Air box design - VFR400

What's the theory on air box design?
Should the Air Box be sealed & have more air coming in than it could
use at peak revs?
How big should that air box be - is there an ideal size?
I've noticed on a VFR list that someone made the air box larger with
better access to cooler air then drilled out holes in the side of the
box. - Why?

Bummers

 

Got an acoustic guitar laying around the house? It has a sound box with
just one big round hole in it, but that curved wooden box does the same
job as the airbox on a motorbike. It resonates at a low frequency...

Your plastic airbox is a Helmholtz Resonator. Google for that. Also
google for " +flask resonance". I posted the formula
for the frequency of a flask and explained how the volume of the box
and the length of the inlet snorkel affected the frequency of the
box...

What you're trying to do with an engine that has camshaft overlap is to
get the resonant frequency of the box in step with the exhaust system's
adverse pressure pulses that try to push air backwards through the
carbs during the overlap period...

A properly designed airbox will make a pressure pulse that opposes the
adverse pressure wave coming from the exhaust system and it will help
to smooth out the depression in the power curve that occurs between
5000 and 7000 RPM...

Got access to a dyno? It would help to be able to get a baseline plot
of the torque curve, so you can compare the effects of your airbox mods
with the stock box...

That seems reasonable. But the airbox does its torque curve smoothing
task at less than half of peak RPM...

Some people have erroneously claimed that the airbox was just a
reservoir of extra air so the engine wouldn't run out of still air at
high RPM. But read up on Helmholtz Resonators and see what they say...

Yes, the ideal size is an airbox that has the right resonant frequency.
The engineers at Honda knew what they were doing when they designed th
box, you can't improve on it much, because it's close to ideal in
volume for street use. If you've changed the exhaust system, and
installed racing cams (or re-degreed the stock cams) and added bigger
carbs and are now running the engine at a higher RPM, you might want to
change the dimensions of the airbox and the snorkel a bit...

If you study organ pipes, you may notice that the small diameter organ
pipes make a higher frequency sound and the larger diameter organ pipes
make a lower frequency sound...

I once worked at a place where they had some large diameter ground
level flares burning off unwanted methane gas. The diameter and volume
of the pipes was so large the pipes would resonate at a subsonic
frequency and annoy the neighbors. Professional civil engineers were
mystifed at the cause of the resonance that made the ground shake. When
I told them what I had calculated the frequency would be, they were
embarassed because I was right and they'd missed the obvious. it was
flask resonance at work...

Why? because the guy had no idea that he was dicking around with a
Helmholtz Resonator? And he arranges to duct cool air in and then
drills holes to let air in from anywhere? Makes no sense at all...

If you drill holes in the sides of the airbox, you change the resonant
frequency and it just doesn't work to smooth out the midrange torque
curve anymore. I learned that the hard way when I removed the snorkel
from my GS-1100's airbox and made it breathe through a big hole in the
airbox...

My 4-into-1 exhaust sytem was able to push air backwards through the
engine at about 6000 RPM, and the engine couldn't inhale fresh mixture,
so it lean surged badly. You cannot re-jet for that condition, because
the air is being pushed backwards out of the carbs. I re-installed the
stock snorkle and the lean surging stopped...

Tell the guy that drilled holes in his box to get some sheets of ABS
plastic and glue them over the holes and play around with the inlet
snarkel's length and diameter. If he wants to reduce the volume of the
box itself and raise the resonant frequency, glue hardwood blocks
inside the box...