In my opinion, I think this is somewhat pointless because mileage from car to car is going to vary due to SO many factors other than turbo size. You just can't compare someone who does stop and go all week with a 35R to someone with a stock TD04 and cruising 100 miles a day. Theroetically, the turbo size shouldn't even make a difference in MPG if you aren't using it. You might want to have people also post what type of driving they do to get the MPG they are reporting. If you can group driving types you might have siomething you can use.

Anyway, I am a nerd. I have kept all of my gas receipts since 2005. Each time I fill up I write down the miles I got on that tank along with the total car miles. I also note any mods or atypical driving that was done on that tank, just to see if it made a difference. I always fill up at one of two 76 stations. I compiled the data and attached a couple charts showing 1) my MPG and price per gallon and 2) the amount I paid at each fillup and that amount averaged to show how much I pay "per year" at that price. I didn't denote my mods on the charts but you will see that they didn't change things much, if at all. The big jumps in MPG are when I took trips with lots of long distance cruise time.

Car info:
04 STi
Stage 2 (with some extras)
Ed tune (I think this actually got me slightly better MPG after I stopped going WOT all the time. Statistically it is not conclusive per the data, though.)
I do mostly commute driving, stop and go for ~20 miles total a day.

I think the worst thing is how much the price of gas changed over the last 2 years! hahaha!

 

1998 Impreza w/ej255 (06 wrx)
stock td04
Aem intake and Hks catback
~21mpg city, havent tried highway yet
No tune

 

07 STi
fpgreen
TBE
about 21-22mpg all driving conditions,
but my commute is mostly city driving less than 10 miles round trip

 

What year Subaru: 2004
What model subaru: FXT
What turbo: VF-41
Supporting mods: Cobb TBE, STi TMIC, Prodrive CAI
MPG (average): 22 (average) regardless of how I drive
Pro-tune or off the shelf map: Pro-tune

 

jeez man, you got your stuff organized! The goal for this thread was not to categorize every detail of DD vs stop n go. Rather, it was intended for people like me, who are considering a turbo upgrade and would like to get a ballpark figure for how it would affect their milage. I figured, at the same time, why not tryn get a huge compilation together and produce a data sheet indicating what types of MPG gains / losses you could expect with certain turbo mods.

Also, i think most ppl are wise of the drastic differences between DD and stop n go driving, and they have actually indicated what type of driving they do in their replys.

Anyways, good lookin out. Better data produces better predictions for the future.

ALL: START ADDING A LINE FOR WHAT TYPE OF DRIVING YOU DO: STREET, HIGHWAY, OR MIXED

 

The #1 factor for fuel use in an upgrade situation is INJECTOR SIZE followed by A/F RATIO. Injectors have a max flow rate but also a min flow rate; 660cc injectors will always put more fuel in the cylinder, regardless of turbo size, than 550cc or 350cc. Obviously to go with a bigger turbo you need bigger injectors, but I have seen many setups where the tuner will super-size the injectors, on the off chance that you might go for a bigger tuner in the future. A VF34 with 800cc injectors is going to burn more gas than a 18g with 500cc injectors. Bigger injectors flow faster and don't shut off as fast. Pick the ideal (smallest possible) injector size for the turbo upgrade and you will minimize the loss in MPG.

A/F ratio is the next biggest factor; you can tune rich for safety, lean for power, this has a dramatic effect on your fuel use. By just leaning out the stock tune of my 02 WRX I gained 40hp and around 30 miles per tank (approximately 1.2 gallons). The stock tune was extremely rich, especially in the first three gears.

Finally, driving style cannot be ignored. Bigger turbo, more power, more often you get the urge to stomp on it, worse MPG. I don't see turbo size itself being as big a factor as the other elements, unless you are really going with a monster that needs 800cc injectors just to avoid cracking your block with it.

 

Your post seems off.

Increasing injector size alone will not change MPG once you get tuned. Assuming there are no other upgrades, fuel delivered by ANY size injector should remain the same for a given AFR. One could drop 800cc injectors into an otherwise stock STi and maintain the MPG once the car is tuned for the larger injectors.

 

get a huge turbo, but you have so much lag that makes you never even reach boost. then you save gas lol when you not using the turbo its the same.

 

so..... what im gathering is that with the proper tune, the turbo shouldnt really be the determining factor in gas MPG.

therefore.. I could run a VF34 w/ 565cc inj, new tmic, tbe, FP, and cobb ap map, and get the same MPG while at the same time significantly increasing HP and torque??

seems too good to be true.. but i understand now, that theres more to it than turbo size, so maybe it is possible.

please advise..

 

What year Subaru: 2005
What model subaru: WRX
What turbo: TD-04
Supporting mods: K&N Typhoon intake, Perrin turbo inlet, BIG AZZ TMIC, Perrin catted DP, Catless up pipe, HKS Carbon TI, Cobb AP stage 2.
MPG (average): 22 (average) and I flog my car constantly
Pro-tune or off the shelf map: off the shelf map.

I could be wrong but if you get a tune your car should be running at almost peak efficiency. If you keep your foot out of it you can expect 18-25 mpg but if you are constantly playing with your new bigger turbo. "See flog your car constantly" your efficiency will go down. Good luck man.

 

No, that isn't true. People assume that tuning is some sort of super-high-tech approach and extremely computerized, but the fuel delivery system on the car is largely mechanical. The system tries to keep a given A/F ratio, but fuel line pressure and injector flow aren't part of the sensors it uses. Also, like I said, bigger injectors are slower to shut off and flow much more per duty cycle. Essentially the bigger the injector the "sloppier" the injector process. A local shop car STI with 1600cc injectors (yes!) gets around 14 MPG, even when it is switched to its street 91-octane tune and just driven normally, according to the shop owner I spoke to.

 

My MPG went up after my tune, not down. However you are taking some safety features out of the tune when you push it higher; the stock tune is extremely rich and does a great job of keeping the cylinder walls cool. When you lean that out, you do gain MPG and power at the same time, but you are also running a hotter exhaust temperature and lowering the detonation threshold.

I should also mention that using your A/C has a much bigger effect than people think, and in my case was more significant than the gain from my tune. I got around 4 MPG increase in mileage from my tune, but lose 3 to 6 MPG if I use my A/C consistently.

 

get a fp green lol

 

and install it in your prius; this is the solution to big turbo/high MPG!

 

2006
WRX TR
FP 20G
All required supporting bolt ons including TMIC, 650cc injectors, full intake, turbo back, and misc etc
24-25 mpg
Mixed driving
GST AP 2 Pro-tune (310 whp 300 wtq)