[wdb]

Originally Posted by TheRipler

I thought you started this thread to discuss fuel economy.

Gasp!

Originally Posted by wdb in post #1

One word you will not see me use (after this) is "[e-word]". Even though the STi gets pretty darn good mileage considering its performance, it will never be an [e-word]ical car -- for a whole lot of reasons besides the price of gas.

It's fine with me if you want to drive like that. For 2MPG it might even be worth it, [e-word]ically speaking.

[TheRipler]

To be honest, I just got the car tuned by Clark a few weeks ago, and I can't stay out of it. I've been averaging ~15mpg, maybe a little less.

Still, when I go down to Goliad for the Texas Mile in October, I'll have the car loaded down with 6-700lbs of junk, and I'll be running the A/C. Last time, I got 26mpg, cruising ~80mph, using these techniques. I'm starting to think I can get better fuel economy cruising at torque peak, but I haven't done enough testing to verify that yet.

If my trunklid ever arrives, I'll be doing it without the wing in October, and hoping for a little better.

I did get 32mpg once, when the car was still stock.

Once I get to the mile, it's a little over 1 gal per run, which works out to just under 3mpg. ...and that's expensive gas. I'm real proud of that stat too.

[smeagol]

Originally Posted by ravent

Getting better than the usual 20-30% efficiency from an internal combustion engine has got to be a GOOD thing and in doing so it also means MORE POWER (usually) so it would seem they go hand in hand. You have to remember that just getting better milage means the car is probably too lean (ie. the Air/Fuel ratio is higher than the 14.7 that is called "optimum") and is in danger of burning it self up

You can run leaner than the 14.7 'magic' air-fuel ratio without damage. Most of these times are crusing at light load, and it isn't comparable to running 15psi of boost at peak torque and seeing lean air fuel ratios. I've tuned vehicles that cruise at 15-17:1 AFR and had no problems, and helped fuel economy. The 14.7 'optimum' AFR isn't optimum under all conditions. It was used mainly for the emissions side of things (having a catalytic converter working in consistent conditions, with a cheap switching/stoich 02 sensor versus an expensive wideband 02 being necessary for an OEM vehicle). Do a search for 'lean cruise mode' to read up on some strategies used in some OEM applications. It'd be fun to disassemble the Subie code to see the strategies used, and if anything custom could be incorporated....or if there was code in place that wasn't used. I've seen some strange things put in place by programmers. Ahh, I wish I had the time.. it'd pay for itself in better fuel economy within a year probably.

[wdb]

Hi all, sorry for slow responses. Blasted real world stuff!

I'll add recent entries to the FAQ I have in the Newbie/FAQ section.

[Gami004pilot]

This is a good topic...
I do alot of city driving and I just filled my tank up (there was only a 1/4 tank left so I put in 3/4 of a tank in which was 11.04 gallons). I spent around $34 and thats the most I every spent on the car. I'm always checking how many miles are on the tripometer before filling up. I drive occasionally with the A/C on and sometimes I push the car around corners
It seem to be averaging around 19-20 city driving and I hope to get alittle more by increasing the tire pressure to around 36psi all around.

I live in hot Florida can anyone suggest a good tire pressure to increase gas mileage? It gets hot here so with the heat I'm afraid of over inflation. Any suggestions?

[avengermike]

what if i use my boost controller to cut the boost will that help?

[wdb]

Okay, the FAQ is updated. There are some recommended tire pressures in there already, see it here:

The STi and Fuel Consumption

[KarlSTI]

I get around 13-14 mpg so I've got no business making recommendations to this thread. So hear goes: How 'bout that "Turbonator Tornado" deally? Hey, with that we can get 30 mpg and an extra 5 HP!!!! Sweeeeet. ~Karl

[Gami004pilot]

WDB? I went to your thread for tire pressure recommendations and you mention 40psi for the front and 36psi for the rear but the maximum pressure on our tires says 40psi. Here in Florida it gets hot and I know that my tire pressure will go over 40psi when driving around on hot roads. thats means that my tire would be over inflated.
Will it be better to put the tire pressure around 37 on all of them or do I still have to have the rear tires with a lower pressure?
Hey.....I'm trying to save gas....okey

[ravent]

Talking about tire pressure, if you fill your tires with 100% Nitrogen then you won't have to worry about how hot it is, or cold! they will stay the same as you set them.

[Enchanter]

Originally Posted by ravent

Talking about tire pressure, if you fill your tires with 100% Nitrogen then you won't have to worry about how hot it is, or cold! they will stay the same as you set them.

Not true. Nothing is exempt from thermal expansion. Nitrogen expands when heated just as everything else does. The reason tires inflated with N2 expand LESS than tires inflated with 'air', is that N2 is usually dry. The moisture in the 'air' is the difference. The moisture expands at a greater rate than the gases, thus effecting the pressure in the tire more so than dry air, N2, 02, CO2, or whatever.

[RexMan]

Originally Posted by RexMan

I'm actually experimenting this week on this very topic. When I can, I'm shifting at 2500 rpm through each gear to see how much of a difference it makes in gas mileage. Sure, there's a couple times when I "clear it out" but I believe I'll be seeing a decent increase in mpg. I'll keep you posted.

Well, this experiment actually hurt gas mileage in the long run.

I was very disciplined and shifted at 2500 when I could and ended up getting 16 mpg. That's about 4-5 mpg less than I usually get at shifting at 3500-4000 rpm.

