quite. A slight increase in stopping power needed. Definitely not "hard" on the brakes that are designed to do one job....stop the car.
him!
Team Owner
Joined: Mar 2006
Posts: 20,172
Likes: 1,818
From: Bakersfield
In some cases, many cases actually, when you're driving in stop and go slower speed traffic, the auto hinders braking performance. It always has a little "push" on the drive wheels so the brakes have to not only slow the car but they have to overcome a little engine torque trying to keep the car moving.
I can't imagine why anyone would ever put an auto in neutral while driving.
And wow, Ron A said that 9 years ago lol. I
I am used to driving manual so I have a habit of shifting to neutral when I approach traffic or long stop.
I find it irritating when my car makes a harsh downshift to third gear to first gear as that's how the grade logic control is deigned.
I have no problem shifting to neutral while driving at 35mph and then shifting back to D while it is moving. I feel no gear hesitation. So I don't see it as a problem or a wear on our transmission. I think its safeguard against harsh downshifts
I find it irritating when my car makes a harsh downshift to third gear to first gear as that's how the grade logic control is deigned.
I have no problem shifting to neutral while driving at 35mph and then shifting back to D while it is moving. I feel no gear hesitation. So I don't see it as a problem or a wear on our transmission. I think its safeguard against harsh downshifts
