Limited Boost Control Due to Relocated Temperature Sensor

A temperature sensor was installed in my intercooler before the throttle body (2011 STI Process West TMIC) and wired into the MAF sensor. This allowed the tuner to protect against heat soak and proactively pull timing as the intercooler heat soaks on track instead of waiting for knock to occur and then having the ECU pull timing. He told me that this method's downside is that boost control becomes more challenging. I've noticed the car will over-boost by ~2psi when outside ambient air temperature drops below 40 F. However, now I have moved to south Texas. During an autocross event, the logs show my car consistently under-boosting around ~2 psi with outside ambient temperatures at 94 F.

-Based on the attached data log, is my speculation that the limitation the tuner described, or is there something else mechanical going on?

-I'm also seeing some Feedback knock above 6000 rpm. The HPA lessons and some tuners I've talked to suggest this is likely a false knock because of the high rpm speed. Another run saw a -4.2 event occur around 7200 rpm, and I'm trying to diagnose what the culprit is.

Thank you for any help any of you can offer!

If this is a Subaru like the one pictured under your name, there are tables specifically for boost compensation based on how hot or cold the intake air temperature is, so adjusting them would account for it being hotter or colder.

In addition, you have the P and I systems which help get boost on target, so even if the intake air temp temperature compensation doesn't do a perfect job on its own, there are other systems to pick up the slack.

I would definitely not treat knock over 6000 RPM as false simply because of the engine speed. That should be diagnosed and resolved. Basics like checking lambda reading, fuel pressure are a start. Check for leaks, take care of any maintenance items like spark plugs as needed, etc.

Thank you for taking the time to reply; yes it is a 2011 Subaru running an IAG short block and bolt-ons. The way I understand the boost control issue is that since the temperature information is coming from post turbo and intercooler, but cold wire in the MAF sensor is preturbo at the OEM MAF location, as the temperature deviates significantly from what the car was tuned at or the ambient temp, the PID system struggles to keep up. Although the temperature is more accurate to what the motor is actually seeing, the air volume calculation for preturbo airflow is inaccurate. I'm probably not explaining this well.

Lamda generally matches the commanded AFR, however, the lambda is lean when the knock appears. It came at a rapid throttle change. The AFR reaches the commanded value after a frame or two, so perhaps it's a tip in compensation issue (a hobbyist's best guess anyway). Maintenance is good with no leaks identified. I'll keep digging. Thanks for the response, I greatly appreciate the help.

Jake I totally understand what you're saying, you've not failed in your explanation at all.

That said, neither air temp location is "ideal" for boost control, and yours isn't making boost control impossible. The location you're using is common and can work fine, not a big deal. Also airflow isn't involved in Subaru's boost control system.

One key word you mentioned is autocross. You're sitting in a hot parking lot baking the entire engine bay. Regardless of air temp location, it's gonna read whatever the engine bay is heat soaked to at the start of the run. As the run gets going the sensor body remains heat soaked and most are not quick to respond. You'll see changed, but they generally won't be that accurate. Keeping boost on target will involve the PI system.

Maybe the tune's IAT WG duty comps are a significant factor in your boost reduction, maybe not, since we don't know what they are, but they can be adjusted.

The AFR behavior you're noticing may be causing the knock, so it sounds like that should be resolved. Tip in enrich settings and wall wetting compensation tables are involved, plus making sure all MAF values the transition runs through are set appropriately, plus load comps.

Thanks again for taking the time to reply. I will dive into ruling out a few mechanical issues over the weekend. Then I'll follow up with the tune. Thank you for your time as always.

Sounds good! You're welcome.

I did some more data logging just doing pulls on the highway and found a couple of issues. 1) The motor was a little lean even without the abrupt throttle movements seen in autocross. 2) I got a prolonged persistent knock at low load (25% throttle) around 3500 rpm. This eventually reached -4.2. It was completely out of character for any knock issues I had seen.

Solution 1: I discovered a broken bracket on the downpipe, creating metal-on-metal contact. Once I fixed the bracket the knock never showed up again on data logs.

Solution 2: I found a nut on the exhaust manifold that had backed off and was allowing a small leak (this seems to happen every few months). Luckily IAG has lock nuts for this application now, so hopefully this will be the last time I have a surprise exhaust leak. This fixed the lean condition I was seeing in the logs.

As far as the boost issue, I still seem to be under-boosting by about 1 to 1.5 psi. I'll keep poking around the engine bay in search of any other mechanical issues.

Ah yea, a tiny leak at the exhaust manifold to head flange can cause engine damage pretty quickly, and I've seen instances where it melted the timing cover quickly too. Good catch!

I logged another autocross event this weekend and I'm still under-boosting by about 1 psi. What is an appropriate tolerance for boost error (i.e. is -1psi from a target of 20 psi a significant concern?)

I'm going to smoke-test the intake again and see if I can find any leaks. I added radiator ducting and pulled the AC condenser before this event and car ran much cooler. The previous events knock events were not present. There was a -1.4 that decremented to -0.7 underload during the second of ten runs, but there was not any other fine knock feedback during the entire event. The car did seem to apply fuel trim during open loop, which perplexed me, however, I posted that question in the appropriate forum.

If at worst you're 1 psi off, I'd say that's good for your current setup.

I'm unclear if you're tuning your own car or not. If you're tuning the car yourself, you can adjust the associated baro, intake temp, and gear based comps if only under certain conditions the boost level is a bit off target. Otherwise PI may need work.

How significant a concern being 1 psi off target is really depends on how critical it is that you have the few additional ft lbs of torque, unless you race in a class requiring a boost limit.

Thanks for taking the time to answer all my questions! I am not tuning myself, yet. Eventually, I want to get there; right now I've been using the courses to begin the learning process and hopefully help diagnose small issues before they become big issues in the data logs. A little under power is of no concern for me at this point. The car isn't built to the point that it's competitive in class yet. Just working on driver mod, having fun, and learning more skills. I'll keep an eye on it and make sure it doesn't get any worse. Thank you for all the help!

You're welcome! That all sounds good. Being more informed so you can keep a better eye on engine health is a great start!