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Old 06-07-2009, 09:14 AM   #1 (permalink)
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Is 42mpg too low a goal?

Greetings everybody.

I just made a 2900 mile trip out to visit friends in Montana. I've been here a few weeks, and I'm here for a few more. While websurfing I stumbled across the Ecomodder forum, and it got me thinking about all that gas I'll be using on the way home. So I read lots and lots of posts until I got an idea of what I'd be capable of, and comfortable with, doing.

My initial question: if I can get a bit over 30 mpg on the highway without any modifications at all, is it out of the question to expect a 10 mpg improvement from some modest aero mods?

Car is a FWD Subaru Legacy sedan. It turns 18 in October (of 2009). The good news is that I (mostly) don't care how it looks at this point (as you can tell by the pop-riveted sheet metal patches in five places that, at the time, saved me $200 in replacement fenders and who knows how much bodyshop payments).

The bad news is it's 17 years old (so no OBDII). The engine's got some wear on it (though not as much as could be, at about 189k miles), it's an automatic transmission, the speedometer sticks and it has no cruise control.

So, please let me show off my progress so far (modest as it will be; no full boat-tails in the near future, and even if I had a manual, engine-off pulse-and-glide is more effort than I feel like spending while driving).

Here's Car in as good an "original" picture as I can find.

Go pop-rivets! I've had a comment from someone that it reminded them of something from Mad Max.

My first thought was, what's the easiest and most reversible mod I could do? After reading plenty of threads on this forum, I decided on a door mirror delete. PA apparently also has that "at least one mirror that can see behind the car" law, so I went for it.

You can see the mirrors are pretty big. Using two different calculation methods, I found that each mirror has a frontal area of about 40 square inches. Folding them in (which the stock mirrors do, glad Subaru realized they're huge) reduces that to about 16 square inches. Good, but not as good as it can be.

Removing the mirrors entirely reduces the frontal area of Car by 3%. Three. Percent. That brings the CdA from 6.81 to 6.61, and that's assuming no reduction in drag.

Here's a comparison pic of my three mirroring options.


They came out very easily, and I'm able to put them back in whenever I want. (which is good for A-B-A testing that will come; also, I believe I need to put them back to pass inspection)

I bent up some sheet metal (to keep with the theme, of course), and with a couple of bolts, nuts and washers, there were no more exposed holes. Also, due to the way I angled the metal in some areas (and folded the edge over in others), there's not a sharp corner to be felt, even when the door's open.

(I won't bother showing a picture of the driver's side here, as it's just a mirror image of this one. Get it? Mirror image? *cheesy grin*)

So what next? Thinking about it, I decided on the mod that would be the easiest (in my opinion) to do while having the least chance of degrading performance, something sure-fire that at the very least would not hurt anything. Rear wheel skirts.

Following the example of several people on this forum, I went with the aluminum bar support at the bottom.

I kept at least an inch of clearance from the tires, figuring that a little too much clearance would be better than too little.

(by the way, don't make the same mistake I did, figuring that steel bars would be stronger for the same dimension of bar, and cost a bit less; I returned them because it was far too difficult to work with)

After I had the support bar and brackets in, I used a piece of cardboard to get a rough outline of the fenders, then traced that onto the galvanized sheet metal I so love and cut them out with tin snips.

For each one, I folded over a bit at the bottom for the smooth, non-dangerous edge (using channel-lock pliers to get to 90 degrees, a flathead screwdriver to get it to an acute angle, then a hammer to get it flat), and drilled the holes in the support bar.

I lined up the slightly oversized sheet metal where I wanted it, leaving about half an inch overlap around the whole curve, then marked where to drill the two center holes in the sheet metal with a pencil while an assistant held it in place for me. Drilled those two holes, then used washers, bolts and nuts to fasten it to the support bar in just those two places.

I then used the pliers to bend the edge of the metal 90 degrees to fit the curve, going an inch at a time, bending then checking the fit, unbending and rebending when necessary, and trimming the little bit I needed to with the tin snips.

I took the skirt off, further folded and hammered the whole edge, put it back in place with the two bolts, marked where to drill the two top holes and the remaining front hole on the support bar, got them drilled, then put the fender securely in place and drilled the final, rear hole that goes into the plastic bumper at the bottom.

