V2Rocket

My car ran with the MAF/runs with the MAP currently using the stock 944 airbox, from the snorkel in the fender through the stock paper filter...although I did use a cone with a large velocity stack at one point, and the sound was great

The AFM is not even close to being part of a 'variable inlet system'...those systems usually have multiple intake runner lengths and the flaps open/close to force the air to travel a certain distance based on the RPM range being operated in. Even the 928S4 has such a system in the manifold.

And while the velocity through the pinched straw may be higher, at a certain point the mass-flow through the restriction will peak and taper off, whereas the open straw will allow much more flow for a lower effort. Pumping losses, perhaps?

And yes, the AFM is worn...so are all the rest of them


On the drive in to work I just remembered that this video exists...taken from inside a 944 airbox. See how most of the time, the door is mostly closed?

924srr27l

Yes looks interesting, sounds like a 944 Turbo?


For sure the door is only opening with higher throttle applications as the clip illustrated when several gears were shifted at higher rpm's and maybe full or 3/4 throttle, the rest of the time it sounded like it was slow street driving in traffic..?


There is a bypass even when the door is fully shut. and I cant' see why this controlled air inlet system wouldn't be seen as a type of variable air inlet, only that's exactly what it's doing...


It's an evolution of the twin choke carburettor..


Many modern engine including Porsche's have variable inlets and / or flaps and valves etc... It provides the characteristics for all street driving conditions...powerful cars can become much more docile and drivable, and efficient (Economy) with these devices...


For Racing is a different scenario which depends on the application.


R

V2Rocket

The AFM bypass is for idle...see below. It is a narrow slit! And notice #8, the adjuster screw with which you can "tune" the idle fuel/air mixture if you have an idle emissions test.

It does not function as a variable intake in any way, shape, or form.
It is ONLY an air-measuring device for a certain range of airflow (based on throttle condition and RPM)...until it is wide open and doesn't measure anymore


I mentioned this once before...the AFM is an electronic adaptation of the earlier Bosch K-jet (aka CIS) mechanical fuel injection system, as seen on the 924, early 928 and 911 among other things. A big flapper pulled open by air flow demand (incoming air mass, actually) from the engine used to determine the proper fuel quantity...

924srr27l

Yeah I understand what your saying, this device is not classed as, or is called by Porsche a variable Intake system, but as you say an incoming air measuring device, however I'm just saying as this tract is closed from idle and then it opens according to load / vacuum the hole / air gap is changing and is varying the amount of incoming air volume..


As I've mentioned many other systems on other cars create similar characteristics, EG the inlet volume is changed with engine conditions..


Bosch and Porsche will be aware the door has an effect of reducing and expanding the amount of air that's coming through and you've mentioned before the dme has pre programmed info to adjust this..


The 8+ hours of Mapping that I was watching involved the mapper changing lot of numbers on files, watching the afr monitor readings, listening to the engine note, watching 3D graphs and changing the ignition timing and many other things (That are beyond me)throughout the full rev range..


I had my AFM checked and serviced by a UK company and the results were it was within spec.


R

Ish_944

Roger, I think you misunderstand the essence of a variable intake system. It changes the length and/or diameter of the intake tract, which the AFM does not.
https://en.wikipedia.org/wiki/Variab...ntake_manifold

About the AFM:
OK, so your mapper managed to get (almost) the same numbers out of the stock system as one could get with a newer one. Fine. It works well at given atmospheric conditions, at a given air temperature, etc. Once you change those, or your injectors get a bit dirty your DME won't know about it and won't be able to adjust. Or do you have a lambda sensor? Knock sensors?
(Due to the small injectors and low fuel pressure I'm guessing you're running rather lean at high rpm. But that's a different story.)

By the way, your "higher air flow velocity due to the reduction in cross-section by the AFM's flap" argument (in one of your earlier posts) is completely flawed, because that only increases the velocity inside the AFM. The cross-section of e.g. the intake runners or the throttle body remains the same, hence the velocity there is not affected.
Actually, this is what a variable intake system does...

So the lower cross-section inside the AFM introduces a so-called orifice loss due to the local pressure drop. (It's discussed in every basic fluid dynamics class at university.)

924srr27l

Yes, so as you've suggested : The flap changes the diameter and the amount of the air coming in which is governed by the engine rpm..


