Thread: SR20DET rich at part throttle under boost

While taking baseline data on my SR I encountered an interesting anomaly during part throttle ramp runs that had me stumped for a while. I was noticing some very rich wideband readings that didn't seem to match the fuel map. At 4800 rpm and only 5psi it is running down into the 9's. At first I thought it may be compensating for knock, but the logs show that the timing is dead on to the timing map demonstrating no evidence of knock compensation.
There is a pattern with lower throttle openings at this RPM and load that the mixtures get very rich. The map sensor is reading 5 psi at 4800 rpm and 40% throttle while at 3200 rpm and 70% throttle it is reading 8-10 psi in the same load columns.

So what is going on? The theory that I have come to is in this rpm and load area the turbo is starting to come up to speed and is flowing a lot of air (this is confirmed by high MAF readings), but the low throttle opening is impeding the flow into the manifold as indicated by the low psi. (pressure drop across the throttle body) Basically the ecu is using the wrong load columns because the maf is reading a higher flow than what is actually making it into the cylinders. When the throttle opens farther the maf reading aligns with the map readings and produces more reasonable afr. Hopefully I am making sense here.

Has anyone else noticed this on boost, part throttle behavior with a maf based ecu? I'm not sure how to approach tuning part throttle under boost.

Stock S13 Red Top
T25
FMIC
Top mount log manifold
Freddy intake manifold

You're going to need a new theory. There is no possible way (without a large leak) that the MAF is reading a different amount of air than is entering the cylinders. The throttle body being a restriction will restrict the air flow of the entire system equally, from the air filter down to the valves.

Valid point at steady state, but if the pressure in the intake tract is continuing to increase then you could have more air entering through the maf than enters the engine through the manifold. Let me explain with an analogy: say you are running an air sander that consumes 7 cfm and your compressor is capable of 10 cfm. You start with the compressor tank at 100 psi. As you run the sander compressed air is consumed until the tank pressure drops to 80 psi and the compressor kicks on. You continue to run the sander consuming 7 cfm but now the compressor is running and producing 10 cfm. Since the sander consumption is restricted to 7 cfm the tank pressure begins to increase due to the 3 cfm extra of air supplied by the compressor. This continues until the compressor reaches the upper set point of 125 psi and turns off.

With a car this phenomenon would likely only be transient with the flow and pressure balancing out or a charge pipe popping off. I suppose I can hook up a map sensor pre-throttle body to determine if there is a large boost leak or if the pressure continues to increase.

(I would post some graphs and data but I don't see any option for adding images to the post)

Sure, but a MAF doesn't measure pressure. It measures flow. The mass of air passing by. Flow is the same before the throttle butterfly as after.
If you had a MAP sensor before the throttle body you'd have a point. (Which is why MAP sensors are plumbed to the plenum).

You can add images by uploading them to a hosting service (imgur.com works well, but I might start recommending imgbb.com instead) and then copying the image URL and placing it between IMG tags in the post.
Like this:

[IMG]www.image-host.com/directory_structure/filename.png[/IMG]

Let's see if this works.

Correct. MAF is measuring flow at the inlet to the system which at steady state (ie constant pressure) will equal the flow out of the system. However while pressure is increasing in the system the inlet flow must be higher than the exit flow from the system or the pressure in the system would never increase. Essentially what I am saying is that while boost is ramping up we have an imbalance were more air is flowing through the maf than is consumed by the engine. Once equilibrium is reached and boost pressure stabilizes (at what ever level that may be) the flow through the maf again matches the mass of air consumed by the engine.

Now that I think about it this would mean that even without any added accel enrichment the ecu would automatically add extra fuel during boost ramp up just by nature of the flow increase measured by the MAF that is not making it into the engine yet. (it is backing up in the intake tract building boost)

Okay, enough with the academia. I did a boost leak test yesterday and found the BOV and the joints near it are leaking. I don't know if the leak is big enough to be the primary factor in the results but the rate of leakage does increase as the pressure goes up so it needs to be addressed.