ecomomy sweet spot

crappyrice · Post by **crappyrice** » Sun Jun 11, 2006 9:40 am

ive got a 91 legacy turbo 4wd, sporty thing. its a auto though so 4 gears. just wondering whihc is the most ecomomic speed or rpm to go by. since the turbo kicks in around 2200 rpm, should i be going before the turbo, so with the turbo on full blast? casue i tried it on highway going 2200 rpm at around 90-100 kph, and then with turbo at around 3300 rpm going about 140-150 kph, it does get me home fastrer, but i noticed that it pretty much took the same amount of fuel. but then again my fuel meter is all messed up, so i have to go by the gas warning light.

also while im at it, how much gas is in the take when that gas light comes on?

btw the car is all stock as far as i know.

Carbix · Post by **Carbix** » Tue Jun 13, 2006 7:54 am

Im with u on this on too. I just got a 93 Legacy SS and well it drinks gas like an SUV. How much do u get per tank? im looking at around 350-400Km. With my sunfire i got about 480-600

crappyrice · Post by **crappyrice** » Tue Jun 13, 2006 11:03 am

humm. now that u meantio it, i dont reacall EVER filing the tank full, usulally put in like 10$ at a tiime. but if my calc's right, im gettin about 300 km per tank
and thats a generous estimate. 10$ of gas gets be about 65-70 km, and at almost a dollar a letre here, its not looking so good.

bmxkelowna · Post by **bmxkelowna** » Tue Jun 13, 2006 6:40 pm

"almost a dollar a letre here"

ha! i can NOT remeber when gas was lower then 1$/L.
its been around 112.9 for a long time, and was 114.9 before that

crappyrice · Post by **crappyrice** » Tue Jun 13, 2006 6:43 pm

ya almost forgot that these cars MUST have premium gas, so ya prettymuch over a dollar fo r sure then.

oh one more question over here i think its shell or sunocco, but they have a 94 octane gas, is it worth the extra money? or stick with the 91

ferrari494 · Post by **ferrari494** » Tue Jun 13, 2006 8:31 pm

god damn metric-standard conversions!!! jk

definitely go with the 94 octane with the turbo, when I switched my mileage increased dramatically compared to what i was getting with 91 octane. i was at about 16-17 miles per gallon and then it jumped to about 21 mpg, and with the new exhaust i'm getting around 24ish.

crappyrice · Post by **crappyrice** » Tue Jun 13, 2006 8:39 pm

how is ur new exaust different? and how come before the exaust ur getting such good milage, on highway best im getting is around 15-16 mpg (i did the conversion yay). everything seems fine with the car, except i havent done oil changes for a while, and never changed oil in differentals or tranny? u think that will make a big difference?

Splinter · Post by **Splinter** » Tue Jun 13, 2006 8:56 pm

A dollar a litre?

Those were the days....

Im paying 130.9 for 92 octane

Carbix · Post by **Carbix** » Wed Jun 14, 2006 5:01 am

i think the best u can do is Chevron right now with the 94oct I also have heard that they put the least amount of extra carp in (thats good for us less ethanol). Stay away from Shell. Im not sure why but i have been told it can lead to seals getting fugled. the best i get with 94 is 400+ after putting in a K&N filter Im still working on what the best way to try. With the turbo or with out. Im going to get some new spark plugs and so on. If anyone lives in Langley BC dont ever go to the Subaru dealer here, they been ripping off my Nana so bad with the Legacy i just got, she had it tunned every 3 months by them and it has gone the last 3 years with out the power steering fluid or air filter getting changed....

but thats another story

rallysam · Post by **rallysam** » Wed Jun 14, 2006 2:28 pm

Wow, holy Canadian takeover j/k

I also would like to know what's the most efficient RPM for cruising, for accelerating gently, etc...

crappyrice · Post by **crappyrice** » Wed Jun 14, 2006 4:33 pm

humm u kow what that might be the reason my car is getting bad milage, casue i dont ever check which gas station iim going to, so i ususally end up at canadian tire , or shell.
i should prolly start another thread on this but which is the best gas station? like company, u said shell sucks, so would cheveron be best?

ferrari494 · Post by **ferrari494** » Wed Jun 14, 2006 4:33 pm

I have the K&N drop in air filter too, I'm not sure why the new exhaust (05 STi turboback) helped, I kinda figured it'd make it worse, but I do the math everytime I go to the pump(unless my odometer is broken...which could be a possibility

), it ranges from 22 mpg when I really accelerate a lot and drive like the stupid 20 yr old that I am, up to high 25s when I drive pretty conservatively and mostly on the freeway.

I also work at Jiffy Lube(awesome part-time college job) so I can take care of it and go over the top with the sythentic oils and frequent other fluid changes if that makes any difference(don't think it would...but you never know).

I also have a manual and manuals always get like 2 mpg more than autos because you shift yourself and don't have overdrive and whatever.

crappyrice · Post by **crappyrice** » Wed Jun 14, 2006 9:49 pm

plus with a manual u have 5 gears, not 4 like us autos, so u can go faster at lower revs. just out of curiousity, how many revs u do at about 100kph on 5th gear.

wiscon_mark · Post by **wiscon_mark** » Thu Jun 15, 2006 4:29 am

Generally, the lower RPM the better, because the injectors are firing less frequently. I really wish I could get 70 mph down around 2200 rpms instead of 3000

oh, btw- 60 mph (100km/hr) is about 2550 rpms. 3.9 FD 5MT

I think the 4EATs might be a lower final drive.

Splinter · Post by **Splinter** » Thu Jun 15, 2006 4:31 am

4EATS are also 3.9

However, the 4EATS require more fuel to maintain the same RPM.

