Bad mileage??? 02 BE Sedan Auto

[ryankenn]

I've just finished upgrading a bunch of stuff on my car, in part to try and improve the mileage, and its gotten me nowhere.

I had Subaru do the major service which included new plugs, wires and filter, as well as reset the ECU to try and optimize the other stuff I changed ( below )

It has a Borla header, a custom 2.5" SS Mandrel bent exhaust with a single brand new Magnaflow cat. I also replaced both O2 sensors.

I tried a full tank of regular driving to ensure it wasn't my lead foot, and got only 17mpg around town. The car does have more spirit with the opened up exhaust, but I thought I'd be able to do better than that.

The only thing to note is right now both O2 sensors are on the upstream side of the cat. Its caused a cel, and I have a bung welded on the proper side of the cat for the 2nd sensor, and will be moving it there shortly. Any thoughts outside of this? I'm thinking of getting a small intake as well, as it can't hurt.

Any tips are welcomed.

Thanks
Ryan

[Wiscon_Mark]

Actually, it can hurt, depending on the one you get.

The second o2 sensor being upstream of the cat probably isn't doing anything to the mileage, but I could be wrong. What about plugs/plug wires, fuel filter, etc?

Are you having any problems with engine knock?

Oh, and what is your idea of bad mileage?

edit: nvm, I see it. Missed that.

You definitely have a problem there.

[bakergtt]

Thats what I get but again I have 550cc injectors and 300 crank HP and typically hit redline so for a semi stock car i would expect more then that.

Could try some octane boost to maybe help clean the inside carbon build up. get something good like redline or something simular.

What type of plugs did they use? Might try better fluids; oil, trans, clutch if you have one. Could always stop lead footing it too see how much better your mileage would get.

[SilentRacer]

I usually get around 17 mpg if im lucky, I believe andivic the same. I think the car is already as economical as it can be, at most you may get 1 mpg more with the mods you have. If you want more then that weight reduction may be the way to go.

[Wiscon_Mark]

17mpg is pathetic, no way around it. Even for city driving.

[SilentRacer]

Would you believe I didn't buy a v6 because i thought this was more economical?

[Wiscon_Mark]

Would you believe I didn't buy a v6 because i thought this was more economical?

Well, considering what you're getting with the 2.5L, I'd say you'd be getting 12 or 13 with the EZ30.

anyways, either it's your driving style, or something is wrong, because I've never gotten worse than 20mpg even in the coldest of winter times on short trips exclusively.

[SilentRacer]

I meant a non-subaru v6. Well i dunno, considering andivic is also getting 17 and he doesn't live too far from me, I'd have to say the BE is terrible in the city.

[Wiscon_Mark]

It's heavy, that's true. But I really don't think you should be getting 17 unless you really lay into the throttle.

[sansMYSTi]

Geeze - I was getting 17 mpg with my STi. I usually get around 23 to 24 mpg with the Leg ('02 BH), which I still think is a bit low.

[ryankenn]

Installed whatever OEM plugs they normally do, which should at least let the car run as intended. The fluids were all changed, and they did add some "fuel cleaner" additive at that service ( for Canucks its the 96,000km service which is the main one ).

As for as driving, I tried no lead foot and the mileage didn't actually change noticeably.

The only thing that's offside with the car is that it will backfire under deceleration. Not big ones, but you can hear them in the pipe. It was much worse before the service, and a quick internet search led me to think that it was unburnt fuel do to improper spark, which I'd hoped the new plugs would have fixed. Now I'm thinking it may be running rich as there is just too much fuel for the combustion cycle to completely get rid of, and would also explain the terrible mileage.

The problem is I don't know how to fix that problem if it actually exists? I'd guess I'd need a ECU mod or aftermarket piggy back which I don't know much about and assume is expensive just to fix this problem.

[Plays_with_Toys]

My mileage has gone down to about 18-20mpg tops. Quite lame in my opinion. It was up around 26 in city last year and 30 on highway. Now I'm lucky to get 24 on the highway.

[Wiscon_Mark]

Installed whatever OEM plugs they normally do, which should at least let the car run as intended. The fluids were all changed, and they did add some "fuel cleaner" additive at that service ( for Canucks its the 96,000km service which is the main one ).

If you drive a BE, I wouldn't go OEM, because OEM is champion plugs and they're absolute junk. NGK FTW.

[ten80]

check plugs, PCV valve, make sure brake pads aren't rubbing, check parking brake settings, wheel bearings, tire pressure, cv boots, differential fluid levels, engine oil, coolant level, thermostat, 02 sensors, intake air temp sensor, and belts (fraying is bad!).

I get 26-30mpg on my 00 BE with 128k miles!

[Huffer]

Anyone check their tire pressures?

First thing I can think of, is that the exhaust is a little too big, so you're probably wasting a lot of energy just forcing gases out the pipe.

What's the gas grade like in Canada? Are you running stock fuel, or a higher octane rating?

[SilentRacer]

I check tire pressure everytime I fill up (once every 2-3 weeks). Everything else on the car is stock. Runs on 87 regular gas.

[ryankenn]

check plugs, PCV valve, make sure brake pads aren't rubbing, check parking brake settings, wheel bearings, tire pressure, cv boots, differential fluid levels, engine oil, coolant level, thermostat, 02 sensors, intake air temp sensor, and belts (fraying is bad!).

I get 26-30mpg on my 00 BE with 128k miles!

Whoah! I'm pretty sure my $1000 service would have them covering the bulk of those. The plugs, valve and brakes are all new. The Parking brake, I think I'm fine, bearing s are good, tire pressure is hard, 02 sensors are brand new, and belts are good. The car is in great shape, but something is amiss inside that engine.

