Factory intake/air box design

[TurboJ]

I'm wondering why the mk2 TD air filter assembly is built the way it is.
The inlet to the air box has this trumpet -style pipe:




Now, the the inner diameter of this pipe is 38 mm ( 1.5 inches ) on its narrowest spot.

On the other hand, the air box - to -turbo pipe has a diameter of 70 mm (2.76 inches) on the air box's end and 60 mm (2.36 inches) on the turbo's end.

What is this all about?
Can someone tell me why the inlet pipe of the air box has this "reverse" trumpet shape?
And how does a 38 mm inlet hole suffice there, when the box-to-turbo pipe is 60mm at the least?

I understand there is some sound deadening effect there, but how does the flow work here?
Knowing also that on many occasions (on VAG air boxes) you actually get lower hp/tq when you remove the inlet trumpet altogether...

[RabbitJockey]

Ha good question.  I am thinking its a combo of air flow and noise reduction, but you have already figured that much.

[gldgti]

By having a small diam inlet to the airbox, the flow velocity through that pipe is very high. When flow velocity is high, relative pressure is low (look up pitot static tubes for some reading on that).

As the air speeds through the inlet tube, it gets to the cavity under the filter and slows down very rapidly. This creates a relative high pressure area.... which helps to 'push' the air through the filter.

Thats about the simplest explanation I can give :-)

[damac]

Maybe off topic but I got mk2 turbo bits all over the place and am changing my personal vehicles over to a rabbit and a truck.  I got everything down except what to do for intake.  I could take the mk2 intake setup to the filter box, but then how do I mimic the extra fitting that went from the box into the fender?  Am I going to have to make some kind of hose from that fitting to maybe a ram air type inlet behind the core support? 

Or I have a cone filter hanging around.  Or do I have to source a mk1 turbo intake setup?

[TurboJ]

By having a small diam inlet to the airbox, the flow velocity through that pipe is very high. When flow velocity is high, relative pressure is low (look up pitot static tubes for some reading on that).

As the air speeds through the inlet tube, it gets to the cavity under the filter and slows down very rapidly. This creates a relative high pressure area.... which helps to 'push' the air through the filter.

Thats about the simplest explanation I can give :-)

How would I go about calculating a suitable trumpet size for my 200-240 hp expected power output?

I plan to make a "replica" of the pictured inlet trumpet, but with a bigger opening to support the extra power.
I know the air box itself is capable of ~350 hp on a diesel engine, but I doubt the OEM inlet trumpet is.

[R.O.R-2.0]

By having a small diam inlet to the airbox, the flow velocity through that pipe is very high. When flow velocity is high, relative pressure is low (look up pitot static tubes for some reading on that).

As the air speeds through the inlet tube, it gets to the cavity under the filter and slows down very rapidly. This creates a relative high pressure area.... which helps to 'push' the air through the filter.

Thats about the simplest explanation I can give :-)

How would I go about calculating a suitable trumpet size for my 200-240 hp expected power output?

I plan to make a "replica" of the pictured inlet trumpet, but with a bigger opening to support the extra power.
I know the air box itself is capable of ~350 hp on a diesel engine, but I doubt the OEM inlet trumpet is.

why tho?

whats wrong with an open element air filter?

[pointynoggin]

That reverse trumpet shape is called a diffuser. The point of it is to slow the air before it hits the air box.

I'm not sure if it is designed for resonance or not but I expect the inlet design is the way it is to distribute the air flow thru the filter uniformly while allowing larger particles to drop out before they hit the filter.

Another point to consider is that it might be made long and narrow to develop the flow and control the degree of turbulence at the entry of the air box.

I'm just guessing at this but if resonance isn't important a cone should work even better being that the dirty side is at free air conditions?

ROR, do you have a cone?

[RabbitJockey]

Hehe has a cone... Head 

I Think I'd just put a big trumpet going in, seems like a lot of engineering into such a part, most people run confikters with great success

[TurboJ]

The reason why I'm not using a cone filter:

1. They are not made of paper which means less effective filtration
2. Using one would require a big amount of work in building air tight heat shields and cold air feed
3. The factory air box is thoroughly engineered and doesn't limit power at the levels that are possible on a 1.6 IDI.
4. Cleaning and oiling a cone filter is hard work and it's difficult to apply just the right amount of oil to assure both filtration and flow.

A cone would be tempting for the relative ease of it, but you can see I have my reasons not using one.

Any help for estimating a good size for the diffuser/trumpet pipe's diameters?

[R.O.R-2.0]

The reason why I'm not using a cone filter:

1. They are not made of paper which means less effective filtration
2. Using one would require a big amount of work in building air tight heat shields and cold air feed
3. The factory air box is thoroughly engineered and doesn't limit power at the levels that are possible on a 1.6 IDI.
4. Cleaning and oiling a cone filter is hard work and it's difficult to apply just the right amount of oil to assure both filtration and flow.

