Tuning for Fuel Economy

I love these kinds of threads. So much interesting information (even if I can't necessarily apply it). Reminds me of a really long FE thread from another forum I was on for a while. I'm not sure how applicable any of it would be for the Subaru platform but some of it might transfer over. Some guy over on a Honda forum got somewhere in the vicinity of 100 mpg in a 400 whp Del Sol.

Here's the link for anyone who might be interested. I didn't understand half of what was going on in the thread.

http://http://www.d-series.org/forums/showcase/117099-i-did-some-fe-testing-64-8mpg.html

I love these kinds of threads. So much interesting information (even if I can't necessarily apply it). Reminds me of a really long FE thread from another forum I was on for a while. I'm not sure how applicable any of it would be for the Subaru platform but some of it might transfer over. Some guy over on a Honda forum got somewhere in the vicinity of 100 mpg in a 400 whp Del Sol.

 

Here's the link for anyone who might be interested. I didn't understand half of what was going on in the thread.

 

http://http://www.d-series.org/forums/showcase/117099-i-did-some-fe-testing-64-8mpg.html

Oh cool! It took me 5 pages to finally find why he's getting such high gas mileage... I'll break it down below.

His main advantage seems to be Lean burning, which Honda's used to have stock. Our engines tend to lean stumble a lot earlier, plus it requires ECU trickery to get it to stay lean (thanks to target AFR style maps). With my Infiniti I can set my closed loop AFR to be anything, and it seems to be more friendly to lean mixtures too, so might try it out. Biggest issue with this is the Cat efficiency/life span will be reduced.

Foe all you guys that want to start going for FE and Perfomance.

 

I got a couple recomendations.

 

When doing a test run try to keep your car under 60mph.

 

Try to make a least a 200+ mile trip, the more miles the better.

This is where most of his games come from, going 60mph or less will help a ton. But, keeping car under 60mph is unrealistic and dangerous on my highways (I-81 and I-64). So that one is out plus I like getting to my destination before I'm 50.

Agreed on trips, fuel pump fill inaccuracy really ramps up below 3.5 gallons.

Adjust your bov to be closed when in vacuum. You need this for fuel atomization and few other reasons I won't get into now;)

The BOV comment is something I've considered playing with myself! My G20 has an adjustable one so I'll play with it more and report back.

Run premium pump fuel only when testing.

Premium Fuel - I'm starting to agree with this one, my 2.5i Outback goes from being happy to cranky on weekly basis with even Shell 87 gas.

Add timing in the cells that you run at while at freeway speed. I use two cells vertical and three cells horizontal for a total of six cells.

This one only applies if you are lean burning. Since I'm not, the opposite seems to be helpful (running less timing).

Try to run high IAT temps. I use a grill block to get the temps up. I'm going to start running with no FMIC when the weather starts to cool down.

I'm iffy on this one.

Run your tire pressure at 55psi to 60psi. Be careful when temps are really hot. Do this at your own risk!!!

This one depends on tires, I'm running 42/40psi, though I'm not sure it's actually helping. Old tires got even wear at 42/40psi, new tires seem to not like it, so I might be going back to 36/34psi

Run with your windows up. Keep some water handy to keep hydrated, if you don't have AC. If you do have AC turn it off.

I sweat really easily, so no dice for me on this one too. The only time I'm not running AC is when it's 50F outside or cooler.

Start keeping a log of all your adjustments. Here is my log on ecomodder.com This is nice because it graphs it for you and calculates a few other items.

 

http://ecomodder.com/forum/em-fuel-log.php?vehicleid=2986

Agreed, I keep a detailed logs in spreadsheets.

What does your current total timing look like in your "cruise" region? In particular, in the .5 to .8 load and 2500 to 3500 rpm region.

I'm getting less than what I'd like during cruise and I'd like to play with my timing in this region to see if I get any improvement.

What does your current total timing look like in your "cruise" region? In particular, in the .5 to .8 load and 2500 to 3500 rpm region.

 

I'm getting less than what I'd like during cruise and I'd like to play with my timing in this region to see if I get any improvement.

That range is pretty narrow and doesn't tell enough story, here is my whole timing map and AVCS map. Notice that regions that I'm running 10* of AVCS I'm running about 2-4* less timing.

