VGT Turbo M20 Sleeper ('87 325 Sedan)

[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]

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]

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]

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]

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]

Back on the ground, coilovers at basically max height:

Untitled by Mikey Antonakakis, on Flickr

[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]

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

[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]

Added new feature to the touchscreen OBC - ability to change the VGT algorithm tuning parameters without having to have a laptop plugged in. This should make the VGT tuning process smoother (previously I had to re-flash touchscreen OBC firmware to make any tuning changes).

This includes changing VGT operating modes, such as the VGT operation during “cruise” (i.e. when there is no boost demand, just cruising around). One mode has the vanes wide open while cruising (for best fuel economy) and the other closes the vanes a good amount to give the turbo a little bit of pre-spooling, so when you do get on the throttle and demand boost you have a little head start on spool-up. The party trick is that it works like an electric exhaust cutout, closing the vanes most of the way makes the exhaust noticeably more quiet.

[Panici]

One mode has the vanes wide open while cruising (for best fuel economy) and the other closes the vanes a good amount to give the turbo a little bit of pre-spooling, so when you do get on the throttle and demand boost you have a little head start on spool-up. The party trick is that it works like an electric exhaust cutout, closing the vanes most of the way makes the exhaust noticeably more quiet.

I have little knowledge of VGT turbos; keeping the vanes closed mimics a smaller exhaust housing?

I always thought that (all things equal) having the turbo sized to spool during cruise would increase engine efficiency and therefore increase fuel economy. Is this incorrect?

[mikey.antonakakis]

I have little knowledge of VGT turbos; keeping the vanes closed mimics a smaller exhaust housing?

I always thought that (all things equal) having the turbo sized to spool during cruise would increase engine efficiency and therefore increase fuel economy. Is this incorrect?

I feel like an imposter a lot of the time, but this is a topic I'm actually qualified to opine on (former Toyota calibration engineer)! Warning: strong opinions and technical stuff to follow. But first, your first question: basically changing vane position changes the AR of the exhaust housing, yes. There are different VGT structures out there, mainly rotating vane and sliding wall. Holset VGT uses sliding wall. Here's my turbo when I was first figuring out how to control the VGT position via CAN bus. Sorry for the poor lighting, but you can see the vanes/nozzle wall moving:

Second and longer answer:
Best thing for fuel economy in terms of exhaust side, all other variables being equal for a given engine, is getting the least back pressure possible, minimizing pumping losses.

Downsized engines with turbos being better for FE is at best intentionally misleading and at worst a bald-faced lie by auto manufacturers, just like "AWD is so much safer!". It's only the down-sizing part that's making an improvement in FE. It can be a really complicated topic, because engine usage varies so much with driving style or fuel economy test cycle, and modern engines have so many tunable systems. But there's a really easy way to "normalize" things: power.

At a given point in time when you're driving a car, your right foot is demanding a certain amount of acceleration from the car, or in other words, longitudinal force at the drive tires' contact patches - i.e. you're asking for a certain amount of power from an engine. You make think "no, you're demanding torque," and while it's a little pedantic I'd insist that power is the key real-time parameter:

• You, as a human in a seat in a car, feel acceleration, and at a given time you are going a given speed.
• The car has mass and drag and rolling resistance, etc., all of which are either speed-dependent or constant.
• F = m*a, and the "a" is what you're actually feeling (although our perception of acceleration is speed-dependent too! But that's almost getting into physiology/psychology or something so we'll ignore that one for now).
• P = F*v, or P = m * a * v. All the "losses" can be calculated for a given speed in terms of force, like aero drag and rolling resistance, which means they can be subtracted out from that equation, which after that's done I'll refer to as "Pnet".
• Solving: a = Pnet / (m * v)

This is not only the "right" way to think about things IMO, it's also super handy for a couple of reasons: one, because almost all cars have some means of changing gear ratios, thereby making a torque-based approach useless. Two, brake-specific fuel consumption, which is a parameter that gives the fuel consumption in terms of power, i.e. power-based fuel efficiency. Fuel flow rate is a useless parameter without additional information (just like torque - an engine with 1000ft-lb that only spins to 1000rpm won't make for a quick car). Since we've established that the best real-time parameter to base things on is power, at a given instant when you're demanding a given power from your engine, a lower BSFC will give you better fuel economy. BSFC maps are plotted against torque and engine speed, and for cars with multi-gear or CVT transmissions, this is great, because you can use that map to decide which gear is best for fuel economy at a given instant. Note the "constant power" line on this plot and the discrete gear ratios. 4th gear is best for FE here:



As you might notice, for the engine in this plot, and most other naturally-aspirated gasoline engines, best BSFC is at relatively low engine speed and relatively high torque. With direct injection and variable valve timing, best BSFC is closer to/at peak torque and low RPM. THIS is the reason for downsizing engines - most of the time when you're driving around, you only have to roll the tires and push air out of the way. You spend very little time increasing the momentum of the car, so your power demand is quite low the majority of the time. And you hardly ever ask for high or peak power. So to get good fuel economy, the car ends up using the tallest reasonable gear ratio to get to the low-speed/high-torque efficiency island. If your vehicle has a big engine to cope with intended usage (like a Corvette or a work truck), this is why cylinder deactivation is a thing. By itself, it loses efficiency since you're spinning a bunch of extra bearings and pumping air for no reason (just creating heat) but it makes a big enough change in BSFC usage that it is a net benefit to FE.

