detuning question / disabling the turbo for fuel economy

[WahooNo2]

I had this idea a while ago when gas was 4 dollars per gallon and wondered what would happen if one were to do the following. sorry in advance if this thread needs to be moved.

 

Background:

The stock wastegate is set to 10 to 12 psi before it opens up. So if you run a 0 psi map eg the cobb fuel economy map max boost would still be wastegate level of 10 to 12 psi.

 

To go further lower in boost you can pop the E clip off and force the wastegate to be stuck open which will put the boost at 0 psi or maybe a little higher. The turbo should be ok with this because it still spins just a lot slower. This requires a lot of intervention with removing the heat shield and playing with the wastegate etc.

 

I have an aftermarket recirculating diverter/by pass valve which recirculates just like the stock one but uses a piston and spring vs a diaphram. My question is, what would happen if the spring and piston were taken out and effectively forced a 1 inch leak that was recirculated.

 

Would the turbo overspool and surge because no major pressure would be made and the turbo would short cycle?

 

Would there be no issue because there would be no added fuel over 0 psi and the turbo would just spin at normal safe speed.

 

Variations of this idea could be performed too with having a smaller orafice on the recirculation side eg 1/2 inch hole instead of 1 inch hole.

 

Just wondering what you guys and gals thought.

[whitrzac]

The turbo would spin itself into oblivion.

 

No resistance to the cold side spinning, hot side spins faster.

 

 

Larger turbo = better MPG. It doesn't boost at normal cruising speeds.

[WahooNo2]

thanks for the quick reply. Thats what i was leaning towards too.

[Phate]

Not sure how well this would work, but you might get close with a very light wastegate spring and a 3 port boost control solenoid.

 

Beyond that, you're getting into electronic wastegates and other fun things like that (great fodder for a senior design project or a bored engineer, but likely not feasible for your typical enthusiast).

[JmP6889928]

The turbo spins the second you turn over the engine-it doesn't have to even be running yet. You can't really change much with the wastegate that way. It's been tried in the past with not good results.

[covertrussian]

I think your going about this wrong, with ECU cars you can have your cake and eat it. No need to detune the whole car, just have high mpg low load zones and high power low mpg WOT zones. At that point your right foot becomes your detuner. I have a thread dedicated to tuning the car for fuel economy, where I explore various mechanical and tuning mods to help with economy: Tuning for Fuel Economy

 

As for disconnecting the Wastegate arm:

When I first bought the Legacy I figured I would test disconnecting the wastegate and see if my highway efficiency went up. It didn't help at all, I got 24.78mpg, a month later I got 24.82 to the same city, infact it might have hurt it slightly, but road conditions can be blamed for that.

 

For a good reason too actually, these cars have low compression motors that are powerless outside of boost. With the wastegate disconnected the turbo will still spin but it won't help reduce pumping losses since it's not building any meaningful boost helping overcome vacuum.

 

What I did notice helping me a bit is zeroing out the Wastegate Duty cycle tables. Stock gate is a ~6psi one, which means with them zeroed out your car would boost around 6psi tops. That also means it would still build boost to overcome the low compression inefficiencies and pumping losses.

[WahooNo2]

Nice thread I may post to it in the future. The only thing that really bothers me about this car is the mpg. When you are crusing as long as you stay below 60 mph you can squeeze out 30 to 32 mpg past that its a rapid drop to 20-25mpg because the turbo is at or near full spool becuase of the gearing. I feel that if the wastegate could open at a lower pressure when at higher speeds or if the cruise contol was you probably get better mileage. The idea of a bigger turbo sounds nice but then i would need to tune it... would a tuner really only tune for something like 280 hp vs max output?

[Phillip J. Fry]

cobb does make a fuel economy map. have you tried that?

[covertrussian]

Nice thread I may post to it in the future. The only thing that really bothers me about this car is the mpg. When you are crusing as long as you stay below 60 mph you can squeeze out 30 to 32 mpg past that its a rapid drop to 20-25mpg because the turbo is at or near full spool becuase of the gearing. I feel that if the wastegate could open at a lower pressure when at higher speeds or if the cruise contol was you probably get better mileage. The idea of a bigger turbo sounds nice but then i would need to tune it... would a tuner really only tune for something like 280 hp vs max output?

 

A good tuner should have no problems doing what the customer asks for, within reason of course. If all mechanical parts are the same, there is no reason a 300whp tune would not get same gas mileage as a 250whp tune. Thus there is no reason to limit yourself on power for the sake of gas mileage alone.

