There has been some discussion on the function of the spring loaded door in the VW airfilter box.
First, there is no way the panel filter will affect the function of the door. Paper, foam or cotton ain’t gonna make a difference. No matter how free flowing or clogged, the panel filter will not affect the function of the spring loaded door. The function of the door depends on the pressure across the door which is the same as the pressure in front of the filter panel, therefore the filter has nothing to do with it. The pressure in front of the filter panel is primarily controlled by the impedance of the main inlet path/snorkel. The spring loaded door has a bleed through flow path with a foam washer filter. This is a small flow path, usually to prevent the door from chattering upon opening.
I did a test. I took a piece of yarn and put it in the secondary air horn. If the door opens, the yarn will be brought into the main air box. Well, I put the yarn in the air horn and then went out for a drive and tested the RPM limit function that is built into the control system. Yep, it limits the engine to 5100 RPM. Still plenty of pedal travel after the engine hits the limiting RPM in 3rd gear. Came back, opened the air box and no yarn. The yarn was still in the air horn. Stock air system all the way. Screen in place. My CAI disabled, stock primary inlet snorkel. So the door didn’t open. The impedance of the stock system did not cause the door to open. Even with the stock diffuser snorkel.
What could possibly cause the door to open? A large pressure drop. What could cause that large pressure drop? A large impedance in the primary inlet path. What could cause a large impedance in the primary inlet path? The clogging of the screen or water ingestion.
It may well be both, but I’m leaning towards protection against water ingestion is the primary reason.
Okay, let’s say the spring loaded door doesn’t exist. There is only the main inlet. As the screen gets clogged, the compressor pressure head gets larger and larger as the screen loads up. Not good. This can lead to surge due to higher compressor head pressure. Now let’s say the spring loaded door is there… it can like act an air make up path intended to protect the turbo from surge. But you have to realize a clogged screen will only affect the upper RPM regime where the high air flows are. Not low RPMs and commensurate low air flows. A clogged screen will be more noticeable at high RPM. And realize, we are talking about the detection of the onset of this restricted screen and it occurs slowly, very slowly making it less observable. An air induction system is designed for max flow conditions. But big deal, a loaded filter panel will produce the same effect and there is no elaborate contingency measure in place for that scenario. So why bother with the spring loaded door to protect against a constricted screen?
The other functional option of the spring loaded door is protection from water ingestion. Let’s say the spring loaded door isn’t there. Also, note the flow path of the primary snorkel is a diffuser. If the snorkel inlet is in the vicinity of high water, the water will readily be inhaled. The engine might hiccup, but it’s over. Now if the spring loaded door was there, something else will happen. The viscosity of water is much greater than air. Therefore, as water is pulled into the small diameter inlet of the snorkel diffuser and increases the pressure drop, that pressure drop will open the spring loaded door to ‘clamp’ the pressure drop and dramatically reduce the possibility of water being ingested past the filter panel. This is not a long term steady state solution. It appears to be intended for the spurious water ingestion event, limited by the water capacity of the air box volume below the filter panel.
If the rubber thingy in front of the primary inlet nozzle does as presented at TDIClub over a year ago where I first learned of it, even better. I haven’t the interest to confirm if the rubber thingy in front of the inlet nozzle does as was posted, but it is a plausible explanation for all the effort that went into putting that spring loaded door in there. Even if it doesn’t, the diffuser shape does, particularly the small area inlet opening.
A paper filter will obviously present a far greater barrier to water than foam or cotton. The paper fibers will allow far superior capillary forces to trap and impede water flow through the media. The foam ‘pores’ are huge compared to paper and capillary forces will be virtually non-existent. The cotton media also has huge ‘pores’ as compared to paper.
First, there is no way the panel filter will affect the function of the door. Paper, foam or cotton ain’t gonna make a difference. No matter how free flowing or clogged, the panel filter will not affect the function of the spring loaded door. The function of the door depends on the pressure across the door which is the same as the pressure in front of the filter panel, therefore the filter has nothing to do with it. The pressure in front of the filter panel is primarily controlled by the impedance of the main inlet path/snorkel. The spring loaded door has a bleed through flow path with a foam washer filter. This is a small flow path, usually to prevent the door from chattering upon opening.
I did a test. I took a piece of yarn and put it in the secondary air horn. If the door opens, the yarn will be brought into the main air box. Well, I put the yarn in the air horn and then went out for a drive and tested the RPM limit function that is built into the control system. Yep, it limits the engine to 5100 RPM. Still plenty of pedal travel after the engine hits the limiting RPM in 3rd gear. Came back, opened the air box and no yarn. The yarn was still in the air horn. Stock air system all the way. Screen in place. My CAI disabled, stock primary inlet snorkel. So the door didn’t open. The impedance of the stock system did not cause the door to open. Even with the stock diffuser snorkel.
What could possibly cause the door to open? A large pressure drop. What could cause that large pressure drop? A large impedance in the primary inlet path. What could cause a large impedance in the primary inlet path? The clogging of the screen or water ingestion.
It may well be both, but I’m leaning towards protection against water ingestion is the primary reason.
Okay, let’s say the spring loaded door doesn’t exist. There is only the main inlet. As the screen gets clogged, the compressor pressure head gets larger and larger as the screen loads up. Not good. This can lead to surge due to higher compressor head pressure. Now let’s say the spring loaded door is there… it can like act an air make up path intended to protect the turbo from surge. But you have to realize a clogged screen will only affect the upper RPM regime where the high air flows are. Not low RPMs and commensurate low air flows. A clogged screen will be more noticeable at high RPM. And realize, we are talking about the detection of the onset of this restricted screen and it occurs slowly, very slowly making it less observable. An air induction system is designed for max flow conditions. But big deal, a loaded filter panel will produce the same effect and there is no elaborate contingency measure in place for that scenario. So why bother with the spring loaded door to protect against a constricted screen?
The other functional option of the spring loaded door is protection from water ingestion. Let’s say the spring loaded door isn’t there. Also, note the flow path of the primary snorkel is a diffuser. If the snorkel inlet is in the vicinity of high water, the water will readily be inhaled. The engine might hiccup, but it’s over. Now if the spring loaded door was there, something else will happen. The viscosity of water is much greater than air. Therefore, as water is pulled into the small diameter inlet of the snorkel diffuser and increases the pressure drop, that pressure drop will open the spring loaded door to ‘clamp’ the pressure drop and dramatically reduce the possibility of water being ingested past the filter panel. This is not a long term steady state solution. It appears to be intended for the spurious water ingestion event, limited by the water capacity of the air box volume below the filter panel.
If the rubber thingy in front of the primary inlet nozzle does as presented at TDIClub over a year ago where I first learned of it, even better. I haven’t the interest to confirm if the rubber thingy in front of the inlet nozzle does as was posted, but it is a plausible explanation for all the effort that went into putting that spring loaded door in there. Even if it doesn’t, the diffuser shape does, particularly the small area inlet opening.
A paper filter will obviously present a far greater barrier to water than foam or cotton. The paper fibers will allow far superior capillary forces to trap and impede water flow through the media. The foam ‘pores’ are huge compared to paper and capillary forces will be virtually non-existent. The cotton media also has huge ‘pores’ as compared to paper.