If you've been around cars for a while you've probably heard that an engine is like an air pump.
The more air in the motor the more fuel you can burn the more power you'll make.
A commonly overlooked essential part of this equation is air density or pounds of air.
Air density will change from several factors including, air temperature, air pressure / elevation, and humidity.
You will not be able to change the air pressure or humidity in the air but you can change the air temperature entering your motor.
I'm not saying you'll be able to change the ambient air temperature.
What I'm saying is the air your engine breathes is all too often HOT air.
I cannot count how many times I go to car shows and see beautiful cars with thousands of dollars into the motor and it has an open air element under the hood.
I have to admit I was one of the guilty and ran the open air element on top of my carburetor robbing power.

How much power are you losing?
From my testing I found with an open air filter under a sealed hood, no scoop I averaged approximately 140°F with a high of 158°F when the ambient air temperature was 80°F.
That's 60°F above ambient air temperature.
Now let's see how that difference in air temperature effects the air density. An Air Flow calculator has an input for air temperature.
Using my intake air temperature measured numbers, 355 ci, 6000 rpm, 90% volumetric efficiency resulted in: 554.69 cfm and 40.9 pounds of air.
Now only changing the air temp to 140 you get 554.69 cfm and 36.81 pounds of air.
NOTE: The cfm did not change but the pounds of air did.
The increase in air temperature from 80°F to 140°F resulted in a loss of 4.09 pounds of air.
That might not sound like a lot but a general rule of thumb is each pound of air equals approx 10 horsepower, therefore I was giving up almost 41 HP.