BMW 3-Series (E90 E92) Forum > E90 / E92 / E93 3-series Powertrain and Drivetrain Discussions > N54 Turbo Engine / Drivetrain / Exhaust Modifications - 335i > Torque Targeting and Throttle Activity-- PART 1

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      12-09-2008, 10:56 PM   #1

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Exclamation Torque Targeting and Throttle Activity-- PART 1

These days it's been tough to post tech data without it being interpreted as an unfair attack on our competition. Quite literally, this is going to change the way we ALL look at tunes, what they do and how they do it. Religions will be questioned. Tuners will get on the defense. Fanboys will shake their heads in disbelief. Flames will ensue. The usual. But hopefully the info presented here will prevail

So I've pretty much given up trying to be kind and gentle about it. If you don't like the data provided, you are encouraged to conduct testing yourself. I'll even tell you what to buy and how to use it at the end of this post.

All modern BMWs (and just about every car these days) relies on Electronic Throttle Control (ETC). This means that instead of having a cable connecting the accelerator pedal to the throttle body, it is the ECU that actually opens and closes the throttle body based upon various sensor input. These inputs include applied throttle (how much the throttle pedal is pressed), manifold pressure, air intake temp, coolant temp, altitude, oil temp, gear selection, road speed, traction control intervention, etc,).

So no longer does your right foot have complete control over throttle opening. These days, the only job of your right foot is sending a signal to the ECU of how much torque you want at that instantaneous moment. Then, the ECU then looks at that applied throttle % with the other sensor data and THEN decides how much to open the throttle. This approach seems overly complicated but it's really not when computers are brought into the picture. While his approach is more expensive than the old-school cable linkage approach, it does provide many advantages. Among these:

1) Soft rev limits. Instead of cutting fuel and spark to limit max revs, now the throttle can gently close at max revs as dictated by the ECU.

2) Integrated cruise control. So no more additional motor required to take the place of your foot.

3) Driving smoothness. Now the ECU will automatically smoothen out driveline slop/backlash by not allowing sudden throttle closure at low speed which would otherwise cause a pronounced engine braking event.

... And a lot more. For additional info about electronic throttle control, check this out.

N54 Specifics
There is nothing especially unique with BMW's implementation of ETC. The principles are the same. But the details are different. For instance, with the presence of two low inertia turbos, the ETC still plays a roll in controlling engine load. That is, the ECU makes sure that calculated torque output remains below the desired torque target. If it begins to exceed this mapped value, you will see throttle closure activity. In a stock car, as you would imagine, there isn't much throttle closure activity going on at max load. Obviously, this is because the factory ECU is properly mapped for the stock engine's volumetric efficiency/boost characteristics.

Example 1
Here is a datalog (RPM vs. Throttle Opening) of a completely stock MSD80 29.2 6AT-equipped 335 during a "dyno style" 3rd gear pull from low rpm to redline. The red dots on the RPM log indicates the period of full throttle pedal application:

Analysis: You can see that actual throttle stays fully open (which is actually 90% throttle) during most of the run (3000rpm to 7000rpm). There are a few dips where the throttle closes slightly. This is caused by the factory ECU intervening when actual load exceeds desired load. No surprises here.

Example 2
Here a log of a MSD81-equipped 6MT running a fully catless exhaust, upgraded FMIC and dual intakes. As with Example 1, it is running a factory ECU with no tune:

Analysis: Here, you can see a very big and prolonged dip between 5000-6000RPM. During this time, actual throttle drops down to approx 50%. This means that despite the pedal being floored to the carpet, the throttle is only half open. Kind of stinks, doesn't it? Not only does this throttle closure make the car feel "choked" and inconsistent, it's also more stressful on the turbos because they now have to push through a large intake restriction (the half closed throttle butterfly). This closure also defeats the purpose of adding free flow mods to your car in the first place. So why is the ECU inducing this nonsense? Well, it has to do with all those mods on the car. Each upgrade improves the volumetric efficiency (breathing) of the engine. Which then, in turn, results in:
-more boost (at the same wastegate DC)
-more torque (at the same boost pressure)
-more fuel consumption (at the same boot pressure)

So the ECU recognizes this implausibility/discrepancy and intervenes by closing the throttle. And as you would expect, this is the major reason that adding bolt-on mods to an otherwise stock car results in very little power gains. Not at all like what we were used to back in the 90s in our modded RX7tt, Supras and 300ZX turbos when the throttle actually stayed open when you put the pedal to the floor

Enter ECU Tuning
Seeing the effects simple and innocuous bolt-ons have on ECU intervention and throttle closure, we can predict that tunes can also make things a lot worse. Many people running tunes (regardless of manufacture) have made the following claims:
"My car makes good power on the dyno but doesn't feel that quick or responsive on the road"
"My car feels inconsistent. Sometimes its fast. Sometimes it is slow."
"My dyno says I have 370lbft of torque at 4000RPM but it sure doesn't feel like it on the road"
"My dyno results show a weird dip at 4500rpm that my tuner blames on a VANOS shift."