Keep in mind this is total timing (just pasted into base timing so it would be easily readable)

http://legacygt.com/forums/attachment.php?attachmentid=259033&stc=1&d=1512009606

I haven't highway tested this map yet so not sure what it would get (it's too cold now for peak MPG testing unfortunately), I was still in stages of retuning for my 3" catback and UEL header when cold weather hit.

That range is pretty narrow and doesn't tell enough story, here is my whole timing map and AVCS map. Notice that regions that I'm running 10* of AVCS I'm running about 2-4* less timing

 

http://legacygt.com/forums/attachment.php?attachmentid=259033&stc=1&d=1512009606

 

I haven't highway tested this map yet so not sure what it would get (it's too cold now for peak MPG testing unfortunately), I was still in stages of retuning for my 3" catback and UEL header when cold weather hit.

It's probably just my right foot as I generally cruise at about 79 mph. I know I can get 27-30 mpg if I stay near 60-65 mph.

That's a total timing table, so for that table Knock Correction would have be 0. What I do is, subtract 10* from 1.15gs + from the base map and add that 10* to the KC table.

79mph is really pushing it in VA (reckless driving is a hefty fine). So I don't go above 75mph. All of my gas mileage testing is at 70mph (~2900rpm) for consistency sake.

Another thought experiment... I've been messing with my timing and AVCS tables to either decrease or increase torque to reduce pumping losses. I finally found a combo that forces me to be at 0psi while slightly accelerating, but I think I went a little overboard, since now I'm actually hitting 1-3psi while trying to accelerate.

The question is, would staying in very low boost (up to 3psi) actually hurt gas mileage, as long as AFR's stay at stoich and not richer?

In theory it will fill the cylinders with less work, but you are flowing more gram/sec which requires more fuel (to even keep it at stoich). Another thing that I just thought of is, cylinders being force filled could increase pumping losses since the piston traveling up is now compressing at higher pressures.

Since this is related to fuel economy. AAA did a study recently, they tested a couple new NA and Turbo cars with Premium and Regular gas and tested their gas mileage difference.

Don’t Be Fueled: Premium Not Always Worth the Price

Engineering Explained does a great job at explaining the results

[ame=http://www.youtube.com/watch?v=dxAQmj3P8xs]New Study Shows Premium Gas Isn't Always Worth It - YouTube[/ame]

Now, I personally was under the impression that premium gas would get worse fuel economy (even with tuned timing), because it's harder to light and burns slower then regular gas. But, my personal experience with the 2.5i Outback is also also showing that premium does better (even though Subaru says 87 only car).

With 93 gas my IAM would stay at 1.0 and I would get 30-33mpg easily. With 87 gas my IAM is usually at 0.5, and I can barely break 30mpg. I started retuning the timing map for 87, which keeps IAM happy at 1.0, but still haven't found the cruising timing combo to get me even 31mpg.

Today I decided to see if could reduce pumping losses by unplugging the TGV motors, which forces them to stay closed, and disable the codes via reflash.

At full throttle the car was much slower, going up hills required a lot more throttle sending me into boost more. To my surprise, going around flatter roads, had just about the same amount of torque, which meant still high manifold vacuum pressure (mainly in 3rd gear going 30mph).

I didn't bother doing a proper MPG test because the engine didn't sound happy and with the holidays it would be a week or two with a dead slow car. But this exercise did me to think outside the box and made me realize that in theory keeping the TGV's closed wouldn't help reduce pumping losses at all. Here why..

Normal operation example, TGV's are open, which means the throttle body is mostly closed, piston tries to go down but the manifold vacuum is pulling it back up.

http://legacygt.com/forums/attachment.php?attachmentid=259993&stc=1&d=1514525780

Now lets close the TGV, to make up for lack of airflow we have to open the throttle body all the way. Sure this reduces the manifold vacuum, but it increases the pre-TGV butterfly vacuum, which still ends up pulling the piston up as it tries to go down.

http://legacygt.com/forums/attachment.php?attachmentid=259994&stc=1&d=1514525780

This is exactly the same results as a dirty or high flow drop in filter. Numerous studies found that on fuel injected cars, having a dirty air filter does NOT reduce MPG, since you simply open the throttle more but since the O2 ingest stays the same mileage doesn't suffer.

The car response was much worse overall, it gave me a taste of what 07+ guys with variable TGV's are experiencing (looks like they stay closed till 2.8-3.3k RPM!). Perhaps TGV delete's would benefit 07+ Legacies the most.