As for Huffer, the exhaust being big makes it easier to expel the gasses, not harder. The mileage was bad before this exhaust, and is part of the reason I had it done, I thought replacing the cats ( don't know if they were bad or not, but they were the originals ) and piping with bigger less restritive stuff couldn't hurt.

I've tried to use 91 for regular driving, but for my mileage woes I've tried up to 104, with no differences in range.

I do run heavier wheels/tires ( 18" 235/40/18) but I don't see that being a reason to loose 5-8 mpg.

[Wiscon_Mark]

There is definitely such a thing as too big of an exhaust. 2.25" is the line we like to draw for most N/A cars around here. 2.5" is usually overkill unless you track the car and run it at 5500 rpms most of the time...

Too big of an exhaust = backpressure, which makes it harder to expel the exhaust gasses, lowering torque.

[MoclovFlop]

Too big of an exhaust = backpressure, which makes it harder to expel the exhaust gasses, lowering torque.

Uh, I'm no expert but i think that's backwards, how can you have back pressure with a free flowing exhaust? Larger diameter=less resistance, hence the reason turbo guys go for something along the lines of 3", more easy to expel the gasses. I know with my volkswagen, you didn't want to go above 2.25 for an exhaust becasue you wanted a little back pressure FOR the low end torque.

[Wiscon_Mark]

I know it sounds backwards, but it's true for both ends of the exhaust spectrum, both small and large. The idea is to get hot exhaust out of the end of the tailpipe, because the hotter it is, the more velocity it has, which creates torque (less restriction on the engine).

So if you have way too big of an exhaust system, the gases cool off inside the pipe and create sort of a "wall" that moves slower, creating backpressure. The deal with too small of an exhaust is that it just doesn't have the volume to handle all of the gas moving, and creates a bottleneck, slowing things down as well.

That's why headers get ceramic coated and heat wrapped, so you keep the heat in where it belongs...

[MoclovFlop]

Huh, sounds ok, Like i said I'm no expert so I'll take your word for it. I've never heard of a large exhaust causeing back pressure but with cars, anythings possible, the heat wrap point makes the most sence. You've got me thinking....

[SilentRacer]

And to illustrate marks point , here is what a zero-loss exhaust system looks like. The exhaust pipe diameter has to be based on the length of your flow system for the best effect.
http://superchevy.com/technical/engines_drivetrain/exhaust/0505phr_exh_22_z.jpg
Picture from superchevy.com

[Huffer]

As for Huffer, the exhaust being big makes it easier to expel the gasses, not harder. The mileage was bad before this exhaust, and is part of the reason I had it done, I thought replacing the cats ( don't know if they were bad or not, but they were the originals ) and piping with bigger less restritive stuff couldn't hurt.

I've tried to use 91 for regular driving, but for my mileage woes I've tried up to 104, with no differences in range.

I do run heavier wheels/tires ( 18" 235/40/18) but I don't see that being a reason to loose 5-8 mpg.

Yeah, ok, the bigger exhaust means the gases cool off faster, their exit velocity isn't as fast... blah blah. Oh well.

How much do the 18s weigh with the tires? Think about your unsprung weight. The BE chassis at best will get 28mpg freeway, it's not the most aerodynamic, and if you're adding mass to that already bulky frame you will suffer somehow.

It's quite possible that you could lose 5mpg, and up to 8mpg because none of us know how you drive - if you shift early, late or if you have the heavier auto.

All those factors, combined with the mods, could leave you with bad fuel economy.

It could also be the brand of gas - do you always buy at the same place?

Who knows!

[ryankenn]

Lots of interesting stuff here. I didn't look at the exhaust that way in terms of loss of heat and gas transmission, but a quick check of my setup and google eased my mind.

He is right that the larger pipe diameter does allow the gas to cool much quicker, but I have a couple of things on my side. The pipe is shorter overall, as the bends have been all but removed. There is only one cat now, and a straight through resonator. Also, its Stainless Steel which conducts heat about 80% less than the factory aluminium one does. ( wikiopedia - thermal conductivity ) I feel confident that I'm probably not any worse off than the factory setup in terms of backpressure, that's for sure.

Also, the reason it is 2.5" is because the axle back I have, I bought off Ebay from jHOT imports, and its an exhaust for our cars direct from Japan. ( http://www.sl-i.net/FORUM/viewtopic.php ... highlight= (http://www.sl-i.net/FORUM/viewtopic.php?t=3433&highlight=) ) Its part of a cat-back you can buy there, and its a 2.5" system. I had assumed it was 2.25" but the exhaust shop chose to match its diameter and it turned out to be 2.5"


Huffer, I do buy my gas at the same place all the time ( Petro Canada ) perhaps I will try another. Long shot but worth it. The wheels and tires are about 7 lbs heavier each than stock.

Here's a quick rundown of what I've got in my head from this mornings drive:

1) The car does mini-backfires within the exhaust after a hard acceleration switches to a gas pedal off deceleration. Under normal driving deceleration doesn't cause this.

2) Between the first gear and the second gear slushbox automatic shift, the car almost has a miss before it takes off in second. Also, on relatively minor grades the auto tranny can't hold it. It will roll back. Maybe the convertor is going, and thus wasting some of my power.

[Wiscon_Mark]

Question: What kind of plugs & wires did you go with?

I've heard that bosch can cause problems (plugs) and champion sparkplugs are the OEM style for some BEs and they're complete junk.