Now the obvious issue with just downsizing is that you lose peak power, making the car slower. This makes the customer unhappy, especially in the USA (not a political thing, it's the nature of our roads/highways/car-centric culture). US drivers legitimately use a lot more power and demand more acceleration in everyday driving than most of the rest of the world. To close this gap, add forced induction! Turbos don't sap mechanical power, great! That said, it does take a serious amount of power to drive a turbo at high rpm and full boost - no free lunch, that power has to come from somewhere, and it comes from exhaust energy, specifically by creating a pressure drop across the turbine wheel, i.e. increasing exhaust manifold pressure. In terms of FE, this increases pumping losses of the engine and therefore reduces BSFC. Add to that the fact that in boost AFR typically needs to go richer than stoichiometric and your FE absolutely plummets in boost. Direct injection helps run leaner by reducing/eliminating knock, but you still have a big pumping loss, since turbine drive power at full boost is somewhere in the range of 10-20% of your engine's power output!

So, downsized turbo engines DO get better FE (thanks only to the downsizing). Maximum FE benefit is when they are putting the least possible power into the turbine since this reduces backpressure/pumping losses - in other words, when it's acting the least like a turbo engine, when the turbo is doing nothing. Knock-on effect is now you're just carrying extra weight around for no reason. OEMs that use this strategy are very careful about calibrating around the EPA/CARB test cycles (which are remarkably low-demand - seriously, if you drove on most US roads like the test cycles, you'd get run off the road). They are able to keep the engine out of boost the majority/all the test cycle. This is why there's often a huge gap in window sticker vs. real-world FE for these engines - although it gets minimized if you drive gently on the highway.

So for my VGT setup, closing the vanes any amount from full open decreases fuel economy. Closing them too aggressively clearly shows up as richer AFR due to less mass flow of air due to increased exhaust manifold pressure (pumping loss). Even if I compensated my fueling to account for this and get AFR back to where it should be, I'm making less power (can feel in the seat of your pants, it's obvious). I'd guarantee my BSFC is lower with closed vanes - although there's probably a range from fully-open to "not very closed" at lower engine speeds that probably only has negligible impact. I'm going to try to document all of this on video/data logs as I go through the tuning process, if I can.

[Panici]

Wow, that's what I call a comprehensive answer. Thank you!

So, downsized turbo engines DO get better FE (thanks only to the downsizing). Maximum FE benefit is when they are putting the least possible power into the turbine since this reduces backpressure/pumping losses

Very interesting, and it does make a lot of sense.


Seems like an interesting application for an electronic wastegate to facilitate an Economy mode.
Keep the gate open all the time in Eco mode to reduce pumping losses at the expense of power.


Think I will try to explore some tuning for fuel economy in my Miata. From what you've said I should be able to make some gains, as it has a LARGE eBay special turbo and a tiny 1.6L engine. The large turbo shouldn't be a restriction under normal driving.

I did finally get a Tuner Nerd Knock Monitor Pro in the mail this week, so time to get my very conservative timing map up to snuff! I've read that more aggressive timing coupled with a leaner then stoich burn in the cruise area can make some significant gains.

[mikey.antonakakis]

Wow, that's what I call a comprehensive answer. Thank you!


Very interesting, and it does make a lot of sense.


Seems like an interesting application for an electronic wastegate to facilitate an Economy mode.
Keep the gate open all the time in Eco mode to reduce pumping losses at the expense of power.


Think I will try to explore some tuning for fuel economy in my Miata. From what you've said I should be able to make some gains, as it has a LARGE eBay special turbo and a tiny 1.6L engine. The large turbo shouldn't be a restriction under normal driving.

I did finally get a Tuner Nerd Knock Monitor Pro in the mail this week, so time to get my very conservative timing map up to snuff! I've read that more aggressive timing coupled with a leaner then stoich burn in the cruise area can make some significant gains.

I should point out that I did not directly calibrate turbo powertrains while at Toyota (did do some evaluation, though - BMW's modern turbo engines are pretty darn awesome, they do some fun stuff to maximize drivability). I also did this circa 2014-2015 and there may be advancements in technology I'm ignorant of. But I think my answer should hold water for older port-injected engines at the very least, and those without variable valve timing/lift or only simple VVT systems.

Yeah, I think a big turbo on a 1.6 should not be any significant detriment to FE for normal driving. And a little knock at light loads might be no big deal, I think plenty of OEM stuff allows for some knock at light load, although I was a transmission engineer, not an engine calibrator, so don't rely on that as sound advice!

Don't know if I mentioned this, but I did build some DIY "knock ears" - since most knock sensors are piezoelectric microphones essentially, you can just amplify the signal and listen it. Pretty cool to listen to in real time, especially on an engine as noisy as an M20 lol.

[Panici]

Don't know if I mentioned this, but I did build some DIY "knock ears" - since most knock sensors are piezoelectric microphones essentially, you can just amplify the signal and listen it. Pretty cool to listen to in real time, especially on an engine as noisy as an M20 lol.

I did try to build those, but my hearing isn't that great with tinnitus. The Knock Monitor Pro I bought has a software interface to detect knock and a set of headphones if I want to listen.