MAF Screens, Or, The Slippery Slope

I'm thinking about removing my MAF screens. Tell me why I shouldn't. VW must have had a reason.

May affect intake turbulance, if anything does make it past the air filter it will protect the MAF, or further down the line, the turbo. The amount of flow that is restricted has not been flow tested to ascertain how much it actually inhibits.

I've had my doubts about removing these screens too. They look hard to remove, and awfully obvious to the VW tech who may end up working on my engine someday. I know they have to prove this mod did the damage, but I don't want to have to prove anything.

On the other hand, the intake screen removal sounds like a good idea. Even if the screen itself doesn't inhibit air flow much, the gunk everybody says gets clogged in there will.

Questions:

1. Will the intake screen removal do no good with the MAF screens intact?
2. Am I right in remembering a post that recommended the intake screen only be removed when using a foam filter (to soak up any water that gets blown in)?
3. Has anybody estimated torque/hp gain, or better yet "dynoed", from this mod alone (i.e., removal of the intake screen and foam filter)?

Everything I've done (+ tuning box in a couple of days) is reversible. This isn't.

I personally think the MAF screens are not very restrictive, and that people take them out because it doesn't cost any money to do so and they think they'll gain 1 or 2 hp.

I did it because of forum peer pressure. I broke the screens and they'll never go back in again.

I think you will only regret taking out the MAF screens if someday your air filter self destructs (very unlikely to occur) and pieces of it hit the MAF sensor - a sensor that I think is rather expensive.

In gas engines they're there to create a more laminar airflow through the sensor. That's especially important when the fuel injection has to maintain a constant air/fuel mixture with a high degree of accuracy. That kind of pinpoint accuracy isn't quite as important in a diesel.

While the increase in power (with a Wetted motor) is very real, I question how much of it is due to increased air flow and how much is due to increased MEASURED air flow. In other words, I think the sensor measures more air with the screens removed precisely because the air is more turbulent. In our engines more measured air flow = more fuel injected = more power and more boost. But who cares? As long as it works!

There is no compelling reason not to remove the screens if you have half a brain, as long as the half a brain you have is capable of properly installing the air filter. With the air filter sealed in place and the intake hose buttoned up there is no danger to the sensor at all. None. The holes in the screen are much larger than the ones in the filter media, so anything that gets through the filter can certainly nail the sensor. (And the turbo, for that matter.)

By the way, it's entirely possible for the screen removal to be reversible if you use care and tiny little screwdrivers. My sensor housing is a bit dinged up, but I could put the screens back if I wanted to.

-mickey

Karl, a good reason for keeping all your screens intact is VW's original reason for installing them. These TDIs end up being parked outside for long periods of time all over the world in a variety of different climates on 6 continents. The screens are there to stop rats and mice from entering the engine once they have successfully chewed through your air cleaner matrix. Keep the screens in and you'll keep the rats out.

P.S. Rats + 162,000rpm compressor blades = dead rat mush + broken blades.

The MAF screens in both my TDI's are happily out and the engine breathing and fuel management improvement is similiar to what Mickey described above. My K&N air filters in the OEM box are wire mesh, however, in case one of the field mice manages to get by my fours cats who sleep on the TDI's in the carport at night. So far the cats are on top of the situation and my MAF screens haven't been missed.

If you are this concerned and hesitant about the MAF screens, you shouldn't mess with them since a new MAF sensor is at least $250.

[This message has been edited by SkyPup (edited May 04, 2000).]

I didn't know there was a wire mesh in the K&N.

Do it Karl! Go over to the dark side. The filter will be there soon.

(I guess I did it for the same reason as HowardZ, with the same result.)

------------------
Rob
2000 Golf GLS TDI (K&N air filter, MAF & airbox screens removed)
2000 Passat GLS Variant 1.8T

BTW, if there is this mcuh misunderstanding and apprehension about the simple MAF screens, forget that I ever mentioned the Garrett VNT15 Turbo Tweak, it is so much more complicated and expensive to replace it isn't funny anymore.

I also had the concern about debris protection, but with so many people using this mod in so many parts of the world - it seemed pretty safe !

A suggestion - remove the screens if you are Wett !! I don't think the stocker chips can use the extra air. Anything more than stock really needs the extra airflow !

With the 3 screens in the stock intake track, I would bet there is a 50% gain in air flow by removing them all.

I agree that rodents can be a problem ! I have seen them in so many types of equipment that is stored for any length of time.