" OK, so your mapper managed to get (almost) the same numbers out of the stock system as one could get with a newer one."


What numbers, you mean the AFR ?


It'll sound Cliché but not many people here in the UK, do what this Mapper does..


R

Ish_944

If changes = restricts, then yes.
However, a variable intake system does something completely different, which is what I was trying to explain. The flap does not change the runner length nor the the air velocity in the runners, hence it is not a variable intake system...

I meant hp & torque.

924srr27l

Yes I understand a variable intake is a change in length of the inlet tract, I was just referring to any AFM flap system as a change in the air supply orifice, it's constantly changing with engine load.


You could also refer to the throttle butterfly as a "restrictive" device as when closed it restricts air to the engine! but as it's opened by the operator the AFM responds accordingly and also opens according to all the calculations from the Bosch designers....


-------------------------------------------------------------------------------------
"Air-flow measurement
The air quantity inducted by the engine is an exact measurement of its operating load. The sensor flap in the air-flow sensor measures the entire air quantity inducted by the engine, thereby serving as the main control quantity for determining the load signal and basic injection quantity. On the basis of the measured air quantity and the engine speed input, the microcomputer calculates the optimum ignition angle and corresponding injection time. Like the ignition angle, injection time is adjusted to specific operating conditions. Air-flow measurement takes into account a variety of changes that could occur in an engine during its service life, such as wear, combustion chamber deposits and changes in valve clearances. Because the inducted air first has to pass through the air-flow sensor before reaching the engine, the resulting signal from the air-flow sensor can be taken into account before the cylinder is actually filled; this makes for optimum mixtures at all operating points during load changes.

Air-flow sensor
The air-flow sensor works on the flap principle, measuring air throughput very precisely and delivering the signal for air quantity per unit of time to the control unit. The principle used here in the air-flow sensor is the measurement of force exerted on the sensor flap by the inducted air passing through it. A helical spring exerts counter force on the flap. For a certain flow value, the flap assumes a certain deflection angle; along with an increase in this angle, the effective cross sectional flow area also increases. To minimize the influence on the flap of fluctuations in the intake system caused by the individual cylinder intake strokes, a compensation flap is connected rigidly with the actual sensor measuring flap. The result is that these fluctuations affect the compensation flap equally but in the opposite direction, so that they cancel out and have no effect on the air-flow measurements. When deflected by the inducted air, the air-flow sensor flap moves the slider of a potentiometer, which translates the flap angle a into a corresponding voltage signal UP and transmits it to the electronic control unit. The potentiometer’s wiper track is composed of eight high-resistance segments, for each of which there is a parallel connected low resistance resistor; wiper track and resistors are calibrated so that the voltage UP increases with increasing flap angle.
The potentiometer of the air-flow sensor is of the thick-film type, with a ceramic plate base. Its high-resistance wiper track as well as the two contact rivets on the wiper itself are of an extremely wear-resistant material. The resistance are connected to the wiper track by very narrow conducting segments, the so-called data points. These data points terminate under the wiper track and thus define the specified voltage steps. In the Motronic, the wiper-type potentiometer is constructed so as to ensure that there is a linear relationship between inducted air quantity and slider output voltage. The high temperatures and abrupt temperature changes that always prevail in the vehicle engine compartment have no negative effects on the behavior of the thick-film resistors. In addition, in order that potentiometer aging and temperature fluctuations do not affect the accuracy of the measurements, the control unit evaluates only resistance ratios. An adjustable bypass diverts a small quantity of air past the air-flow sensor flap. Variations in this bypass air quantity, which is independent of the sensor-flap position, are applied as the basis for correction of the air-fuel mixture for idle adjustment. In the Motronic system, the control unit switches on the fuel pump by way of an external pump relay; for reasons of safety, no fuel is pumped if the ignition is on and the engine stationary. "
from 'Automotive Electric/Electronic Systems ©Robert BOSCH GmbH, 1988'





R

Ish_944

This is where we have the issue! The butterfly valve is a restriction, yes. Your right foot controls it.
Let's say that you're cruising at 2500 rpm. Now your butterfly valve is partially open, restricting flow so that your engine does not rev higher.
Suddenly you want to overtake somebody as fast as possible so you floor the accelerator. Now the butterfly valve is fully open but the afm flap is not, still restricting flow, so your engine cannot rev up as fast as it would be without the flap.

