Ehhhhhhh... I disagree. Let me explain.
Let's take a step back and look at some fluid dynamics. We're going to look at calculating flow through an orifice. The flow rate of a fluid through a hole is a function of the area of the hole and the velocity of the fluid. The velocity is then function of the fluid is based on the pressure differential across the orifice. McNally Insitute writes textbooks and has a good explaination of the math here:
http://www.mcnallyinstitute.com/13-html/13-12.htm
Now let's look at the carburetor. We have a CV, or Constant Velocity, carburetor. Based on the convergent shape of the carb, air is accelerated through the carb body, and therefore loses pressure as it goes. This is known as the venturi effect and is similar to how and airplane produces lift. This means that at the inlet to the carb (without airbox), air is at atmospheric pressure, while you have slightly less than atmospheric pressure at the outlet of the carb. This means we have a slight pressure differential across the carburetor. This is where our vacuum comes from for things like sucking fuel through the jets, opening the diaphragm in the petcock, etc. It's not "vacuum" so much as a low pressure area in the carb.
Since we have CV carbs, their claim to fame is that they attempt to keep a constant air velocity at all operating conditions. How does this happen? We know that when the engine revs faster and we suck air in faster, if we had a fixed size hole, we would create a larger and larger pressure differential across the carb and our engine would find itself limited for air. So one hole size that would work for idle intake would not likely work for full song intake hole size. This is where the slides come in. As your engine revs up and wants more air, instead of creating a bunch of restriction at the carb, the slide lifts, effectively increasing the size of the hole your engine is sucking through. Increasing the size of this hole as your pressure differential goes up flows more air and keeps the air speed constant. This is why the carbs are called constant velocity carbs; they do their best to keep the intake air velocity constant at all operating ranges so that you can meter fuel consistently through jets (using the same theory described above. Pressure differential, orifice size, etc).
Now lets move further from the engine and look at the airbox intake and apply our fluid dynamics to that setup. Your pistons move, pull air in. Again, your intake snorkel is a fixed size orifice through which air flows. This means your flow rate can be calculated the same way as above, based on orifice size and pressure differential. However, unlike our CV carbs, this orifice is constant and therefore, while it works well at idle, it presents a restriction as the engine revs faster and faster. At this point (we'll say 12,000 rpm) we have three distinct pressure zones in our intake (ignoring intake pulses due to the valves): we have atmospheric pressure outside, slightly lower pressure in the airbox, and slightly lower pressure than that in the outlet of the carb. The overall limit to flow rate is the airbox snorkel hole. As we both know the limit to power is fuel, the limit to burning fuel is air. To lift this limit, we have two options: 1) modify the airbox to have a bigger hole, or 2) ditch the airbox completely and use a pod filter. The biggest benefit of a basic airbox like this (I say basic because I doubt these cheap airboxes on these cheap bikes have been tuned for intake resonance to pressure the intake. This is where the Hemholtz theory you linked earlier comes in in a perfect MotoGP intake that has been tuned to perfection) is that the airbox supplies a nice consistent flow of air that is in a relatively smooth flow pattern. This is what CV carbs like because of how they work. Remember how the slides lift to keep a constant velocity through the throat of the carb based on pressure differential across the carb? If we have turbulent flow, this pressure differential is chaotic and inconsistent, where if we have a nice clean, perfect flow, the pressure differential is consistent and uniform, leading to smoother slide movement and consistent vacuum in the carb for sucking in fuel.
So!!! This means that in a basic air box, the benefit is smooth, laminar flow to create a constant pressure differential across the carb, allowing for more consistency with fueling. Let's say we want to get more air flow, and we cut more holes in our stock air box. Let's say we do this to an extreme and cut off everything past the filter. We basically now have a 5 sided box with a filter making up the sixth side, right? Tell me; what is the difference between this setup and a pod filter on a tube? To further the curiousity, look to Leo Vince, who recommends a pair of 50mm holes be cut in the backside of the stock airbox when their exhaust is installed. This is practically the same setup as I have suggested for thought, an open ended box with a filter. How is this different than a pod filter on an intake runner?
I will voice my thoughts here: pod filters are not the bane of a CV carb's existance. Turbulent flow is. CV carbs like smooth flow. So if you can figure out how to reduce the restriction in the intake but keep the air flow smooth, you'll be fine. These airboxes are so simple that I really am not considering resonance to be a major consideration. heck, the pregen was designed in the late 80's and back then the box was stuck on there just to hold the air filter, forget about optimizing intake pressure based on intake pulses
Quote:
Originally Posted by Linkin
The restricted size of the intake on the airbox compared to the size of airbox creates a vacuum inside.uhhhhh... lost me. Comparing area to volume? Airboxes are designed so that there is enough vacuum to feed the engine the correct amount of air in relation to the stock jet sizes inside the carbs. vacuum feeds the engine? Also, you're backwards. The jets and carb settings are chosen to fuel based on the available flow through the given airbox. When we're talking about intake changes, forget the stock jets. Rejetting is assumed here.Pod filters change this. No airbox, no vacuum, different amount of air, different jets and mixture settings required to get running properly again.yes, pods flow more. That's how they're able to make more power and quicken throttle response, of course they're not going to have the same mixture using stock jetting. See above note about rejetting
No airbox = improper improper? how so? amount of air = harder to tune. Impossible to tune if your bike has constant velocity carbs as my FZR250R does. see above, my thoughts are that air flow turbulence is more important than total amount of air. If total air flow rate was what made tuning hard, CV carbs would be rated based on how much power they are limited to. Like you said, fuel limits power, air limits how much fuel you can burn. To change your fueling to match a change in airflow, you HAVE TO rejet
You can also mess with the airbox boots. A longer inlet tract (longer airbox boot) will move the peak power and torque to lower in the rev range. The reverse is also true.in my reading, this is more the case with the boots between the carbs and the head, not the boots between the carbs and the filter.
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EDIT: I will say, in a modern superbike or supersport, where the intake is tuned for a specific characteristic of the engine and creates positive pressure in the intake to force air into the engine at a certain speed, I would NOT touch the intake past putting a high flow filter in the box. Those are optimized for lamiar flow, intake resonance, sound, etc etc etc and no average joe is going to be able to touch that. You could put a team of engineers with CFD software at their disposal, but even then, you'll get a different solution out of each team to work on that problem. With old/simpler/cheaper bikes like ours, go for it. Experiment, play, and use the butt-dyno all you want. Slap on a different filter, and change one thing at a time until it runs well. Heck, keep it stock, just don't tell people that the engine runs on vacuum and that ZOMG OEM IS BESTTTTT. You CAN tune these bikes well with a pod filter and rejetting. Reliable, consistent, perfect fueling. Its possible.