Post by **Legacy777** » Thu Jun 15, 2006 7:34 pm

crappyrice wrote:plus with a manual u have 5 gears, not 4 like us autos, so u can go faster at lower revs. just out of curiousity, how many revs u do at about 100kph on 5th gear.

Actually, that's not necessarily the case.

With a 3.90 final drive ratio, the automatics should have a higher combined final drive ratio then the 5spds do.

I can double check when I get access to my manuals.

Also when at steady state crusing an automatic with lockup torque converter should have the same (near same) power transfer through the transmission as a manual transmission. However, as soon as you unlock the torque converter, things change.

crappyrice · Post by **crappyrice** » Thu Jun 15, 2006 9:51 pm

how do u lock or unlock it, sometimes when i go at faste er speeds, like past 120 kph, and if i suddenly take the foot off the throttle, the car will jerk, almost like it just shifted gears, although at those speeds it should be sticking on 120, so i s that the TC locking? also noones answered the orignal questions, we went a little off topic

Post by **Legacy777** » Thu Jun 15, 2006 11:54 pm

The torque converter locks up via the TCU, nothing you can really do to change that. I don't think higher speed necessarily changes the lockup. Constant throttle input, and steady speed should have the torque converter locked up.

Checked the gear ratios. On the automatic, 4th gear has a 0.684 gear ratio. While on the manual, 5th gear has a 0.738 gear ratio.

There's more overdrive on the automatic. So a car with the same differential will have lower rpms with the automatic then the manual trans.

What you are probably feeling is engine braking. Can you describe the "jerk" a little better.

Post by **Legacy777** » Thu Jun 15, 2006 11:57 pm

Engine load is really what determines fuel efficiency. On a turbocharged engine, the turbo does come into play as well.

Staying out of boost will obviously net you the best fuel economy. Typically, higher torque (more engine load) situations will give better fuel economy. So staying at lower rpms is a good idea for that.

Bottom line....if you only have so many gears, and you're traveling at a higher speed, you're going to get worse gas mileage then if you slow down to a lower speed. Plus wind resistance is less.....etc, etc.

The fuel light is supposed to come on when 2.5 gal is left in the tank.

crappyrice · Post by **crappyrice** » Fri Jun 16, 2006 1:04 am

so stick with 2200 rpm,going about 100kph, rather then 3000 rpm, with the turbo going nicely. casue i was starting to think that since the turbo is goin im gettin more air in, and shouldent that impact economy? i noticed yesterday travelling at around 3200 rpm, 130-140 kph, that i got really good ceconomy? withing 30km, my gas needle almost didnt move at all.

as for that jerk, i really think its that lockup thing, going out of lock, casue i usually cruise nicely at about 100 kph, but it happens when i take my foot off the trottle and then quickly apply it again, to speed up. also its better to be locked right? fuel economy wise?
also could it be casue i havent changged any fluids in the car for a VERY longt time - over 6 months for oil, and about a year for the transmission, never for the diff's front and rear. ive only had the car for about 2 years now 208 000 km

Post by **Legacy777** » Fri Jun 16, 2006 4:00 am

You should get a boost gauge.

If you're just cruising and light on the throttle, an 800 rpm change isn't going to make a big difference in the turbo doing anything. Your manifold vacuum will be slightly less at the higher rpm though.

Higher rpm in general will consume more fuel per mile because you're spinning the motor faster, and it's firing more times per mile. That's just how things work. Plus you have the wind resistance increase with higher speeds. So more drag on the car, which requires more power to overcome that drag, which is more fuel.

Wind resistance/drag is not linear with speed. I don't know if it's quite an exponential growth, but as speed increases above a certain point the amount of power required to overcome drag increase substantially.

The torque converter is probably disengaging from lock when you get back on the throttle again. As you ease off on the throttle from accelerating, do the rpms drop.....which sort of feels like the transmission changing gears. This is the torque converter locking up.

Yes, it's better to have the torque converter locked. You don't lose power through the transmission fluid.

Changing the fluids could help. You should check your owner's manual for the proper fluid change interval. If you are unaware of the the last time the previous owner changed the fluids, it'd probably be a good idea to do them at some point.

Manarius · Post by **Manarius** » Fri Jun 16, 2006 3:05 pm

I get 22-23mpg around town on a 4EAT. This means ~350-360 miles to one tank. Now, I did do a highway test coming down to NJ so we'll see how well I did (and that was doing between 55 and 90 mph)

Splinter · Post by **Splinter** » Fri Jun 16, 2006 10:21 pm

For the record, wind resistance goes up as velocity squared

If it were exponential you wouldnt be able to run

Post by **Legacy777** » Sat Jun 17, 2006 11:43 pm

Splinter wrote:For the record, wind resistance goes up as velocity squared

If it were exponential you wouldnt be able to run

Thanks for the correction.

Manarius · Post by **Manarius** » Sun Jun 18, 2006 3:58 am

So, here are my results. Driving to Jersey, I went 174 miles with no AC (2 people in car, two peoples' worth of stuff). I proceeded to do the next 170 with moderate city driving (little use of the AC and varying # of people in the car). MPG for that tank was 25.7 mpg (this is 4EAT at 4.11 gearing; highway driving was between 55 and 90 mph). This tank contained MTBE. Combination of HESS and Exxon-Mobil gas.

Coming back, I had 4 people in the car total, two peoples' worth of stuff. A/C was used most of the time. It was mostly highway, with a few more stops than on the way down. Coming back, the mpg was ~25 mpg. This tank was also 10% ethanol. This was "WaWa" gas.

Note: AWD 1991 Legacy L with 150k miles running 20-50w oil.

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ecomomy sweet spot

ecomomy sweet spot