Question about air flow/induction

Originally Posted by Sid Squid

Very generally speaking the airbox of a modern engine will ideally be about seven times the swept cylinder volume of the engine as a minimum. This is the best compromise between big enough to work as a still air reservoir*, but not so big it will be difficult to find anywhere to fit it.

The airbox has a very specific function, it's not just there to hold the airfilter, it's there to provide the engine intake with a volume of air at as great a fraction of atmospheric pressure as is possible, (atmospheric pressure is roughly 1 bar, approx 14.5 psi, varying a bit with temperature and weather conditions).
As you will likely know a moving gas, (or liquid), exerts a lower pressure, and the faster it moves the lower the pressure it exerts, (hence the term 'static pressure'), the job of the airbox is to slow the air as it flows into the airbox such as to keep it's pressure as high as is possible, this means the pressure differential between intake and cylinder will be the greatest it can be, so that the cylinder will fill as best it can.
The measure of success, (or not, depending), of cylinder filling is referred to as volumetric efficiency.

Pod filters are the worst of both worlds - a physically small area for gas flow and out in the air rushing past the bike as you move, (remember that bit above about moving gas exerting a lower pressure?), compromising cylinder filling.

*Unsurprisingly it's never still in there, but stillness is the ideal - that would give the best VE.

I would disagree, with a cone/ pod filter there will still be atmospheric pressure on the outside of the filter, just because it's not in a box doesn't mean it goes away. Assuming a motorcycle speed of 70mph in an idealised sense the air will still be at 0.994 atm. I would say the primary role of airboxes is intake noise supression and in some cases providing performance improvements through resonance gains. Advantages of reduced pumping losses when using a pod filter should outweigh any losses from a 0.006 atm pressure differential loss (assuming the airbox in question is capable of providing air at 1atm).

[johnnyrod]

You'r enot dealing with steady flwo though, so every time the intake valve opens on one cylinder of (say) ZRX1100, it tries to suck in approx 275ml during the induction stroke (swept volume). Even if the pod holds this much, part way through the stroke, it'll be half full and will be being replenished by air coming through the filter. Thus it's at a lower pressure than if it were a larger reservoir like an airbox so you're feeding the carbs with air at lower pressure. Thus lower flow and poorer volumetric efficiency.

[embee]

Mostly reliable previous info.

Big airboxes are much to do with noise these days, plus having a "stable" atmosphere to draw from means consistent calibrations for emission reasons and consequently consistent performance. Exposed intakes are subject to all sorts of turbulence so not as consistent or predictable and can cause all sorts of issues with air meters or pressure sensors (as well as carbs). I'm talking from a manufacturers point of view, you can make more or less anything run but not necessarily to meet regs.

One big air filter generally is less restrictive than smaller individual ones, each cylinder sees the big filter in turn. Aftermarket foam type filters usually don't filter all that much, they keep the bricks and dead birds out and are better than open intakes, but not generally all that good for extended engine life. Yes, users will now tell me their engine has done a zillion miles on them and is in perfect nick. European air quality is pretty good these days as long as you don't go into quarries etc.

Road dust is very abrasive however, and the silicon (sand) causes all sorts of problems with hot wire/film air meters.

A big airbox reduces the pulse amplitudes so the intake spout can be smaller diameter for a given typical pressure drop (smoother flow) and thus achieve better noise control. The minimum size depends on how many cylinders it's supplying to a large extent, as well as the overall engine size (i.e. the individual cylider swept vol. gives the pulse size). Usually bigger volume = better, space is the limitation.

More noise is an easy way of reinforcing the impression of more power, not that there's necessarily anything wrong with that if it pleases the rider. A reasonably well designed big airbox and air filter will give negligible power loss compared to open intakes, a few percent or so usually, but a quiet intake is much less impressive than a noisy one.

Hello all, first post and all that, had an '04 pointy for just over a month now.
I won't pretend to know much about this, although my understanding of the airbox is that one of its functions is to allow a good throttle response and generally speaking, a bigger airbox is better at this-I guess that in racing its better to have cones as you are aiming to be in the powerband at all times where constant airflow is preferred.

Horse poo? Maybe, but next time you're in a car stick your head right out into the airflow and see if you can breath faster!

[mikerj]

Originally Posted by yorkie_chris

So how come an SV needs a 137.5 for the p*ss poor stock un, a 160-165 for a nice BMC race, and some dirty great 220-250 for a pod filter then if it flows badly?

Much more likely down to pulse tuning effects - cone filters put a nice flat reflecting surface not far out from the throttle body/carb.

An air box means the high, peak air demands of the engine are smoothed out into a lower and more constant flow. This gives less pressure drop across a given sized filter. It also damps the pulses caused by the inlet valves opening and closing which can cause havoc with fuelling, and it will contain any stand off which short cones filters aren't great at doing.

As was pointed out above, a carb running a tea strainer can be as efficent as either a pod or conventionally flowed (ie airbox or airbox modded) delivery. All you need is unobstructed air to suck in. Thats yer induction roar/howl which journos bang on about. You don't need air to be 'flowed' directly to the carb. The thing just needs to be allowed to breathe in on behalf of the lump.

Pressure only comes into it via climate or altitude, which why dyno software takes into account barometric conditions.... at least, that's the impression I've been under for the last twenty five years

[mikerj]

Originally Posted by saintnick

As was pointed out above, a carb running a tea strainer can be as efficent as either a pod or conventionally flowed (ie airbox or airbox modded) delivery.

A carb with a tea strainer won't be very useful at all. Often you see carbs running on classic bikes and cars with a gauze over the air inlet, these are massively restrictive.

[yorkie_chris]

Mikerj, I suppose only way we'd settle this one is to see some dyno curves, but I don't think anyone's done a direct comparison of pods vs a decent standard shaped one.

I'm still fairly convinced the jet size increase from stock - race filter (normal shape and position) is due to reduced pressure drop across filter and this is what gives resulting power gain.

Originally Posted by saintnick

Pressure only comes into it via climate or altitude, which why dyno software takes into account barometric conditions.... at least, that's the impression I've been under for the last twenty five years

Whaaat?

Pressure is what makes the pistons move and turns the wheels, pressure is what forces the fuel through the jets, pressure is what picks the carb slide up to control the fuel flow. Pressure has been around longer than 25 years and longer than dyno software!

Originally Posted by mikerj

A carb with a tea strainer won't be very useful at all. Often you see carbs running on classic bikes and cars with a gauze over the air inlet, these are massively restrictive.

You can get perfectly decent dyno figures with mesh over bellmouths or simply exposed bellmouths. Obviously no one in their right mind would apply an incredibly restrictive mesh... unless for kinky reasons.....

Originally Posted by yorkie_chris

Pressure is what makes the pistons move and turns the wheels, pressure is what forces the fuel through the jets, pressure is what picks the carb slide up to control the fuel flow. Pressure has been around longer than 25 years and longer than dyno software!

yes yes but what was originally being implied on here was that there was a difference between pressure on one side of the filter and on the other: surely the difference is about airflow up to the point where - for what ever reason and through whatever mechanism - pressure is raised or lowered....

[yorkie_chris]

Aye but fluid mechanics says a pressure drop will reduce flow, a more restrictive "thing" (pipe, valve, bend, filter, whatever) will reduce flow more.