 

During warm months (above 60f) I usually get 29-31mpg and colder months are usually 26-28mpg highway going 70mph. This is with 120k miles on the car, stock VF40 turbo, stage 2 downpipe, up pipe, crank pulley, inlet, tune and side gapped plugs. The reason colder temps hurt fuel economy so much is due to: thicker fluids and mechanical resistance, winter gas, more wind resistance (cold air is dense), longer & richer cold starts, etc.

 

Gearing:

Gearing is about normal for Japanese cars, 70mph at 3,000 rpm, heck my Nissan's were 3,500 at 70mph. I think they do this due to small engines and Americans hating to downshift to pass cars. My 6 speed outback sits at 2600rpm at 70mph and constantly needs downshifting up hills.

 

With these motors I think keeping RPM's up actually helps the gas mileage. Case and point is STI's and 07+ Spec B's. Both of those have 6 speeds and rev much lower then 3000 at 70mph and get pretty bad gas mileage. EPA rates 07 LGT's and Spec B's as having the same gas mileage. As for STI's, I've seen threads on STI forums saying that they get best gas mileage at 75-80mph (which would set them at 3,000-3,300rpm range ).

 

Low RPM's doesn't equal to good gas mileage alone, there are other factors at play.

 

Bigger Turbos:

From personal experience bigger turbo doesn't necessarily help with fuel economy. I went from VF40 to a Big 16G and my fuel economy stayed the same so far. I'm a little bummed, but will wait for it to be warmer before I cast any stones. Though it does feel like the 16g hits boost a bit too easily at highway speeds, this could be due to the 13psi wastegate (vs 6psi stock vf40 one).

 

While the stock turbo might seem small and bosting too easily, that's actually Subaru's boost controller and tune at play, in the tune section I'll cover it more. Moral of the story, don't go running out for a bigger turbo just for gas mileage, stock turbo is more then capable.

 

Tune:

Best for last, tune is really what helps the most even on a fairly stock car I've gotten right under 30mpg. If I didn't run over a deer I would have done more stock car tuning, but that deer destroyed my downpipe so went to Stage 2 downpipe at that point. With stage 2 I did hit 31mpg at best, but only after I reworked the tune (mainly with AVCS).

 

The stock turbo boosts easily not because it's small, but because of the electronic boost controller and the tuning methodology (to appease V6/V8 crowd). Once I retuned the wastegate duty cycle tables and AVCS table the turbo started feeling lagy, which is exactly what I wanted. Car became less aggressive when you used cruise control too.

 

05-06's use wastegate duty cycles based on throttle position, at 0 WGC your running spring pressure, as you increase the WGC value the EBC bleeds air and wastegate stays shut at longer increasing boost.

 

On stock tune your running about 50% WGDC, on a stock car that's enough to build about 11psi (what mine was running stock).

http://legacygt.com/forums/attachment.php?attachmentid=259599&stc=1&d=1513484056

 

I made the WGC tables run spring pressure until 30% throttle, but honestly I'll probably will raise it to run 0% WGC till about 50-60% throttle on next tune.

http://legacygt.com/forums/attachment.php?attachmentid=259600&stc=1&d=1513484056

 

AVCS is another beast to play with, stock AVCS is around 0* at highway RPM's. I found that makes the car have way too much power (get pushed into the seat just from tapping up on cruise control). With stage 2 downpipe my car likes 10* avcs and gives me the best mpg with least amount of knock (15* avcs had a good amount of knock on my current timing map).

 

 

As you see with proper mods and tune you should be getting closer to 30mpg if your car is well maintained (when was the last time you changed the spark plugs, diff fluids, made sure brakes are not sticking?).

 

I hope this helps because took me about 3 hours to write it .

[WahooNo2]

Spark plugs are about due, i have around 30k on them they are ngk iridium ix. Front brakes i just changed in december, periodically I get out my temp gun and check that they are heating evenly. Transmission oil was changed also in december. Rear diff fluid is on my list and has 30k on it. I have an access port and am only on stage 1 with original pipes. My mileage is crappy usually because my commute is short (not enough time for cats to warm up), pretty hilly and im usually rushing in stop and go which equals my 19.8 or lower winter mileage or 21-22 summer mileage. I do have access to 91 octane ethanol free gas but its 30 to 50 cents over the cost of premium 93 e10 and that can bump the mileage by 1or 2 mpg.

 

i did write a custom cobb fuel economy map using access tuner race by zeroing out the boost tables on the stage 1 93 map and changing the open closed loop points to be at the points of the stock map. The mpg didnt go up too much for me around town and then i obviously had the consequence of the car being slower. The avcs tables honestly scare me to touch since i dont have proper knowellege of tuning avcs because touching any of the avcs values seems like it would change fueling requirements because of more or less air in the cylinders. i wish we could get some sort of aftermarket heads with direct injection, more agressive timing could be used with less chance of detonation.

