VGT Turbo M20 Sleeper ('87 325 Sedan)

[mikey.antonakakis]

Quick spin to the dentist: car feels so much better with the new shocks/springs. Definitely stiffer but ride quality has improved significantly (and NVH? Interesting that’d I’d notice, considering I have solid aluminum motor/trans mounts and 80a bushings everywhere). None of this is a surprise, but I think I like driving the car a lot more now. Might dial back the damper adjustment, roughly midway front and rear at the moment.

Garagistic order placed a month ago should be here tomorrow, including 95a trans mounts and the parts to swap a Z3 rack to replace my worn stock rack (a few weeks ago alignment shop pointed out steering slop between the front wheels with essentially all new front end). Car should finally drive nice and tight after that.

[mikey.antonakakis]

375 front, 475 rear. Only drove it about a mile, but definitely feels more like a car now!

[Panici]

Back on the ground, coilovers at basically max height:

How does it ride with the GC/Koni setup compared to the stock suspension you had before? What spring rates did you spec?

[mikey.antonakakis]

Back on the ground, coilovers at basically max height:

Untitled by Mikey Antonakakis, on Flickr

[mikey.antonakakis]

Everything was going smoothly with the GC/Koni install until the last corner - strut housing had some boogers on the inside from too much weld penetration from stock spring perch welds, ugh. The thick paint on the Konis had them totally stuck about 6” in. Thankfully had a die grinder just long enough to get in there and clean it up, but what a pain.

[mikey.antonakakis]

In non-turbo news, finally replacing my possibly-original shocks and springs, going with Koni yellows and GC coilover conversion. Pulled the struts out just now, guess I was overdue:

[mikey.antonakakis]

Took the car to work today (65 miles each way). Turbo gets a lot more responsive after the heat soak of highway driving. Found another little mistake in the VGT code on the way in - vane calculation is based on real-time engine speed, MAP, and VE - except my math for each of those parameters was referencing RPM for all three. That explains why the VGT actuation wasn't going exactly as expected. That was a super easy fix and helped with spool - so much so at mid rpm that with all settings unchanged I hit my boost limiter (200kPa, at 185kPa boost target).

160kPa MAP at a ~3000rpm on a 60mm turbo at 6000' above sea level, I'll take it! I think there's room for more. Makes highway cruising really nice, no need to downshift from 5th, just spool the turbo and make the pass. Here's about 90mph to 105mph in 5th. I think wastegate may have actually been limiting the boost here, and I can get more from the VGT too. 4 seconds from tip-in to 160kPa MAP (~80kPa/11psi gauge boost pressure).

I am hoping to get some video to go with all the data this weekend. Either need to take a friend for a ride or make a gopro mount.

5th_90-105_2022-06-03 by Mikey Antonakakis, on Flickr

[mikey.antonakakis]

Another test run tonight, turned boost target up to 185kPa and the dome control is doing great. Did a little seat-of-the-pants VGT tuning too, but need to do more with it. I have a new problem now: traction!

Here's a light 2-step launch from a dead stop, launched at about 130kPa map and 3400rpm, built that boost in ~2 seconds at 3500rpm.

• 0-60 ~5.2s, spinning and lifting
• 0-100 ~10.7s
• 60-100 ~5.3s

This is despite big wheelspin and throttle lifts in first and second gear. The spin in second scared me - usually this car drives straight as an arrow through wheelspin, but this time it got very sideways very quickly.

Note the very quick ramps in RPM followed by throttle lifts before the 1-2 and 2-3 upshifts. Hooked in 3rd except for the one little blip in RPM, must have hit a big crack in the road or something and got some very brief wheelspin. Shows up as a disturbance in dome pressure and turbo speed, interesting.

launch_wheelspin_2022-06-02 by Mikey Antonakakis, on Flickr

[mikey.antonakakis]

I think it's working!! I tried to record two quick pulls, but missed the fun one (1st-3rd all the way to redline and part of 4th, so maybe 20-120mph). Here's the less-fun one, just a 2nd gear pull, MAP nearly perfectly on boost target. VGT tuning can be more aggressive, and I think I can finally stop debugging it and start tuning it!

There's still a little blip in the dome target on tip-in, that one will be a little harder to avoid but it's not a big deal.

dome and VGT working 2022-06-02 by Mikey Antonakakis, on Flickr

[mikey.antonakakis]

Made a major addition to the dome control algorithm today: a lookup table of steady-state target MAP vs. dome pressure. This replaces the idea that dome pressure target should be boost target minus spring-only boost, which is not a great model for a real wastegate. This makes the dome control basically one or two steps less advanced than the most fancy options out there, and should work really well. Easy to add the last step or two: add RPM dependency to that look-up table as well (not a huge influence), and go to true MAP target PID control, with outer PID loop monitoring MAP error to determine target dome pressure, and inner loop making sure the dome control meets that target pressure.

