Cold air VS Warm Air Intakes - what's the difference?

[Taylor95]

I have some data for the WAI vs. CAI debate.

I recently took a 1000 mile trip in a 2001 Outback. For the first 500 miles I had the factory CAI on. My average was 25 mpg. On the way back I removed the CAI and averaged 27.5 mpg. Typically the return trip should be less efficient because you are going up in elevation. I should add that removing the CAI also reduced drag. My average speed was similar both times (80+ mph most of the time). So the WAI definitely was good for efficiency.

[hotshoetom]

Originally Posted by oldschool

The cold air vs warm air issue will be more of a factor in carbed engines, then in FI engines, and again more of an issue in outside temps below 50 degrees F. In carbed engines cold air may even present drivability issues in extremely cold climates.

On a carbed engine , hot air allows the engine to run a leaner air/fuel mix, as the hot air tends to allow more fuel vaporization, and less of a need for the combustion chamber to use the actual burn in converting raw fuel into vapor , or useable fuel. Deterents to this process are the fact that the air and fuel mixed past the carb rarely spend more then .6 seconds in the intake before entering the combustion chamber,(hardly enough time to cause an appreciablle change of state in the fuel).

Hot air on an FI engine could easily help if the fuel is also heated,particularly on a wet manifold system or TBI, and this is doable because of the increased fuel pressure before the line, that will inhibit vapor lock. When the hot fuel is introduced to the air stream, the hot air will further help to keep it in a close to vaporous state as long as the puter does nothing to dump extra fuel into the mix.

Systems using hot air tend to run in a smoother fashion again because the air/ fuel mix is normally more homogenous.

This part of your post reminds me of Smokey Yunick's "homogenizer" that he used to such good effect on a 2.5 liter iron duke in a fiero.

[hotshoetom]

Originally Posted by Taylor95

I have some data for the WAI vs. CAI debate.

I recently took a 1000 mile trip in a 2001 Outback. For the first 500 miles I had the factory CAI on. My average was 25 mpg. On the way back I removed the CAI and averaged 27.5 mpg. Typically the return trip should be less efficient because you are going up in elevation. I should add that removing the CAI also reduced drag. My average speed was similar both times (80+ mph most of the time). So the WAI definitely was good for efficiency.

As a fellow subie owner interested in improving efficiency, (I have two leggy wagons with normally aspirated sohc 2.5 liter engines in them and a wrx for fuelish fun) I am always interested in the mods that improve performance and fuel economy too because those are truly a win-win situation. Particularly since the NA subarus are so dang slow (compared to their turbo brothers). But having said all of that, I'm also quite happy to improve efficiency if it reduces operating expenses. So now to my questions:

When you took your trip was there a net gain or loss of altitude in either direction? I am sure the road wasnt dead flat. Case in point, when I go visit my lady friend who lives 2.5 hours from me, I always get better mileage going there than I get coming back because the terrain is more favorable in the going direction than on the return trip. I have done it enough and measured the results enough under different conditions that its a valid measurement.

If you were traveling east west on your trip and the return trip was going east, I could easily believe you got better mileage simply because you were traveling with the prevailing winds that grace our planet.

So if you could clarify the conditions under which you made your trip and verify elevation changes that you encountered that might explain the differences more than simply pulling the cold air horn off. But given that it is so simple to do, I am going to try it on my next road trip and see what happens.

[Taylor95]

Originally Posted by hotshoetom

As a fellow subie owner interested in improving efficiency, (I have two leggy wagons with normally aspirated sohc 2.5 liter engines in them and a wrx for fuelish fun) I am always interested in the mods that improve performance and fuel economy too because those are truly a win-win situation. Particularly since the NA subarus are so dang slow (compared to their turbo brothers). But having said all of that, I'm also quite happy to improve efficiency if it reduces operating expenses. So now to my questions:

When you took your trip was there a net gain or loss of altitude in either direction? I am sure the road wasnt dead flat. Case in point, when I go visit my lady friend who lives 2.5 hours from me, I always get better mileage going there than I get coming back because the terrain is more favorable in the going direction than on the return trip. I have done it enough and measured the results enough under different conditions that its a valid measurement.

If you were traveling east west on your trip and the return trip was going east, I could easily believe you got better mileage simply because you were traveling with the prevailing winds that grace our planet.

So if you could clarify the conditions under which you made your trip and verify elevation changes that you encountered that might explain the differences more than simply pulling the cold air horn off. But given that it is so simple to do, I am going to try it on my next road trip and see what happens.

Someone finally understands how slow these cars are!

The first half of the trip was west to east. On that part of the trip there was a loss in elevation. I might have been driving slower on the way back, but I was still in the 80s.

So on the way back I had a gain in elevation and I still saw an increase in mpg. The weather was also worse on the way back.