Larger (BMW) Bosch Air Flow Meter Installation



By Scott Goodwin Austin, TX

First some background. My car is a 1982 Balocco SE with a 3.0 engine from a Verde (with typical street modifications like ported heads, cams etc...).

In researching performance modifications I ran across a guy who builds racing Porsches for a living and specializes in fuel injection. The cars he builds race at Sebring. Anyway, this guy just happens to be an Alfa nut and has built some seriously fast GTV/6's and Milano's. One of the first things he told me was the Bosch AFM was a giant restriction to the intake of the engine. The best thing to do was to find a way to get rid of it all together, but that would require an upgrade to a Haltech system (or similar).

Second best would be to replace it with a similar but larger Bosch AFM. Best place to get one is from an old BMW 63X or 73X. So I get one of these AFMs from a BMW 733 at the junk yard for about $50. It looks pretty dirty so I decide to tear the thing apart into little pieces to clean it, paint it and lubricate any moving parts.

An important message I'd like to get across to my fellow Alfa brothers is this: DO NOT FEAR YOUR AFM!!! It is a ridiculously simple contraption. I've read dozens of posts from people warning others not to open or adjust your AFM -- black Voodoo magic inside. I say BS. Tear one apart for yourself and you'll see what I mean. The thing is basically a box with a spring-loaded flap on the inside. Attached to the top of the flap is a variable resistor which has a number of positions on it.

The important thing to know about the resistor is that it is not infinitely variable between its lowest and highest values. I think it has about 10 possible values it can report back to the ECU. It also has a built-in air temperature sensor. I'm sure you've read a few posts about making adjustments to the spring settings inside your AFM. "Loosen it a few notches" etc... All this adjustment is doing is changing how fast the air moving through your AFM will push the little resistor over to a higher or lower value setting for a given amount of air -- which equates to a richer or leaner mixture (theoretically).

I'm jumping ahead here, but it is also important to know that your ECU has the power to adjust the mixture itself up or down as it sees fit. It does, however, have a limit to how much it can adjust the mixture based on the information it is given. As a result, a minor change to your AFM spring control may or may not have any impact on your mixture. The bottom of the AFM is held on by little rivets that are cast into the assembly. If you take a little Dremel tool (grinder) you can grind these little nubs off yourself. Carefully pry the bottom off by sticking a screwdriver or something into both openings of the AFM to work it loose.

You can re-attach the bottom of the box with a hot glue gun to place little spots of glue where you ground out the rivets. As for the electrical parts, I tried two different configurations. I tried the BMW components as is, and I also tried the components from my Alfa AFM transplanted into the BMW AFM body. Guess what? They both worked for me. I even tried combinations of my Alfa ECU and a BMW ECU (which I found to work best). You will notice that the BMW unit has more connectors than the Alfa unit. Don't worry about it.

If you want excruciating detail about how to diagnose and troubleshoot the Bosch L-jetronic system and/or AFM, I refer you to a great book called "How to Understand, service and modify Bosch Fuel Injection & Management by Charles O. Probst. Robert Bentley Publishers. ISBN 0-8376-0300-5 To get the larger AFM to fit into the Alfa intake hose, you'll need to heat up the hose. One guy I talked to put the whole thing in the microwave to heat it up. My solution was to boil a large pot of water and submerge the end that I was trying to heat up. With the hose very hot, stuff the AFM into the hose and let it cool down. A little Vaseline will help too. You'll notice from my pictures that I painted the BMW unit black with spray paint just for kicks.

You'll also notice from the pictures that the BMW unit is significantly larger than the Alfa unit. See for yourself. Another important tip that I want to communicate is that you will most definitely need an air/fuel mixture gauge before you try this modification. Buy one and mount it on your dash. They are about $100 and are really easy to install. 12V Positive, Ground and one more wire to splice in to the wire coming from your O2 sensor in your exhaust system. If you think you can safely screw around with your fuel system without some kind of gauge to tell you what you have done to your fuel mixture -- think again. Don't do it. Your original air cleaner assembly won't work any more because the hose won't line up with the opening to the air cleaner. Take the top part of the air cleaner off and throw it in your garage.

Get some kind of padding (like a piece of foam rubber) for the new AFM to sit on top of the bottom portion of your existing air cleaner -- until you get really creative and figure out how to permanently mount the new AFM. There is a K&N filter available (I don't know the model number) that has the exact opening size as the BMW AFM. It costs about $50. I love the added "intake noise." The car sounds faster...

Once installed you will also have to modify your idle speed and your idle mixture. The idle mixture is an Allen screw in the AFM itself. Using your Air/Fuel gauge, let the car idle (at temp), then adjust this screw until the mixture is correct. Hope this is all helpful to anyone wanting to try this modification. It's really very easy compared to other mods. And if you don't like it, you can always put the original unit back on. Good luck and happy motoring!


afm connected to the k&n filter


innards of the AFM - black cap removed

inside details of the afm and flapper removed

size comparisons

size comparizons