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Old 04-22-2008, 06:45 PM   #1
yarsec
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Default Improving MPG on 2008 Impreza

Hello all. I am interested in purchasing a 2008 Impreza Hatch 5 speed for commuting, but would love to get the mileage up to around 30 mpg highway (I drive 500 miles per week).

Doing some searching on the forum, it looks like the common things to do is intake, exhaust, and crank pulley. It also seems like the biggest area to improve would be to lean out the engine during light throttle cruise conditions around 3000 rpm. The problem being, it seems that programmable ECU/piggyback systems are rare for NA 2008 imprezas. Does anyone know of any new ones are coming down the pipeline? The only one I have seen is the Rallitek P6. Are there other ones out there?

Are there other things that would help boosting mpg?

Thanks
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Old 04-22-2008, 07:33 PM   #2
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Remember weight reduction - remove your spare tire and jack. Pump those tires up close to maximum operating specs to reduce roll resistance.

And adjust your driving style! Use hills to your advantage and don't be going WOT all the time for daily driving. Now, when it's the weekend and it's time to have fun, then you can do whatever.

As far as mods, a lighten crank pulley will net you the most gains. Intake and exhaust probably won't get you much though.. a bit of power and fun, but not much MPG.

Good luck! I love my Impreza as a communter for it's AWD and decent gas mileage, but remember, it's no Geo Metro or Hybrid. Still it can get fine MPG.
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Old 04-22-2008, 11:04 PM   #3
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Much of your MPG is from
1. Mood
2. Your weight (tubby )

In other words, don't step on it all the time. Try to cruise all the time. Slow acceleration, slow braking.

Too much air pressure in the tires can cause lack of traction, and therefore dangerous situations.

Removing spare tire and jack might not be the best idea for the daily road. You never know what's going to happen.
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Old 04-23-2008, 02:04 AM   #4
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If you really want to save gas, NA subie is better than turbo subie for sure, but you are really lookin at the wrong car......

Get a Toyota Yaris or Honda Fit, those are way better for gas saving.
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Old 04-23-2008, 10:00 AM   #5
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You don't want to lean out cruising AFR, CL target is stoich and should remain there for optimal balance of fuel economy, power, long-term reliability.

Haven't heard much about the PP6 on the 08. The Romraider team is working the new CAN-BUS ECU (I think all 08 Subaru's have this type of ECU- someone correct me if I'm not right) and will probably have an open source option available in the next year. Might just have to be patient...

Bolt on mods such as exhaust and intake will help the engine breath better but net gains in fuel economy are pretty small. I average 2-3 MPG better with intake, engine back exhaust, and an agressive 92 octane tune. Impreza's are naturally going to lage a bit in fuel economy as a result of AWD and a heavier chasis (think top rated five-star safety).
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Old 04-23-2008, 10:38 AM   #6
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Really, I don't want to lean out AFR's under cruise? I always thought the impreza's ran very rich at cruise.

Chris
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Old 04-23-2008, 10:53 AM   #7
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OEM maps are typically very rich @ WOT in the higher RPMs, maybe thats what you are thinking of. The CL cruising AFR is 14.7 (stoich) and should be left as is. (unless for some ungodly reason '08s are different, I highly doubt it though)
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Old 04-23-2008, 11:01 AM   #8
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Great, thanks for the clarification. So it sounds like leaning AFR is not a solution. So I guess intake, exhaust with high flow cat and crank pulley are my best bets.
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Old 04-23-2008, 11:09 AM   #9
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Make sure you run a 5 or 0W30 engine oil with a low HTHS and low viscosity. Also, switch your tranny/diff fluid to synthetic. Although that opens up a new problem. Which synthetic gear oil?

-Dennis
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Old 04-23-2008, 11:22 AM   #10
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A crank pulley might help acceleration a bit but I doubt it would make any tangible difference on fuel economy. I'd save the coin and spend it on: http://forums.nasioc.com/forums/show....php?t=1499873

Lubrication is an interesting topic. Does Subaru still recommend 5w-30 for 08's, I know others (Ford, Honda, etc) are transitioning to 'ultra fuel efficent' 0w-20 in new passenger cars. I burn through M1 5w-30 a bit to quickly for comfort (1.5qt per 3k) and am spending the summer in steamy-hot southern AZ, RTS 5w-40 to the rescue!

