No, TT is not wrong, his last post was maybe a little difficult to read and understand but perfectly correct AFAIK.
Jimbo, I can be done the way that you suggest but it will not be perfect. Since the transfer function is incorrect for the MAF, the DME will not calculate the correct airflow. It's not just a scaling problem, since a MAF's output is not linear to the flow, the calculation error will be unpredictable. You can somewhat correct this with changes in the fuel maps but it will be a band aid solution. Your AFR will almost guaranteed be off in certain driving conditions and if you for instance change boost you will have to change the maps.

Tomas

 

Since you remove the temp sensor that's in the VAF, you have to replace it with a new sensor. Or you can remove the old one from the VAF and reuse it.
I think the temp sensor is used for cold start enrichment as well as for air density corrections.
If it was only used for air density corrections you could replace it with a fixed resistor when you install a MAF.
Can someone confirm that the cold start enrichment also uses the IAT sensor?

Tomas

 

Ok I see where tt has gone wrong in his calculations. He is comparing a stock afm. When the maf puts out a voltage the piggy back is transferring to a afm signal. But not to the same afm signal that the stock afm would put out. when the signal changes over it dose so much more broadly. For instance. When a maf outputs 2.5 volts it's at half capacity 300 cfm. That doesn't mean that the afm signal out of the piggy back will be maxed out at 5 because the afm maxes at 300 cfm. The equation doesn't work that way.

 

Jimbo, I don't know what to do:

?
?
?
?
?
?
?

or all of the above???

 

This is getting so ridiculous that I'm beginning to think jimbo is just doing this to jerk your chains.

 

Max, you are correct with the caveat about the temp sensor.

Cold start uses the engine temp sensor, hot start uses the IAT.

What other meter was the 951 Motronic programmed for?

To use a piggyback to convert MAF to VAF, it must be converting to some input standard, in this case the stock VAF (Vane Air Flow) afm. Voltage output MAFs (excluding the Delphi frequency output meters) output a 0 - 5V signal that relates output voltage to air mass flow through the meter. Fortunately that corresponds to the range of the stock afm, 0 - 5V. The relationship of output voltage to the air flow differs greatly between the MAF and the stock afm. The stock DME with the stock code is restricted to the preprogrammed flow curve that translates the stock afm voltage to air flow. Again for the purposes of this thread, lets fix the stock DME with stock code to show that the stock afm is flowing 600 SCFM when the voltage output is 5V. Even if it flows more than 600 SCFM, the output voltage cannot go above 5V, it won't happen. So lets create an imaginary MAF and use a piggyback to convert it to the equivalent VAF voltage.

MAF Flow (SCFM) -- MAF Output Voltage
11 SCFM -- 0.4V
200 SCFM -- 2.5V
400 SCFM -- 3.5V
600 SCFM -- 4.5V
700 SCFM -- 5.0V

VAF Flow (SCFM) -- VAF Output Voltage
11 SCFM -- 0.8V
200 SCFM -- 3.7V
400 SCFM -- 4.4V
600 SCFM -- 5.0V

Piggyback input voltage -- Piggyback output voltage
0.4V -- 0.8V
2.5V -- 3.7V
3.5V -- 4.4V
4.5V -- 5.0V
5.0V -- ???V

How is the piggyback going to convert the MAF voltage for flows >= 600 SCFM to the equivalent stock afm voltage for the DME to interpret?

 

Well, what I dont get is that he asked for people to shed some light, but then refused to believe anything anyone else said.

 

Ok tt you seem to forget that porsche had that limitation already. That is why they added a raise rate fpr. Remember that the dme is almost identical to the na. The only thing added was a klr. What I was saying earlier was that you don't have to max out the afm signal to achieve what i'm after. If you scale back the large injectors when needed. You will achieve the same results. I have seen it done with my own 2 eyes. Perfect a/f and timing. You are using air to do it I'm using fuel.

 

I smell a dyno shootout in the near future.

 

This all makes complete sense to me...but than again I don't know anything about this stuff!

Thanks for all the in depth explanations TT.

It is my understanding that systems built by SFR and Lindsey use MAFs as I described earlier. They are calibrated to mimic our exact AFM (VAF) so that AFM chips can be used. So a 150cfm = 3.0V for both the AFM and MAF (made up number). It sounds like this type of kit is pretty adequate until you hit the upper limit of the flow of the MAF which is limited by this calibration. They could use a 2 ft diameter MAF and it wouldn't flow any higher.

Vitesse on the other hand, could scale their MAF to the maximum flow the MAF can convert to a voltage simply by adjusting the chips, correct?

Max

Max

 

Correct. With TT's help I am able to support many MAFs..

 

Not sure what you mean by that. Porsche had no limitation, they selected the VAF sized for the HP and air flow requirements they wanted the 951 to achieve. The fpr used for the stock setup is a 1:1 rising rate regulator. It only equalizes the pressure so that the fuel pressure relative to the intake manifold pressure remains constant (2.5bar for the stock setup, 3.0bar for most other setups). You are confusing a 1:1 FPR with FPRs that have a higher ratio. Many aftermarket setups use a higher ratio rising rate FPR to add more fuel based on boost. This is NOT what Porsche did for the 951. There is nothing wrong with that approach and it works with many aftermarket kits.

Yes you can do that, but that is when you SHOULD readjust the 3D fuel and ignition maps because you changed the DME's interpretation of load. Its much easier to change the afm flow calculation and/or change the FQS for the injectors and keep the maps constant.

There are many ways to achieve an end result that will work. The effort involved differs greatly. I can swap out different type MAFs, injectors, and/or FPRs and have it work in a matter of minutes with my approach.

 

TT your approach is good and well thought out but I will throw this on at you.

I will explain it so simply that everybody will understand.

You mentioned in a earlier post that a afm signal could never grow but I will show you how it can.


Lets look at a stock 944t for example witch has a afm. My only mod would be to drop in a set of 55 lb injectors. I just increased the effective range of my afm. You ask how. Because the limit of the afm signal never has to reach 5 volts because I am getting the same fueling 37% earlier. So I just increased the effective range with no other mod. Now if I dropped in a set of 72lb I have increased the effective range 35% above that. Now I have enough fuel and effective range for my amf for over 500 hp. This approach is very easy. All I need now is a set of chips for 55 lb injector specific. This only has to be done once as well. The only problem that comes to play is the barn door on the afm. Witch is taken care of by a maf conversion. Simple isn't it. Now back to the main ? on page 1. Could someone burn me a chip for this configuration? Thanks

 

Jimbo, using larger injectors increases the fuel capacity, that's it. Of course you can trim it with PB to attain correct AFR. This is nothing new, early ARC2 users were foing it. Now How about timing?

You will have to alter the MAF signal to get the correct AFR. The MAF signal indicates the LOAD for the DME.. So the DME is getting the incorrect data (which is a must for AFR with larger injectors), how about timing?


With larger injectors, you will be telling the DME that it's runining at a lower load than actual, and the DME will give you a lower pulse width so you endup with acceptable AFR. Also the DME will give you a timing value for the lower tahn actual load.. This is best noticed as hesitation/stumble at PT (we can hope for no blown HG).

It looks that you are having fun while learning, this is the goal!!!

 

John the whole purpose of this thread is to gather info as to how I can get a chip set made up for this configuration. It's now just a timing issue. All that has to be done is the timing values have to be shifted back 37% to correspond with the fuel. Thats it. No magic dust.