Impending Yellows, Floor it or Stop? - Fuelly Forums

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Old 03-11-2006, 03:36 PM   #1
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Impending Yellows, Floor it or Stop?

So, I come across this situation almost daily. When one light turns green (I'm usually the only car around) I graudually get up to speed, then I can tell the next light is going to turn yellow because of the timing and the car at the intersection is tripping the signal to change.

I have a few rules, I don't run red lights and an officer once told me that if your back tires cross the stop bar before the light turns red, then you're fine.

So, I know the light is going to turn yellow, so I floor it and then sure enough, the light turns yellow. I get through the intersection with plenty of time 'til red. Now for the question (speed limits aside):

In an automatic, would it take more fuel to quickly speed up from the momentum you have, or; stop, (shift to N, if you're inclined), wait, (back to D, if applicable) and then overcome the inertial forces to get moving up to speed again? I usually don't redline, but I get up to 5000 RPM at WOT then let off when I go through the intersection. I don't have a scanguage, so I playing with theory here. My guess is that it really eats up the fuel in my situation to start from a stop instead of temporarily WOT'ing it. Any thoughts?

RH77
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Old 03-11-2006, 04:11 PM   #2
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Re: Impending Yellows, Floor it or Stop?

Quote:
Originally Posted by rh77
In an automatic, would it take more fuel to quickly speed up from the momentum you have, or; stop, (shift to N, if you're inclined), wait, (back to D, if applicable) and then overcome the inertial forces to get moving up to speed again? I usually don't redline, but I get up to 5000 RPM at WOT then let off when I go through the intersection. I don't have a scanguage, so I playing with theory here. My guess is that it really eats up the fuel in my situation to start from a stop instead of temporarily WOT'ing it. Any thoughts?


RH77
Two ways to answer that question, theory and experience. Both say WOT thru the light.



Theory: Even though your engine may be less efficient at WOT, the resulting energy will go into increasing your momentum, which is an investment in kinetic energy that you will use after passing the light. If you were to stop instead, by braking you are converting kinetic enregy into heat, which you cannot use.

Experience: I have a light on my commute at the bottom of a hill. After the light is a steep uphill climb. Normally I approach the light with engine off at 45MPH due to the downhill approach. I have measured the extra fuel it takes if I have to stop vs if I make the light, approx 0.016L more if I had to stop.

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Old 03-11-2006, 04:13 PM   #3
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I believe the stop does eat

I believe the stop does eat much more than just going WOT, especially if you can adjust your kickdown cable to not shift down into second when you slam it, . WOT will not really use that much more gas, but high RPMs will.
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Old 03-11-2006, 04:19 PM   #4
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I would think that a

I would think that a shortburst of WOT would be better(even if it is higher RPM's) would be better than a longer period of starting from a dead stop. According to my Scanguage, my mpg's drop by a few tenths when accelerating while rolling and by as much 1-3 when going from a dead stop.
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Old 03-11-2006, 04:20 PM   #5
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I would think that a

I would think that a short burst of WOT would be better(even if it is higher RPM's) would be better than a longer period of starting from a dead stop. According to my Scanguage, my mpg's drop by a few tenths when accelerating while rolling and by as much 1-3mpg when going from a dead stop.
Edit: Darn double posts :-(
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Old 03-11-2006, 07:17 PM   #6
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double elbuob

Yeah watch the double clicks - need a message delete - or edit it to something completely different!!

I think there should be a line on the road prior to a light that if you are past and the light turns yellow you don't have to stop assuming you are going the posted speed. Kills me when I am going slower and I have to stop when really close to the light. The problem is they make the yellow different time lengths and judging distance to stop gets to be tricky if you are not sure of your speed and stopping distance at that speed. Just think if you take yoru eyes off the road for 2 seconds for a 4 second yellow light just as it changed.
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Old 03-12-2006, 08:33 AM   #7
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Re: I believe the stop does eat

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Originally Posted by SVOboy
I believe the stop does eat much more than just going WOT, especially if you can adjust your kickdown cable to not shift down into second when you slam it, . WOT will not really use that much more gas, but high RPMs will.
I checked under around the throttle body, and there are only 2 cables: one for cruise control, and the other for the pedal. I think my kickdown may be a result of the throttle position sensor communicating with the ECU -- just a guess though. Any OBD-II folks out there?

JanGeo- to reply to your statement, they just installed these signs that light up on a highway I take -- it says "Red light ahead" as it's over a crest. There are 3 of them and when the sign lights up, you have probably 5 seconds to get through the light -- but they've engineered it so you would have to really get on it to make it (they probably knew exactly what people would do). The yellow times vary here. If the road sensors find a car speeding into the intersection, the light stays yellow longer (more for safety than ticketing). We have cameras in place, but it's un-Constitutional to ticket based on camera observation. Maybe they'll get a description of the car, radio it to an officer and catch up to you if you're speeding.

So, it sounds like the theory may hold some water. Unfortunately, I need second to get the power up because of the HAI. Otherwise, it would slowly accelerate and I'd never make it -- need to wind it up. I'd still like to figure out how to loosen the criteria for kickdown on the Teg. When I drive the TL, I use the Manu-Matic shift gate, which is great - holds the gear you're in, upshifts automatically when you slow down.

Thanks all...

RH77
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Old 03-12-2006, 10:18 AM   #8
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shift down

I think the shift down point is controlled by a pressure regulator in the transmission hydralic pump circuit and not the engine. It may be sensing torque load on the converter and at a certain point change gears to reduce the torque or and increase engine rpm. Why not just ease up on the gas so it does not down shift ? ? ?

I'm having more problems keeping my foot off the gas on bumpy roads - not enough spring in my throttle - was thinking of a dial variable gas pedal tension step that adjust where in its travel it adds spring tension to hold the pedal up. Then I could very accurately vary the throttle by hand but still have full pedal control.
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Old 03-12-2006, 11:53 AM   #9
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Quote:I checked under around

Quote:
I checked under around the throttle body, and there are only 2 cables: one for cruise control, and the other for the pedal. I think my kickdown may be a result of the throttle position sensor communicating with the ECU -- just a guess though. Any OBD-II folks out there?
Do you have a seperate TCU on your car? I have a hard time thinking the ecu does it or some people that convert to obd1 might be getting screwed, but I can't really remember the last auto obd2 to obd1 I saw,
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Old 03-12-2006, 06:42 PM   #10
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Re: Quote:I checked under around

Quote:
Originally Posted by SVOboy
Quote:
I checked under around the throttle body, and there are only 2 cables: one for cruise control, and the other for the pedal. I think my kickdown may be a result of the throttle position sensor communicating with the ECU -- just a guess though. Any OBD-II folks out there?
Do you have a seperate TCU on your car? I have a hard time thinking the ecu does it or some people that convert to obd1 might be getting screwed, but I can't really remember the last auto obd2 to obd1 I saw,
It may have a TCU -- there are 2 shift points for the TC based on speed. I used to experience it all the time in my old neighborhood. I'd cruising along at 50 in full TC lockup. I'd keep the same pressure on the gas going up a hill -- about less than half. The transmission would shift about 500 RPM when the car slowed to around 45, then it would downshift slightly again at 40-45. I would really try to keep it from jolting into 3rd.

Also, what folks may not realize is that it takes lots more throttle input with the HAI to get up to and stay at speed. I also recall reading in the owner's manual that the transmission "learns" the driver's habits and shifts accordingly. Would that indicate a TCU?

-RH77
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