[covertrussian]

Crap another 3 hour reply coming.... just kidding... I really enjoy MPG talk for performance cars especially when it comes down to ECU tuning, since everyone else on the web is too busy playing with cardboard box mods . I'm more passionate about it then for tuning for power, 300whp is easy 40mpg is not .

 

Spark plugs are about due, i have around 30k on them they are ngk iridium ix. Front brakes i just changed in december, periodically I get out my temp gun and check that they are heating evenly. Transmission oil was changed also in december. Rear diff fluid is on my list and has 30k on it. I have an access port and am only on stage 1 with original pipes.

 

That all seems good and shouldn't be hurting the economy, I would make sure all of your caliper guide pins are not stuck and easily gliding (Sil-glyde form Napa is good stuff). Also make sure your rear drum is not sticking. It's really hard test for friction on these cars, all 4's need to be up in the air. On my FWD, I just spin each side and see how long it takes the wheel to slow down

 

My mileage is crappy usually because my commute is short (not enough time for cats to warm up), pretty hilly and im usually rushing in stop and go which equals my 19.8 or lower winter mileage or 21-22 summer mileage. I do have access to 91 octane ethanol free gas but its 30 to 50 cents over the cost of premium 93 e10 and that can bump the mileage by 1or 2 mpg.

 

That's good mileage, guess my commute is shorter, but I have lots of hills. My daily average is 8 miles at 19mph average speed. I run the car for 4 miles then shut it off, luckily it hits 180-190F even in winter before I get to work. Stockish my gas mileage was 16-17 winter, 17-18 summer .

 

Avoid 91 at all costs, even if it's ethanol free, these cars will knock even with 93 gas. Which means you will get worse gas mileage on 91 due to retarded timing.

 

i did write a custom cobb fuel economy map using access tuner race by zeroing out the boost tables on the stage 1 93 map and changing the open closed loop points to be at the points of the stock map. The mpg didnt go up too much for me around town and then i obviously had the consequence of the car being slower.

 

Open loop and closed loop points, do you mean the Cloosed loop to open loop delay? I personally saw gains in disabling that delay, WRX guys seem see mpg loss. I think our motors/cars don't like to be lean (just because your lean doesn't mean your save on gas). These motors loose a lot of power when going lean, which means struggling to keep speed up.

 

I personally hated the fact that I had to feather the throttle, even tried to remap the DBW tables. Reducing power by messing with WGDC tables make the car be more linear and easier to drive overall, when I wanna go fast I just set the throttle to be above 50% .

 

The avcs tables honestly scare me to touch since i dont have proper knowellege of tuning avcs because touching any of the avcs values seems like it would change fueling requirements because of more or less air in the cylinders.

 

You can try out my AVCS tables, it's one of the first mods I've done and saw MPG gains right of way. Just make sure to check learned timing to make sure it's not knocking after driving for a bit.

 

Stock:

http://i160.photobucket.com/albums/t188/covertrussian/Cars/05%20LGT/ECU/AVCS/05LGTStockAVCS.png~original

 

Just simply fix the hole with 10's.

http://i160.photobucket.com/albums/t188/covertrussian/Cars/05%20LGT/ECU/AVCS/AVCS10.png~original

 

Now my Stage 2 AVCS maps has a lot less AVCS, that's because the free flow of the downpipe made the engine more efficient thus it went over MBT easier. Thus to keep using similar timing I reduced AVCS:

http://i160.photobucket.com/albums/t188/covertrussian/Cars/05%20LGT/ECU/AVCS/AVCS10DPv2.png~original

 

The first row being zeroed out is based on 07+ LGT's having it zero (thanks to throttlehappy for talking about it on RR forums). It supposedly helps with the brakes by reducing overlap = more vacuum in the manifold while decelerating. Which also means more engine braking and slowing down faster, which could be detrimental to fuel economy.

 

i wish we could get some sort of aftermarket heads with direct injection, more agressive timing could be used with less chance of detonation.

 

These cars already run stupid aggressive timing, compared to similar turbo 4's, I'm not sure more aggressive would help. Direct injection helps with economy because you can run high compression and not have to be supid rich to avoid detonation.

[WahooNo2]

The Open loop and closed loop points were these attached one is for minimum throttle position and the other is the delay values at each rpm range. I will give your avcs tables a try at some point. I know this is leaning towards the land of tuning but covertrussian what is an acceptable amount of knock for you after the car has gotten used to the map its running for a trip? On longer trips i see my knock sum reset after 30-60 min. The highest knock ive seen on the highway over that time was about 100-200 knocks but that is likely attributed to acceleration and then engine braking.

