Please do not take this as bashing, I am just confused now. Edjamacte Me! :Flash:
That and I thought intake pressure was bad, too.
Well, I'm kind of drunk, but I'll see if I can be coherent here.
You're right when you said that air velocity is good.
Intake velocity IS important, in the right place. This is why the diameter of the runner leading up the the throttle body matters. A narrower runner will give higher velocity, which has the effect of packing extra air into the cylinder just as the intake valve is closing. If you go with a narrow runner, you'll gain low end power, but at high revs the narrow passage becomes restrictive, and you lose top end power. So you want a high air velocity in the intake passages leading to the valves.
For example, on the BMW oilhead motors, owners will swap out the runners between different models. The sportier models had a wider runner, while the GS had a narrow runner to give it low end power. By swapping out simple parts, the behavior of the bike could be changed.
It's a balancing act. Too little velocity kills the motor. Too much kills the motor. Just enough makes the motor produce more power, but only within a certain RPM range.
But that's just on the intake runners, throttle bodies and the intake passages leading into the head. Those draw air from the airbox, which can for our purposes be considered a chamber full of stationary air. It's what the engine sucks its air from, and the chamber refills itself after each cylinder sucks in its air.
We want that chamber to refill as fast as possible, and to get as full as possible (higher air pressure.) That's the theory behind all the airbox mods. The people who have done that are opening up the airbox so that it's flowing more air into it, keeping the entire chamber at a higher volume. Does it work? Maybe. There's more factors involved here, for instance airbox resonance, which I cannot easily explain as I don't totally understand it.
But back to ram air. Ram air, like turbocharging and supercharging, is a form of forced induction. That's a fancy way of saying "Cram in as much air as you can."
Turbos and superchargers do it by shoving in the air via a pump. This heats the air up a lot, which is why these machines generally have intercoolers to cool the air back down.
Ram air does it by using the high velocity air at the front of the bike. That air has a lot of energy, but it's all kinetic energy. You were referencing Bernouli's Principle when you talked about air pressure and velocity being related. High velocity = low pressure, and vice versa.
Since you want high pressure in the airbox, what you need to do is take that high velocity, low pressure air and turn it into low velocity, high pressure air. The way to do that is to decelerate it gradually and smoothly, without letting the air flow become turbulent.
So a ram air opening on the front of a vehicle is going to be large enough to admit the needed air, but it's not going to be huge. And as the duct heads back to the airbox, it's going to gradually get larger. As it enlarges, the incoming air slows down, and pressure increases. By the time it gets to the airbox, it's moving slower than it was when it entered the ram air system, and that energy has been converted into air pressure.
The important thing about ram air is that the energy to compress the air is essentially free. A turbo creates a major exhaust restriction. A supercharger creates drag on the crankshaft. A ram air system is taking air that's hitting the bike and slowing it down, and diverts some of it to force feed the engine. So it's taking a force that works against the bike's motion and making it assist the bike in going faster.