With the current state of things, the pure dome control PID is working pretty well at this point and just needs a little fine tuning with real driving. A quick test drive this afternoon identified two bugs causing some crazy operation of dome target and VGT, both should be fixed now.

First datalog shows a fixed dome pressure control, with a target dome pressure of 105kPa all the time. The dome control is pretty good here, but in this log you can see the major VGT bug: as soon as MAP hits the boost target, the VGT closes fully. Had a small mistake in one line of code that does some math to taper the VGT position to fully open just before the engine hits target boost. It caused a crazy position command to fully closed once target boost was reached. This in turn probably caused huge exhaust manifold pressure that pushed the wastegate open, increasing dome pressure. The dome pressure control actually handled this crazy condition pretty well:

VGT_glitch_at_full_boost by Mikey Antonakakis, on Flickr


Next datalog shows pretty stable operation of the VGT, but a major issue with the dome target. This time it's using the new lookup table to determine dome target. Very simple user input error here caused dome pressure target to be 13kPa absolute lol. The dome PID did what it could, but obviously can't get it down below barometric pressure (80kPa). So here we are seeing the boost level at wastegate spring only. Dome control PID settings are a little too agressive, hence the brief oscillation on step changes in target. But it's close!

dome_control_glitch_2022-06-02 by Mikey Antonakakis, on Flickr

[mikey.antonakakis]

PID is definitely snappier than the last drive, so that should help with the overboost. Once I update the dome target math to include that real-world offset from “target boost - spring boost” I think it’ll be working pretty well!

[Panici]

Love the plot video, nothing like some live data to start the morning!
Interested to see if the new valve makes a noticeable difference when driving.

[mikey.antonakakis]

Old DIYAutoTune solenoid replaced with another MAC valve, this time with no filter on the vent port. It tuned in really nicely in some quick bench testing tonight with engine off! Higher voltage with the engine running at cruising speed may shift my deadbands a little, but that's easy to recalibrate for at this point. The PID tuning is getting close to dialed into where I want it, as shown in the video below.

I also calibrated the dome pressure sensor to the Megasquirt's MAP sensor. Happens to be that 10-bit ADC counts are very close to being the same as kPa, and very very linear. I'll still need to add a correction factor into the code to account for the exhaust manifold pressure on the wastegate valve - basically need to just shift dome pressure target to be about 20kPa higher than the math would suggest.

Untitled by Mikey Antonakakis, on Flickr

[mikey.antonakakis]

Some details on the solenoid plumbing modifications:

Here are the solenoids as set up previously, nevermind the ugly mount. Eventually they'll get something nicer. Lefthand solenoid is fill (with hose going back to compressor in the trunk), the other is vent - vent solenoid was purchased 10+ years ago from DIYAutotune (knockoff Humphrey 310 I think?), fill (genuine MAC valve, or a believable knock-off) was a more recent purchase. I've got another matching MAC valve arriving soon to replace the fill. The lower brass tee fitting splits the solenoids' output - one side goes to wastegate dome, other side is the pressure sensor.

20220531_115122 by Mikey Antonakakis, on Flickr



The changes were twofold: move pressure sensor directly to the dome to get the most accurate signal and maybe to dampen the pressure fluctuations from the low-frequency solenoid PWM operation (the solenoid valves bounce between full open and full close, they don't "float"). This is how Holley recommends to set things up, for what it's worth. My JGS wastegate only has one port on each side of the piston, so I had to drill and tap another, 1/8" NPT. Other main change: re-plumb the vent solenoid to work as normally-closed instead of normally-open. Less iditot-proof this way, but again it's how Holley recommends it. Also means that the transition from blocked to venting happens at a low duty cycle now. Maybe these valves are happier running that way? Sure seemed like it from the test results.

20220531_113757 by Mikey Antonakakis, on Flickr

20220531_115918 by Mikey Antonakakis, on Flickr


Back together, pressure sensor relocated a bit and vent solenoid converted:

20220531_133917 by Mikey Antonakakis, on Flickr

[mikey.antonakakis]

Dome control is close! Changed up the plumbing a bit - vent solenoid changed from normally open to normally closed. Pressure sensor moved from the tee at the solenoids to the dome directly (had to drill/tap another port in the dome). PID is a little slow, needs more tuning, but it's stable and hitting its targets. Need to add a little more hysteresis to prevent the boost fluctuations seen. This is due to the "boost = spring pressure + dome pressure" not being totally true combined with the slow PID operation in this log. Also my dome pressure sensor is not calibrated well (will do a good calibration on it this evening).

dome_control_almost_2022-05-31 by Mikey Antonakakis, on Flickr