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Old 10-27-2009, 04:46 AM   This thread is in the EcoModder Project Library | #1 (permalink)
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Experiment: What do I lose with AWD?

It's almost a given you're sacrificing a few MPG when driving an All Wheel Drive Vehicle. (AWD) I mean, that's 2 more wheels to push and a lot more weight right?

I've always heard it, but never saw any actual tests. I kinda lucked out when I realized I could manually force my car to drive in Front Wheel Drive (FWD) mode by way of a fuse. It disables the rear transfer clutch completely so the car acts like a regular FWD car (albeit, still toting around a rear end, drive shaft, hubs, and axles necessary to be AWD)

So being a good little ecomodder I set out on my test course tonight to see what I could come up with. Test were done by setting cruise control at 50 mph (speedometer and scangauge confirmed) and driving 6 runs, then putting the FWD fuse in, then 6 more runs, then back to normal mode for another 4 runs. Readings were taken with scangauge and after the testing I filled up at 13.9 gallons and the scangauge was within 1% accurate. Car was well warmed up as I had already been driving an hour when I decided to do this.

The course is 1.1 mile straight and level as can be around here, hardly any traffic in the middle of the night.

Temperature was 51 degrees, humidity was at 52% with no reported wind.

AWD:

A: 33.2
B: 33.3
A: 33.2
B: 33.2
A: 33.1
B: 33.2

AVERAGE: 33.2 MPG

FWD:

A: 32.9
B: 34.6
A: 32.9
B: 34.9
A: 32.9
B: 33.4

AVERAGE: 33.6 MPG

AWD 2:

A: 31.1
B: 32.0
A: 31.7
B: 32.1

AVERAGE: 31.725 MPG


This is some really odd data to me. A lot of it is consistent is the strangest part.

The first set of AWD runs I was really happy with how consistent they were. I usually don't get those consistent of numbers on the course.

Then the FWD runs were very consistent (all were identical even) in one direction, but not so consistent in the other. They were all a few degrees higher.

THEN the final AWD run the average dropped again to below the original test average.


Right now however, with that data, I can say that the FWD fuse on an automatic transmission Subaru doesn't do anything to really help gas mileage. Its all within the noise it seems.

Strictly speaking to test this, I should remove all the rear drive train components, but I'm not doing that for a simple test. This really is just to test the fuse itself.

ITS also worth mentioning here, that its my understanding that the transfer clutches give something like 5% to the rear wheels when cruising. I imagine this helps them overcome the drag when lugging themselves. So maybe that's more efficient than just letting them drag behind?


Last edited by brucey; 10-27-2009 at 02:24 PM..
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Old 10-27-2009, 05:32 AM   #2 (permalink)
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With a viscous coupled AWD or similar system, most of the time the AWD isn't doing much. It's only when there's wheelspin at either end that the VC (or electronic equivalent) locks and you get torque transferred.
So I'd say in normal driving the difference between having the rear end switched in or out will be minimal.
Physically removing the AWD parts will show the most gains. I did this on my little Nissan and there was more usable power and mpg, but to be honest nothing startling.
Landrover Freelander owners here in the UK do this too and call it 'mondo mode'.
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Old 10-27-2009, 06:34 AM   #3 (permalink)
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Back in the day I read a comment from an Audi engineer saying the AWD doesn't hurt fe because a powered wheel has less rolling resistance than an unpowered wheel and that pretty much cancelled out the extra weight and mechanical transmission losses. They made it sound like an fe wash.

That said, I wouldn't build an ultimate fe car with AWD.

For your fuse thing to work I'd think you'd have to disconnect the rear axles too so all that junk isn't spinning.
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Old 10-27-2009, 09:54 AM   #4 (permalink)
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Given the fact that I
1) Almost never go offroad
2) Live in a part of the country where snow is only something so see on TV
3) Took it to an independent mechanic

...makes me wonder if that fuse is something I should look for under the hood of my Forester. I took it to the place I bought it - independent used car dealer, sells mostly Subies and has his own tame wrench wrangler - when it started clunking in sharp turns. He said it needed the center diff - his words, not mine - fluid changed, and I simply took him at his word.

Now, I'm pretty sure that's really a viscous coupling in there, which acts as a sort of center diff while also turning some torque to the stern. And it seems to me that when I floor it on wet roads, only the front end spins. But maybe it's electronic? So hmm, maybe he's pulled a fast one? I'll have to check for that fuse.
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Old 10-27-2009, 10:03 AM   #5 (permalink)
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Hey, I just went to some of the Scooby forums and read up a bit - it looks like activating the solenoid that is fired by that FWD fuse isn't a great thing to do - it's not designed for 100% duty cycle for extended periods.

I guess that's Subaru's way of saying that if you wanted 2WD you should've bought a motorcycle.
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Old 10-27-2009, 10:23 AM   #6 (permalink)
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While not a "scientific" as using the scan gauge, I can tell you switching from 2WD on my Jeep to the Fulltime 4WD mode nets me at least a 1 MPG loss. But mine is a different system than the Subies...
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Old 10-27-2009, 11:28 AM   #7 (permalink)
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I suspect the extra drop is the heat soak into the parts, you finally got your tires up to full temp or the drive train.

The ford Escape comes in 2wd and is way cheaper than most of the Subarus.

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Old 10-27-2009, 11:46 AM   #8 (permalink)
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This is about the same results I had on my dsm.

You still have to consider that if you could remove the transfer case drive-line and all the AWD rear parts and and replace them with a lighter FWD rear end parts you would increase your FE by a lot. All these parts have more mass and parasitic drag, so by reducing their mass and parasitic drag you would increase you FE.

This is the same as when testing a car in awd on a chassis dyno verse fwd mode on a chassis dyno.
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Old 10-27-2009, 12:01 PM   #9 (permalink)
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Too bad you only did A-B-A testing and not A-B-A-B. But I know how time consuming it can be, so thanks anyway

I'm with Mark that there shouldn't be noticeable difference while cruising. Better would be city driving, with lots of turns and stop-go, but this would be much harder to test.

Without the fuse, have you noticed any difference in handling?
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Old 10-27-2009, 12:02 PM   #10 (permalink)
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The "clutch" that connects the front and rear wheels in an AWD car is typically a viscus coupler. The reason is that as you turn sharply the front and rear wheels go a different distance. with a solid connection like in a transfer case would give you binding and jerking motion as one or two tires slipped a bit to release pressure in the drive lines. It is likely that although you are no longer getting power to the rear wheels with the fuse pulled the rear wheels are still spinning a torque converter.

On a side note I gain about 1-2mpg when I have the Differential in 2wd and the hubs unlocked on my Suzuki Samurai. Unfortunately I hardly ever get to do that as we have snow/ice in the winter and mud in the summer.

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