US 4.7L Air Flow or Not to

[Fixer]

While cleaning the my MY 1983 (US) mass air flow meter the other day, i noticed the sealing frame between the lower air cleaner and mass air flow meter looked very similar to the sealing frame on my 3.2L Carrera.

Sure enough the sealing frame part # 928 110 201 01 is the same part used on 3.2L Carreras from 1984 to 1986.

The size of the inlet to the MAFM of the smaller 911 motor is the same size as the much larger 928.

These MAFMs are known to restrict air on the much smaller 3.2L 911 motor.

I also noted that the O-ring to the air guide housing on my L- Jetronic 1983, is the same size and same part #(999 701 436 40) as the O-ring to Air Guide Housing on a Lh-Jetronic (1985 1986) 32 V motor.

Seems my 1983 L-Jetronic has a serious case of AFM limited HP

due to the size of the inlet at the mass air flow meter. Euro hot wire AFM made of unobtanium?

Has anyone tackled this issue with something home machining?

For starters note the misalignment of paper gasket btwn AFM and airbox plastic opening, very lousy tolerances for a Porsche. This gasket hangs into airflow about 1mm on 2 sides. Trimmed, filed as a unit together (air box plastic) for clean flow into AFM, this work should have been done by the Apprentices at the factory. You should re install the AFM with the airbox already aligned to it. Order the AFM O Ring too, this should be replaced if original.
There was a lot of machining on the drill press. I did not do a good job of taking pics of the final product i was too curious to see if it made a difference. But it is common sense when you get into it and see the rough casting edges, misallighment of component parts which need smoothing.
The space of the air chambers can be machined to flow into the next shape more efficiently,
*Hold barn door fully open and note the AFM interior metal hanging into air flow! Marker it and file it away, barn door stop can be reduced in size by 50%, it is directly in the air flow path. Use common sense.
I remove easily a packed 'dixie' cup worth of aluminum shaving with more on the floor.
Be very careful, don't use machine lube, don't need it and it will make it difficult to blow out the light shavings with compressed air when done. Be sure to blow out all shavings! They blow out very easily and cleanly,. sand with 220 gt sanding paper finish up to 320 grit will suffice. Finish cleaning by shoot carb cleaner thoughout liberally letting it flow into a suitable bowl.. Now adjust spring tension of barn door first marking factory setting with a white paint dot. I did 10 clicks. You will see which direction spring is tensioned in.

[Fixer]

Originally Posted by Opelotus
I believe you are correct, I recall reading in several threads that the AFM used in the US-spec L-Jet system is a known airflow restriction. IIRC the Euro 16v cars use a heated wire type MAF sensor which is a larger diameter (or at least the same as the inlet diameter to the lower intake "U"). I thought someone had mentioned a way to convert to a better flowing unit, but I can't imagine it would be cost effective?

Me:
Have you noticed the Daytona Lightweight's intake is directly above the air cleaner?
I'd like to see that motor in that area.
When it is warm and starting cold was not such an issue couldn't we remove the AFM completely and make up something to connect the air cleaner to lower intake?
This could be done easily using a K&N and a hood like that on the Lightweight.
This is when a parts car would be great to have to experiment with.

[Fixer]

[Danglerb]

All of the hot wire MAF's for Euro and US 928's are the same part, but a few people have put Euro or Euro hybrid motors in Ljet (flap afm not hot wire) cars without much drop in expected HP so its not clear how much of a restriction they are.

Throttle body on a Euro S is also same size as US 85/86, but linkage is different.

[XLR8928]

I believe the MAF opening on a US 4.5/4.7 is about 2 x3" (6 sq ") That should be the equivelent of a 63mm throttle body. I would guesstimate that to be good for around 525cfm, plenty for a mildly tuned small motor. I am building a 4.7us motor with some cleanup porting,
euro cams, long tube headers, and a 3" exh system. I wonder if the stock injection has the range to keep up , or if I should install larger injectors?
-K

[Fixer]

Thanks Danglerb and XLR, great info.

The inlet size is 50cm X 65cm or roughly 1-11/16" x 2-9/16". So it is roughly the same size as a 63mm throttle body.

But the shape looks like an air flow disruption. All new cold air intakes are simpler and round straight through.

I made a few $40 offers on ebay on 3 "not tested" AFMs (81-84) to experiment with.

Maybe some of you could offer $30 to help with this and maybe get your own to play with. There is a ton of 3.25" aluminum intake tube to play with on ebay.

Thinking of trying to fit the air flow sensor to a section of 3.25" alum tubing (in place of the ancient Bosche unit)

The throttle body could be machined a few mms larger probably although I haven't looked to see. Then you may have something that could net 15HP.


If i can get a AFM cheap enough it could be a fun experiment.

[Pete R]

This is probably why the Euros went straight from CIS to the LH fuel management. Intake components are MUCH larger for the Euros form intake runner, throttle body, heads, valves ect Unless you do all of the components it's probably not worth doing 1 or 2 as you will still have restrictions in there

[Fixer]

Originally Posted by Pete R

This is probably why the Euros went straight from CIS to the LH fuel management. Intake components are MUCH larger for the Euros form intake runner, throttle body, heads, valves ect Unless you do all of the components it's probably not worth doing 1 or 2 as you will still have restrictions in there

Your definitely right but still i wonder about the Bosche AFM and whether a small gain could be made on such a large displacement motor?
Is the throttle body bigger on US '85-'86 cars, the connecting piece btwn the throttle body and intake manifolds are very close in size.