Next I'm planning on doing the TGV deletes and testing them out, it will probably be a couple months before it gets warm enough to bother.

Today I decided to see if could reduce pumping losses by unplugging the TGV motors, which forces them to stay closed, and disable the codes via reflash.

 

At full throttle the car was much slower, going up hills required a lot more throttle sending me into boost more. To my surprise, going around flatter roads, had just about the same amount of torque, which meant still high manifold vacuum pressure (mainly in 3rd gear going 30mph).

 

I didn't bother doing a proper MPG test because the engine didn't sound happy and with the holidays it would be a week or two with a dead slow car. But this exercise did me to think outside the box and made me realize that in theory keeping the TGV's closed wouldn't help reduce pumping losses at all. Here why..

 

Normal operation example, TGV's are open, which means the throttle body is mostly closed, piston tries to go down but the manifold vacuum is pulling it back up.

 

 

Now lets close the TGV, to make up for lack of airflow we have to open the throttle body all the way. Sure this reduces the manifold vacuum, but it increases the pre-TGV butterfly vacuum, which still ends up pulling the piston up as it tries to go down.

 

 

This is exactly the same results as a dirty or high flow drop in filter. Numerous studies found that on fuel injected cars, having a dirty air filter does NOT reduce MPG, since you simply open the throttle more but since the O2 ingest stays the same mileage doesn't suffer.

 

The car response was much worse overall, it gave me a taste of what 07+ guys with variable TGV's are experiencing (looks like they stay closed till 2.8-3.3k RPM!). Perhaps TGV delete's would benefit 07+ Legacies the most.

 

Next I'm planning on doing the TGV deletes and testing them out, it will probably be a couple months before it gets warm enough to bother.

I was under the impression that unplugging the TGVs left them open, not closed.

For what it's worth, it's possible to tune the TGV position maps. It's a function of engine speed and requested torque. If you give above a certain throttle, the stock map will open the TGV, even below 2800RPM.

I was under the impression that unplugging the TGVs left them open, not closed.

All pre-2007 default to closed when not powered, even going back to 2002 2.0 WRX's. A friend mentioned that he was down on power one time, they later found out that one TGV motor failed and was stuck closed. I can't speak for 2007+ since I haven't played with those .

For what it's worth, it's possible to tune the TGV position maps. It's a function of engine speed and requested torque. If you give above a certain throttle, the stock map will open the TGV, even below 2800RPM.

Those maps seem to be on 2007+ only sadly. If 05's had tunable TGV's I would definitely play with them, I think there might be some MPG gains in the swirl. But then again 07+ legacies are rated at same gas mileage as 05's even without catted Up Pipes.

I'm really curious on the TGV logic for 07+, does anyone have any logs with calm to aggressive driving with either of the "Tumble Valve Position Sensor's" logged? I basically wanna see at what TPS and Engine load it switches over.

Don't have any logs, but here's the tables from my stock ROM (only showing the S tables since the I and S# tables are identical).

The open-above 50g/s roughly equates to 1g/rev load in my experience.

Yup that's sufficient, guess that explains the super high AVCS and ignition timing under 1g/s. There is barely any airflow to cause lots of heat or knock.

Btw thanks for your definitions!

I've been pretty damn silent in this thread, but I've actually been doing a LOT of MPG testing with different mods (doing highway drives almost every weekend, which is starting to add up in $$ ).

First thing's first, I had to change how far I drive, the usual city I got to is just too close, ~120miles, which is usually around 4.0 gallons (30mpg). This would cause my city MPG to be down. Basically at 4 gallons the gas pump doesn't fill the tank up accurately enough. When I usually would get 18mpg city, I would get 16mpg after a 4.0 gallon highway fill.

In my testing I found that filling up 5 gallons does not negatively impact city gas mileage after. Thus I had to add an additional 27 miles to get above 5 gallons at 30mpg. Sadly this means more of my time is wasted and money ($5 more per test).

But the real sad thing is, all my previews testing is suspect and not comparable to current data .

Here's what's in the pipeline:

TGV Deletes

3" Hard Inlet

Spark Plugs: SILFR6B8

Have you ever repeated your test with bikes on the roof + yakima type cargo box, and add like 150 lbs of stuff in the trunk?

Then, you do the exact same drive, same speed and stuff. I'd be curious to see how much of a mpg hit you'd get.

