Is 42mpg too low a goal?

[Istas]

McTimson: yeah, I want to pick up an MPGuino, but one thing worries me about it; my speedometer sticks sporadically. I'm not sure if this problem is at the instrument panel, or if it's at the sensor (wherever it is). If the problem is with the sensor, getting an MPGuino won't do me much good for testing mods, and its effectiveness will be reduced for altering driving style.

Yup, I found that 0.33 as well, through wikipedia, through a link on that energy/mpg calculator that's on this site. It's not too bad to begin with, definitely, but there's also room for lots of improvement (as with most cars). Thanks for the encouragement.

Wonderboy: well, I'm still going to try for a 10mpg improvement. I am willing to put it in neutral for long hills, although that means kicking it out of torque converter lock. I do pay a lot of attention to that, and have gotten to the point where I know when it will kick in. I'm also trying to drive with load recently, too, which is not always easy because of traffic, also not easy because of the automatic transmission, and I'm already going at, or slightly below, the speed limit (which is of course slower than 99% of people on the highway).

For AWD, I think it was 1993 that Subaru started putting that on all their vehicles. Mine's FWD, so at least I don't have the extra weight and drivetrain losses of AWD.

*nods* I am planning on the wiper spoiler/deflector. I don't quite want to take the wiper off, wouldn't feel comfortable losing visibility for that much of the windshield when it's raining/snowing. I'll be doing tuft testing on the hood and windshield (more from curiosity than anything at this point), as well as the rear window and trunk, so I'll be able to do that again after I put the wiper deflector on and see what's changed.

As for the door mirror, as I read the PA law, the only thing I can find on it is "at least one mirror that can see directly behind the car for 200 feet", as long as it's not obstructed or broken, and I don't think other states can enforce their state-specific laws like that for out-of-state vehicles. However, it is something I'm still a bit nervous about it. Even if I can't get a ticket for it, I don't like the idea of getting pulled over and hassled.

You are correct, fortunately for me: Subaru mirrors, at least on this car, do fold in quite forgivingly. Observe.


So perhaps that's what I'll do, just as insurance against PITA fuzz. 16 square inches for the driver side mirror, hugging close to the car, is still better than having 24 square inches in addition to that, sticking way out.

[McTimson]

The speedometer is probably mechanically driven by a cable from the transmission. Most cars have a separate VSS electrical signal used for things like cruise control - I doubt that the VSS signal is 'sticking', because it's rare for an electrical signal to stick like that.

[Istas]

Ahh, that is very encouraging to know. I was worried the electrical signal was sent to the dashboard needle, and that the electrical sensor was faulty. But from what you said, that doesn't make a whole lot of sense.

I'll try to get an MPGuino to me before the 2500 mile trip home. Though I still have the fun of getting to puzzle out the wiring. >.< Thank you very much for the information.

I guess that would mean I might have success installing a cruise control too, though I'm not sure I'd have use for it other than quantifying aero mods.

[Clev]

I wonder if there's a way to scavenge and retrofit a power mirror folding mechanism from a car that has it. It would be cool to have mirrors that stay folded until you signal in that direction, and then have them automatically unfold.

[Istas]

That would be a cool mod indeed. They'd have to move pretty quickly to be useful, though, and I would think it would be good to check the mirror before putting the signal on. Still a cool idea. And it would be nice to have some way to fold and unfold the mirror without needing to reach outside, especially in cold weather. Maybe a separate switch? Hand crank? Lever?

[Istas]

I just went out and took pics of the underbody relative to the bottom of the front bumper, as NeilBlanchard was asking. The oil pan is lower than the front lip, but lower still by a bit are the two exhaust headers, and the thing I use for the rear mounting point of the skidplate.

It's hard to say just how much further down from the lip the car parts are, but I'm going to guesstimate two or three inches lower.

So I'm thinking, an air dam that extends down about 2.5 inches from the bottom of the bottom lip of the bumper, all the way around from one wheel well to the other, and then a partial underbody tray that goes from the bottom edge of that to the skidplate mounting points at the lower-back of the engine.

What does anyone think of that idea?

And I think I discovered some more cause of the road noise I experience.

It looks like the bottom of the car just before the front wheel wells actually encourages the flow to go up into the wheel well. C'mon, Subaru, yer killin' me! This is not a race car, there doesn't need to be that much airflow into the wheel well!

I'll plan out some front-of-wheel fairings (splitter? spoiler?) to go with the air dam, as well.

[aerohead]

Istas,I tossed your numbers around on the calculator.A jump from 30-mpg to say 40-mpg would be a 33% increase.If say,your mpg were at 55-mph,then to get an extra 10-mpg would require a 66.6% drag reduction if you were going to use streamlining exclusively.That would require dropping the Subarus drag coefficient down to Cd0.11.If you can imagine GM's Sunraycer of 1987,that's what we're talking about.------- Low rolling-resistance tires would help remedy some of the challenge.------- I like the mods you've done.Consider a bi-wing,or tri-wing rear deck to clean up some of the turbulence behind the roof and extend it out even with the length of the rear bumper.Ford's German Merkur Xr4Ti will give you some idea of a multi-vane spoiler.And use Search EcoModder to find Project: permanent Kamm back for Metro to find a template to gauge angles and curvature for the spoiler.

[Istas]

Aerohead: Thanks for bringing that to my attention.

