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A different kind of performance question


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14 replies to this topic

#1 chaz345

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Posted 13 February 2013 - 03:54 PM

There's lot here about how to get more power out of our beloved Subies, but I have a question about the other direction. What, if anything can I do to maximize fuel economy, even if it comes at the expense of reduced performance?  It may be helpful to know that my driving is 95% freeway.  I suppose I could get larger wheels and tires which would bump up MPG slighly at the expense of acceleration.  Somewhat reduced performance is OK, reduced reliability/longevity, not so much.

 

 

I suppose it's probably also helpful to know that I drive a totally stock 93 Legacy Wagon.


Thanks



#2 tallwelder81

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Posted 13 February 2013 - 04:22 PM

well, an EASY 5 minute answer?  nothing will help fuel economy, easily.

but if you want it bad, then yeah....

all comes down to this:

how MANY hundreds of dollars are you willing to spend, to save a dollar per day?

if you get like, 24mpg NOW, and you get a turbocharger for somewhat cheap, used,

for say...  400 dollars, and it gets you up to 27 or even 29mpg, how long will it

take before it SAVES you money?? one thing that would probably affect your mpg

noticeably, good or bad,  would be taller or shorter gears.  or, if you don't use your

4wd mode, like, EVER, converting it to full fwd mode would help a lot. even if the

transfer case isn't engaged, it still creates a slight drag. parasitic loss.  and the rear

axle drive capability adds a lot of weight. like, as much as another grown man.

I think this is most likely your daily driver, if you care about a few extra mpg.  so that

rules out my other suggestion.  porting and polishing your intake and heads and exh.

it can be done for basically free. it just means taking apart your engine for a week.

and a new set of gaskets to re-assemble. people usually associate a ort and polish

with more performance. not mpgs. but, think about it, anytime you add more power,

without adding more fuel thirst, you always have the option to put a little less foot

on that old pedal. don't look at it as more power at the same rpm. look it at equal power

at less rpm. 

 

a nice exhaust can help a lot with midrange power. and, surprisingly, freeway isn't

high-end rpm. unless you are really out of tune or tired, or tragically underpowered.

most likely, you have enough power that, once the weight gets moving, 55-65mph,

your engine is not working at full throttle, its simply maintaining that already earned

inertia. hence, big heavy V8 trucks, being able to shut down half their cylinders even

going 60mph with 6,000lbs.  I bet your legacy doesn't weigh 6,000 lbs. 

sure, your legacy doesn't have "variable displacement" capability. but the logic is the

same. once you get going, its not working very hard to KEEP going. 

so if you get an exhaust and a cam profile that helps MIDrange, freeway driving is

improved a LOT.



#3 rpholz

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Posted 13 February 2013 - 04:37 PM

2 EASY steps:

Sell Lego
Buy 2wd ea82

Enjoy 40mpg.

Other than that, better air filter/muffler and shed as much weight as possible.

#4 ShawnW

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Posted 13 February 2013 - 05:48 PM

I have been toying with doing a legacy with an ej18 or ej16 and a turbo, but a large enough turbo that it doesn't kick on until around 33-3500RPM.  I am looking at some of the Haltek stand alone units and maybe a corn map along with an 87 octane map.  Then a tune to as lean as possible.  Even considering having a split gas tank and 2 fuel pumps so I could go long distances on 87 and have a small tank with e85 for around town with the thought that I can go on a trip leaving town on the e85, drain that half of the tank, switch the map to 87, drain that and then fill with 87 for the trip since e85 can be pretty scarce in places like Wyoming, Western Colorado, Utah, etc.  where I like to go.  

 

The problem with the projects to me is spending the kind of money it takes to do a project like this is probably as expensive as the savings you would see in the car for a lot of miles.  

 

Gearing is also a consideration.  A conversion to 3.7 gears and appropriate reductions in the trans should help too.  Also not cheap but a start.  



#5 NorthWet

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Posted 13 February 2013 - 06:04 PM

Manual or automatic trans?



#6 chaz345

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Posted 13 February 2013 - 06:39 PM

Automatic transmission.

 

As for how long it will take anything I do to pay for itself, my commute is 160 miles (round trip) every day. So we're looking at spending about $425/month in gas. Even a 1 or 2 mpg difference could  pay for itself in a reasonable amount of time.

 

In terms of gearing, since I may need to replace the tranny soon, I'll keep that in mind.