If I store my car for awhile, I think I will plug the Kamei bumper ducts. They have a 3" intake connected to them to feed the airbox, and they would be attractive -



------------------
98 Jetta TDI, MAF De-screened, Ram-Air, Boost Guage, Kamei/Naylor air intake, Wetterauer

Hey, a fellow NY'er!
Does that scoop get clogged with snow?
Is that the only place the engine gets air?

BTW, I have another direct screen analogy to relate.

I installed a totally trick ceramic lined exhaust header and extractor on my high performance motorcycle this past month and then spent the next month dialing in the pilot jet, needle jet, jet needle, and main jet. The first thing that I did was totally remove the identical stainless steel screen from the intake tract of the airbox to the mouth of the carb to increase the max airflow, primarily on the top end. After much trial and error and many high speed top end runs from A->Z, I finally am pleased with the throttle response and power upgrades from about 55HP OEM to 66HP customized (on a 350 lb two wheel dirt bike no less! ).

Anyhow, the exhaust setup is a full race and the fiberglass packed muffler tube is straight through, great for power but very hard on the eardrums (both mine & the neighbors). Worried that I soon would be shot by an irrate who had her peace and quiet disrupted by my precision tuning experiments, I fretted over how to tone down the exhaust note I was sending out.

The simple solution was taking the stainless steel screen that I had previously removed from the airbox intake and stretching it over the final muffler outlet and attaching it with a stainless steel clamp. The decrease in exhaust flow exiting the pipe resulted in much less "BARK" and a much subdued audible tone from my bike at all speeds, especially while I was gassing it under load. Of course, I had to move the main jet down 5 complete sizes from a 175 to a 165 to accomodate the DECREASE in airflow out the exhaust.

Simple here to understand how much that stainless steel screen is inhibiting airflow here, both in the intake air stream and in the exhaust air stream? Seems rather simple and straighforward to me, plus it sounds much more mellower too!

It is interesting that a few weeks ago I installed a BMC air filter and removed the two post filter screens. I have not removed the pre-filter one yet (couldn’t find it at the time). Otherwise the engine is stock. With a stock engine there seemed to be no noticeable increase in raw power, but there was a noticeable change in low-rpm performance with a cold engine. It seems much smoother and more powerful when first driving away. Maybe my imagination but the car runs real smooth now whether accelerating or cruising on the highway. Maybe pure placebo, but on the other hand I have found no detrimental issues whatsoever from removing them.

Put D-con pellets in the bottom of the airbox if you're worried about renegade rodentiae. As they decompose they'll give off methane, thus boosting your fuel economy.

-mickey

My perception of the result of removing the screens (& replacing the stock air filter) on a stock engine is similar to Craig's. The power seems to come on smoother & at lower rpms.

However, at about the same time, I started using a certain cetane booster in addition to Power Service additive, so that may also play a role. Also, my current tank of fuel (the only one since I've done this) was labelled "premium" diesel (Texaco).

I'm not getting a tuning box for a while anyway, because of a shipping screw up at New Dim..

Maybe it's a sign.

Hello Karl - Yes, the bumper duct is the only intake. I removed the high restriction OEM air horn and plumbed a 3" line from the Kamei directly to the box. Ran it this way in snow, sleet, rain, etc. Never a problem. I have to block the duct when I wash the car, thats all. There is a least 12-16" drop between the airbox and the bumber duct, and moisture seems to dry out before it gets to the box. In addition to the power increase, I am getting consistently higher mileage. What a car, modify it for more power and you gain fuel efficiency hard to take.

------------------
98 Jetta TDI, MAF De-screened, Ram-Air, Boost Guage, Kamei/Naylor air intake, Wetterauer

I have removed the MAF screens and feel an increase in power (this without Wett) but with a GMR air filter(foam filter).
The car acts more sensitive on the pedal when pulling and feels quicker.
Before this mod i could soot a white car black at traffic lights.
I wonder what fuel consumption will do with this mod.
Anyhow there is no soot anymore.
I am thinking of Paul's bumper duct but how will the car act on high speed, isn't there a major increase of air going to the air-box more than the car can handle, anyone?

I agree with what Paul has done, because I have also experienced positive results from removing the 3 screens and running 2 extra 2 1/2" intake lines to each side of the front grill, in addition to the OEM air intake tube. This results in average MPG of always over 50.

[This message has been edited by Turbo Steve (edited May 05, 2000).]

