Sorry to start like this, but this post contains information on an important safety system in the BMW. I am merely posting my observations and assume no responsibility for any other individual's use of this information. In other words you are accepting full responsibility if you read past this line. 



Are you still there?

Okay, now for the good stuff. There has been a lot of discussion about the ASC throttle body on the message boards. A few famous vendors offer over-sized main throttle bodies for the 3.2L M engine and there are questions as to how this "bored-out throttle body" works in a car that as a second throttle body used to control the ASC. In the name of science I ripped my ASC apart to get a better look at the ASC throttle body.

In order to look at the intake tract, lets imagine an air particle who gets a ride through the engine.  The air particle starts the journey by being sucked through the airbox (stock, Fogged, Confortied, ECISed, Dinaned or whatever). The airboxes for BMWs are the specialty of Saint Fogg... so I will leave them out of my discussion.

Once through the airbox an air particle must then pass through a device known as an HFM. The HFM measures a small portion of the air to extrapolate the total amount of air entering the engine. I'll save the conversation on laminar flow for another day, but I did get some good pics of the HFM I have on my car. It is a euro-HFM that is .5" bigger then the US HFM (88.9mm vs. 77.2mm).  Note that running a non-stock HFM requires software modifications (in this case Dinan Stage 4 or 5) because with a larger HFM there is a lot more air flowing through the HFM when the sensor samples the same amount of air (Remember the HFM is bigger...  so while the sensor sees same amount of air there really is a larger amount of air traveling through the HFM).
HFM - Notice the sensor in middle it measures the amount of air at just the sensor.  Assuming a specified shape of airflow, the software can calculate the total airflow.  Change the shape of airflow and you need new software Stock HFM on the left and euro-HFM on the right.
Side view of the euro-HFM on the left and stock HFM on the right

After the HFM, an air particle hits the ASC throttle body on '99+ M Roadsters and Coupes.  The ASC throttle body is used by the ASC system to reduce engine power in a traction control event by closing the throttle plate.  The ASC throttle body has roughly a 2.5" plate which is 63.5mm.  The following pictures show how the ASC throttle body would look to the air particle just as it has left the HFM.

The view of the ASC throttle body that an air particle would see.  The throttle plate and swivel is in the way! A different view of the ASC throttle body.  You can see the regular throttle plate beyond the ASC. 

The next step for the air particle is the regular throttle body.  This is the throttle body that opens when you "step on the gas".  The stock throttle body has a 64.5mm throttle plate.  The throttle body pictured here is a Dinan throttle body.  Dinan enlarges the bore and plate to 66.5mm.  As far as throttle body modifications go this is a relatively mild modification.  The better machine shops will shave the swivel down, countersink the screws and sharpen the edge of the plate to a knife edge for better flow.

Dinan Throttle Body

So now we have a dilemma.  The HFM is 89mm (but has a large sensor in the middle), the ASC throttle body is only 63.5mm and the throttle body is 66.5mm.  Air is going to be restricted by the smallest part of the intake tract... which is the ASC throttle body.  So why would you buy a bored-out throttle body if the ASC throttle body is still the stock size?  I don't know the answer to that... but be sure to ask the famous tuners before you plunk down the change to buy a modified throttle body.

Having seen the '98 M Roadster setup in John Huestis' car I decided to tinker a little bit.  The '98 M Roadster did not have traction control.

The ASC unit has two connections, one Bowden cable and one electrical connection. The connecting elbow has one tube that comes out the bottom... it looks to be the same as on the non-ASC elbow I have seen in the ETK and on John's car.

First of all, the car will run with the ASC electrical connection disconnected without setting a MIL (check engine light). It appears to disable the traction control system entirely because the ASC light comes on, but I have no way of knowing for sure.  You are definitely operating at your own risk at this point!

It is probably a good thing that the ASC system shuts off if the ASC throttle body is off because if I swap out the ASC throttle body with a rubber boot I want ASC to be off so there is no weird interference with the braking aspect of the ASC system (ASC can help in a traction control event by reducing the engine power and/or touching the brakes).  Note that after I re-assembled the system, the ASC light is still on...  I probably set some kind of fault in the ASC system.  I'll post details when I figure that out.

The ASC throttle plate attaches with a roughly 3/8" wide full length swivel (you can see it in the pictures above). So at best case there is a 3/8" blockage that runs the diameter of the opening. Considering that the plate is actually at an angle and air probably doesn't flow exactly down through the bore it is most likely worse then that. The plate can easily be removed with two screws.  This is an interesting mod because you can keep the ASC unit in place (maintaining a stock look and keeping the ASC light off) and remove the throttle plate. You can judge for yourself if this will increase flow by comparing the picture of the ASC throttle body sans plate and screws with the previous pictures of the ASC throttle body. I personally don't like this because I worry about weird by-products of disabling the throttle ASC function and not the brake. But it might not be an issue and will look better to a dealer at service time.

ASC Throttle Body ASC Throttle Body With Plate Removed.

The idea I am going to pursue is to remove the ASC throttle body totally and switch to the '98 M Roadster rubber elbow.  It looks like it will be an easy retrofit (that is said without having the piece in hand). I have on on order and will update the page when I have more information. The elbow will most certainly provide the best flow because of it's larger size (the ASC throttle body still has the smaller 63.5mm bore) and total lack of a plate swivel.

For now I would have to recommend that you don't buy an enlarged throttle body if you have the traction control system.

Update - ASC-Less Rubber Boot Installed

Thanks to the generosity of Jae Sung, another'er I was able to get a '98 M Roadster boot. The install was very straight- forward - it was a perfect swap for the ASC rubber boot and throttle body. The traction control light is now always on. I tried to observe additional airflow using Vehicle Explorer and MAF measurements (airflow at the HFM). The problem is that my car appears to be maxing out some sort of software-induced airflow limit of 26.9lbs/min. So I can't directly plot before and after airflow curves because the curves flat line at about 5500 rpms.

The next best idea was to monitor my long term fuel trim to see if the car is running relatively richer. While it appears that the car does now have a tendency towards richer mixtures, I can't say for sure that difference is from the mod... it could be from any number of other environmental factors.

So ultimately I don't know if this modification makes a single horsepower. But it does provide a much cleaner air flow path. And I don't have to turn my damned ASC off evertime I start the car! I will try to get a dyno done with and without the ASC in place to see if it makes any difference, but no guarantees that I will have the time to do it both ways.

Intake System Without ASC-Throttle Body

Taken from