Took the car out for a test drive after moving the wastegate reference line from upstream of the throttle to the intake plenum, and reenabling closed-loop boost control. The problem is still present.
Through some helpful posts on K20A.org, I now better understand the nuances of wastegates and boost control. The 3-port boost control valve I’m using can’t keep the wastegate shut if high back pressure in the exhaust tries pushing it open. However, there are several different ways to plumb boost control valves, and one does just that, actively keeps the wastegate shut until boost target is reached. The downside is that when configured this way, if the valve ever fails, boost will attempt to increase without limit. Fortunately the ECU can protect the engine; if MAP gets too high, the ECU can instantly cut fuel, spark, or both.
I have a feeling that this boost-drop issue has been present for a long time. I remember feeling the same drop in power back in January when still was running the KPro. At the time though I attributed it to the water/meth system cutting out – apparently that wasn’t what was going on. I never noticed it on the street because I don’t rev to much more than 6000 rpm, and the problem doesn’t happen until about 7400 rpm.
I can either reconfigure the 3-port valve, or switch to a 4-port unit which allows both positive wastegate control and a wider pressure range, but the first time I tried the 4-port, its control “finesse” was worse (more sensitive) than the 3-port. However, I now know that that’s due to the efficient exhaust manifold design, which allows way more flow to the wastegate than most manifolds do.
On the list of things to do: 1. Switch to a 4-way boost control valve – but only if research shows that it can actively hold the wastegate shut. Or, just replumb the 3-port to do the same. I rather do it this way than swap in a stiffer spring. Currently, spring pressure is about 14 psi, and both first and second gear are nearly useless (second is good to around 70 mph…). I much rather start with a ~7 psi spring as a baseline.
2. Replace the leaking dual wastegates with a smaller unit. Right now I’m running two 38mm units, which is the equivalent of one 54 mm unit. The plan is to go with a single 38 or 44 mm wastegate, which has the benefit of less parts, less weight, cheaper, and will increase the resolution of boost pressure.
3. As a separate issue, now that I’m logging air temperature out of the intercooler, it’s looking like the intercooler’s not doing a whole lot. Though I haven’t tested it yet, it appears that air temperature will get above 60 degrees C after about 5 seconds of full throttle. On the street it’s not a big deal because in just a few seconds, I’m breaking whatever the speed limit happens to be. On the track though, that’s not going to be acceptable. An electric fan may be added to the intercooler.
My brother was giving me grief about how I keep fussing with the turbo engine. (So far) I don’t see it that way; I built a turbo engine partly to learn about them – and I certainly am. No doubt his comments were also designed to try and get me over to the V8 side of the camp. Meh.