It is usually a tube that collects colder air from outside the car and deposits it to the air filter.
On the fortwo and Roadster it connects the air box to the air inlet on the left side of the car.
Cold air is more dense so it increases performance of your car very slightly.
Well, that's the problem I'm having. Yes, performance is nice but for every person who buys a smart to increase performance, there are 100 people who buy the smart for its fuel efficiency.
What happens when you have a cold air feed is the inlet air is cooler and therefore more dense, therefore, to balance the combustion mixture the injectors add more fuel.
More fuel means more performance but less MPG.
So therefore the opposite must apply. The warmer the air, the less dense it is,
less fuel is added, performance goes down and MPG rises.
My 451 71bhp Passion has the standard air intake into the air box.
2010 had 2 different extremes of temperature, very cold and very hot (for the UK).
In January when it got down to -10 degrees I was getting about 260 miles per tank.
In July when it went up to 32 degrees I was getting about 360 miles per tank.
This was doing the same drive to and from work in the same manner at the same speeds.
The heater was on in the winter and the A/C was on in the summer.
No modifications had been done between January and July so the main factor was the temperature.
I could feel the performance loss on hot days, I'd have to use lower gears for longer to get up to speed but it still kept up with traffic and could still break the speed limit should I wish it to.
Absolutely! Instead of collecting air from the outside of the car so it's cold as possible,
why not collect air from the warmer engine bay?
If you remove the air box and fit a cone style filter then that's good but if you want to keep your standard air box you just have to disconnect the pipe that goes to the outside of the car.
On the 450 and Roadster you can remove the air box lid and run the car with it off.
You could remove the intercooler scoop or try blocking the intercooler radiator on turbo models to see what sort of fuel efficiency increase you get.
Attaching a heat gun to the inlet isn't going to get you 100MPG.
Sucking air from the cabin isn't a great idea, it'll be noisy.
No, you can't use the hot air from the exhaust.
A heat exchanger over the exhaust probably wouldn't be a good idea either.
Just stick to ambient temperatures if you want to give it a go.