Maximize Your Engine's Power with Improved Air Intake Surface Area

[will6131]

I have been thinking about the large number of "cold air intakes" and why they are such an overwhelming trend. I question their validity for a couple of reasons. First, the stock airbox on all new cars brings cold air in, usually from the fender, and is sealed off from the engine bay, usually better than the "cold air intake." This is especially seen on trucks, where a heat shield is all that is separating the filter from the engine bay on most "cold air intakes." Secondly, on many old musclecars and hot rods, both those built today and in the past, a simple open air cleaner is all that is used, and these engines make tons of power and torque. These people are not bound by anything (cost, noise, etc ) and would most likely make the intake work as well as possible, at least in most instances. Granted, looks may be a factor in some cases.

I am assuming that what we are really interested in is the mass flow rate (or, mass air flow) of the intake. Assuming pressure to be a constant (i.e. no forced induction or nitrous), and air being an ideal gas, we can then say

mass flow rate = ((air velocity)(area of intake opening))/temperature

this equation shows, if you can improve the area significantly, (which is easy to do over the stock setup) temperature can increase and mass flow rate will still increase. For example, with mine, I increased my area by a factor of 48 while increasing my temp by a factor of 1.4

However, I don't know how to determine the velocity change. I simply opened up my stock intake to the filter (i.e. removed the air box surrounding it).

My overall hypothesis is that "cold air intakes" work not by giving you cold air, but by giving you more air, and the reason stock intakes are less efficient is noise reduction only.

 

[pantaz]

http://en.wikipedia.org/wiki/Cold_air_intake" has a reasonable definition of the principle:

All cold air intakes operate on the principle of increasing the amount of oxygen available for combustion with fuel. Because cooler air has a higher density (greater mass per unit volume), cold air intakes generally work by introducing cooler air from outside the hot engine bay.​

You are partially correct in that "cold air intakes" generally increase mass flow over stock configurations. However, the temperature reduction by itself can add significant improvement.

 

[brewnog]

From where did you get that equation? Why do you assume that the air intake is the limiting factor, and able to give a 48x improvement in air flow?

 

[jack action]

However, I don't know how to determine the velocity change.

This is what's wrong with your reasoning. The volumetric flow rate is defined by your engine characteristics (assuming no force induction), i.e. engine displacement, rpm and intake & exhaust tuning.

The volumetric flow rate is air velocity times area. Simplifying, if the area increases somewhere in the intake passages, velocity will drop equally, keeping the same volumetric flow rate.

Where temperature plays a role is that mass flow rate equals the volumetric flow rate times air density. Since volumetric flow rate is driven by engine size and rpm, lowering the temperature increases the density, giving you more air (mass wise) for the same volume displaced by the engine.

 

[rethunk]

A related question:
How quickly does your nice cool air start warming up when it comes into contact with the interior of the cylinder? Is there enough time before the inlet valve closes for air heating up as it's sucked into the cylinder to expand and offset the vacuum towards the bottom of the stroke? - Does this negate the lower temperature of air in a 'cold-air' system?

 

[will6131]

A related question:
How quickly does your nice cool air start warming up when it comes into contact with the interior of the cylinder? Is there enough time before the inlet valve closes for air heating up as it's sucked into the cylinder to expand and offset the vacuum towards the bottom of the stroke? - Does this negate the lower temperature of air in a 'cold-air' system?

I think the expansion in the cylinder is desirable, as power is directly related to compression...

i didn't say 48x air flow improvement, i said 48x area increase...although it is a moot point to some degree, as the mass air flow sensor housing ultimately determines the available area.

i suppose the velocity vs. area idea is valid...but the whole idea that got me thinking about this was the fact that on carbed engines, a simple open air cleaner is used in most performance applications...maybe that works only because the intake path is no longer limited by the mass air flow sensor?

as for the idea that volumetric flow rate is defined by engine size and rpm, wouldn't it make sense to increase available area?

the main idea of my hypothesis is that the underhood temperature increase over ambient has negligible effect on the density of the air (I know it does affect it in some way) but the restriction on the stock intakes area, whatever the reason, has more detrimental effect than the increased temperature.

 

[jack action]

as for the idea that volumetric flow rate is defined by engine size and rpm, wouldn't it make sense to increase available area?

Ultimately, the area that will limit your mass flow rate will be the throat area at the inlet valve (minus the valve stem area). Making the area a lot bigger before that point is useless (a strategically placed area increase may help intake tuning and a slightly larger area will compensate for some losses due to friction and bends but that's it).

If the air box and the tubing leading to it are large enough to carry the needed flow rate, you will not gain anything by increasing their size (the only effect will be a drop in air velocity in those components).

 

[pantaz]

...
the main idea of my hypothesis is that the underhood temperature increase over ambient has negligible effect on the density of the air (I know it does affect it in some way) but the restriction on the stock intakes area, whatever the reason, has more detrimental effect than the increased temperature.

You seem to be looking at it as an either/or proposition. Generally speaking, increasing inlet area will be the primary improvement. However, as others have pointed out, there is a hard limit -- a maximum usable inlet size. Reducing intake air temperature is simply an additional, and proven, method for performance improvement.