Air Boxes, Air Filters

[Speedy5]

Can anybody out there give me feedback on whether they have had a distinct improvement in power by adopting one of these fancy air filters you see in the brochures? These things like "Carbon Dynamic Airbox" or "Conical Direct Induction Filters" or "Venturi Admission System" (who thinks of these names!).
Also, do these fancy air filters only work with injection systems, or can they work with carbs also?

[tzei]

Originally Posted by Speedy5

Can anybody out there give me feedback on whether they have had a distinct improvement in power by adopting one of these fancy air filters you see in the brochures? These things like "Carbon Dynamic Airbox" or "Conical Direct Induction Filters" or "Venturi Admission System" (who thinks of these names!).
Also, do these fancy air filters only work with injection systems, or can they work with carbs also?

Not sure which filters you´re referring to but some makes will flow as promised and then some and some doesn´t. If it´s good i think "fancy" doesn´t apply. Same goes for airboxes - it may be evolved from competition use or not (just for looks?). Anyway it (decent) should be tuned on dyno for particular engine IMO. Can´t see why airfilters would differ between inj. or carbs.

[Speedy5]

Have a look at the GPR catalogue Pages 132-133 or the DT catalogue Pages 245-247. There's a lot of different options on filters, and I wanted to know if any are substantially better than others, and are they worth the money?

[tzei]

Got links for those? K&N and Pipercross are good filters, ITG flows just about what is promised AFAIK, dunno about other brands.

[JohnD]

Speedy,
Cone filters are just a way of packing a greater area of filter into the same volume, especially if there is a reverse cone inside the outer one. More filter area=less resistance at a given flow.
Some Piper or K&N filters, especially 'socks', have a derisory filter area for the intake crosssection.

An airbox works in two ways:
1/ It allows air to be drawn in towards the engine from outside the engine bay. These fancy filters are often just plonked on top of the lump, to be admired by all, and to draw in nice, hot, less dense air containing less oxygen/litre.
2/ Fresh, cool air travelling down a duct towards the engine is fast air=low pressure air (Venturi's principle). An airbox, of as large a volume as is practical, allows that air to slow down right outside the intakes. Slow air=high pressure air, so more will enter the engine with each stroke.

Place that filter as far into the fresh air flow as you can, and surround the intake manifold with as large an airbox as you can. After that, please yourself about the filter! But leave the bling to those as like it!

John

[dtype38]

Hi Speedy, an excellent and very straight forward question, but I'm afraid there isn't a straight forward answer. If you ask a guy with a standard car with a rediculously restrictive induction system (for reasons of fuel economy at 56mph say) what improvement he got by fitting "Induction System X" he might say he got a power jump of 15%. What you don't know is that although completely true, the same might have been achieved by just removing the original filter and fitting nothing!

So... before looking at new systems, you need to know what you have now and what its strengths and limitations are. Some of the factors to consider are...

Filter area/orientation... a big area filter can potentially flow a lot of air very efficiently, but if the air has to make tortuous turns to get through it the dynamic pressure loss might mean that you don't actually get any benefit.

Inlet tract lenght/size... the air flowing in the induction system has momentum by virtue of its speed and mass. The more mass of air in the tract (volume) and the faster it moves, the more momentum it has, which can give better cylinder filling and increased power. But the faster the air moves, the more friction losses you get and this can hurt maximum power. The relationship needs to be tuned for your engine. Just making the tract short and wide might give you more top end power, but at the expense of a great big hole in the mid range torque... not good!

Thing is, your original question has too many variables. Could I suggest, therefore, you give us an idea of exactly what you have now, and what it is you are looking for from a new induction system. Then we might well have some experts here who have had a similar issue on a similar engine, and can tell you what they've tried and whats worked.

[Speedy5]

Thanks for the info guys. But am I correct in saying that there is a contradiction here? JohnD has said that you should have a big airbox to slow the air down, which means higher pressure in the airbox, which means the engine sucks in more air due to the pressure difference between cylinder and airbox. Then dtype38 says that the more mass of air in the tract (volume) and the faster it moves, the more momentum it has, which can give better cylinder filling.
So who's right? I must admit I thought it was JohnD's theory which was correct. Anyway, don't want to start an argument!
BTW, I have a 4 cylinder engine which is going to have twin-choke webers fitted.
Final question: am I correct in saying the design of an airbox is much more important to power output, than fitting any sort of fancy airfilter?

[Dave Brand]

Originally Posted by JohnD

2/ Fresh, cool air travelling down a duct towards the engine is fast air=low pressure air (Venturi's principle). An airbox, of as large a volume as is practical, allows that air to slow down right outside the intakes. Slow air=high pressure air, so more will enter the engine with each stroke.

