Tips are submitted by fellow users, and have a voting system. Just vote negative.

DTMAce
Sep 30, 2011
12:30 PM

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

rallyr
Oct 2, 2011
4:45 AM

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.

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.

>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).

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.

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).

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 ass, and that I am making sense.

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.

Actually, we normally do see increased economy from Cold Air Intakes, because the motor is running more efficiently. Same reason we see better economy from free flowing exhaust systems. The more efficient the motor is running, the better its economy and the better its performance.

Been modding cars for 17 years, and have had V8s in 4000lb sedans get over 30mpg on the highway. No stocker could do that, but a modded motor can.

Gasoline is more volatile in warm air then in cold air. That is why most air intake systems are provided with a heat source. Warmer air gives better milage.

Warmer air also has a tendancy to detonate in highly tuned motors. I have never seen a heat source on any performance intake.

Any Turbocharger or Belt driven Blower has a built in heat source, it is called the heat of compression. When you compress a gas it builds heat which in some applications is great enough that the blower or turbo requires an intercooler to keep it from overheating.

As for warmer air giving better mileage, that is true to a point, warmer air is less dense at any given pressure than cooler air. Less density means less free oxygen for combustion which in a fuel injected application means less fuel delivered in order to maintain a proper fuel / air mixture that will fall into the combustable range for that application. But as the density of the air charge decreases so does the available horsepower which helps mileage up to the point where the engine can not provide adequate power to maintain a constant speed.

Mike1nyny. Performance intake manifolds are available with or without a heat source. I have owned both. Don't confuse tuning an engine for best mpg or for max HP. They are not the same.