A cone would be tempting for the relative ease of it, but you can see I have my reasons not using one.

Any help for estimating a good size for the diffuser/trumpet pipe's diameters?

lol, the stock TD air box isnt a restriction because there are other MUCH LARGER restrictions in the intake system..

the top half of the box is great, i imagine it flows amazing..

the bottom half tho, it sucks greatly.. there is only a 1" hole in that velocity stack.. THATS A RESTRICTION.

i guarantee you that you could take a 100hp TD, run it on the dyno twice, with the stock intake on one run, and the cone on another run..

i GUARANTEE it would make more power with the cone..

and its not rocket science.. oiling a filter is EASY..

K&N filters tell you EXACTLY how much oil to use (mine takes 1.42 ozs of oil) so, if you cant figure that out, i feel bad for you.

not all cone filters are created equal tho.. you get what you pay for..

something cheap, like a SPECTRE, or anything similar that they carry at napa auto, or vato zone, those are NOT GOOD.. they really dont filter well..

K&N filters on the other hand, those are quite nice filters.. you can tell the difference in quality just by setting the 2 filters side by side..

ive been running my K&N for lots of miles, and my entire intake system (from filter to intake valves) is still SPOTLESS.. light coating of oil, but NOTHING else.. just clean.

also, cone filters actually filter better, the dirtier they get.
they are re-usable, and for someone like me, that lives in the country, it doesnt take long for the K&N to pay for its self..
they are not hard to service. drying the filter is the hardest, and most time consuming bit.. people try and dry them with compressed air, but end up ruining the filter media..
you dont NEED to fabricate anything to run a cone filter. i plumbed mine down into the inner fender, where the original charcoal canister used to be.. BAM! cold air intake without fabricating anything..

VW diesel filters are quite large, they will flow lots of air.. the actual air box tho, that is the restriction.

[RabbitJockey]

I was thinking instead of making one maybe find one from another bag car that makes your power goal stock like an Audi v6 or v8 car.  That or maybe make a trumpet dimensionally 3 times the size of the stock piece

[gldgti]

The reason why I'm not using a cone filter:

1. They are not made of paper which means less effective filtration
2. Using one would require a big amount of work in building air tight heat shields and cold air feed
3. The factory air box is thoroughly engineered and doesn't limit power at the levels that are possible on a 1.6 IDI.
4. Cleaning and oiling a cone filter is hard work and it's difficult to apply just the right amount of oil to assure both filtration and flow.

A cone would be tempting for the relative ease of it, but you can see I have my reasons not using one.

Any help for estimating a good size for the diffuser/trumpet pipe's diameters?

lol, the stock TD air box isnt a restriction because there are other MUCH LARGER restrictions in the intake system..

the top half of the box is great, i imagine it flows amazing..

the bottom half tho, it sucks greatly.. there is only a 1" hole in that velocity stack.. THATS A RESTRICTION.

i guarantee you that you could take a 100hp TD, run it on the dyno twice, with the stock intake on one run, and the cone on another run..

i GUARANTEE it would make more power with the cone..

and its not rocket science.. oiling a filter is EASY..

K&N filters tell you EXACTLY how much oil to use (mine takes 1.42 ozs of oil) so, if you cant figure that out, i feel bad for you.

not all cone filters are created equal tho.. you get what you pay for..

something cheap, like a SPECTRE, or anything similar that they carry at napa auto, or vato zone, those are NOT GOOD.. they really dont filter well..

K&N filters on the other hand, those are quite nice filters.. you can tell the difference in quality just by setting the 2 filters side by side..

ive been running my K&N for lots of miles, and my entire intake system (from filter to intake valves) is still SPOTLESS.. light coating of oil, but NOTHING else.. just clean.

also, cone filters actually filter better, the dirtier they get.
they are re-usable, and for someone like me, that lives in the country, it doesnt take long for the K&N to pay for its self..
they are not hard to service. drying the filter is the hardest, and most time consuming bit.. people try and dry them with compressed air, but end up ruining the filter media..
you dont NEED to fabricate anything to run a cone filter. i plumbed mine down into the inner fender, where the original charcoal canister used to be.. BAM! cold air intake without fabricating anything..

VW diesel filters are quite large, they will flow lots of air.. the actual air box tho, that is the restriction.

With respect ROR, I'm not sure you can state that the TD airbox underside 1" hole is a restriction and expect anyone to believe it unless you have some proof to back it up.

The factory setups from VW are very well designed devices... all the way from the mk1 n/a diesel intake to the current TDI airboxes... and the nature of aerodynamics is that you can't always just look at something and correctly guess whether its good or rubbish.

