The most commonly spouted bollex about ram air is that the bigger the scoop, the better the air pressure. Utter shite I'm afraid.
Your bikes' airbox exists as a kind of air reservoir - purely there to provide the engine with cool clean fresh air 'on tap'. Imagine if you didn't have an air box - the air would be rushing over the carbs or throttle bodies at anything up to 200 mp - bloody difficult to get it in the engine when it's in the engine when it's whizzing past at that speed. Hence the airbox holds a near-still supply of pressurised air directily over the carbs to feed them effortlessly with as much air as they need, every time they need it.
The airbox has to be much larger than the capacity of the engine too. If you take the GSX-R1000 as an example: Each cylinder as it fires will need 250cc of air to be sitting there ready to flow into the cylinder, and as the next cylinder fires it will also need another 250cc of air, and so on.
Now, obviously with each 250cc gulp of nice fresh air disappearing from the airbox - there needs to be enough to sustain each of the next 'draws' too, which means that the flow of fresh air into the airbox must be sufficiently high that the rapidly diminishing air can be replenished beforre the pressure drops significantly inside the airbox. Therefore the airbox needs to inflow more than it can outflow - but no more, although even that's a lot of air. Each cylinder uses 250cc , which is 1 litre of air per four revolutions. At 11,500 rpm that's 2,875 litres of air a minute, or 48 litres a second. Going back to the size of the intake and, taking the example of the Gixer Thou again: it has 42mm throttle bodies, which means that the intake needs to be bigger than 42mm to flow more that the cylinder can take - but literally 43mm would do the job, it doesn't need to look like a '50s hot-rod.
The total pressurisation of the airbox is dependant purely on the speed at which the bike is travelling - again, nothing to do with the size of the intaks. Static air has higher pressure that air moving at high speed because air has fixed amount of energy - raise the air speed (kinetic energy) and the pressure energy drops to keep the total volume constant. This is why bikes have airboxes.
In all reality the maximum pressure you're gonna see in the airbox at normal riding speeds is significantly less that 1 psi - so you might be wondering 'why bother?' Well, if you consider that the effect of not having an airbox which even only held the air still would mean that you'd actually be losing power the faster you went - the effect of having positive pressure in the airbox is simply a boost - literally.
Fast Bikes - Apr 03