The AFM
- under 4500 measures air flow and restricts air flow
- above 4500 measures nothing and does not restrict air flow.

Now imagine a system that can measure airflow at any rpm without restricting the flow! Those are called MAF/MAP.

Sure, the AFM is better than a carb but is not the ultimate solution.
Sure, with a lot of work you can tune it pretty well. The problem is that they are 30+ years old and expensive to replace. So why not get a more accurate and feature rich system for that price?

924srr27l

Pity your not close, or anyone else for that matter....


Pop across to the UK,.... jump in the seat of my 924SRR27L and I'm 99.9% sure you'll never slag an old system again in your life!


The words "Restricts" accurate and problem don't feature !


It's all works exceptionally well....


You as many have bought into the idea it's terrible and the buzz words as you suggest "feature rich" and "Modern" have lured you in this direction...


R

924srr27l

Yes, this sounds contradictory ?


The smaller the flap opening the higher the velocity in the AFM, it's like a variable orifice for the air to enter, which is always dictated by the buttery's position further up the tract....


So if the AFM was taken off completely the size of the hole would not change, and this Motronic system has been designed so that the ECU works with the varying gap, and calculates the fueling and ignition accordingly.


It measures temp and air flow, but also varies the inlet flow for better torque as the flap shuts when the engine is off load.


I or anyone else will never convince you and the other minority of doubters that's it's very good and does a far better job than it's reputation and poor PR from a handful suggests.


R

Ish_944

Hehe, I'd love to drive your car! I'll surely get in touch next time I'm in the UK.

What "lured" me was my constant idle issues. It would randomly not idle anymore for days or weeks, of course mostly not in traffic jams. I got fed up and threw it away for a working system which gives me more control, too.

To give you examples: I use the stock ignition, throttle body, complete intake tract down to the cheap panel filter (not even a K&N yet). I don't have cross-drilled brake discs nor a huge diameter exhaust. My engine bay looks more stock then 99% here.
I have, however, installed Turbo valve springs and injectors for good measure.

So I don't think I easily buy into the new stuff. If I was after modern I wouldn't have had bought a 28 year old car without ABS & power steering to begin with...

924srr27l

OK, I'm assuming your Idle issues were on a total stock car before you changed anything and bought the ECU Upgrade and AFM delete gear from AA ?


I've had (7) 2.5 8v N/A Cars all with no Idle issues so whatever was wrong with yours could of been fixed as these car's are NOT known for having such issues....


My new Build would not idle at all, but that's because the ECU needed setting up for all the changes, after an hour of Mapping it's now perfect.


You could always change at any point and fit all the stock items back on the car even get a stock ECU from Ebay and then have live mapping as I did.


However, I was lucky and put on to the best in the UK to do this, and It's a rare thing to find many people that will do this, partly due to lack of expertise & knowledge, the equipment required and the patience.


Not many places would spend 10am to 8pm on a Mapping session!


R

Ish_944

It is not. The velocity increases inside the AFM which causes a loss due to pressure drop. But nothing else.

On the other hand, a variable intake increases velocity in the intake manifold's runners and thus in the head and combustion chamber. The higher velocity there increases turbulence, thus mixing, which in turn results in a better burn and lowers the chance for knocking.

Moreover, the flap is not connected to the butterfly valve, it opens with higher air flow. So when you floor it, the butterfly valve opens immediately, while the flap will only slowly follow as airflow builds up. Hence it causes a lag. This is why people with a MAF/MAP report improved throttle response.

Better torque is achieved by tuning the length of the intake system.
If you shorten it by connecting your afm to the throttle body directly and put a cone filter over the afm you'll lose low-end torque. The afm doesn't influence this.

You want to give a feature to the afm that it was never intended to have. The flap is there merely to measure air flow.

I'll accept that it can be tuned pretty well and a MAF/MAP will only have a small advantage if everything else is equal. However, the afms are dying due to old age. Moreover, Rogue tuning has shown how different voltage curves good ones have. So once you have to replace yours, your tune will be off.

924srr27l

We'll agree to disagree that the AFM is dead!


If I ever to decide to make any changes in the future, it'll either be a head change to the 2.7 bigger valve head and a Turbo, or a 3.0 Twin cam head and throttle bodies...but none of this may never happen.


R