#7 ferox

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Posted 13 February 2013 - 06:56 PM

I have a '92 Lego, and I don't know a lot about them yet, but any car that uses an O2 sensor for engine regulation will experience less than optimal fuel economy if the sensor is worn out or wearing out.  

 

For myself, I think an Air/Fuel gauge or something that gave me a direct or indirect instantaneous reading of fuel economy would help a lot because it would indicate what conditions produce the highest efficiency and I could modify my driving to match.  You just need an O2 bung appropriately placed, which any muffler shop could do for you if you don't have the set-up at home to install one.



#8 ivans imports

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Posted 13 February 2013 - 09:42 PM

I would find a way to overide the o2 and force it lean would have to watch the stoich ratio closly to pervent damage to engine but i'm shure you could tune it for milage if you dont mind losing power. Or find a way to cut out cly at speed like the newer cars do droping injector pulse or somting like that



#9 NorthWet

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Posted 14 February 2013 - 07:45 PM

Your gearing is probably pretty good right now.  What is your rpm@cruising-speed?  As long as your TC is locking-up, the AT might actually be good for your mileage.

 

I would think that your best investment would be in proper tune/tire pressures.  After that, speed and aerodynamics will have the biggest effect. 

 

Speed's effect on fuel consuption is moderated by the engine's characteristics, plus the body's aerodynamics. You have increasing power needed to overcome aero drag as the speed increases, plus increasing fuel needed to maintain the engine at higher rpms, and the engine's power curve that varies how much fuel is needed per HP at different engine speeds.  Best fuel consumption will probably be with engine speed 500-1000 rpm below torque peak.  (WAG)

 

Aerodynamic improvements might be worth pursuing.  At the very least, make sure that the front of the car is not higher than the rear of the car.  Lowering can help.  Making a front air-dam, or better yet a "splitter", would be worth looking into.  The splitter helps reduce turbulent flow underneath the car, reducing drag.



#10 NorthWet

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Posted 15 February 2013 - 02:29 AM

A couple thoughts along the lines of your engine-mod question:

 

Try making a hot-air intake.  Same concept as a cold-air intake, but for a much different purpose.

 

More expensive and involved:  Raise the compression ratio.

 

For a more challenging engineering solution, look into the Atkinson Cycle, a modified version of which is used in the Prius.  Basically (if I understand it mostly correctly), it reduces pumping losses by holding the intake valve open for a portion of the compression cycle.  It can improve fuel consumption at the cost of power density... not a real concern at freeway cruise.  Experimenting might be more easily doable on a DOHC...



#11 chaz345

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Posted 19 February 2013 - 12:11 PM

Your gearing is probably pretty good right now.  What is your rpm@cruising-speed?  As long as your TC is locking-up, the AT might actually be good for your mileage.

 

I would think that your best investment would be in proper tune/tire pressures.  After that, speed and aerodynamics will have the biggest effect. 

 

Speed's effect on fuel consuption is moderated by the engine's characteristics, plus the body's aerodynamics. You have increasing power needed to overcome aero drag as the speed increases, plus increasing fuel needed to maintain the engine at higher rpms, and the engine's power curve that varies how much fuel is needed per HP at different engine speeds.  Best fuel consumption will probably be with engine speed 500-1000 rpm below torque peak.  (WAG)

 

Aerodynamic improvements might be worth pursuing.  At the very least, make sure that the front of the car is not higher than the rear of the car.  Lowering can help.  Making a front air-dam, or better yet a "splitter", would be worth looking into.  The splitter helps reduce turbulent flow underneath the car, reducing drag.

RPM @ 70 (legal limit for most of my drive) is a shade over 2500.  And given that the tires are near the end of their life I'm definitely checking them more than average, probably every other fill up which for me means I check pressure once to twice a week.



#12 NorthWet

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Posted 19 February 2013 - 01:33 PM

Sound good.  The only thing i could suggest is dropping down to 65 or 60 to see if it is worthwhile, and I suspect that you have already considered this and dealt with it. :)



#13 ivans imports

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Posted 20 February 2013 - 10:31 AM

taller narrower tire with high air preshure



#14 rpholz

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Posted 20 February 2013 - 10:35 AM

taller narrower tire with high air preshure


Could always Prius tires...

#15 Subruise

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Posted 21 February 2013 - 09:28 PM

heres some decent reading I came across   http://ecomodder.com/forum/


Edited by Subruise, 21 February 2013 - 09:30 PM.





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