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Old 10-03-2014, 01:43 PM   #1
bobdole888
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Default Simple mod to improve MPG

I'm doing a simple mod with hopes to improve MPG on '12 impreza.
I need to wait to see how the mod affects MPG over a few tanks of gas to confirm the usefulness, but so far with half tank used, it seems to have improved by 1 MPG.

The mod is just simply removing the air duct to funnels cold air from front grill to the air filter. This can be done by popping up the 2 connectors on top of the grill and just lift up the duct for removal.

The idea is to get warm air to the engine instead of cold air so the ECU adjusts the air/fuel mixture to be leaner due to the warmer air.

I've been checking the air intake temperature using Torque App and find that the max temp is ~110F, but typically 90-100F. With seasonal cooling coming, I'm not concerned that the air would ever get too hot to risk normal operation. So even when it's 60F outside, my engine is taking in 90F air.

I found that in the first 1/4 tank, my car didn't behave any different. However, within the 2nd 1/4 of the tank, the ECU seems to have adapted to the warmer air and started to burn less fuel. It could be placebo effect, but the MPG on the dash is reporting 1 MPG higher than usual.

If anyone else wants to try it and report back that would be great.

Unless you live in a very hot area(I live in California), I don't think there's any risk as the car is just getting air temperature typical of summer times. Your are not exposing the car to any additional stress.
Do so at your own risk, I shall not be held liable for any mod that you chose to do on your car.

Data showing 1.1 MPG improvement after 6 tanks using this mod (compared to all other fill ups in 2014):
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Last edited by bobdole888; 01-04-2015 at 01:44 PM.
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Old 10-03-2014, 02:31 PM   #2
Haunty
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Someone else posted a while back doing the same thing, they stuffed the intake in closer to the engine
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Old 10-03-2014, 02:36 PM   #3
flyboy1100
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get an OBD2 scanner, you will see your intake temps are exactly the same.......

the only way to get warmer air to do what CLL_OBP or I have done, which is reduct it somewhere hotter.

trust me I did this last winter, made a huge difference, but come spring time it has to be removed (at least in my case) because it raised IAT's 50F over ambient and kills all performance, actually makes it a bit dangerous to drive. oh and it had a negative effect because the air was so hot it took a lot more effort to get to speed and therefore used more gas
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Old 10-03-2014, 04:28 PM   #4
bobdole888
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Quote:
Originally Posted by flyboy1100 View Post
get an OBD2 scanner, you will see your intake temps are exactly the same.......

the only way to get warmer air to do what CLL_OBP or I have done, which is reduct it somewhere hotter.

trust me I did this last winter, made a huge difference, but come spring time it has to be removed (at least in my case) because it raised IAT's 50F over ambient and kills all performance, actually makes it a bit dangerous to drive. oh and it had a negative effect because the air was so hot it took a lot more effort to get to speed and therefore used more gas
It was probably getting too hot because you added ducting to get HOT air. So far with my simple mod, temperature only gets to 110F. I wouldn't think that there's observable performance degradation at that temperature.
Also as you said, it helps during cool weather, so shouldn't you be putting it back as weather is getting colder now?
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Old 10-03-2014, 04:54 PM   #5
flyboy1100
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Quote:
Originally Posted by bobdole888 View Post
It was probably getting too hot because you added ducting to get HOT air. So far with my simple mod, temperature only gets to 110F. I wouldn't think that there's observable performance degradation at that temperature.
Also as you said, it helps during cool weather, so shouldn't you be putting it back as weather is getting colder now?
no, once temps are consistently in the 40s i will put it back.

80% of my driving is interstate w/o much other traffic. just removing the duct raised temps while at idle, but once moving for more than 20 seconds, temps were the same as ambient.

if the engine bay were better sealed you would see better temp control with that simple mod
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Old 10-07-2014, 09:22 AM   #6
Thefifth
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I tried it this morning. Even with the engine completely warmed up you don't get better temps. If you want warm air, you need to make your own intake.
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Old 10-07-2014, 09:48 AM   #7
auskip07
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first time i have ever heard of wanting warmer air in the engine. typically every 10 deg lower equates to 1 hp
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Old 10-07-2014, 09:52 AM   #8
cris90
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btw the hotter the air the worse HP you make, for very 10 degrees I believe it comes to about 1.5% of power you lose.
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Old 10-07-2014, 09:56 AM   #9
Thefifth
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I don't think it would actually do much that having a lighter foot wouldn't do. The afr is changed based on coolant temp, not air intake temp.
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Old 10-07-2014, 11:09 AM   #10
flyboy1100
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In the bitter cold winter where the air is so dense warmer air really helps mpg and really doesn't hurt performance. Honestly who is out racing in the winter.

I posted all the numbers somewhere
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Old 10-07-2014, 11:10 AM   #11
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Quote:
Originally Posted by Thefifth View Post
The afr is changed based on coolant temp, not air intake temp.
That makes no sense, not even a little. Air temp & density & flow rate are the A part of AFR
The only thing that coolant temp would do it's closed(variable afr) vs open loop(fixed)

Last edited by flyboy1100; 10-07-2014 at 11:36 AM.
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Old 10-07-2014, 11:35 AM   #12
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Found it, all discussion starts here http://forums.nasioc.com/forums/show...php?p=41138293
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Old 10-07-2014, 11:35 AM   #13
Thefifth
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So what is high idle mode doing then? And what is causing high idle mode? Is it just spilling my gasoline on the roadside to be funny? No. It's lowering the afr because the engine (and coolant) is cold and the lower afr warms the engine faster. I believe the effect is even more intense in the pzev models.
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Old 10-07-2014, 11:49 AM   #14
flyboy1100
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Quote:
Originally Posted by Thefifth View Post
So what is high idle mode doing then? And what is causing high idle mode? Is it just spilling my gasoline on the roadside to be funny? No. It's lowering the afr because the engine (and coolant) is cold and the lower afr warms the engine faster. I believe the effect is even more intense in the pzev models.
Yes, but that is preprogrammed open loop to get engine and emissions equip up to temp faster.