It's a little bit more complicated than that, I think! The volume of air drawn into the engine depends on the depression (partial vacuum) in the cylinder. Changes in cross-section of the induction system will create local variations in velocity & static pressure, but the mass flow will be the same. I suspect there's an optimum relationship between airbox volume & induction tract volume for a given engine/fuel system set-up. It's common practice on bikes now to use ram effect to pressurise (very slightly!) the airbox - said to be good for three or four BHP at 100mph on a 600.

By the way, It's Bernoulli's Principle!

[Dave Brand]

I should, perhaps, point out that it's a long time since I studied fluid dynamics, so my hazy recollections should not be considered authoritative!

[JohnD]

Oops,
Bernoulli has it!
And as a wholly self taught aero-ist, I am glad to be so corrected.

I don't think, Speedy that there is actual conflict - aero is so complex that art comes into it almost as much as science. But I agree with dtype, so that an engine setup with forward facing carb/throttle bodies could benefit from ram. If they face the side, slow the air down when it turns the corner.

John

[dtype38]

Originally Posted by Speedy5

But am I correct in saying that there is a contradiction here? JohnD has said that you should have a big airbox to slow the air down, which means higher pressure in the airbox, which means the engine sucks in more air due to the pressure difference between cylinder and airbox. Then dtype38 says that the more mass of air in the tract (volume) and the faster it moves, the more momentum it has, which can give better cylinder filling.
So who's right? I must admit I thought it was JohnD's theory which was correct. Anyway, don't want to start an argument!

No problemmo. Yes sounds contradictory, but isn't really. An airbox gives the engine a big chunk of slow moving air at as close to atmospheric pressure as possible to draw on. This gives the best possible distribution of air to each of the carb intakes and good static pressure. Any ductwork or air filters between the outside of the car and the airbox are just excess restriction to the airflow and are normally made as short in length and big in cross section as possible to avoid frictional pressure loss. But this has to be traded off against the location of the inlet which should ideally be outside air scooped into the duct at a high pressure position at the front of the car.

From the airbox to the engine is another matter though. If you can arrange to have quite long straight inlet ducts to each carb from the airbox, this allows the air to build up momentum as it heads towards the inlet valves. When the valve shuts the weight of air rushing down the tract can't stop instantly so slightly compresses the air behind the valve. This means that when the valve opens the pressure difference pushing air into the cylinder is slightly higher and helps fill the cylinder. Momentum equals the mass of air in the duct times its speed, so the longer and narrower the duct the better, but going too long or too fast increases frictional losses so is counterproductive. As as a rule of thumb, a short duct will give better maximum power (on a Weber 45 we're talking stub stacks up to say 45mm long bellmouth) but a long duct will give better maximum torque (on the Weber 45 say 50mm to 100mm long bellmouths). Which gives a better lap time is open to a lot of differences of opinion

[JohnD]

Dtype, Speedy,
It's always difficult to put these things into words. IMHO, a better analogy is an organ pipe. Long pipes for high notes, long pipes for low. A 'right' lengthed pipe will resonate with a fixed high pressure at the valve, promoting induction when the valve is open. But only at a certain revs, with next to no benefit at any other.

Such lengths are usually much too long for practical use. A 45mm stub will not resonate at any revs that an internal combustion engine can achieve! A 450mm inlet tube will boost induction at 6000rpm. To improve your low down torque, say at 3K, you need an inlet pipe 3/4 of a meter long!

John

[dtype38]

Interesting, John, that you suggest the improvement is more due to resonance than purely momentum effects. Would it be true to say, though, that the resonant length isn't just the bellmouth, but includes the carb, the manifold and the cylinder head inlet port. On my car that adds up to about 470mm.

[johnny yuma]

A cold air box for racing is a bit of a bad marriage with any kind of proper filter.A leaky,loose fitting airbox [to prevent buffeting]fed by a big duct with a mesh stone guard on the front is what the smarts seem to have.

It's not like you're going to do the Baja Rally and need to protect the engine from dust clouds.

[TEAM78]

from the little I know, air boxes on turbocharged engines do little in the way of performance, so long as its cold air the turbocharger collects, thats all it needs, but I guess you aint gonna stop a turbocharger sucking in the atmosphere when its on song!
as for nasps, the resonance is of more importance than flow, am I right here??
so what is right on nasps? whats more important?
pressure difference or momentum?