I'm happy with how they turned out. As with the mirrors, there's not a sharp edge to be felt. They sit right inside the edge of the fenders, and they're quite sturdy, no wobble at all.



The bottom of each skirt undulates a little, and the trailing curve at the bottom is a little severe for aero, but on the whole it has to be doing more good than harm, I think.


Last edited by Istas; 06-07-2009 at 09:25 AM..
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Old 06-07-2009, 10:13 AM   #2 (permalink)
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Next up, simple to do and likely an improvement: partial grille block.

You may have noticed I had Car sporting some nice cardboard-and-duct-tape in the last pic; I proof-of-concepted a grille block before I worked on the skirts, but I got the skirts complete while the cardboard was still on, so I posted them first.

Car has quite a bit of area open in the front, some of it completely useless.


After a tip from RobertSmalls, and also looking at other posts on here, I knew that the top openings were the ones to block off completely. I did a quick patch job with cardboard and plenty of duct tape.


After sustained highway driving on a warm, sunny summer's day, I was stopped 30 seconds at a light on an off-ramp before the radiator fans kicked in (and of course the thermostat needle didn't budge), so I'm fairly confident I left enough of the bottom vent open (basically all of it: what I covered on the bottom was the non-functional parts).

I'm definitely going to block the middle and top radiator vents, and it will be pretty easy to do (hook the top over the back of that plastic grille, screw the bottom into the license plate mount (my resident state of PA doesn't require a front plate)). I do have a bit of a question about the bottom, though.

The bottom lip angles in in the front, so the center vent is the furthest out.

The tempting way to do it is the way I have it in the cardboard mockup, just follow the slant from the upper lip to the bottom. (I'll also duct the inner sides of the opening when I do that, unlike the cardboard)

I worry that this will funnel air under the car, though I still think a tapered angle pushing air down is better than the useless parachutes that the edges of the bottom vent are right now.

Another option I'm thinking of is a short, vertical air dam extending from the furthest-point-forward part of the bumper, down level with the bottom lip, and ducting a radiator-wide, one-quarter-high-as-the-radiator opening in to the radiator. It'd be short enough to not even need support bars, just screw it to plastic bumper.

Still another option is doing a full air dam, extending vertically from the tip of the bumper down to the level of the lowest protrusion of the underbody. I worry about the lack of clearance with this option, though.

Car has pretty high ground clearance for a sedan, I seem to remember reading somewhere on here that high-ground-clearance cars should avoid big air dams?

Any and all feedback welcome. I've got about three weeks before the return almost-cross-country trip.
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Old 06-07-2009, 10:56 AM   #3 (permalink)
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Mods look awesome; I really like the look.
Make sure you do all the easy stuff, like maxinflating your tires, etc. How fast did you drive on the way out? Slowing down by 5 mph will be huge.
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Old 06-07-2009, 11:48 AM   #4 (permalink)
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Thanks! I take pride in Car's "intentionally unfinished" look on the patches and mods. If I'm not going to take the time and money to make everything look stock, it might as well look so bad it's good. *grin* I really don't want to try matching Car's color, and I think a not-quite-matching-and-likely-to-fade brown would look worse than bare sheet metal.

One of the reasons I got my current tires was that they had max 44 psi, so that's what I'm running them at.

A problem I've noticed though is that the centerline of the front tires seem to be wearing down a bit more than either the outside or inside edges. I've seen other people say that they've never experienced uneven or increased wear, but maybe that's because their tires are more narrow? Stock tires on Car are 185/70R14. Next time I need tires I'll be getting 175/70R14 HTR T4's. (I'm having trouble finding narrower tires than that for 14" rims that are still all-seasons and -also- don't sacrifice tire radius by a decreased height/width ratio) A centimeter narrower and 7psi higher than my current tires.

I tried to stick around 60mph on the way out, when I could. I-44, though, in Missouri into Oklahoma, is insane. I'm not exaggerating when I say that traffic even in the slow lane on that highway tends to move about 85mph, at least at the time of day I went through. And even when it wasn't congested, it seemed like people tended to drive beside each other, not planning ahead at all for slower traffic in the slower lane, so that I constantly had cars and pickups riding up on my bumper. I sped up more than a little there, but still didn't suffer much in fuel economy according to the fill-ups.

