poormans9fitty1

I am very confused. On the 951s, do they have a self adjusting a/f ratio? What I mean is if you put on a cone filter instead of the stock box, will the car run lean because the cone brings in much more air? Or will the comp measure that and correct the a/f ratio? OR if i put on a maf will it correct that because it is such a large amount of air? Also, with a fpr, does that correct the fuel for adding on a maf? Sorry, i am very confused. Any info would be geat thanks!!

Bones944

A MAF uses it's own software in conjunction with the DME to control fuel ratio. Custom mapped chips are recommended for YOUR unique setup with a MAF sensor, but you'll also have to fine tune the ratios using the MAF's software. That's the easiest way I know to explain it though it is much more involved! There has never been any conclusive evidence that a cone filter in place of the stock airbox offers any additional performance, and may in fact hinder performance due to the intake of hot air from the motor compartment.
I'm sure much more detailed posts will appear in this thread shortly!

Charlotte944

OK, class is in session.

During normal "closed loop" operation the DME uses the AFM (or MAF) signal to determine engine load, and then uses engine load to adjust injector duty cycle (how long the injectors are open) to maintain an average Air/Fuel ratio of 14.7:1.

Any change in the engine load signal regardless of what caused the change will cause the DME to change injector duty cycle.

For example, if you have a vacuum leak (which is always downstream of the AFM), the engine will run lean because the DME does not know about the additional air flow.

AFMs and MAFs do the same thing. In theory, by replacing a "restrictive" AFM with a "less restirctive" MAF you should see some additional ponies, and that is true up to a point.

The problem I have with the "K&N" style filter is why it is less restircitive: The holes in the filter media are bigger and there is less surface area than in a stock paper filter.

You see, the size of the holes in the stock filter media is based on how big a "particle" of "dirt" can be without doing any internal damage to the engine. Any particle larger than that size should be kept out of the engine. The surface area of the filter is based on how many holes of a given size you need in order to flow a given amount of air. The rest is just simple math.

OK, now for the FPR.

This little goodie is very simple. It maintains fuel rail pressure within a specific range. This is necessary because the DME calculates injector duty cycle based on injector flow rate which is a function of rail pressure and the designed flow rate of the injectors.

led

I don't see how changing the filter will cause the engine to run lean. It's the engine that pulls air in and the afm/maf/whatever will/should measure the amount going in regardless of the restriction caused by the filter.
just my 2 cents.

ninefiveone

There's no measurement going on at WOT. It runs off tables. This is why custom chips are needed for different setups.

jimbo1111

Posted by Ninefiveone:
There's no measurement going on at WOT. It runs off tables. This is why custom chips are needed for different setups.
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I don't think this is true because a piggy back changes your fuel curve at wot by altering the voltage output of the maf.

In regards to replacing the stock filter with a cone. There will be no differents between the two. In fact most believe that a cone will harm hp because it draws hot air in from the radiator. The main restriction is the flapper door on the afm. That is why most go to a maf/map

ninefiveone

True, a piggyback will change the fuel curve at WOT but it's still running off tables. So far as I know no piggyback is running closed loop (or is it open loop?) at WOT because they're still using the stock DME to handle engine management. So the shift in voltage merely causes different cells to be looked up.

So you obviate the need for custom chips by using a piggyback (or other tuning device) to "fool" the DME into the desired response. In a way, you're creating your own custom chip.

In any case, it's still table lookups.

Ben Z.

Wouldn't it be awesome if you could just wire in a wideband sensor (and/or egt) to your computer and it would adjust fuel/timing based on the values it sees from the wideband/tps/engine speed/inlet temp? Maybe some fuzzy logic so that it would learn the perfect settings after time. You could program it to make optimum power when at full throtle, etc. Maybe have some conservative tables to fall back on if a sensor fails. I realize I probably don't know enough about this stuff...

Charlotte944

The Porsche DME will go to open loop control when the O2 sensor signal is invalid: Bad sensor, cold sensor, WOT, and when you jumper pins B and C on the diagnostic connector.

The DME is in closed loop when the O2 sensor signal is valid and the throttle plate is NOT at WOT (part throttle operation).