Air Sensor Plate Height - Why?

[Funracer]

Starting a new thread because I don’t want to interfere with the other sensor plate thread that is happening concurrently.

Can someone explain to me why the sensor plate height setting is so critical? Not saying its not critical, I just don’t understand why.

Thanks

[gled49]

If the plate isn’t t centered in the Venturi, and the plate drop under pressure in spec, false air, not being measured, will throw off the fuel mixture. Not to mention the plate sticking in the housing. The amount of air moving the sensor plate is how the system works.

[Schulisco]

The sensor plate height defines the way the lever can make and related to this it defines the amount of fuel being injected. Saying that - the lower the sensor plate is adjusted, the more way it will travel based on the same amount of sucked air. In other words: the mixture gets richer on lower rpms and over the whole rpm band. On the other hand: the higher position of the sensor plate , the more lean the mixture will be, because the lever has less way to travel.
This relates directly to this is the basic adjustment setup of the CIS : If the basic setup of the CIS (my personal wording is "injection begin") is not correct, this also affects the mixture in the same way, but in finer resolution: The CO screw is basically nothing else than a fine tuning mechanism for the sensor plate height. The goal is now to find the right point that the injection at the cylinders begins precisely exact then, when the lever is barely lifted by the sucked air of a) the starting engine or b) to keep the mixture especially on idle as lean as possible and as rich as required.
The rest is pretty easy because there's nothing more to adjust, because every component of the particular CIS system is adjusted to the engine. The curve which defines the fuel amount to be injected is "hard coded" in the air flow sensor housing and its components (fuel distributor, sensor plate housing, venturi size etc) to match the engine's individual requirements.
This explains why you can't simply replace a fuel distributor from on a 911 SC with one from e.g. a Mercedes. The curve of injected fuel according to the sucked air is mechanically hard wired in the mechanics of sensor plate housing, fuel distributor, injectors etc. This is a math function which in todays systems programmed in a ECU.

You can imagine the way the sensor plate lever travels (and causes a particular amount of fuel being injected) as a "sliding window". This window is simply the direct relation of lever travel and fuel amount. This is basically not linear, because the mixture requires to be richer on higher revs. If the window is either too low or too high, the mixture always is partly correct only, but never over the full rpm band! This explains why some engines then will start good, but having poor performance and really bad fuel economy. Others start really bad, but run better, have bad low rpm behaviour etc.
On top of that we have the WUR (and AAV) to enable cold start and running which enriches the mixture by lowering the control pressure. And on US SCs from 1980 model year on we have the lambda control, which also influences this "mechanically programmed curve" to make the catalyst working.

Thomas

[Funracer]

Originally Posted by gled49

If the plate isn’t t centered in the Venturi, and the plate drop under pressure in spec, false air, not being measured, will throw off the fuel mixture. Not to mention the plate sticking in the housing. The amount of air moving the sensor plate is how the system works.

I know and understand that the plate has to be centered. Only asking about the adjustment of the plate height at rest in the Venturi.

[spuggy]

Originally Posted by gled49

The amount of air moving the sensor plate is how the system works.

^^^ This, what he said.

Known quantity of metered air needs to move the sensor plate the correct/anticipated amount - because the plate moves the fuel distributor piston, to which it is directly (mechanically) connected. Otherwise, incorrect amount of fuel delivered.

A misaligned sensor plate with too large a gap will allow air to pass without moving the plate as much as it should. Lean mixture, stubborn high idle - or both. Perhaps even binding/wear in the venturi.

If there is a safety cutoff fitted and the plate doesn't sit where expected to activate the switch, this can also result in "fuel pump doesn't switch on when motor runs".

[Funracer]

Originally Posted by Schulisco

The sensor plate height defines the way the lever can make and related to this it defines the amount of fuel being injected. Will say - the lower the sensor plate is adjusted the more way it will make based on the same amount of sucked air. In other words the mixture gets richer on lower rpms over the rpm band. The opposite is the higher position of the sensor plate will lean the mixture because the lever has less way to travel. According to this the basic setup is directly connected. If the basic(my personal wording "injection begin") setup is not correct, this affects the mixture in the same way but finer resolution: The CO screw is nothing else than a fine tuning mechanism for the sensor plate height. The goal is to find the right point that the injection begins precisely exact then when the lever is lifted by the sucked air of the starting engine (on engine startup) or to keep the mixture especially on idle as lean as possible and as rich as required. The rest is easy: The CIS and the setup of every component is "hard coded" to match the engine's requiremnents. Will say you cannot exchange freely a fuel distributor from e.g. a Mercedes 6cyl engine with 911SC one.
You can imagine the way the sensor plate lever travels as a "sliding window". If the window is either too low or too high, the mixture is partly correct only, but never over the full rpm band! This explains why some engines will start good, but having poor performance and really bad fuel economy. Others start really bad, but run better, have bad low rpm behaviour etc.

Thomas

You are talking about the CO 3mm mixture screw moving the FD plunger. I have a good understanding of how this effects everything. It affects plunger height in the FD at all times.