Had 2 adults and 2 kids + 2 mountain bikes (on the roof) + full 16 cu ft yakima cargo box + full trunk (with a 55 lbs fridge+20 lbs AGM battery, camping gear, food, tools, etc...). Drove on I 70 at ~75-80 mph. Guess what my gas mileage was?

Probably below 20mpg, due to speed and all the crap on the roof. In the past when I would load the trunk to the brink my MPG was fairly similar as unloaded (once object is in motion, it stays in motion).

My legacy is a commuter car and not a hauler anymore, with 2 kids, it's impossible to fit everything we need, so Outback is the trip car now.

Actually. I did not want to know my mpg... What I know though is I used ~half a tank and just did ~120 miles.

I will be curious to see if my mpg gets better once I place the bikes on a hitch rack. but then the hitch rack, I will use, will add another 40 lbs .

I probably need a truck... However, the outback with the BTS kit handles so well even with all of that stuff. Amazing.

In the past when I would load the trunk to the brink my MPG was fairly similar as unloaded (once object is in motion, it stays in motion).

I attest to that.

I wanted ambient temps to stabilize for the spring before doing the TGV deletes, but I sprung a fuel line leak, which required me to act soon since fuel leaks tend to make MPG be worse . Full Details of install, tune, and power gains can be found in my build thread.

City MPG Testing

Before:

37-76F: 19.59mpg

35-70F: 20.77mpg

Average: 20.18mpg

After:

43-81F: 21.63

37-75F: 19.13

Average: 20.38mpg

Max Gain: 0.86mpg or 4.14% increase

AVG Gain: 0.20mpg or 0.99% increase

Highway MPG Testing

I couldn't do as many back to back highway MPG tests, simply because of time and cost. City is easy since I have to drive to work everyday .

Before:

54-64F: 29.83mpg (Lights 0%)

After:

53-64F: 29.22mpg (Night/Lights for 16% of miles)

MPG Gain: -0.62mpg, 2.04% loss

Since it's a rather small dataset I can't say with certainty that TGV's made me lose highway gas mileage or if having the lights on for 16% was the cuplrit, but what I can say is it didn't gain anything either.

For 2010+ Subaru changed to the newer style NGK spark plugs. These spark plugs contain a much thinner ground strap, which helps reduce spark quenching.

These are as close to factory side gaped plugs as I've seen so far. While I love side gaped plugs, with fine point center tips, they wear out way too quickly compared to coppers. Thus these should be best of both worlds, efficient and long-lived.

Onto the Testing...

Highway MPG:

SILFR6A: 29.68mpg - Temps: 74-88F

SILFR6B8: 30.01mpg - Temps: 70-91F

Total Highway Gains: +0.33mpg, +1.10% improvement

City MPG:

Sadly the temps did go up, which probably lead to a slight improvement too

SILFR6A:

19.94mpg - Temps: 46-89F

21.04mpg - Temps: 55-82F

Average: 20.49mpg

SILFR6B8:

21.76mpg - Temps: 64-83F

21.43mpg - Temps: 67-83F

21.67mpg - Temps: 67-87F

Average: 21.62mpg

Peak City MPG Gains: +0.72mpg, +3.31% improvement

Average City MPG Gains: +1.13mpg, +5.23% improvement

I didn't expect average MPG gain to be higher then peak, I guess that's why averaging is good, fishes out the outliers. One thing I will say is, weather did get much warmer, but I don't see 55F vs 65F making that big of a difference around town especially when the amount of cold starts is the same.

Overall I would highly recommend these next time you need to replace your spark plugs anyway. On my setup I didn't need to do anything to the tune to realize the gains, when I tried retarding the city timing by 2*, I lost little more then half an MPG.

So we can put these in our 2005+ LGT/OBXT then?

Well I am running them on my 2005 LGT

I'm running them in my '06.

If you want something more exotic, they also have the Ruthenium LFR6ARX-P of the same design.

Well I am running them on my 2005 LGT

Somehow I thought you were referring to your 10+ outback. How much did you pay for these?

Somehow I thought you were referring to your 10+ outback. How much did you pay for these?

Ha nah! I should take that as a compliment, you see 30mpg and automatically assume it's not a turbo Subaru . Outback uses BKRE series spark plugs, which are shorter. I paid under $40 shipped from Rock Auto.