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Old 10-21-2008, 10:58 PM   #1 (permalink)
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Question about shutting down at red lights

So I was at the dragstrip the other day (how many threads on Ecomodder start like that?) and while waiting in line to make a run I would shut the engine off, wait until the cars ahead of me had moved up several spaces, start it back up and move forward, etc etc, to help keep the engine cool. A friend of mine suggested that this was ultimately counter productive because the alternator would have to do extra work to replace the charge lost by cranking the starter. This got me thinking about one hypermiling technique I use, which is to shut the car off when stopped at red lights. Is it possible that it might actually take more fuel to replace the battery power used to restart the car than is saved by shutting the engine off? I read a statistic once that the threshold for whether you save fuel by shutting the engine off is six seconds, but I have no idea how that was calculated or whether it took this variable into account. There would also clearly be a large difference between the energy needed to start the 2 litre four cylinder engine in my Escort vs the 5.7 litre V8 with 10.4/1 compression in my Firebird. Would an amperage gauge possibly be useful in determining this? Let hear some theories.

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Old 10-21-2008, 11:13 PM   #2 (permalink)
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nah with in 3 seconds my voltmeter levels off at 13.8ish, i would be more worried about heat soak
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Old 10-22-2008, 06:23 AM   #3 (permalink)
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Quote:
Originally Posted by Formula413 View Post
So I was at the dragstrip the other day (how many threads on Ecomodder start like that?) and while waiting in line to make a run I would shut the engine off, wait until the cars ahead of me had moved up several spaces, start it back up and move forward, etc etc, to help keep the engine cool. A friend of mine suggested that this was ultimately counter productive because the alternator would have to do extra work to replace the charge lost by cranking the starter.........There would also clearly be a large difference between the energy needed to start the 2 litre four cylinder engine in my Escort vs the 5.7 litre V8 with 10.4/1 compression in my Firebird. Would an amperage gauge possibly be useful in determining this? Let hear some theories.
Hey, I'm the guy that made money last weekend drag racing my 3 cylinder bone stock Geo. Lots of racers do what you do or simply get out and push, not to save gas, but to keep their motors cool. From what I have garnered from these economy forums cars like my Geo use as much gas to restart as 10 seconds of being at idle while a V8 might take over 30 seconds worth. Heat soak would not be much of a problem since every restart will circulate coolant that was in the rad in place of that in the hotter motor. I like many racers have a aswitch to turn on the fan even with the engine off. If you are bracket racing and want the car to be consistant you should keep it at your normal highway coolant temp and keep it running a couple of minutes or more before you hit the starting line. On computer controlled cars sometimes this helps out a lot.....then to keep it cool I turn on my defroster that automatically runs the fan....or turn on the AC....BUT....if you run the AC make sure you turn it off way before you get to the ready line since you'll be dripping condesation and not be allowed to rum since you are dripping liquids onto the track. I also turn my fan on every time that I am getting ready to stage, even though if not needed it will cause alternator charging drag and slow the car down, byt because it might come on during the run....remember that consistancy is the name of the game. My Horizon used to slow down nearly .2 seconds with the fan on, and it would slow down also about that much if the time trials were in the day but the racing was at night....I took months to figure this out, while all my V8 oppoments were going faster at night with cooler air I was going slower! I also tried an alternater kill switch to simply race on the battery and have no alt drag....the computer in the Horizon did not like the 12 v from the battery and the car ran like crap. On my 98 Neon the car loved running with the alt shut off. I guess it all boils down to the individual car.
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Old 10-22-2008, 06:14 PM   #4 (permalink)
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I do have a manual fan switch hooked up in the Formula. It is a pretty consistent car, if it hooks up good most runs will be within a tenth, if not less.

The guy who told me that I should skip shutting it down over and over said that it takes twenty miles of driving to replace the charge used to start the car. I have no idea where he got that statistic, it seems very high to me.
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Old 10-22-2008, 07:32 PM   #5 (permalink)
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Quote:
Originally Posted by Formula413 View Post
The guy who told me that I should skip shutting it down over and over said that it takes twenty miles of driving to replace the charge used to start the car. I have no idea where he got that statistic, it seems very high to me.
Typical manufacturer's specifications for starter draw on a medium to large V-8 gas engine are in the range of 100-300 amps at an ambient temperature of xxF. Let's figure a 300 amps at 75F. There's usually no manufacturer's spec. for -30F, but experience would suggest 350-500 amps for a healthy starter. If it's that cold, you might have to crank the engine for longer than normal. Let's say 500 amps for 20 seconds.

500 amps x 20 seconds = 10,000 amp seconds
10,000 amps seconds / 60 seconds per minute = 167 amp minutes.

Least amp rating I found in a quick search for an OEM replacement for you '97 Firebird was 105 Amps. Let's assume wipers, headlights, rear defroster, heater motor and stereo consume 85 of those amps - leaving 30 amps to recharge the battery.

167 amp minutes / 30 amps = 5.6 minutes

Now lets assume more normal weather, warm engine and just stereo and heater fan (so at least 50 charging amps)


((300 amps x 5 seconds) / 60 seconds) 50 amps = 0.5 minutes.

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