Air flow through hood scoop

[ItBites]

I run a fuel injected BBC bored to 440ci that is shifted at 7500 RPM (or above). I do not run a turkey pan. I have modified a drop-base 14" filter base with a 2.5" element. This is all I can get in the space I have. I'd prefer more.

According to K&N I suspect I'd be short on filtration area too. Understand their size recommendations are likely based on a particular max velocity thru the element that, based on the mechanisms involved in any good filtration system (I forget the three mechanisms - entrapment? impingement? diffusion?yatayata?). Anyhow, beyond a certain air velocity, the filter is not as effective at removing particles of various size categories (bins).

So what? Well, if you run a smaller than recommended filter, the filtration will not be as good. OK, we may have to live with that. I suspect that you could get away with a filter of about half to 2/3 of what they recommend and still not get the velocity way too high thru the element. More frequent filter service might offset the decreased filtration efficieny.

The other downside (maybe more interesting to us) is similar to that discussed in my earlier post regarding carburetor size. If you have to increase the velocity of the air at the filter boundary because the cross-sectional area of the boundary is small, this velocity comes at a price. The price is that the pressure drop from atmospheric to the other side of the boundary (filter) must be higher (it takes energy to move the air faster, remember momentum = 1/2 * M * v^2) so energy required is a function of the square of the velocity. Lower pressure just above the carburetor (inside the filter housing) will result in lower manifold pressure and then we're back to the density that is available to fill the cylinders discussion above...

Just food for thought: Fuel Injection can be designed to be capable of producing more power than carburetion with ALL other things being equal, because carburation requires signal (low pressure) at the venturis to meter fuel, resulting in manifold densities always below atmospheric pressure, whereas injection can be designed to allow manifold pressure to approach or be at atmospheric pressure and still meter the correct fuel amount. Think about it.