Cold air intake tip is incorrect! - Fuelly Forums

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Old 09-30-2011, 12:42 AM   #1
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Cold air intake tip is incorrect!

The tip says that a cold air intake will improve gas mileage. This is incorrect. In fact, cold air is more dense and the fuel injection system will inject MORE fuel at any given throttle setting. Now, this does produce more power at full throttle, but the gas/air ratio is constant and it will have no effect on mileage.

Further, in a modern, fuel injected car, even a dirty and clogged air filter won't have an effect on fuel mileage. It will restrict airflow, but the fuel injection system will only inject enough fuel for the air present. So, total available power will be less, but gas mileage will remain constant for a given power setting.

In fact, if you have a heavy foot, a clogged air filter or standard air intake vs cold air systm can improve gas mileage since you won't have as much power to play with and your jack rabbit starts will less severe.

Older cars with carburetors are another story. If the air intake is not free it will cause the carb to siphon gas and run rich, which will waist gas - and lots of it! That's exactly how the cold starting choke works to enrich the mixture.

I guess it's time to update those "rules of thumb" to the 21st century!
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Old 09-30-2011, 04:08 AM   #2
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Tips are submitted by fellow users, and have a voting system. Just vote negative.
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Old 09-30-2011, 05:30 AM   #3
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Sometimes people mix up performance vs mileage.

Warmer air vs colder air is a valid point. However keep in mind too, in times such as winter, ANY car will suffer a drop in MPG, even if you do not change your driving habits/style. I have proven this for 2 years. Whether I let the engine warm up, or just go when its completely cold makes no difference, it simply uses more fuel in the winter.

So saying the colder air vs warmer are does not impact MPG is not exactly true. Applied to the particular instance you are referring to however, I would agree that it shouldn't matter, the only thing that should be affected is performance providing the temp of the outside air is constant to the vehicle under both types of intake conditions.

But as MMUK stated, you can simply not agree with the tip. lol
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Old 10-01-2011, 09:45 PM   #4
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Sorry, I am new here and figured a simple vote would not have much impact without an explanation. As an engineer that's very important to me. Also, I'd hate to think that someone would read that and run out and buy an expensive kit for the wrong reasons. The marketing on this sort of stuff is misleading enough without the added impact of misinformed promotion.

As for winter driving, there are a lot of reasons for the reduces gas miles. Dense air has more drag, the gasoline in many areas has different additives in winter, sloshing through slush and snow is draggy too, longer warm-up times keep the engine running a little richer and the oil a little more viscous. All very small impacts, but they do add up. The "cold air intake" systems reduce intake temps a few degrees to maybe 10 degrees. Winter vs summer can be 50 and more degrees different.
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Old 10-03-2011, 07:13 AM   #5
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Another way of looking at it is whatever the density of the air, the fuelling, emissions etc are monitored by of late by various sensors from the intake to exhaust. These report back info to ECU which in turn alters fuelling, timing dependant upon the throttle usage.

Therefore if more air is able to be mixed you back off the throttle and get more mpg.

Did notice a drop in mpg when it rains, whether thats due to less actual air or just wet roads with more friction, who knows.
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Old 10-03-2011, 09:01 AM   #6
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>Did notice a drop in mpg when it rains, whether thats due

>to less actual air or just wet roads with more friction,

>who knows.

Probably two main factors - firstly, as you guessed, higher rolling resistance due to the water on the road, and secondly, the extra loss of heat out of the engine (since water has a vastly higher heat capacity than air).
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Old 10-08-2011, 10:50 AM   #7
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Perhaps I'm in the wrong forum, but I think another thing to factor in for cars getting worse mileage in the winter is different blends of fuel used in the winter. I've heard that refiners have to add certain things to prevent the fuel from evaporating during the winter, and boiling off during the summer. Those different additives have differing energy contents which would make your car burn more fuel.

I could very well be wrong here, but this has always been my belief.
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Old 10-08-2011, 10:59 PM   #8
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For diesel at least, the winter additive is to prevent gelling (i.e., the fuel turning solid when it is very cold). The winters here (Southern UK) aren't cold enough to do this, so our fuel is the same year-round.

But yes, on the TDI club forum the general consensus is that the winter diesel fuel does cause MPG to drop. (Personally I think the effect of that is less than the effect of the cold on the engine).
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Old 10-09-2011, 02:33 PM   #9
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As mentioned in one of the earlier posters, since colder air has greater density, the throttle position for a given load will be less. I believe that the reduced throttle position increases pumping losses.

Also, cold air might hurt fuel atomization for some cars (perhaps TBI and carbs for sure) at the typical part throttle operation engines see. Fuel atomization is not as much of an issue at WOT because the charge speed through the manifold/ports/ into combustion chamber will be at its highest(at least depending on a particular engine's geometry).

For port injected cars with high horsepower motors, I would make a gamble that colder air has greater effect on economy, as large injectors (at least from what I read from performance books) will not have the greatest atomization at the low outputs to make the 20 - 30 hp going down the road.

I hope it doesn't sound like I am talking out of my ***, and that I am making sense.
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Old 10-10-2011, 12:21 AM   #10
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Plus in winter you'll have the heater on, increasing your warm up time, and the extra load on the alternator from the heated screen(s) and the headlights + wipers will be on more often.
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