You're using this calculator, here? Aerodynamic & rolling resistance, power & MPG calculator - EcoModder.com

I put my car's Cd (0.33) and weight (2830, plus 200 pounds for the driver) in there, along with the original area (20.63 sq ft), as well as the rolling resistance measurement I got last week (0.0103, I'll be re-doing this to check), then dropped the engine efficiency to .21 to bring the numbers more in line with my experienced fuel mileage. (derived by fill-the-tank-and-calculate, but better than nothing)

Using that as a baseline, it spits out 35.96 mpg at 60mph (my preferred compromise highway speed). When I drop the area to 20.13 (which is what it is after passenger mirror removal and driver mirror fold-in), and plug in a Cd of 0.23, the calculator spits out a fuel economy of 45.71 mpg at 60 mph.

I know all of these numbers are very rough, but a Cd of 0.23 is a lot less intimidating than 0.11, heh heh.

I've also taken the air conditioner belt off, and will, when next I need tires, be switching from 185cm-wide 44psi-max tires to 175cm-wide 51psi-max tires.

I'm likely going to pick up some airtabs and see how that helps the airflow down and over the rear window and trunk.

I've also pondered just riveting a flat sheet-metal spoiler onto the back of the trunk, to help airflow separation.

That Merkur looks interesting, I think if I was going to try that route I might just opt for a full Kammback instead. Which I might do, but that would be a daunting task for me at this point.

[aerohead]

Originally Posted by Istas

Aerohead: Thanks for bringing that to my attention.

You're using this calculator, here? Aerodynamic & rolling resistance, power & MPG calculator - EcoModder.com

I put my car's Cd (0.33) and weight (2830, plus 200 pounds for the driver) in there, along with the original area (20.63 sq ft), as well as the rolling resistance measurement I got last week (0.0103, I'll be re-doing this to check), then dropped the engine efficiency to .21 to bring the numbers more in line with my experienced fuel mileage. (derived by fill-the-tank-and-calculate, but better than nothing)

Using that as a baseline, it spits out 35.96 mpg at 60mph (my preferred compromise highway speed). When I drop the area to 20.13 (which is what it is after passenger mirror removal and driver mirror fold-in), and plug in a Cd of 0.23, the calculator spits out a fuel economy of 45.71 mpg at 60 mph.

I know all of these numbers are very rough, but a Cd of 0.23 is a lot less intimidating than 0.11, heh heh.

I've also taken the air conditioner belt off, and will, when next I need tires, be switching from 185cm-wide 44psi-max tires to 175cm-wide 51psi-max tires.

I'm likely going to pick up some airtabs and see how that helps the airflow down and over the rear window and trunk.

I've also pondered just riveting a flat sheet-metal spoiler onto the back of the trunk, to help airflow separation.

That Merkur looks interesting, I think if I was going to try that route I might just opt for a full Kammback instead. Which I might do, but that would be a daunting task for me at this point.

Istas,I didn't use the calculator,just ran numbers for aero.Cd 0.23 is realistic goal as I hit this with the CRX.I hope you've seen basjoos AeroCivic,he's at Cd0.17(wickedly delicious!).I did a thousand dollars in wheels and LLR Michelins and saw absolutely no improvement in MPG.I also did the AC belt delete with no reward.The airtabs might help you,as they were developed for cars like yours.I don't have any first hand experience with them.The flat sheet extending back would have to help.The Kammback with all compound surfaces are the most complicated to form,so I thought the multi-wing spoiler might be more doable.I don't understand the mention of engine efficiency at 0.21,can you help out there?------ Of course the stickys are a good source for all the mods we know can contribute to better mpg,so indulge yourself there if you haven't already.The Legacy is a great car.I've got a lot of time in a wagon model and I think it will be a great project for you.If you can get to 45-mpg it would be a real coup!Keep us posted.

[Istas]

Well, the default engine efficiency that the calculator lists is 22%. I don't have any idea on what my engine efficiency is (or the drivetrain efficiency, or 'parasitic overhead'), but when I put in the other numbers I have a rough idea for (weight, Crr, Cd, A), the gas mileage seems to be a bit higher than what I find I get. So I adjusted the engine efficiency down a tad to make the estimates more in line with my observations.

I'm also assuming some loss due to inefficient driving tactics, as I don't yet have an MPG readout to help train and guide my foot on the gas pedal. I'm intimidated by working with Car's wiring, but I'm going to get an MPGuino and try it out.

I have read basjoos's Aerocivic thread and website, and I find it amazing. I never would have thought somebody could get away with doing something like that to a vehicle. His efforts are instruction and inspiration wrapped together. (so is AndrewJ's boattail, also very nice looking)

Hmm, you did standardized, instrument-backed testing and the AC belt removal didn't help at all? In that case I think I'll put it back on. (I hear that having it not rotate for an extended time can harm the compressor)

I'm still a bit leery about doing the aerotabs without having a way to quantify testing them, but I'm going to just take a leap and assume (until I can test them) that having just enough of them to reattach airflow to my rear window will do more good than harm.

The flat sheetmetal ledge spoiler will look interesting, and be easy to do. Your words are encouragement. A multi-spoiler would certainly be interesting.

If I do decide to go for a boattail, at least I have this advantage: the back edge of my car is already lower than the back edge of a hatchback.

I look over those two stickies every once in a while, re-evaluating what I'm willing to do as my boldness waxes and wanes. I better order the rollers for the front wheel skirts now, so I have them in time to work with them before the trip home.

I also am working on getting some CFD software working on my computer. I have a start, but it's tough diving into programs that are so complex and powerful... Overload of information to deal with. It would be excellent to test the trunk spoiler(s).