I've seen threads go round and round about suitable synthetic gear/transmission lubes for Subaru's but found no solid recommendations other than OEM.
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Old 04-23-2008, 11:41 AM   #11
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We should get a MPG sticky once and for all... with a nice mythbusting section.

Unabomber?
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Old 04-23-2008, 11:42 AM   #12
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The n/a cars come from 5W20 from the factory, but I think the recommendations are still 5W30. If you go thin, oil brand choice become more important IMO. For thin, I would only use something like Redline, Motul or Amsoil in a turbo. The n/a cars generally we well on any synthetic, depending on your driving style and location. There's a guy on bobistheoilguy.com running 5W20 in an n/a Sube.

I just got 27 mpg's on a trip back from SC in my FXT. Your right foot has a lot to do with mpg's. No traffic helped as well.

-Dennis
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Old 04-23-2008, 12:30 PM   #13
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A sticky on improving MPG on NA Subarus would be great! Especially with gas pushing $4/gallon.
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Old 04-23-2008, 01:44 PM   #14
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We don't have to worry about oils being thinner due to the fact that we don't have any bearings to blow out on turbos.

This is generally why a lot of turbo'd cars run thicker oils.

I run M1 currently, and I think it runs smooth, but it does burn off quite a bit.
I may switch to amsoil asl soon.

Last edited by formula91; 04-23-2008 at 02:02 PM.
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Old 04-23-2008, 01:50 PM   #15
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A couple of thoughts:

-Yars is a skinny mofo. "Tubby" may apply to me, but hardly to him.

- Leaner than stoich (14.7) fuel trims are encouraged under cruise; especially light throttle applications (think highway). A means to monitor the EGT's are highly recommended.

- Someone already mentioned tire pressure. Lower profile sidewalls would also help to decrease tire deflection and energy loss.

- Clean air. Lowering the vehicle will clean up some of the air flow and help with aerodynamic resistance.

- A lightened crank pulley will only help rotational acceleration. Any gains that will effect mpg are minimal.

- Lighter weight fluids are an interesting discussion but necessarily something I'd like to try on a new still under warranty vehicle.

Fitz
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Old 04-23-2008, 02:25 PM   #16
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Quote:
Originally Posted by LTracer View Post
and am spending the summer in steamy-hot southern AZ,
AZ is anything BUT steamy.
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Old 04-23-2008, 02:25 PM   #17
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Quote:
Originally Posted by Fitz View Post
A couple of thoughts:

- Leaner than stoich (14.7) fuel trims are encouraged under cruise; especially light throttle applications (think highway). A means to monitor the EGT's are highly recommended.
Don't you risk introducing knock (and the corresponding retardation of timing), ...especially with poor fuel and hot climates? Probably need a wideband O2 as well... Also, what are the long term effects on cats of running higher than normal EGT's? Seems too impracticle IMHO.

1+ to sticky, crude is near $120/barrel!
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Old 04-23-2008, 02:27 PM   #18
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Quote:
Originally Posted by Counterfit View Post
AZ is anything BUT steamy.
Sorry, just plain HOT.
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Old 04-23-2008, 02:43 PM   #19
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Quote:
Originally Posted by Fitz View Post
Lighter weight fluids are an interesting discussion but necessarily something I'd like to try on a new still under warranty vehicle.
When I say go thin, I'm referring to the actual viscosity of an oil (e.g. 9.5 cst's at 100C) and an oil with a low High Temp High Shear (e.g. 2.9 - 3.0%).

Here is Pennzoil Plantinum's info . You have to look for the Product Data Sheets to get this info. Here is Valvoline . HTHS isn't always listed, but if an oil only meets ACEA A1 (and not A5) it will have a lower HTHS.

