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Old 09-23-2010, 06:14 PM   #1 (permalink)
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What is MPG penalty for a running radiator fan?

I've started working on my grill block, and so far so good, I can already tell the difference with only half of it blocked. Was getting ready to work on blocking more when the adage of not letting the rad fan come on came to mind.

My question is, how much is the cost of letting the fan kick on at temp as necessary vs the aero drag of a half opened grill? I know the conventional wisdom is to make the fan stay off, but why? Especially if the temp the fan comes on is raised a bit?

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I think you missed the point I was trying to make, which is that it's not rational to do either speed or fuel economy mods for economic reasons. You do it as a form of recreation, for the fun and for the challenge.
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Old 09-23-2010, 06:53 PM   #2 (permalink)
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i blocked off the full upper what does yours look like? i did not notice any differance with temps with the upper blocked. and i believe the temp thing is it gets too hot in the engine bay and melts things plus if the fan is running it is using electricity which mean the more the alternator has to work i e more gas used
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Old 09-23-2010, 07:07 PM   #3 (permalink)
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I'm certain the underhood temps are high anyway, so not too concerned with that. But what is the cost of running the fan? The Max has a 30 amp fuse for the fans, so it would have to be less than that, so about 325 watts? (25 amps @ 13 volts) Assuming alternator efficiency of 50%, that would be a 750 watt pull, or close to 1 hp of the 50 I use at crusing speed.

I'm just not seeing a huge drop in mpgs from a radiator fan running in theory.....
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Old 09-23-2010, 07:11 PM   #4 (permalink)
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it would be interesting to have a thermometer under the hood in the engine bay to see if it really got that bad.
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Old 09-23-2010, 07:14 PM   #5 (permalink)
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On a side note, by using the current mpg guage on my scanguage, which is an average of the current drive, my wife and I have been perfecting our driving technique and have been averaging 30 mpg for the tank thus far. And this is on several trips of about 10 miles each. So far so good! Both of us getting into the spirit of things helps a lot!
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Old 09-23-2010, 07:18 PM   #6 (permalink)
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it would be good to check engine bay temps just to see if it is really that bad but i am sure it may be a very small percentage loss in mpg more for true hypermillers not us in v6 sedans. sweet i am excited mine is around 26 - 28 here in branson which i see as impressive feet since we all know how traffic and hills are here lol.
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Old 09-23-2010, 07:19 PM   #7 (permalink)
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I can guarantee it gets a bit warm. Ever tried to work on the top of the engine after it had been running?

Hmm, maybe unhooking the fresh air snorkel and using the Intake Air Temp gauge on the SG would give an indication......
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I think you missed the point I was trying to make, which is that it's not rational to do either speed or fuel economy mods for economic reasons. You do it as a form of recreation, for the fun and for the challenge.
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Old 09-23-2010, 07:21 PM   #8 (permalink)
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it would be good to check engine bay temps just to see if it is really that bad but i am sure it may be a very small percentage loss in mpg more for true hypermillers not us in v6 sedans
You're right I'm sure. But I sure do love our v6 sedan. I saw a gold '98 Max at a dealership with a 5 speed and nearly started crying from desire!

Luckily I convinced the wife that replacing the auto with a manual was a good idea....now just need some parts
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Old 09-23-2010, 07:41 PM   #9 (permalink)
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ShadetreeMech, regarding the MPG penalty for running the fan I will be doing a test on current draw vs MPG for my car when i finally find a good enough test site.

but from previous tests (that weren't ideal) i think there was only a few percent penalty for running with full head lights compared to parkers. I have the standard fillament lights not LED so the current draw of my head lights is probably comparable to the fan current draw.

So i wouldn't expect to see more than 2 or 3 percent for running a fan also the fan wont always be on.
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Old 09-23-2010, 07:59 PM   #10 (permalink)
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we will have to compare grill blocks once we both finish. same car, similar driving environment. let the testing begin lol

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