2008-09-21 - UK - Pics From A Very Sunny Santa Pod

[TerryY]

Here are pictures of the factory straighteners used for the V8. I believe that the M70 screens are around the same diameter as the standard 6 cylinder screens but I only have M60 and M62 screens in hand.

One picture shows both an inlet and a part of the larger opening outlet straightener. The other picture is a closeup as straight on as possible of the inlet straightener that the Gizmo replaces.

The adaptive reset is done by disconnecting the connector of the ECU for a few seconds. All of the adaptive values are reset as well as any stored error codes. The engine will re-adapt in a short period of time -a few minutes and a few miles.

[martinw]

The Gizmo has certainly got to be less restrictive than the factory item shown ! and with the rear screen removed ....

I don't have any background in airflow but this has got me thinking ...
Is it a common thing to have an air straightener/screen somewhere in the inlet behind the airfilter ?
If so, what is it doing exactly ? I could imagine a straightener perhaps being necessary to reduce the "swirl" of the airflow, but this is after the air has passed through the airfilter. Is there going to be that much swirl there ?
If so, then to straighten the airflow, one would expect the depth of the fins in the straightener to have to be longer (certainly longer than the factory item) to have any effect ?
If its intended more as a coarse strainer/filter - whats the point ? The air has already passed through a far better filter (the air filter).

Perhaps there's more to this than meets the (uneducated) eye ?
Has anybody tried just removing the front and rear screens altogether (no "straighteners" at all) ?

[TerryY]

It takes 12 times the diameter of the pipe for air flow to naturally straighten after a 90 degree bend. Until that distance there is a steadily diminishing amount of turbulence and swirl in the pipe.

The AFM is measuring the speed of the air as it passes that point. The computer knows the area of the pipe and the speed data allows it to calculate the total volume of air flowing in the pipe as a function of time.

The MAF uses the same data but adds the "weight" of the air to calculate mass over time rather than volume.

The distance to the meter is far less than 12 diameters but the turn is fairly gentle and the air flow is measured on the side of the tube. The sensor itself is fairly long so it is averaging over the wire length. The side of the tube will be the most stable area so it should need a lot less help than the stock straightener. Hence the Gizmo which is a truly minimalist piece of "art".

[martinw]

Thanks for that Terry ! Very interesting.
Wow! I knew I needed educating but .... just shows the average guy's ignorance on the subject.

I hope the company selling the Gizmo give a similar explanation of the benefits otherwise the average owner could easily ignore this useful (and well designed) device.

So, straight pipe runs to the AFM would be a good thing ? Difficult to achieve though, and maybe more expensive than the Gizmo.

[TerryY]

Fun stuff ? It is amazing what you have to learn when 80,000 Cubic Feet per minute of building exhaust from a 135,000 square foot building comes up inadequate for point of use applications. Then you learn how to cheat with what you have

Yes a straight tube would be ideal before the meter.

The next problem is the flex tube angle into the DK motor on the 850 and then the throttle plate in the DK.

The 840 has a secondary throttle plate for the traction control and then the main throttle plate after that.

The total system flow can't be better than the flow thru worst spot so there are several points of restriction any direction you go. The usual fix is to oversize everything and let the flow be "lazy" rather than to try for "optimal".

I just thought that a smoother less restrictive flow thru the point of measurement can not hurt.

This kind of thing is why the new BMW engines have no throttle plates and use the valve opening to control the air flow into the engine.

[martinw]

Interesting you say the flow cannot be better than the flow through the worst spot....
Had a quick look at the AFM on mine today and measured the screen diameter at approx 67mm.
Later was looking at the moulded plastic inlet air tube behind the grille in the nosecone. the start of the tube is at 90 deg to the direction of the air flow into the nosecone (not ideal, would have thought) but I was also surprised at how small the start of this tube was (and roughly square in shape) - difficult to measure but could only just get two fingers in (!). Gotta be a smaller area than the entry to the AFM.
Not good if the airflow is restricted right at the point of entry - or am I making another uneducated assumption ?