A Quest for 35mpg (Subaru Outback Limited Sedan 2.5L auto)

[euromodder]

Nice work

Originally Posted by Zerohour

So the material needs to be heat resistant. Not to mention being that low to the ground, it will need to withstand a pelting of gravel, rocks, sticks, etc.

Sounds like a case for polycarbonate / Makrolon ® - the stuff that many fancy headlights are made of these days.

[Zerohour]

@320:

Actually I use the fogs for driving quite a bit. Not so much for fog, but for road hazards. Pennsylvania is infamously know for have terrible roads. We have pot holes that will bend a wheel if you hit them. Its absolutely terrible in this state, and even worse the road crews "fix" something by doing half ass repairs. They install storm drains without properly settling the ground. So we have water run offs that drop off the side of the road ~6"-8". The fogs help catch these tire eating holes, especially at night in unfamiliar areas.

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So for the news, took some videos while I was out doing my coast tests. After reviewing the videos, timing the changes, doing some map checking, and rough estimations, here are my results!

Baseline for the Coast Tests:
Average 60-50 Time: 8.11 Seconds (out of 9 runs)
Average 50-40 Time: 9.07 Seconds (out of 8 runs, one run hit an uphill and vastly skewed time)

Front Bumper Block (without wheel/suspension blocks installed)
Average 60-50 Time: 8.76 Seconds (out of 10 runs)
Average 50-40 Time: 9.63 Seconds (out of 9 runs, lost 50-40 because someone pulled out in front while I was coasting.)

The accuracy of these times should be around +/- 0.25 seconds. I tried to make all the runs in the same locations along the roads, weather was fairly consistent Saturday as well. I took multiple timings of the videos and then used those to get a average time for each test.

Other Details:

Baseline Test:
70*F, mid morning (9 am) 8/14/2010
Rural Highway, start to end ~125ft change in elevation (net result its uphill the whole way)
Tire pressure - not recorded
~1/2 Tank of gas

Front Bumper Block Test
74*F, Mid day (noon) 8/14/2010
Rural Highway, start to end ~125ft change in elevation (net result its uphill the whole way)
Tire pressure - not recorded, but unchanged
~1/2 Tank of gas (about 1 gallon less, but I had a bag of tools on this run)

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The interesting bit was I never knew the road I travel frequently was moderately uphill/downhill depending on direction of travel. Found this out in an interesting sort of way!

Without the bumper block, I took more video. I actually filled my SD card on my camera, so I had no return runs with the bumper block installed. So the trip out was 8.11 seconds/9.07 seconds without bumper block. The return trip was 10.04 seconds/10.72 seconds without bumper block. I was actually pretty confused by this result at first. Trying to find a solution I checked into the topography of the area and found out where my route started at ~525 feet above sea level. The end of the trip is ~650ft above sea level. So in the end I could only compare data for the outward trips.

The real test will be coming up this week for work. I'm at ~300 miles on my current tank and will fill up on my way home from work Monday.

[Zerohour]

I may or may not have accidentally ordered black ABS 12" x 12" squares through McMasterCarr. All I really wanted to see was the shipping charges. :-/ God damn one click shopping...

I guess I'll be switching to a different project. Anyone have a toaster oven I could borrow? and by borrow I mean warm plastic with?

[RobertSmalls]

Can you set your conventional oven low enough, or use a pot of boiling water to heat the plastic?

If you did a few 25-15 and 20-10mph runs on the same road in the same conditions, we could get an idea of your CRR and delta CdA.

It looks really good so far.

[flander]

I haven't read this whole thread, so I don't know if anybody has suggested this but one option is to lower the car by using Legacy sedan suspension. It will lower your car by about 1 inch over the stock suspension. If the 3rd gen Legacy/Outbacks are anything like 4th gen, the suspensions are almost the same. The difference I think is the Outbacks have maybe different springs. I would suggest checking out Legacy GT Forum for more info on your suspension.

[Zerohour]

@ Smalls

Nah I won't use the regular cooking oven to melt plastic. I would catch waaaaayyyy too much flack for doing such a thing. Toaster ovens are cheap and we have a BigLots nearby (close out retailer, so cheap price everything).

@ Flander

Yes I could go through the trouble of putting in legacy springs on my outback, and the drop would be ~1", but to yank the suspension apart and spend 200+ just for springs would not really be a worth while cause at this point. Plus I hate pulling apart a suspension that is in a working condition. The AWD just makes it worse as well because of dealing with the four half shafts as the components come apart. And on top of it, there are two local shops that have the equipment to do a AWD alignment, neither of which are cheap.

Not to mention the 04-05 Turbo Baja springs fit the outback and would yield ~1.5" lift @ $175 dollars. hahaha And these springs carry a higher spring rate and give better handling than the factory outback springs! I'm not sure how the legacy springs are from factory, but I would venture to guess they are as soft as the outback springs.

[Zerohour]

New high score 28.55 MPG! Thats with the accelerating and coasting runs. lol

[Zerohour]

Well my green, but not green, but still green mod was an epic fail tonight. It helps to not leave an online shopping cart reset, you may end up the the wrong items.

I'm 9 Miles into the first tank with the bumper block and wheel guards. I also finally took the plunge and pumped my tire pressure to 41 PSI (10 PSI below maximum for the tire).

We'll see how it goes.

ABS Plastic should be in route for delivery. Tomorrow or Wednesday I may be looking for a toaster oven. :-p

I need to determine how to make the molds to do the thermal-form. Not quite sure yet. I know it's going to involve building a mold using something that will require prep time. :-( here starts another 40 hour project. :-p

Brucey got almost an entire MPG by removing his subaru's truck mirrors. I'm hoping to make them aerodynamically neutral. Neither providing lift nor downforce. I want to try to split the mirror in the middle and guide air around it. I need to make a curved leading edge, or else the design will be far, far too boxy. I could do box much quicker, but the subaru is round and soft. Box just doesn't blend.

[RobertSmalls]

The biggest problem with mirrors is the trailing end. For good aerodynamics, you need a boat tail. But in order to see out of them, they need to be flat. The best you can do is decrease their size.

The round leading edge of the mirror is actually pretty close to optimal. That's not intuitive, but take a look at all the low-drag record cars out there, and look at streamline illustrations for various nose shapes.

When you're designing your mirrors, don't forget that airflow in that region has an upward component as well. That may explain the flat bottom, round top of most automotive mirrors.

[brucey]

I've put my mirrors back on recently, and I can still average 35 a tank pretty regularly.

This is only a few % lower than my best without the mirrors. That said, I tested at nearly 1.5 mpg difference at 55 mph without the mirrors.

There were other problems though with not having them. Like pulling out of a parallel parking spot in traffic, or backing up into a garage. The biggest one for me was above 65 mph my windshield seal made a strange buzzing noise with the mirrors gone. I guess the air was still turbulent there or something? Then after 4000 miles my camera mirror just quit working. I believe the camera works fine still, but the screen does not. Was cheap anyway.

But anyways, it's an awful lot of work to reshape them for not a lot of improvement. Driving style will make a much more meaningful increase. And at the same time, I still want to see you do it, just seeing how nicely you did the grill block. Go for it.