Bumping up the viscosity and HTHS may give you more protection and maybe less mileage. That's why German Castrol doesn't meet energy conserving requirements. It has a high HTHS and viscosity at 100C to meet ACEA A3 requirements.

HTHS discussion on bobistheoilguy. And another is here.

-Dennis

Last edited by bluesubie; 04-23-2008 at 02:50 PM.
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Old 04-23-2008, 04:01 PM   #20
formula91
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Quote:
Originally Posted by bluesubie View Post
When I say go thin, I'm referring to the actual viscosity of an oil (e.g. 9.5 cst's at 100C) and an oil with a low High Temp High Shear (e.g. 2.9 - 3.0%).

Here is Pennzoil Plantinum's info . You have to look for the Product Data Sheets to get this info. Here is Valvoline . HTHS isn't always listed, but if an oil only meets ACEA A1 (and not A5) it will have a lower HTHS.

Bumping up the viscosity and HTHS may give you more protection and maybe less mileage. That's why German Castrol doesn't meet energy conserving requirements. It has a high HTHS and viscosity at 100C to meet ACEA A3 requirements.

HTHS discussion on bobistheoilguy. And another is here.

-Dennis
Man I'll never understand all that motor oil jargon. But I do understand the brand names.

Note: you'll need a wideband o2 sensor (LC-1) in order to have way different A/F Values in CL. You introduce higher chances of knock with too (keyword here) lean values. The problem is in finding that target lean A/F value in CL. It's not as bad of a problem with N/A because the chances of lean detonation are significantly less than that of forced induction, but the problem nevertheless is there. Timing retardation can cause greater loss in MPG than having leaner A/F values. It's reasonable to try to go leaner, but it's a very cautious play with numbers.
This is even worse for 06 and 07 models because the ECU is designed to stay in CL for SO LONG, and up to a much higher load value. It could mean a faster degradation of your engine.
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Old 04-23-2008, 09:48 PM   #21
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Quote:
Originally Posted by bluesubie View Post
Make sure you run a 5 or 0W30 engine oil with a low HTHS and low viscosity. Also, switch your tranny/diff fluid to synthetic. Although that opens up a new problem. Which synthetic gear oil?

-Dennis
I always change the tranny fluid in a new car after 1000 miles or so and then never again. I'm going to use the Mobil1 75w-140 in my 5spd.

The syn will even make the tranny shift better, I've seen this in other cars.
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Old 04-24-2008, 12:49 AM   #22
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Quote:
Originally Posted by saber787 View Post
I always change the tranny fluid in a new car after 1000 miles or so and then never again. I'm going to use the Mobil1 75w-140 in my 5spd.

The syn will even make the tranny shift better, I've seen this in other cars.
You need to check/replace the tranny oil every 30,000 miles.
Try out Scottie's Cocktail. It's supposed to be one of the best.

ATF is a different story.
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Old 04-24-2008, 01:14 AM   #23
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Mine gets 31mpg at 65mph stock, which seems about what others are getting as well. Perhaps driving more conservatively.
Also, if you have < 1000 miles or so, you won't get very good gas mileage. Wait a bit.
Power mods won't help.
Reducing weight, drag, rolling resistance, and friction will help.
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Old 04-24-2008, 02:27 AM   #24
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Quote:
Originally Posted by 2008OBS View Post
Mine gets 31mpg at 65mph stock, which seems about what others are getting as well. Perhaps driving more conservatively.
Also, if you have < 1000 miles or so, you won't get very good gas mileage. Wait a bit.
Power mods won't help.
Reducing weight, drag, rolling resistance, and friction will help.
Are there even any power mods for the 08?

Even so I disagree. An intake should improve your mileage by a bit, even if it is a bit. The frankentake should give a noticeable improvement in MPG simply because the engine breathes better.

From what I understand, exhaust gives more power but no MPG.

But, like I said, hands down the best way to improve MPG is to drive conservatively.
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Old 04-24-2008, 02:34 AM   #25
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zomg double post somehow

Last edited by formula91; 04-24-2008 at 02:35 AM. Reason: baleeted
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