[Mike07LGT]

Most people worried about mpg buy a non turbo car. Why would you want to disable the turbo to gain a few mpg? What's the point in having a turbo car to begin with? Go buy a honda or a prius!

[WahooNo2]

nah... i love my car, to the point of possibly an obsession and eventually will probably put another engine in when this one dies.... random when i bought it in 09 i could only find 4 manual wagons in the whole country and i flew from pa to chicago to buy my current car.

 

i just want best of everything... why cant we have decent gas mileage with good power?

 

ford has eco boost, turbo audis get decent mpg, my old 04 wrx wagon also got ok mileage.

 

i understand that this is a 3700 lbs car with awd but if we can figure out how to extract over 300hp out of 2.5 liters of displacement with a turbo, surely we can figure out how to extend the range to something higher than 25 highway

[Phate]

Most people worried about mpg buy a non turbo car. Why would you want to disable the turbo to gain a few mpg? What's the point in having a turbo car to begin with? Go buy a honda or a prius!

 

Because turbocharging lets you have power when you want it, and fuel economy when you don't. Like when cruising on the highway. And I already have a honda.

 

Covertrussian: I've bookmarked several threads that you've dropped knowledge on.

[covertrussian]

The Open loop and closed loop points were these attached one is for minimum throttle position and the other is the delay values at each rpm range. I will give your avcs tables a try at some point. I know this is leaning towards the land of tuning but covertrussian what is an acceptable amount of knock for you after the car has gotten used to the map its running for a trip? On longer trips i see my knock sum reset after 30-60 min. The highest knock ive seen on the highway over that time was about 100-200 knocks but that is likely attributed to acceleration and then engine braking.

 

Gotcha, that table looks pretty good, if you wanna make your life easier you can just zero the whole table out.

 

It's very common to see knock at low end, that's because engine vibrations under 1k rpm are very high, coupled with road noise etc. I honestly don't pay attention to knock sum unless I'm doing a WOT run, but at that point I also pay attention to Feedback Knock and Fine Knock Learning.

 

I also watch the learning views over periods of time and make sure I don't have any more then 1 knock count per cell. If I have more then 1 count I start adjusting the tune. If your getting knock at warm temps but not cold temps, it might be time to adjust the the IAT based timing compensation.

 

 

Most people worried about mpg buy a non turbo car. Why would you want to disable the turbo to gain a few mpg? What's the point in having a turbo car to begin with? Go buy a honda or a prius!

 

You know, it's this kind of mindset is what's leading us to not have any fast affordable cars. Car manufactures think no one wants a car that's fast and gets great economy, and rightfully so as you demonstrated. Thus they start building high efficiency cars instead (thanks to federal regulations too). If people demanded 30mpg out of a fast car like a Legacy or WRX, manufactures would follow the money trail and build them. But since people are fine with one or the other, we are now stuck with handful of fun cars but all get piss poor economy.

 

Just because we want to be throw less money to bill oil corporations, doesn't mean we hate fast cars nor want to have a car for every occasion. It's like telling your wife; honey you don't have a dress for this party, thus I'm gonna take the girl next door who does have the right dress.

[Merc6]

Most people worried about mpg buy a non turbo car. Why would you want to disable the turbo to gain a few mpg? What's the point in having a turbo car to begin with? Go buy a honda or a prius!

 

Reason most cars like mine exist now with tiny motors and turbos. 50 mpg highway tuned but 15 MPG if I beat on it. Cruse control is used to switch from stock and race tune.

[Slickvic]

Current technology has addressed this. Remember that the 4th gen Legacy 2.5 engine has a low compression ratio of around 8.5:1 and port fuel injection.

It was never designed with economy in mind.

 

Now lets compare that to the 2015 WRX. It is rated at Up to 20 city / 27 highway.

It also has direct injection, high compression (10.6:1), and a twin-scroll turbocharger.

 

As a result, the 2.0 DIT unleashes 268 hp and 258 lb-ft over a 2,000-5,200 rpm engine-speed range with respectable fuel mileage.

 

So maybe consider a newer Subaru with the DIT?

 

 

Edit for sauce: http://www.torquenews.com/1084/two-reasons-2015-subaru-wrx-goes-back-20-liter-engine

[covertrussian]

Current technology has addressed this. Remember that the 4th gen Legacy 2.5 engine has a low compression ratio of around 8.5:1 and port fuel injection.

It was never designed with economy in mind.

 

Actually 2005's have 8.2:1 compression ratio.