Once it goes to closed loop, the afr is constantly being adjusted by maf & o2 and a few other things. If you do some digging starting where i just posted you will see subaru has programmed our cars to a 14:1 afr which is slightly richer then ideal, but probably due to the engine requirements vs 87 programming...
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Old 10-07-2014, 01:11 PM   #15
Thefifth
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I don't know why you think a warmer intake temp will magically make your car more efficient. All it does is require more air to be pulled in to produce the same power. (read: same as driving with a lighter foot)
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Old 10-07-2014, 03:55 PM   #16
flyboy1100
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Quote:
Originally Posted by Thefifth View Post
I don't know why you think a warmer intake temp will magically make your car more efficient. All it does is require more air to be pulled in to produce the same power. (read: same as driving with a lighter foot)
read through the other thread.....i have 10K+ miles proving the concept last winter. I
there is a point where too much cold and too dense of air causes issues. more fuel is required, the air causes more drag on the car, among the other things that take way longer to warm up (transmission, differentials, etc). with my WAI car drives the same as it does on a spring/fall day.....same mpg even. I can't wait until winter when you start to complain about how bad your mpg has gotten

like i said i only use mine when it gets COLD out.

not to mention it is also a proven mod made by people who hypermile, i don't but i also don't like averaging 24mpg in the winter when I can average 28. 70%+ of my driving is interstate at speeds above 70mph, how am i suppose to use less throttle when the cruise is set for 33min of my 40min commute (only includes 3 stop signs and 3 stop lights of which only 1 is ever red always)

Last edited by flyboy1100; 10-07-2014 at 04:15 PM.
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Old 10-07-2014, 04:16 PM   #17
bobdole888
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Quote:
Originally Posted by Thefifth View Post
I don't know why you think a warmer intake temp will magically make your car more efficient. All it does is require more air to be pulled in to produce the same power. (read: same as driving with a lighter foot)
I'm also seeing MPG improvement with WAI (warm air intake), that's why I started the thread. My drive is mostly local, so I think maybe this mod is more effective on local driving than freeway driving.
The temperature differential I typically see is 30-50F between outside temp and WAI mod.
If you don't believe the theoretical explanations of why it works that's fine, but fact is that it does work. I don't have long term data that flyboy has, but based on 1/2 tank of driving with it I do see that it's working.

For someone who's trying this, keep in mind that the ECU is 'smart', so it takes some time for it to 'learn' before you see the benefit. So don't dismiss it just because you don't see immediate improvement. Give it a couple of tanks to run through before you draw your conclusion.
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Old 10-07-2014, 04:18 PM   #18
flyboy1100
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Quote:
Originally Posted by bobdole888 View Post
I'm also seeing MPG improvement with WAI (warm air intake), that's why I started the thread. My drive is mostly local, so I think maybe this mod is more effective on local driving than freeway driving.
The temperature differential I typically see is 30-50F between outside temp and WAI mod.
If you don't believe the theoretical explanations of why it works that's fine, but fact is that it does work. I don't have long term data that flyboy has, but based on 1/2 tank of driving with it I do see that it's working.

For someone who's trying this, keep in mind that the ECU is 'smart', so it takes some time for it to 'learn' before you see the benefit. So don't dismiss it just because you don't see immediate improvement. Give it a couple of tanks to run through before you draw your conclusion.
mine took a few tanks to see full benefits last year.

our VW has a factory WAI (technically it draws cold air from outside, but with the routing under the cover and near the warm engine it warms a lot before entering the engine.

her mpg is worse in the winter, but no where near as bad as my Subaru until the WAI mod.
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Old 10-07-2014, 05:08 PM   #19
bradze
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hmm, interesting i may try this. especially since my wife puts on at least 500 miles a week.

Last edited by bradze; 10-07-2014 at 05:18 PM.
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Old 10-07-2014, 05:40 PM   #20
flyboy1100
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Quote:
Originally Posted by bradze View Post
hmm, interesting i may try this. especially since my wife puts on at least 500 miles a week.
If it is mostly highway do something like me or CLL did.

I put on 480/week
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Old 10-07-2014, 05:52 PM   #21
bradze
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she drives backroads in a mountainous area. im amazed she sees an average of 36 mpg, especially considering her complete disregard for driving with any type of fuel economy in mind. she also leaves her defrost on all the time, which i think has some type of effect on the air conditioning system. im trying to maximize the fuel economy of the little clown car. my '11 outback only avgs about 25mpg so if something works on her car it may work on mine.
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Old 10-07-2014, 06:39 PM   #22
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That is good. My mpg has started to drop with cooler temps.

I would leave hers alone and just try it on yours
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Old 10-07-2014, 09:40 PM   #23
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I did the snorkel removal on both cars, makes perfect sense. She normally averages about 33 mpg in winter. I'm also going to take out the spare tire and tools and some of the unused ****. If I keep on her about her driving habits I think she can get close to 40mpg.
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Old 10-07-2014, 09:52 PM   #24
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No thanks, but good luck.
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Old 10-13-2014, 09:16 AM   #25
Thefifth
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Air duct on: 20c
Off: 12c

This mod has exactly the opposite effect that you want it to in my experience.
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