I am asking about the AFSP zero position at rest prior to start. This is altered by the 7mm nut and screw underneath the FD inside the air box. Porsche says the edge has to be level with the venturi or up to .5mm below.

After the start the plate floats in the air well above the screw so Im not sure how the “zero position”, as its called in the Bosch book, effects a running engine. My thinking is that the sensor plate height (and really, we are talking about the position of the plunger resting on the arm) effects the initial start for a second or two and then has no effect.

Having said that, the AFSP height at rest IS important. But only to put the plunger in the right place for start. After that the plate moves with the air column.



This screw changes the resting height of the plate. I have moved it all over the place and have never noticed any change in the AFM or how it runs after start except at the very extremes.

[E Sully]

It has no effect once the engine is running. It's the zero point Bosch used for the base position a in figures 12 and 14 when the engine is off. The air plate is at the narrowest point, and the fuel distributor piston is blocking fuel flow.
If the plate were set above that position, fuel could flow with the engine off due to a higher starting point of the plunger. If set too low then the plate would not be at the narrowest part of the venturi, it might leave a gap allowing for air to bypass the plate when starting, perhaps preventing the plate from lifting during startup.
The screw (4) on the small arm adjusts the fuel distributor piston to get the best fuel air mixture when the engine is running.

[Schulisco]

Originally Posted by Funracer

You are talking about the CO 3mm mixture screw moving the FD plunger. I have a good understanding of how this effects everything. It affects plunger height in the FD at all times.

I am asking about the AFSP zero position at rest prior to start. This is altered by the 7mm nut and screw underneath the FD inside the air box. Porsche says the edge has to be level with the venturi or up to .5mm below.

After the start the plate floats in the air well above the screw so Im not sure how the “zero position”, as its called in the Bosch book, effects a running engine. My thinking is that the sensor plate height (and really, we are talking about the position of the plunger resting on the arm) effects the initial start for a second or two and then has no effect.

Having said that, the AFSP height at rest IS important. But only to put the plunger in the right place for start. After that the plate moves with the air column.



This screw changes the resting height of the plate. I have moved it all over the place and have never noticed any change in the AFM or how it runs after start except at the very extremes.

My explanation on that:

The lever is not just a simple lever, it's more like a control arm system consisting of two levers. Furthermore both levers are not simply statically 1:1 connected. There is a variable gear ratio being set by the CO / idle mixture screw! Additionally the cone of the funnel also implements a non linear behaviour for the delivered fuel amount. See a picture from the yellow technical instruction to the CIS from Bosch:



The rotating point, where the control arm system hangs in the sensor plate housing is the red marked #8. This point is also slightly out of the center of the fuel distributor plunger to be able to lift it. #5 is a pivot outside that rotating point #8, which movably interconnects the levers #6 and #9. #10 is the counter weight for the sensor plate to balance it's weight for the air draw.
Basically you're right - the CO / idle mixture screw mainly sets the idle mixture. But at the same time the screw also sets a transmission ratio (with narrow limits) for the control arm system! When the air draw moves the lever system, it moves as a single lever. I suppose Bosch was forced to do this to split the mixture enrichment into two parameters to realize the altitude correction: first is the travel of the control lever by the air draw and second is to detect the air density and get it into the equation (which decreases on higher altitude). That transmission ratio is fixed after being set by the idle mixture screw / CO screw.

If now the sensor plate height is out of spec this will also bring the gear ratio of the control arm system out of range - with the consequences I described above.

Thomas

[Funracer]

Originally Posted by E Sully

It has no effect once the engine is running. It's the zero point Bosch used for the base position a in figures 12 and 14 when the engine is off. The air plate is at the narrowest point, and the fuel distributor piston is blocking fuel flow.
If the plate were set above that position, fuel could flow with the engine off due to a higher starting point of the plunger. If set too low then the plate would not be at the narrowest part of the venturi, it might leave a gap allowing for air to bypass the plate when starting, perhaps preventing the plate from lifting during startup.
The screw (4) on the small arm adjusts the fuel distributor piston to get the best fuel air mixture when the engine is running.

Outstanding explanation with illustrations thank so much. Thanks too for defining the two extremes- too high would allow fuel flow too soon, too low the plate would not lift.

So on to my next question. There is a long ways, relatively speaking, between those two extremes. I’ve been to both ends. Off the top of my head over 1/4 inch between high and low (6mm) and maybe more. About 8-10 turns of the screw. The engine starts and runs the same as far as I can tell over a pretty wide area in the middle of that range.

It’s clear why Porsche would not want the plate too high for safety reasons.

Porsche says the plate can only be .5mm below the venturi edge. Anyone know where this lower limit comes from?

Thanks to all for the interesting discussion so far.

[fanaudical]

I believe that lower limit is to make sure the plate is centered in the smallest diameter of the venturi. Much lower than that and the venturi gets larger and then smaller as the plate rises. Image the response of the plate to small air flow in that condition - it won't be a nice, linear rise (and you'll get funky small load throttle response).