 

Just because it wasn't designed with economy in mind doesn't mean it's not capable of it. These cars were designed to make 190whp and 13.5psi max, how many on these boards stuck to that? Furthermore these cars were designed with Up Pipe cats, Banjo Filters that clog and kill turbos, stupid lean tunes that kill motors. Just because it's designed one way doesn't mean it's the correct way either .

 

Now lets compare that to the 2016 WRX. It is rated at Up to 20 city / 27 highway.

It also has direct injection, high compression (10.6:1), and a twin-scroll turbocharger.

 

As a result, the 2.0 DIT unleashes 268 hp and 258 lb-ft over a 2,000-5,200 rpm engine-speed range with respectable fuel mileage.

 

So maybe consider a newer Subaru with the DIT?

 

Your saying I should upgrade my comfortable luxury car that makes ~310BHP and gets 20/30mpg, for an awesome looking boy racer car that's really stiff had 270bhp and gets 28mpg? All for a small $600 payment a month that I can't afford?

 

It's actually really dumb to buy another car to get better gas mileage, even if your buying a Prius. The amount of time it takes to payback going from ~25mpg to 40mpg takes years. The only time it makes sense is if it's same value or your getting 10mpg.

 

 

Furthermore, can we please stop derailing this informative thread with buy another car BS? It's helping no one and distracting us engineers from getting any real brainstorming done.

[Slickvic]

 

Furthermore, can we please stop derailing this informative thread with buy another car BS? It's helping no one and distracting us engineers from getting any real brainstorming done.

 

 

Sure, I can stop being helpful.

[covertrussian]

WahooNo2, what kind of tires are you running (size, model)? That all plays into gas mileage calculations (even manual calculations), plus tires your running could suck at rolling easily

 

 

Sure, I can stop being helpful.

 

How is telling someone to get a new car, and get into complete debt, a helpful suggestion, especially when the goals he's shooting for are more then achievable?

[coldsoda]

Slickvic - at first, I had the same thought "this isn't a prius forum!!" HOWEVER; if you take the time to read all of covertrussian's posts, he has a reason for discussing this - and it makes sense.

 

Think about the Subaru brand - the Legacy in particular - it was not built as a high performance race car, but a fun to drive family sedan. Due to the diverse audience this appeals to, the modification path or ownership decisions will vary quite a bit (read post 11 top paragraph).

 

Lastly, he is going about this the right way, and posting a lot of his hard work right here for others to see and collaborate with. It might not be your approach, but that doesn't make it wrong or a waste of time. I find the technical discussion quite interesting and will continue following the thread

[Slickvic]

 

How is telling someone to get a new car, and get into complete debt, a helpful suggestion, especially when the goals he's shooting for are more then achievable?

 

Well you can be as snarky as you want, but if you look back at my posts from the past 10+ years, I do genuinely try to help people here.

 

Just because you didn't like my answer doesn't mean it could not be helpful for somebody else.

 

Again, I apologize to everyone I mislead.

[covertrussian]

Well you can be as snarky as you want, but if you look back at my posts from the past 10+ years, I do genuinely try to help people here.

 

Just because you didn't like my answer doesn't mean it could not be helpful for somebody else.

 

Again, I apologize to everyone I mislead.

 

How am I being snarky? I'm not questioning your 10+ years of help anywhere, I'm questioning your off topic "help" in this thread. It's the same as going into a thread that talks about increasing power on the Legacy and telling the posters that their car was never engineered to be as fast as an STI or GTR, thus they should give up and buy one of those cars instead.

 

Another analogy time:

OP: My house is costing me $500/mo to heat in the winter.

You and Mike: Your house was never designed to be efficient, buy a new 2016 house if you want to pay $200 a month (forgetting that mortgage will be $2,000/mo, while current house is $1,000/mo).

Me: First air seal the attic and basement, then add a few inches of insulation, finally if you must replace windows with modern double pane windows. Those mods should get you down to $225 a month, with around $1k investment.

 

How is your and Mike's advice helpful? It's plain rude to OP and to those that spent hours gathering information to help OP and build useful searchable information. If OP said, I have major engine issues, but I would like to improve gas mileage, I would agree with you guys. But OP's car is perfectly operational, he just wants to improve it.

[WahooNo2]

https://www.fueleconomy.gov/feg/Find.do?action=sbs&id=19505&id=34668&id=21062&id=20778

 

using the magical epa numbers the low and high numbers are only 3mpg higher (which they lowered to probably make it look like newer cars used less fuel)....from 2004wrx to 2015wrx it gained about 40hp 227 to 268 with the direct injection.

 

as for my tires i still have my snow tires on 205 50 r17 and my summer tires are 215 45 r17