Air passing into the lower intake faster and more freely could do something. The Bosche AFM can be ruined easily, it all works as a calibrated unit so any
mods need to be done very carefully re the wiper action etc. If the entire mechanism could be repackaged with a better designed inlet for flow that would be the ticket.

Pete thanks you're a great Wrench please share your ideas.

[Pete R]

Originally Posted by The Fixer

Is the throttle body bigger on US '85-'86 cars, the connecting piece btwn the throttle body and intake manifolds are very close in size.

The US 85-86 throttle body's are about the same size as the Euro 85-86. In fact the connecting boot for the euro is NLA and the US one fits perfect. The Euro throttle body is massively bigger than the original in my car, my other tb actually fit in it.
Personally if my current engine management ever fails I will probably look into buying a stand alone complete system like a Megasquirt or something like that. They are getting pretty reasonably priced and they are endlessly tunable.

[Danglerb]

Quite a few "reasonable" options;

Switch to a 85+ 928 MAF and use a level translation device, been done, known to work.

Switch to complete 85+ injection system and brains, then sharktune.

Switch to a complete 88+ injection system and brains, then shartune with knock sensors.

Switch to aftermarket ala megasquirt and spend some time making it work. IMHO ok for racing, but not that good for a street car.

I think one can go a long way in upping the 928 power using the second choice, US 85+ brains etc., but ultimately the later model brains have much more capacity for control, but they aren't as reliable and it will cost double the older brains etc.

[Danglerb]

BTW none of those choices are cheap, a grand to three easy by the time all is working well, so fair incentive I think to make the stock system work as long as you can get away with it.

[Fixer]

Thanks Danglerb,

I got to thinking i should just improve what i have also and took a look at the air passage of the AFM. There is a lot that can be done to smooth out this passage.

I milled all of the casting imperfections and ridges between parts, smoothed out all areas that air could hang up,
I.e.: With the barn door open fully there was a very decent ridge in the air passage (above the barn door, see Sharpie marked area) removed and smoothed a lot of material and matched the port between the lower airbox and AFM almost 2mm of material did not line up correctly even the paper seal hung into the air flow.
I also made the barn door stopper 1/2 the size where it is in the air passage.
I can't go into everything smoothed out but there is a lot that can be done if you take a look.

This is something which should have been done by the Porsche technicians or as soon as the car was purchased. At the inlet there are many burrs and lips between the castings as well as the outlet. A lot of material was removed, 1/2 is in a pile shown the other half on the floor. At the outlet where the air passage transitions to the 3"opening i removed over 2mm and shaped it to this opening.

Then i lessened the spring pressure on the 'barn door' by 10 positions to get more fuel. I think it worked very well.

Boy I am happy with the 3 hours spent, i noticed a difference in the driveway just revving it up. The motor is more responsive without a doubt.
Very noticeable small power gain

[Danglerb]

Looks like you donated some blood to the cause, that always helps.

[AirtekHVAC]

Nice Matt!

[Fixer]

You should do this w/ your US '83 Danglerb, it's most likely the barn door spring tension change that is giving the boost in power. Porsche must set them for economy at factory.
Just got home, had the four of us in the car (The Wife and Kids extra 275lbs) and it still had more power. Very, very pleased.
Thanks Ron, I hope you're having some luck, will read in a moment.

[Fixer]

Busy all day, more '81- '84 Air Flow Meter interior pics,'before' and some process.

See how crude the interior is and how much can be done to improve flow.

The corners have shelves that you can lift the AFM up from, even they can be shaped to taper smoothly into the next area as everywhere else. There is plenty of material to work with.

Hope you all had a great weekend.








Reduce the size of barn door stop by 50%, much material to work with to increase flow and there is so much to smooth out to increase air flow. Don't bother polishing beyond 320 gt. will do nothing. Less Spring tension allowing barn door to fully open faster gives more fuel, not just more air. I know this because it pulls harder all the way up. No leaning out wall hit. Plugs look normal after hard use. Is Not Leaning Out. Starts as Usual Hot or Cold, Idles exactly the same. Absolutely no ill effects. Win-win. Fuel consumption seems the same. Porsche set this AFM for fuel economy no doubt about it.

[Danglerb]

Originally Posted by The Fixer

You should do this w/ your US '83 Danglerb, it's most likely the barn door spring tension change that is giving the boost in power.

Not a path I am likely to pursue. I like the driveability of the US 83 stock, which is mostly about torque, and have the Euro 85 with a hotwire MAF to play with. For more HP I have a much hotter than Euro S set of cams to go with a Euro hybrid 5.0 block, just waiting to see if a good donor 5.0L block turns up so I can bore it to fit a set of 104mm 968 pistons I have.

[Fixer]

I hear you. Driveability is better i think though (at least the way i drive) It definitely has more power, it doesn't have that flatness it used to have.
Makes me want more as in Euro intake and cams.

[Danglerb]

Its funny having both, I've got so I like each for what it is. Nice to have a choice.

[Fixer]

Driveability was not affected at all. It starts hot or cold the same, no bad effects i can see, just more power. Idles the same, etc.
A lot of time adding more power increases driveability.

So will i ever get to see pics of your 928s Danglerb? They look great to me in any condition so long as they are cared about.