My answer to this question on Quora: Why does a cold air intake add better throttle response?
“In addition to Mr. Berson’s excellent succinct answer, I would like to add a bit about some lesser factors.
Where throttle response is measured in terms of how much throttle angle produces a particular quality of change in engine speed, then his answer alone is entirely correct.
However, where throttle response is measured in terms of how rapidly an engine speeds up, it may be worth considering the ability of an engine to rapidly accommodate more intake air volume. For example, consider how quickly you can rapidly force air in and out of your mouth.
Now consider blowing down a paper towel tube.
Now consider a drinking straw.
Now consider a garden hose.
Each of these things has a particular resistance to allowing air to flow. There is no very simple and entirely comprehensive explanation for which sorts of shapes and lengths and diameters of conduit are most restrictive or not, But in general, flow is negatively impacted by narrow, long, and obstructed passages.
So, let’s consider what a cold air intake really is. It IS the case that drawing air from closer to the front of the engine compartment of a front-engined vehicle, or immediately in the path of air coming into the engine compartment, will intake colder air than a conduit that pulls air from a place which is occluded from a path of high velocity air. This is less because the air is itself colder as it enters the vehicle than it is that the OEM intake is likely installed in a place where the immediate surroundings are warmer.
Original equipment intakes make compromises to maximum performance, though sometimes negligible, because they address several matters that the aftermarket does not care about. Stock intake passages are designed to pull air at the lowest pressure, or with the best filtering, or with the least variation as a component affecting the rev-band’s volumetric efficiencies, or to help regulate the vacuum assisted systems, or to simply minimize noise. More speculatively, they may frequently be located in less efficient places simply because it makes them easier to service or assemble or to get them away from other systems that may have other concerns, which would be negatively impacted by locating the intake elsewhere.
To accommodate any number of the above design goals or concerns, it is quite possible that the stock intake simply cannot provide the shortest, straightest, widest, least obstructed path. By comparison, a cold air intake … is almost always a large diameter metal tube with minimal bending, which routes from a place on the top of the motor, to a space that was already not otherwise obstructed, near the front of the engine bay, or even straight down.
It is reasonable to presume, that such a shorter, smooth, wide diameter conduit, without many bends, and which is positioned in the path of oncoming air, and which typically has a naked conical filter (a symmetric and large but thin and straightly-air-permeable obstruction, is much more likely to have lower flow restriction.
But… why does lower intake conduit airflow restriction help throttle response?
Because “throttle,” means to pinch flow. The throttle pedal is the control input for varying the degree to which the intake flow is throttled, by being correllated with a particular angle of the throttle plate, with respect to the direction of airflow, through its housing, the throttle body.
The throttle plate pinches flow two ways.
- The smaller the angle of offset from the direction of airflow, the smaller the frontal area of the plate presents to the intake air. Less frontal area means less obstruction. In this respect, a wider throttle body angle is similar to a tube which has increased in cross section area/ became a wider pipe.
- The throttle plate, at all-but completely wide-open angles, leans into the airflow at one edge of the round periphery, and leans away from the oncoming airflow at the opposite end of the round plate. This means that air approaching the plate is deflected toward the edge which is further away, relative to the edge which is “into to the wind.” Like the wings of an airplane taking off, this large obstruction, with a biased resistance to flow, this creates an area of low pressure on the backside of the plate. On a plane wing, this low pressure is able to generate lift. Inside the closed environment of an intake conduit, this low pressure pulls the air which passes it back toward the plate, and the intake charge becomes turbulent, decreasing the intake air velocity downstream of the plate (closer to the intake valves). Because air is a fluid, its behavior is not uniformly scalable. A smaller conduit typically has less than proportional flow than the ordinary scale decrease, with respect to any particular larger conduit. Increasing throttle plate angle means turning the plate away from the low pressure environment that formed behind it, as flow increased velocity,
These things mean… the throttle body is something that is an obstruction far more restrictive than the difference in restriction between contorted and straight line intake conduits, and is also a source of its own resistance to opening. BUT… the degree to which this lower pressure environment and greater drag fight the throttle plate increasing its angle can reach an inflection point in its angular offset from the direction of flow at which increasing throttle angle begins to increase obstruction by less and less incremental amount, per degree.
This inflection point, at which the incremental resistance begins to decrease, occurs when the velocity of the air is able to return to a rate at which the intake air begins to straighten, and is less turbulent. This velocity… is lower when there is greater restriction upstream of the throttle plate. Even though the conduit upstream of the plate is less restrictive than the plate itself at most angles, the obstructions and resistance upstream of the throttle affect the incoming air in a way that affects whether the throttle plate is able to close or open at any particular rate.
- shortening, widening, and decreasing the length of the conduit upstream of the throttle plate, and using a less obstructed filter, decrease the resistance of air coming into the engine.
- decreasing the resistance of air coming into the engine decreases the pressure differential between the front of the throttle plate and behind the throttle plate
- the decreased intake pressure increases velocity of the incoming air
- the angle at which flow through the throttle body/ past the throttle plate begins to dispel the resistance to its motion occurs at an angle which had previously been correllated with a larger throttle opening,
- changes in throttle input, correllated with any particular angle / increase in throttle plate angle are thereby performed in an environment with lower resistance, for some part of the range of throttle angles
- and therefore:
for a given range of throttle input angle, the throttle plate is able to open with lower resistance than it did with a more obstructive intake… and…
throttle response to throttle input is more rapid.
note: Ultimately, though… the overall ability of the intake air to be colder is determined by whether air is colder at the front of the engine compartment or near the wheelwell or under the hood. On any particular day, the static air in the engine bay will get hot according to the state of the engine, reduced only by the temperature of the air from where it is brought-in, which is typically the wheelwell. The air near the grille will still be in the engine bay, but will have ambient air blowing on it. It IS colder, but only insofar as air at that location is air that has not been near the brakes or obstructed from the wind. it’s the difference between standing behind a tree on a windy and sunny day and stepping to the side and facing the wind.
The degree to which performance is increased by this relatively small (but real) cooling advantage will likely only be available where the car is actually moving at cruising velocities, and has not been sitting for very long, and only for speeds at which the engine would otherwise be running a richer-than-stoichiometric air:fuel ratio.
By comparison, the increase in throttle response, due to the decrease in flow restriction, is like standing behind the tree and breathing through your nose, as opposed to breathing through your mouth. You can take a deeper breath more quickly.
And, the degree to which a cold air intake is more exposed to the wind, making it easier to take the breath through your mouth, is like stepping out from behind the tree, and opening your mouth into the wind.
Note, though: it is harder to breathe shallowly with your mouth open, facing into the wind, than merely breathing through your mouth, behind the tree. If airflow is too fast for an expected throttle angle, by the OEM setup, then the amount of fuel arriving in the cylinder may amount to a lean intake charge, by the time it is in the cylinder.
Hope that explains it!