I think it was because I had to use the boost more than usual at low rpm.

So...shifting at 3500-3750 rpm looks to be the sweet spot.

[wdb]

Originally Posted by Gami004pilot

Here in Florida it gets hot and I know that my tire pressure will go over 40psi when driving around on hot roads. thats means that my tire would be over inflated.

All tires get hot when they are driven, and the air in all tires expands when the tires get hot. Sure, they might get a little hotter in Florida than they do in Alaska, but they heat up in both cases. If the tires in Florida and Alaska both start out at the same pressure, and they both increase in temperature the same amount as they drive, the increase in pressure due to driving will be roughly the same in the Florida tire as it will be in the Alaska tire.

So you are just starting out at a higher ambient temperature. If you were talking about racing tire pressures, for example trying to dial in the handling by adjusting tire pressures in fractions of a pound, then you can be justified in thinking about about ambient temperature. Otherwise I wouldn't sweat it.

The recommended tire pressures on the tire and on the door of the car are COLD pressures. COLD does not mean "in the freezer", it just means an undriven tire. If you want to put 40lbs in your tires in Florida, go right ahead. And don't worry about them being overinflated when they heat up. Just be sure to check your tire pressures on a regular basis. Once a week should be a minimum.

[wundej]

Originally Posted by RexMan

Well, this experiment actually hurt gas mileage in the long run.

I was very disciplined and shifted at 2500 when I could and ended up getting 16 mpg. That's about 4-5 mpg less than I usually get at shifting at 3500-4000 rpm.

I think it was because I had to use the boost more than usual at low rpm.

So...shifting at 3500-3750 rpm looks to be the sweet spot.

Anytime you are on the positive side of the pressure gauge under heavier load (ie lower rpms, moving slowly in 3rd/4th etc, or low rpm/up hill) you are going to be using more gas than if you were only pulling vacuum. At least this is how I've noticed my mileage behave. The engine seems to be much more efficient reving a little further 3-3.5k than when pulling positive pressures due to heavy load.

Stay out of boost if you want to save gas, that's the bottom line. I would be interested to know if we are using more gas going up a hill in 5th gear at 3k rpms or going up the same hill in 6th a few hundred rpm's lower but under a higher load. I'm guessing you'd be using less gas running in 5th gear, but not sure.

It'd be nice to hear someone who knows a bit about how the ecu uses it's fuel maps in relation to engine load and rpm. This would really tell us what driving techniques would get the best mileage. Anyone ?

[donmei]

The most effective way to increase mileage is with the boost limiter connected to your right foot.

If you go easy and shift at lower rpms when just following mommy in her Tahoe you'll do better. You can shift as early as 1800 rpm or less in the lower gears. There's no point in winding it out when you're barely off idle.

As it is, at low manifold pressures (off boost) our cars run much richer than is necessary to avoid detonation. Also, ignition advance at low manifold pressures is not as aggressive as it could be. With that said, a custom tune should get you better mileage. Remember that these things also get you more power. So just keep your foot out of it.
Here are a couple of engineering generalities.
1) weight or mass, will affect your mileage most when starting and stopping. When at a steady speed, the weight of the car, or any of its rotating components (like drive shafts) has ZERO effect.
2) At a steady speed drag has the largest effect. There are 2 kinds of drag:
a) mechanical or frictional drag. This is all the drag associated with our cars 3 differentials, other driveline components, bearings, transmission, etc. Remember that mechanical drag increases linearly with speed. In otherwords, if you double the speed, mechanical drag doubles.
b) aerodynamic drag. This is the drag associated with pushing your car through the air. Total aerodynamic drag = frontal area x Cd. Since we can't make the frontal area of our car appreciably smaller, we need to concentrate on lowering the CD. Another point is that aerodynamic drag increase as a square funtion of speed. In other words, if you double the speed, your aerodynamic drag goes up by FOUR!. Whats this mean? It means if you want to better on gas, drive slower.

I have an example of this from when I used to own a Passat. I used to average 29 to 30 mpg in mixed driving. One day I had to follow a friend for a 100+ mile trip. He was pulling a boat trailer and went between 55 and 60 mph the whole way. I had never driven my car that slowly. I was astonished as I went over 450 miles on my 16 gallon tank. When i did the math I had gotten better than 35 mpg.

So there you have it.
1) get a custom tune
2) drive slow
3) accelerate easily.
If you do those things I guarantee you will add 5 mpg to your average.
I don't see the point in it, but thats your decision.

Don
p.s. I've said it before. The STi has several thing working against it in the fuel economy area.

1) aerodynamic drag - tall, upright, with lots of protrusions. we lose on the frontal area side and the Cd side
2) turbo engine - the low compression ratio necessated by the high boost turbo system we have greatly decreases thermodynamic engine efficiency. This hurts mileage. (we have an 8.5:1 CR, while the new direct injection VW turbos have a 10:1 CR and the new corvette has a 11:1)
3) complex, draggy, heavy driveline. While AWD may be magic in some ways, it does nothing but hurt when it comes to fuel economy

In contrast a Corvette gets much better fuel economy in the real world. Why?
1) simple drivetrain
2) very small frontal area
3) good Cd
4) high CR engine. (remember, an engine burns fuel in relation to how much power it is being asked to make, not how much power it CAN make. A 400 hp engine being asked to produce 25 hp will burn less than a 200 hp engine being asked to