ClubWRX Forum banner

261 - 280 of 284 Posts

·
Registered
Joined
·
18 Posts
does getting an exhaust and/or cold air intake increase or decrease fuel efficiency assuming your driving practices remain the same?
 

·
UnBanned
Joined
·
17,654 Posts
Discussion Starter #262
does getting an exhaust and/or cold air intake increase or decrease fuel efficiency assuming your driving practices remain the same?
Tricky question. Short side of it, if you drive conservatively now... You'll improve mileage. If you drive aggressively now... It'll get worse. Below are my explanations.


Cold air intake brings in colder air, colder air means leaner condition which the ECU compensates for by injecting more fuel. (that's why we have an IAT bulb in the MAF housing).

The cold air intake is less restrictive. That means less resistance, which means more power at lower RPMs with less throttle.

So... With conservative habits, the mileage should be better. With aggressive habits, it should be worse.

Less restrictive exhaust means more flow, more scavenging and therefore more volumetric efficiency. More efficiency means more gas mileage. But, it also means at higher throttle percentages, more air and fuel can be injected.
 

·
Registered
Joined
·
18 Posts
that's more or less what I thought ; your explanation makes it clearer. I want to get them but wanted the reality of what I was in for with regards to fuel consumption.
 

·
The Maestro
Joined
·
153 Posts
I haven't read all of this thread but I average about 24 in the winter, and 26-27 in the summer. I used fuelly, and my numbers are all based on calculation, not the gauge in the car. My best mpg on a tank was 28.5. my average over 37K miles is 25.1 mpg. My average price per gallon over that time has been 3.60, making my car cost 14.8 cents per mile in fuel.
 

·
Registered
Joined
·
31 Posts
According to my gauge, I managed to get 30.8 today on my 2015. This was a 34 mile drive and mostly highway, so yeah I was feathering the throttle quite a bit maintaining average speed around ~70mph

Sent from my HTC One_M8 using Tapatalk
 

·
Registered
Joined
·
1 Posts
Home Maintenance


Wow!! Nice article..I appreciated that work..I got the really useful info about the wood window repair..Your info is useful for me..But My friend suggest me for visiting on handyman home pros site..When i visit on this site i got the really great info about the wood window repair service..they provide the nice service of the wood window repair..Thanks to my friend and you...I suggest you for visiting on this site for more better info about the wood window repair:
Handyman | Home Repair | Remodeling
 

·
Registered
Joined
·
7 Posts
Regardin the WRX gas mileage, I have a 2014 sedan WRX (Past model) and I'm a little worried regarding the bad mpg that I'm having. I drive mostly in the city and I cannot have a better mpg than 17-17,5. More weird is that I don't drive like a crazy, standing on the gas pedal all the time, in fact, I drive more like a grandpa. I care all the time trying to have a good mpg, but I'm still having bad numbers.
Is there something, apart from the common gas saving tips, that may be causing a bad mpg?

Thank you in advance!
 

·
Registered
Joined
·
86 Posts
Pepezeta, stop and go traffic in the city definitely makes your mpg worse. I mostly drive in the city as well and i get the same mpg as you.

Sent from my C6806 using Tapatalk
 

·
Premium Member
Joined
·
4,856 Posts
City driving will absolutely kill your mileage especially stop and go. Highway if you keep easy on the throttle around 65 mph you'll see around 26 or 27

Sent from my SCH-I545 using Tapatalk
 

·
Registered
Joined
·
7 Posts
Thanks for the replies. I was a bit surprised because at fuelly.com my mileage is one of the worse and I was thinking "how the hell does these people do that mpg"? That's why I thought something was wrong.
Anyway, I enjoy my ride everywhere I go, so screw my mpg.
 

·
UnBanned
Joined
·
17,654 Posts
Discussion Starter #273
Pepezeta. Published city mileage for your year is 17-19mpg. Glad you're having fun with it though! ;)
 

·
Super Modulator
Joined
·
19,391 Posts
Tricky question. Short side of it, if you drive conservatively now... You'll improve mileage. If you drive aggressively now... It'll get worse. Below are my explanations.


Cold air intake brings in colder air, colder air means leaner condition which the ECU compensates for by injecting more fuel. (that's why we have an IAT bulb in the MAF housing).

The cold air intake is less restrictive. That means less resistance, which means more power at lower RPMs with less throttle.

So... With conservative habits, the mileage should be better. With aggressive habits, it should be worse.

Less restrictive exhaust means more flow, more scavenging and therefore more volumetric efficiency. More efficiency means more gas mileage. But, it also means at higher throttle percentages, more air and fuel can be injected.
Sorry Kevin, this is where I disagree. While improved flow does indeed increase VE, warm air increases fuel atomization which (in term) leads to a more complete burn. A CAI reduces air temps (at the injection point), reduces the fuel atomization, and should net worse economy.
 

·
UnBanned
Joined
·
17,654 Posts
Discussion Starter #275
Sorry Kevin, this is where I disagree. While improved flow does indeed increase VE, warm air increases fuel atomization which (in term) leads to a more complete burn. A CAI reduces air temps (at the injection point), reduces the fuel atomization, and should net worse economy.
I'll bite. You're wrong. ;)

Your cold air goes through a hot turbo. Our coolest intake temps (after warm-up) are still higher than the highest intake temps that street driven NA cars make.

The best drag times are in the late evening after a drag strip was heated up all day, but the air has cooled down from the sun going down. Or on a warm day that a cold front just moved in.

The difference in fuel atomization from heat isn't enough to matter for anything. Within normal operating temps. If it were... Then the inter cooler efficiency wouldn't matter to us, or inter cooler sprayers. Nor would water and methanol injection, or "wet" nitrous injection.

As far as fuel atomization... Age and condition of your injectors will matter a lot more than intake temps.

Love you sweet-t!ts,
Kevin
 

·
Super Modulator
Joined
·
19,391 Posts
I will admit that winter-driving tends to have worse fuel economy for a variety of reasons:
1. Increased ethanol content in fuel
2. Lower pressure in tires OR softer winter tires
3. More time spent in Cold Start enrichment

However I do believe that lower intake temps will lead to reduced fuel atomization. You are correct about the TMIC, but since the compressor follows adiabatic expansion and radiative cooling, delta T in = delta T out (rough simplification).
 

·
UnBanned
Joined
·
17,654 Posts
Discussion Starter #277
All I heard were a bunch of smart person words and "you are correct".

;)
 

·
Registered
Joined
·
9 Posts
I saw better gas mileage after I installed my AOS. I have a 2011 STi sedan stage 2+ Cobb OTS with 11.3 lb flywheel. Went from 22ish to 24ish city. Probably from the lack of degradation of the octane by cutting the oil vapor out of the intake air supply; sounds legit.


Sent from my iPad using Tapatalk
 

·
Water.....sometimes waves
Joined
·
2,172 Posts
GREAT write-up Sinister. I do all that stuff you spoke of, to get better mpg. A side affect of keeping my 2000 Regal tire pressures on the high side was that I got 135,500 miles on the OEM Firestones before ever buying a new set of tires for the car! I posted recently about getting 31.2 mpg on my new 15 WRX. Today on arriving at work the screen is showing 31.8 mpg. If I slow down to the speed limit everywhere I am sure I could get close to 33 mpg. I might try that next tank, just to see how high I can get it. I love your write-up; its dead on.
 
261 - 280 of 284 Posts
Top