REVIEW OF OPERATING DATA ACQUISITION FOR THE TDI ECU VIA THE MAF SENSOR IN THE INTAKE TRACT

One of the most important variables used for determining injection quantity and timing in the Volkswagen TDI direct
injection diesel engine is the engine's load state. An air flow sensor is the main load sensor to monitor engine load.
The MASS Air Flow Sensor (MAF) is located between the air filter and the intake tract where it
monitors the volumetric flow rate [m3/h] of the air being drawn into the engine. Air Mass Meters in Direct Injection
Diesel engines fall into two categories, the hot-wire and the hot-film air-mass methods (although some substandard
rice machines have been known to use Hot-Pants Air-Head meters instead! hehe). The VW TDI utilizes the latest in
BOSCH Motronic electronic engine management systems, which is the Hot-Film Air Mass method. Previously, the
Hot-Wire Air-Mass method had been in use prior to the development of the latest Volkswagen TDI engine design.
Either of these Air Mass Meters operate according to a common principle, which is an electrically heated element is
mounted in the intake-air stream, where it is cooled by the incoming air. A control circuit modulates the flow of
heating current to maintain the temperature differential between the heated wire (or film) and the intake air at a
constant level. The amount of heating current required to maintain the temperature of the wire or film thus
provides an index for the Mass Air Flow (MAF). This concept automatically compensates for variation in air density,
as this is one of the factors that determines the amount of warmth that the surrounding air absorbs from the
heated element.

HOT-WIRE AIR-MASS METER:

Previous diesel engine designs used this hot-wire method. The heated element on the hot-wire air-mass meter is a
platinium wire 70 microns in diameter. A temperature sensor is integrated within the hot-wire air-mass meter
to provide compensation data for intake-air temperature. The main components in the control circuit are a bridge
circuit and an amplifier. The heated wire and the intake-air temperature both act as temperature-sensitive resistors
within the bridge. The heating current generates a voltage signal, proportional to the mass air flow, as a precision
resistor. This is the signal transmitted to the ECU. To prevent "drift" that could result from contaminant deposits on
the platinum wire, the wire is heated up to a "burn off" temperature for one second each time the engine is
switched off. This process vaporizes and/or splits off the deposits and cleans the wire.

HOT-FILM AIR-MASS METER

This is the current MAF design in use on the Volkswagen TDI engines. The heated element of the hot-film air-mass
meter is a platinum film resistor (heater). It is located on a ceramic plate together with the other elements in the
bridge circuit. The temperature of the heater is monitored by a temperature-sensitive resistor (flow sensor) also
included in the bridge. The separation of the heater and flow sensor facilitates the design of the control circuitry.
Saw cuts in the ceramic are employed to ensure thermal decoupling between the heating sensor and the intake-air
temperature sensor. The complete control circuitry is located on a single layer. The voltage at the heater provides
the index for the mass air flow. The hot-film air-mass meter's electronic circuitry then converts the voltage to a
level suitable for processing in the ECU. This device does not need a burn-off
procedure to maintain it measuring precision over an extended period of time, as does the hot-wire air-mass sensor.
In recognition of the fact that deposits collect on the sensor element's leading edge, the essential thermal-transfer
elements are located downstream on the ceramic layer. The sensor element is also specifically designed to ensure
that deposits will not influence the flow pattern around the sensor.

The VW TDI ECU receives the input from the hot-film air mass sensor (MAF) and along with the engine speed
sensor's signal (ESS or rpms) calculates a load signal corresponding to the air mass inducted into the engine during
each stroke of the piston based or its internal programmed fuel maps. This load signal serves as the basis for
calculations of injection quantity, timing, and duration by the BOSCH VP VE 37 rotary distributor injection pump.
These signals from the MAF hot-film air mass sensor and the engine speed sensor are further modified in
accordance with the coolant temperature sensor signal, fuel temperature sensor signal, manifold air pressure signal,
and the intake air temperature signal. Although the primary input circuitry controlling the fuel delivery, timing, and
duration pulse to the fuel pump is derived from the MAF and ESS, the other sensors mentioned are
also monitored to fine tune the fuel output signals from the ECU in order to achieve the highest levels of
performance, economy, and emissions.

Please note from this discussion, that the VW TDI hot-film air mass sensor does not need to be cleaned each
time of use as was the previous version of hot-wire air mass sensor. This means that a K&N type oiled cotton
gauze air filter should not affect the operation of the hot-film air mass sensor as was previously speculated in
another post about this suspected problem.