Why your "cold air intake" is mostly a waste of money

[YotaLac]

Engines are complicated air pumps. The more air you can shove in there, while supplying ample fuel and spark, the more power.

 

[snivilous]

In modern computer controlled engine, the PCM is going to do everything it can to kept the ideal ratio (stoichiometric (sp??) mixture) of 14.7:1 or close to it. So, based on the amount of air, it will adjust fuel metering accordingly. More air (O2), more fuel, less air, less fuel.

Given this, it seem that aftermarket system will do the opposite...if more air is indeed coming in, the PCM will add more fuel to keep the ratio...more fuel used, worse gas mileage.

This is true at the same RPM, but that also means the AM intake setup is getting more power. To compare fuel economy in the way you are talking, you need to have the amount of fuel used as the control variable, in this case the AM setup is at a lower RPM with the same power/fuel intake as the stock setup is at a higher RPM. So the gist would be, you would theoretically get a higher overall power but your fuel economy wouldn't change until you exceeded what the stock power levels are. Though as you said, tons of stuff modify those numbers.

 

[jwyshrek]

I can kind of see what you are saying...by allowing the engine to 'breath' easier...it is effectively 'shifting the power curve'...this would give the effect of 'more power'.

However, I guess the argument is whether or not that 'shift' is significant or worth it...

But consider this...for highway driving, you will be going a set speed...say 65mph. Once you cruise at that speed, the PCM will 'sense' a steady state and lock the torque converter...or let's say you have a manual transmission.

So, at 65mph, the speed of the engine is fixed as long as the drive train is 'lock' at a steady cruise. The relationship between the speed of the vehicle and engine rpm is fixed by the gear ratios throughout the drive train.

An AM intake is not going to change this...even if more power is made. The only effect I can see is it allows the engine to work less to draw in the necessary air to maintain that RPM...but how much less???