I'm thinking the gas through Indiana and Missouri is just better for high mileage. Or maybe they've already switched to summer gas, whereas PA hasn't. Who knows, I'm not about to take samples of gasoline to have it analyzed.
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Old 06-07-2009, 05:18 PM   #5 (permalink)
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Istas -

I really like the workmanship. I may copy your side view mirror metal look. The rivets make me think of unpainted aluminum planes from the 1930's, Spirit of St. Louis and all that. I also understand the "intentionally unfinished" look. It's kind of an anti-cool cool statement.

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Old 06-07-2009, 07:13 PM   #6 (permalink)
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Thank you for the compliment! I never thought of that comparison before. And I'm glad to see I'm not the only one who appreciates that look. (I get ribbing from my one friend all the time)
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Old 06-07-2009, 09:23 PM   #7 (permalink)
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Hello,

Is the undercarriage in the engine area lower than the bottom of the lower grill? Maybe you can integrate a chin spoiler/air dam under there? It might help a smidgen to angle the upper block back to the headlights, eliminating the side gusset triangle; forming a bit of a "point" on the upper part of the nose?

What is your tire pressure? I would try a foam gasket all along the front edge of the hood to seal the gap.
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Old 06-08-2009, 01:04 AM   #8 (permalink)
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The oil pan is a bit lower than the lowest lip of the bumper. (I have a skid plate I made for the car out of some thick stainless steel, but I didn't put it on for this trip :/ ) Parts of the undercarriage are too. It's hard to get a good picture right now as the driveway I'm using is gravel and woodchip, but I'll try to get some good pics detailing the underside tomorrow.

And hmm, yeah, a chin spoiler/air dam/whatnot would alleviate some of the clearance issues I was worrying about, but still give some effect. I do hesitate to do anything along those lines, though, without having an MPG gauge to see if what I'm trying helps or hurts. I would also like to get some lower spoilers just in front of the front tires to shove the air off to the outside of the car.

I'm not really sure what you mean by angling the upper block back to the headlights. I can post a pic without the cardboard on at all if you want to sketch something quick and easy in Paint, if you wish. I'm interested in what you mean.

I have tire pressure at max sidewall of 44 psi, and am experiencing a bit of uneven wear (center of front tires wearing out faster than edges, characteristic, I believe, of too much pressure for the application).

The foam gasket is a good idea, I'm planning on getting that for the front and side gaps of the hood. I'm also going to get some clear caulk and seal up the gaps between the bottoms of the headlights and the bumper.

I'm going to do tuft testing as soon as we get a rain-free day to do so (there's a stationary front stalled over us)

Last edited by Istas; 06-08-2009 at 01:46 AM..
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Old 06-08-2009, 09:17 AM   #9 (permalink)
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Your car looks great! I like the look as well, as long as it works, who cares what it looks like

You should definitely get some instrumentation. You might not be able to use a Scangauge, but as long as the engine is fuel injected, you'll be able to use an MPGuino, which is just as good for older cars.

It looks like your car has a CD of .33 (source), which is pretty good, and with your mods, you've definitely lowered it. So, aerodynamically, I think you're doing great, now with some instrumentation, I think you'll be able to notice even greater improvements.
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Old 06-08-2009, 09:59 AM   #10 (permalink)
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This is pretty good work, but I don't think you can expect a 10mpg increase if you're not willing to do some EOC and other techniques. You can get away with it on the highway if you are willing to at least put it in neutral instead. Any combination of the mods I recently did gave me about a 10mpg increase from my previous norm, but I also pay very close attention to techniques while I'm driving. You might see a 5mpg or so increase, not to be a naysayer. More power to you if you can get an increase of 10. I don't know how well you can do with an AWD car either (pretty much all subies except for a few like the justy are AWD, right?) I'd suggest doing what I may end up doing, which is making a little wiper spoiler so the wipers don't create any drag - I'm currently running with only the driver side wiper, which I'm perfectly comfortable with.

As for the mirror thing, you may want to opt for the driver's side, folded in setting. Many state laws require that mirror there, but don't say anything explicit about it being folded in, so if you get stopped by smokey just apologize and fold it back out. You could even make a nice little fairing for when it's folded in like MetroMPG has (although I believe subie mirrors fold in more gracefully):

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