mike77

Can the wire mesh on the MAF be replaced? I see pictures with it removed. It's a 1990 S4.

Managed to jab my finger through the mesh when I was putting the air box back on Doesn't take much force. Don't see how dirt can get past the filter but I have found little bits of grit in there in the past.

ammonman

They are available from multiple sources and are necessary for more than just protecting the hot wire and other bits.

Mike

SeanR

Yep, Roger keeps them on hand for when I do stick my finger in them.

FredR

Mike,

There is a school of thought that they are needed to get a more accurate measure of air flow by "straightening out" the streamlines. I am a bit suspect of this concept given the air is invariably going to be in a turbulent flow condition but nevertheless I would advocate installation of the top mesh for mechanical protection against ingress of debris should something punch through the filter matrix.

I have run without the lower screen for a long time [yes I managed to poke a hole in the top one as well] with seemingly no ill effect however I have wondered if it is functionally justified as a flame arrestor in the event of a backfire - something I have never experienced in my 928.

Of course my use of the shark tuner may help mask anything untoward but I tend to view it as a necessary barrier to air flow. I believe the root of the problem lies in the shallow air box design wherein two streams of air hit each other head on and try to fight for the "priviledge" of entering the MAF at a sharp angle- poor fluid mechanics! For this reason I run the bifurcated inlet system where the two inlet streams are more complimentary to each other flow wise. the only thing I do not like about my system is the surface area of metal and the potential for heat pick up. One of these days i will fit a temp sensor in the air stream to see if it makes a measurable difference to the inlet charge temperature.

I have a feeling John Speake also stocks these screens.

Rgds

Fred

PorKen

Fred - I would replace the lower one. I found that both screens are necessary for proper metering. Oddly the lower one was more important in my testing. Perhaps it reduces or homogenizes reverse pulses?

FredR

Hi Ken,

I believe the reversion pulses do impact the output of the MAF and thus why your MAF smoothing algorithm's will help stop the thing from jumping around too much. Based on what I have seen fluid dynamics wise I would be surprised if the reversion pulses would even notice whether the screen was there- more a problem on the higher lift cams I suspect hence why John developed the MAP sensor approach to eliminate the MAF altogether and give more stable control with hot cams.

By the way I am in the process of testing your "simpler" MAF smoothing patch-I can always add a lower screen from one of my spare MAFs. The dammed heat has been in the 47C region recently and I have some issues to sort out at the moment before I can continue [one of my half shaft gaiters has now let go- doubtless the others are about to- grrrh!].

Rgds

Fred

dr bob

Ken, I need to add that word to my vocabulary to help me describe possible causes of turbulent flow. I love it! Shaken not stirred...

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The hot wire element sits in that little venturi. My guess is that the screens help maintain a fairly even flow across the whole MAF cross section, so that flow through the venturi is more typical of the flow in the other areas of the MAF. Ideally that would be with both screens in place, especially with the butterfly-type valve (throttle) underneath. Butterfly is always my first choice for flow-control... Anyway, flow through the areas of the butterfly varies wildly as DP, fluid density and position change.

ROG100

I stock both Porsche $25.30 and aftermarket $12 MAF screens - I know which I use 8>).
Also stock the locking rings.
Also rebuilt MAF's for $315.

Mrmerlin

FWIW if you want an extra few Horse Powers simply try removing the screens for some reason this will result in more HP it has been proven on the dyno,
I would also suggest that you install a fresh factory air filter and the hump goes to the sky.
NOTE do not use the HnM filter it passes too much dirt

John Speake

Hi Mike
I have plenty in stock....

ptuomov

On the reverse pulses: Aren't all eight pulses combined with their reflections in the throttle body element? There shouldn't be any meaningful pulses coming from the cylinders themselves. The flappy closing and opening is going to shock the MAF somewhat, and suddenly closing the throttle will majorly shock the MAF. I am sure that JDS guys have some data on those shocks. I don't see how the bottom screen would help with either of them, however -- not to say that it wouldn't, just that I don't understand how it would.

The flow thru the MAF sensor is influenced by both upstream and downstream pressures. Getting the upstream and downstream pressures to be as consistent as possible should result in consistent flow thru the sensor element. What makes this kind of a difficult issue with the 928 is that the hood line forces the air box very close to the MAF sensor so there's no way of putting a sufficient straight pipe length upstream and downstream of the MAF sensor. So some kind of flow straightener is probably going to be beneficial in getting the MAF sensor readings to be as consistent as possible.

Does removing the screens increase power with stock chips only or also with the tune optimzed separately for screens and no-screens configurations? Just thinking out loud some of the power improvement from removing the screens may come from the indirect effect of the altered MAF signal getting the fuel and spark of the stock chips closer to the power optimum. Just a thought, I've never done A-B-A dyno with and without the screens installed. I've blown a mangled screen into the throttle blade with the turbos, however! ;-)

John Speake

I don't subscribe to the view that removing the screen increases power. The slight restriction of the screens is not a factor compared to all the other restrictions in the 928 intake system.

It is important to keep the flow over the venture smooth with little turbulence. There have been plenty of threads on hear of people fitting "cheese graters" before the MAF to achieve this.

It is true that intake reversion does upset the MAF, and this gets much worse with hotter than GT cams. This is why the GT has a higher idle speed.

I was not surprised that a stroker race cars with hot cams had a very docile idle when used with the MAF-less Alpha-N system.

Mrmerlin

that bit of info was from word of mouth from a few different old guys that used to work on these cars when they were still serviced by the dealer,
I am fortunate to ride motorcycles with one such fellow,
its interesting to pick his brain to get the tidbits.
Some of the other resources included info gleaned while the new X pipe that Louie made was being redeveloped and dyno time was included in this.

John Speake

Until I see back to back dyno runs with/without screens PLUS data logs of air/fuel ratio for both runs then I am yet to be convinced there is any benefit in removing the screens. In fact there are several negatives in removing them.

mike77

Ok looks like I'm going to need to buy a long screwdriver and dig my maf out. I'll also get to use the maf cleaner spray I bought in anticipation of pulling the maf.

John, I'll be in touch