You asked before whats wrong with an open paper filter... well nothing is 'wrong' with it really. Thats essentially what I am running at the moment in my cabriolet (I have no space where the filter is mounted to have a bottom on the airbox at all, unfortunately :-( )

However, by having no plumbing whatsoever 'upstream' of the filter element, you forgo any further possibility of some gain by clever aerodynamic design.

[pointynoggin]

I have a 91 and no trumpet, my air box inlet is piped into the fender.

If you wanted to design one the inputs would be frequency and amplitude to get a standing wave in the intake air volume.  It would be a trick, you need a vibs book and fluid mechanics book with a compressible section.  Maybe there is a engineering student on this site who would take a interest.

Otherwise you could experiment with shapes and volumes using a slide whistle as your inspiration i.e. vary resonator length/volume to harmonize with intake freq.  Maybe a trombone rather than a trumpet lol.

[R.O.R-2.0]

The reason why I'm not using a cone filter:

1. They are not made of paper which means less effective filtration
2. Using one would require a big amount of work in building air tight heat shields and cold air feed
3. The factory air box is thoroughly engineered and doesn't limit power at the levels that are possible on a 1.6 IDI.
4. Cleaning and oiling a cone filter is hard work and it's difficult to apply just the right amount of oil to assure both filtration and flow.

A cone would be tempting for the relative ease of it, but you can see I have my reasons not using one.

Any help for estimating a good size for the diffuser/trumpet pipe's diameters?

lol, the stock TD air box isnt a restriction because there are other MUCH LARGER restrictions in the intake system..

the top half of the box is great, i imagine it flows amazing..

the bottom half tho, it sucks greatly.. there is only a 1" hole in that velocity stack.. THATS A RESTRICTION.

i guarantee you that you could take a 100hp TD, run it on the dyno twice, with the stock intake on one run, and the cone on another run..

i GUARANTEE it would make more power with the cone..

and its not rocket science.. oiling a filter is EASY..

K&N filters tell you EXACTLY how much oil to use (mine takes 1.42 ozs of oil) so, if you cant figure that out, i feel bad for you.

not all cone filters are created equal tho.. you get what you pay for..

something cheap, like a SPECTRE, or anything similar that they carry at napa auto, or vato zone, those are NOT GOOD.. they really dont filter well..

K&N filters on the other hand, those are quite nice filters.. you can tell the difference in quality just by setting the 2 filters side by side..

ive been running my K&N for lots of miles, and my entire intake system (from filter to intake valves) is still SPOTLESS.. light coating of oil, but NOTHING else.. just clean.

also, cone filters actually filter better, the dirtier they get.
they are re-usable, and for someone like me, that lives in the country, it doesnt take long for the K&N to pay for its self..
they are not hard to service. drying the filter is the hardest, and most time consuming bit.. people try and dry them with compressed air, but end up ruining the filter media..
you dont NEED to fabricate anything to run a cone filter. i plumbed mine down into the inner fender, where the original charcoal canister used to be.. BAM! cold air intake without fabricating anything..

VW diesel filters are quite large, they will flow lots of air.. the actual air box tho, that is the restriction.

With respect ROR, I'm not sure you can state that the TD airbox underside 1" hole is a restriction and expect anyone to believe it unless you have some proof to back it up.

The factory setups from VW are very well designed devices... all the way from the mk1 n/a diesel intake to the current TDI airboxes... and the nature of aerodynamics is that you can't always just look at something and correctly guess whether its good or rubbish.

You asked before whats wrong with an open paper filter... well nothing is 'wrong' with it really. Thats essentially what I am running at the moment in my cabriolet (I have no space where the filter is mounted to have a bottom on the airbox at all, unfortunately :-( )

However, by having no plumbing whatsoever 'upstream' of the filter element, you forgo any further possibility of some gain by clever aerodynamic design.

for a stock engine, im sure its more than good enough..

if you want 5 or 6x stock power tho, i doubt it would keep your engine happy..

if Aki, or andy2 used a stock air box, it would definitely negate from power..

thats all im saying.. it may very well NOT be a restriction on a 100% stock engine, because there are other BIGGER restrictions, like the intake..

[TurboJ]

I was thinking instead of making one maybe find one from another bag car that makes your power goal stock like an Audi v6 or v8 car.  That or maybe make a trumpet dimensionally 3 times the size of the stock piece

I have thought about this many times, but this isn't an option for me as the mk2 EcoTD airbox is the biggest one that will fit the space I have available.
However, I'm looking into fitting a trumpet from a ~200 hp diesel car - that would be kind of easy and safe.

Currently I'm thinking making a plastic system out of a funnel of sorts, and some bass reflex tube ends. An a cold air feed to that. Lots of work still..