Odd Change in MAP During Engine Speed Sweep

Wondering if anybody has any insight as to what I am seeing below.

Car: 1986 Porsche 944 Turbo. Only modifications are -> Slightly bigger turbo, 3.0" cat-less exhaust, NA camshaft, turbo smart wastegate, inlet to the turbo ducted from the wheel well, ID1050X injectors. The intake manifold, exhaust manifold, exhaust cross over pipe to turbo inlet, and downpipe are all stock.

ECU: Link G4X, sequential injection and ignition.

I've tuned most of the VE and ignition maps on the dyno, and have been cleaning up the area's I could not reach on the dyno via some road tuning when I have an open empty road available. At one point I decided to do some sweeps from ~2krpm to 6.5krpm across a single load row. While doing that at my 120kpa load row, I noticed that around a certain RPM (4350rpm - 4550rpm) I was feeling some change in acceleration (torque), at the same time I noticed the active cell tracking box in the VE table jump from 120kpa to 100kpa then back to 120kpa as I passed through that rpm area.

Screen shot below shows what I'm describing. Is it possible this is some resonance in the intake manifold causing this? Has anybody else ever seen this?

There is no variable cam timing with this engine, and the only modification related to cam timing from OEM is that I'm running a camshaft from a naturally aspirated 944 as I found my turbo camshaft to have some pitting on some of the lobes while doing maintenance. Since I had an NA camshaft sitting on the shelve I installed it knowing that it would not be ideal given there is some small amount of overlap with the NA cam that is not present with the turbo cam. So based on the small amount of overlap and the greater exhaust manifold pressure than the intake manifold (exhaust is gage pressure in the screen shot above), I assume I have some small amount of internal EGR going on. So I'm wondering if it's possible that this specific RPM area allows for some pressure waves from the intake manifold runner design, maybe combined with some small overlap to cause MAP to suddenly drop like this??

What do the experts think?

What is your maximum boost pressure? Are you sure you're not getting Overboost, as your boost pressure was climbing -- then it wasn't. I think the boost control massively reducing the boost is what made your MAP dip. Is the Boost Status being logged (or updated) at the same rate as the MAP and BoostPressure? If not, you can't really know which was the cause, and which was the result.

Is there anyway you can target a lower boost level when the throttle is only 26% -- I'm sure the car will be easier to drive if it isn't trying to make maximum boost (judging by the WG Duty Cycle) on part throttle.

I have a switch for different boost targets but during that log it was 210kpa.

This was "PC Logging" which is set fairly slow at 40Hz (25ms). With the Link SW I believe everything is logging at the same 40Hz when using PC logging as apposed to internal logging where you can configure each channel. So I agree with your comment regarding it being difficult to know for sure which cause and result and sort of had the same thought at first. However, the time from MAP starting to drop and boost control turning off was ~374ms and it seemed to only be as soon as MAP dropped below the 105kpa threshold.

That being said my next step was to log at a faster rate and to drop the boost control threshold to 103 or 100kpa and try again to see if the dip was the same amount or less, but I suspect it will be the same.

Also, yes I can modify the boost target to be a function of throttle pedal amount.

Not sure what the turbo intake is like on those, but a problem I have come across before that presents similar to what your log shows is where at high flow the compressor intake pipe collapses. Quite common with silicone intake pipes when they get hot. I dont think it is likely to be resonance - I think even at 40Hz you would see noise on the MAP if there was significant resonance going on.

Hey Adam. The inlet to the turbo isn't OE, it is primarily two sections of hard pipes with short silicone joiners between, so nothing really to collapse. Also, it doesn't act strangely during higher load situation or across other RPM ranges. Seems to only be around the mid rpm range and when just barely above atmospheric.

Currently driving a different car but I'll get some more data to start ruling things out shortly.