944M3

To be clear, I'm in no way blaming A-Tune for this and I don't know enough to even draw theories. As I've stated, the A-Tune feels good, the car is smooth building boost. Coming from a stock chip and k27/6 (which I had to run for a few weeks while I figured out A-Tune install), the A-Tune was an improvement in drivability. I did try to find if anyone was running A-Tune with a bigger turbo before I installed but wasn't really able to find much.

I do have an WB O2 installed towards the cat (recommended location per LR). I can view AFR via Bluetooth on my phone but I can't log. I need to do some runs with a friend to note the values during a pull. That being said, idle is good at about 14.5 or so and highway cruising is also about 14. My understanding is that those numbers are good. During WOT, I've been able to glance down and seen 11 and 12 at times. But again, I should have a friend with me watching AFR during some test runs.

I've thought about knock as well. I don't have a visual knock signal/alert. BTW, I only use Shell or Exxon 93. New stock fuel pump and filter.

I do think I've heard enough to warrant purchasing a MAF.

divil

This pdf contains a pretty comprehensive explanation of what the KLR does. As John says it responds to knock by pulling ignition timing. But if you have the stock cycling valve setup still in place, then it will also reduce boost in small steps, if knock persists after 6 deg of timing retard. That document contains one misleading bit of info though: the section titled "Precaution Against Excessive Air Charging Pressure" is really describing the DME's overboost protection, not the KLR's.

944M3

Will never stop learning new things about these cars!

You mention that this is how a stock DME/chip would behave. I do remember experiencing the "cut off" with the stock chip as well (using the k27/6). Because this happened on the stock chip as well let's take the A-Tune out of the picture. So it sounds that on long pulls, I'm maxing out the AFM (meaning it can't measure air at WOT after a certain point), so the DME sees the RPMs remain "high" for a specific period of time, it then makes the "guess" that overboost is "happening". Is my line of thought on the right track?

jimbo1111

No two gears will have the same air fuel ratio at corresponding rpms. If say for example you are in third and see 11: 1 chances are in forth you will run leaner under a higher load. if you have a stumble it could be rich or lean, could also be spark related or intake leaks.
You have to first find out if its lean or rich mixture. If its lean then you will need to move up to large injectors. Rich is probably an intake leak.
You did mention you have a cat right? it may be clogged. If so dump it or replace it for a more efficient model. 30 plus hp there.

Mike Goebel

Well hurry up already. hahaha

Mike G.

944M3

Mike Goebel

If the engine is mapped for load vs RPM. It doesn't care what gear you're in. Look up load value vs RPM in table and inject.

Mike G.

fast951

Maxing out the AFM is different than overboost/overload.

The AFM measures air flow and generates a 0-5Vdc signal to correspond with the air flow. The AFM, by design measures a limited kg/hr flow. Ideally, at high RPM and under max boost you want the AFM signal to stay <= 5Vdc. With the K26 style turbos you are fine.
Once you upgrade your turbo to a high flowing one, and you turn up the boost, you can flow much more air than thd AFM is designed to handle. There is a good possibility the AFM signal reaches 5Vdc at RPM way below your max RPM. At a constant boost level, the engine air flow increases with RPM (of course head and cam affect the flow and it's not a linear % increase).

​​​​​Once the AFM generates its max value 5Vdc, with the increase of RPM so does the air flow, but the DME is no longer capturing the exact/correct air flow. This is what we refer to as "AFM maxed out".
The MAF can be maxed out as well!

If the factory overload is enabled, you can hit overload fuel cut way before you max out the AFM.
As mentioned earlier, the factory overboost/overload is based on load (flow / rpm). You can trigger overload if you are flowing enough air and your RPMs are low (the lower the RPM, the higher the low for the same air flow).

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jimbo1111

Technically yes, but on the 944 turbo the afm will max out eventually and not be able to perform that function efficiently. So tuners use the wot maps and just add fuel based on rpm. More sophisticated systems use the map sensor and fill in the blanks . So in essence you have a hybrid afm/maf to map which makes up for the short fall.
The a- tune is actually limited by the injectors. it can actually be tuned for larger ones, but that would cut into maf sales.

Dan Martinic

Since we're learning.. What's the temp sensor in the AFM for? Does it provide data for the DME to determine mass, load, or something else?

Mike Goebel

So the big question is, does the Raptor K27/6 flow enough to max out the reading on the AFM and and require larger injectors. At 3 Bar FPR you can get slightly over 300HP with stock injectors at 100% duty cycle. So what does the Raptor flow installed on a 2.5L 944 engine.

Mike G.

fast951

Hot & cold start, target AFRs

jimbo1111

With a cam Yes.

fast951

It depends on boost and how much the compressor is able to flow before backpressure skyrockets due to the size of the #6 hot housing.

I doubt there is an air flow model available. A dyno chart can give us an idea of the air flow. Or if anyone running a Vitesse MAF system + MoMonitor can log the air flow across the Rev range and boost.

I do know, the AFM works with the vanilla flavor k27/6 @15psi, but you are on the ragged edge AFM & injectors (+3bar FPR). "if" the raptor flows more, then the AFM is not the best option.
Turbo size and injectors capacity are the main reasons I refuse to sell an AFM chip when I know the AFM & injectors cannot handle the increased air flow capability of the turbo.

Droops83

IAT is a very important factor, because as we all know the density decreases as temperature rises, which means less actual air molecules entering the engine and thus affects the accuracy of an AFM reading. IAT is also a huge determinant of an engine's knock sensitivity, so it is a very important factor to consider in a "tuned" application.

However, the IAT sensor is often built into the AFM or MAF sensor, which is not 100% ideal in the case of a turbocharged engine like the 951---what is really important is the temperature of the air actually entering the intake manifold after it has been compressed by the turbocharger and then somewhat cooled by the intercooler. With the stock 951, Porsche likely modeled this and built it into the the stock DME maps based on the pre-turbo IAT and other factors. I am not sure how the aftermarket 951 MAF kits accomplish this.

Beginning with the 993 Turbo, Porsche mounted the IAT (more accurately called a manifold air temp sensor/MAT in this case) sensor in the intake manifold to measure the actual temp of the intake charge, and 996TT and later use a 4-pin combination IAT and MAP sensor (also known as a TMAP sensor) to accurately determine the temperature and density of the intake charge (996TT and 997TT also had MAF sensors pre-turbo, 991TT and newer are speed/density only)

For my 951 with VEMS speed/density setup, I have the IAT/MAT sensor mounted in the charge pipe between the intercooler and throttle body. This ensures accurate measurement along with being somewhat insulated from heat-soak due to the intercooler and throttle boots. I have the system tuned to progressively pull timing if the MAT is too high, which can help save the engine in an extreme case!

I'd be curious to hear how John/ @fast951 and others incorporate IAT/MAT into their MAF setups.