[norman-normal]

A cold air box (properly constructed) will make a very real difference, a TURBO will benefit the most. Under the bonnet temps on turbo engines are much higher on a turbo engine. An engine intake is an indiscriminate sucker; it should be obvious to anyone, that a supply of cool air will be more dense than hot air.
You are not convinced and need proof? easy, with a cold car, drive it a few min then put your hand on the AF regulator then do the same thing with a well designed and constructed cold air box. You won't need a thermometer, the temp difference will be quite obvious.

[TEAM78]

Originally Posted by TEAM78

from the little I know, air boxes on turbocharged engines do little in the way of performance, so long as its cold air the turbocharger collects,

my statement assumed that the turbocharger collected cold air irrispective hence improvements in inlet temps before the turbo were ignored.
Im discussing regarding to flow that increase the pressure in the airbox, this makes little impact to the turbocharger, I cant explain why or provide proof except I have made nice airboxs for turbod cars and it has not made one jot to performance on the track in any way.
as to why is hard to explain and this does seem to correlate with some manufacturers findings that I have heard of

[dtype38]

Chill out guys, Speedy is after info on airboxes for his twin webber intakes.

Anyone else got any experience on with-vs-without airboxes for this sort of intake?

[norman-normal]

Flow that increases pressure in the intake has been proven to have negligible effect many years ago,(1930's) thats why it isn't promoted now days.
If you need a greater authority for cold air boxes for turbos, you need only pay a visit to a new car dealer, if they sell turbo-charged cars they will have factory cold air boxes as standard equipment.
I did not ignore your statement, it just did not seem that you were aware of the trouble the engineers go to to insure the coolest possible air (inter coolers, cold air boxes). Getting cooler air doesn't just happen.
My daily driver is a toyota supra turbo, it looses about 30 of its 330 HP. with out the cold air box. The turbo acting as an air compressor heats the incoming charge, as compression does. On the supra they have knock sensors as well as incoming charge temp sensors and engine temp sensors all of which cause the engine computer to retard the timing. With out them, destruction would be assured.
It has become obvious that turbos have not achieved the popularity in the UK that they have here. It is not my goal to belittle anyone, but misleading information information can cause a lot of problem$.

[Chris Y]

I can only speak from my own limited experience, but on my normally aspirated car (1600), the carbs are at the top, and quite close to the bonnet. The carbs are sat right on top of the exhaust manifold, so I figured that a cold air box was the right way to go.

However, no one else running these cars uses an airbox, and now I know why - it's just not practical to get a large enough volume of air into the carbs without having an enormous airbox that takes up most of the engine bay!

I did two dyno runs back to back, and the car was losing a good 10-15bhp with the airbox on. Admittedly I don't think the trunking was quite big enough, but the dyno operator thought the airbox was a little too small too.

The airbox was made from the outer 'shell' of an ITG foam filter, which is normally stuck on top of the twin Weber 40s. Trunking was then added to a cooler region of the engine bay, but I think the Webers sucked so much air down that I'd need a much bigger box than my bonnet will currently allow, so that project is on the 'back burner' for now

[JohnD]

Chris,
Although you don't give the dimensions of the trunking to the airbox, I'm sure you're right that it was too small. My engine has as standard a tubular plenum, feeding all six cylinders, totalling 2.5litres. The plenum tube is 3" in diameter, and it is easy to calculate that an air flow of over 60mph is needed through that at 6K. That ignores the boundary layer and turbulence in the plenum. To mitigate that, I use 4" ducting from the remote filter to the plenum tube, and I plan to make that the plenum much wider.

John

[norman-normal]

Pardon me, my statements relate to electronic fuel injection as used on modern automobiles, my bad.

[dtype38]

Just for comparison, I have tripple Weber 50s on my 3.8lt engine and I'm using a 5" air duct from the front of the bonnet up to the carb inlets. I use this to supply plenty of cold air when running, but I don't actually use an air box, which I feel would be "restrictive" at high revs in a low gear.

[norman-normal]

You don't need one, a cold air box to house the air filter and provide about 2" clearance and supply it with cool, fresh, air from outside.

[johnny yuma]

Best cold air feed I've seen on a 66 Mustang [ex Geoghegan] in Sydney has
specially fabricated aluminium box with 8 holes each a loose fit over the upright fuel injector trompets,attached to the underside of the bonnet[hood]
When you lift the lid,the air box is out of the way. Ducting facing up to airbox inlets feeds cold air from the plenum chamber below the windscreen and from a point at the front of the engine bay sourcing cold air.There is no air filter.
All unions between air box and in-out points are fairly close fit but accomodate movement.