Well, here's the real truth:
Tuning options, until now, didn't address this issue of dynamic throttle closure. And this can be proven by anyone running a tune and access to a simple OBD2 data reader/logger. Instead, tuners have focused on dyno results and 1/4 drag times. Both of which are largely insensitive to the effects of throttle closure. Dynos offer a slow enough ramp-up rate and heavy enough rollers that sharp dips in torque caused by sudden throttle closure are often (but not always) blurred/hidden. On the dragstrip, torque dips in the low end and mid-range aren't even noticed because revs rarely drop below 5000rpm.

Example 3
Here's a 3rd gear "dyno style" log of a MSD80/29.2 equipped 6MT taken this past weekend in LA. The car is equipped with dual intakes, catless downpipes and an un-named tune:

Analysis: There is a big and prolonged throttle closure at 4500rpm. it is prolonged enough that it would even be noticeable on a dynojet. There is additional throttle closure at 5500rpm and then again above 6000rpm. Boost, as measured on a boost gauge, would stay at the desired target of 14-15psi. But actual torque output would be inconsistent and turbos would be forced to work unnecessarily hard to push through that semi-closed throttle butterfly.

Example 4
Here's a log of the same car during a 3rd to 4th gear WOT run:

Analysis: Notice that during dynamic conditions that involve changes in applied throttle and sudden bursts of accleration, undesired throttle closure becomes even more of an issue. The car may make good power once boost hits the target but due to the unintended throttle gymnastics, the way it achieves this power results in unnecessary stress of the turbos, inconsistency, unresponsiveness and extra heat. Again, you would see no evidence of this on the boost gauge as it would stay on target. But you would feel it. Especially when it's finally gone. If there is ever an example of not seeing a problem until it's finally solved, this is it.

Last edited by shiv@vishnu; 12-09-2008 at 11:18 PM.
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      12-09-2008, 10:56 PM   #2

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Torque Targeting and Throttle Activity-- PART 2

Reserved for Part 2

Sneak Preview:

Last edited by shiv@vishnu; 12-09-2008 at 11:14 PM.
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      12-09-2008, 10:59 PM   #3
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I like to read this stuff
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      12-09-2008, 11:07 PM   #4
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Love good reading...very informative...thanks for a good read....I just printed it and going to the toilet for some quality reading time!!!
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      12-09-2008, 11:12 PM   #5
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Bring on part 2. Very very interesting

And, if you aren't offended by me asking. Is this "problem" we're describing so far a characteristic of "all" the tunes as of a month ago? Is this why the pre-beta map is so "amazing"? I feel like, after reading your description, I can "feel" what you're talking about on my car.
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      12-09-2008, 11:13 PM   #6

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Very interesting stuff.
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      12-09-2008, 11:16 PM   #7

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Originally Posted by TheAcAvenger View Post
Bring on part 2. Very very interesting

And, if you aren't offended by me asking. Is this "problem" we're describing so far a characteristic of "all" the tunes as of a month ago? Is this why the pre-beta map is so "amazing"? I feel like, after reading your description, I can "feel" what you're talking about on my car.
Yes, Yes and Absolutely.
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      12-09-2008, 11:21 PM   #8
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I want my throttle cable control back. Maybe carbs should be enlisted as well!
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      12-09-2008, 11:22 PM   #9
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Why do the dips in the first graph appear much less analog? Is there a difference in resolution?
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      12-09-2008, 11:24 PM   #10

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Originally Posted by Hit_Apex View Post
Why do the dips in the first graph appear much less analog? Is there a difference in resolution?
It's from a stock 6AT while the others are from modded and tuned 6MTs. The corrective throttle activity will be very different.
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      12-09-2008, 11:26 PM   #11

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So let me ask you this, what if in the specific case of N54 the ECU was modified to open the throttle to exactly match the gas pedal input... sort of simulating as if there was a physical cable from your right foot to the engine. So putting cruise control, smooth engine and soft limiter issues aside would this approach make N54 a more responsive engine?
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      12-09-2008, 11:42 PM   #12
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Good info...

This is good stuff and I'm sorry to jump ahead but it looks like the 'after' in part 2 still has a little throttle wiggle at 4.5K RPM (but looks *much* better IMO)

I was wondering if that's an artifact or if it's just not possible to hold the throttle completely open (yet) without a little more 'tweaking'?
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      12-09-2008, 11:43 PM   #13
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Interesting; looking forward to experiencing the updates myself.

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      12-10-2008, 12:11 AM   #14
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...'bout time you divulged the info that some of us have been sitting on
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      12-10-2008, 12:24 AM   #15
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      12-10-2008, 12:24 AM   #16

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Just so I'm clear of what your talking about, are you saying your new tune can fix this throttle activity problem? If yes is it the rev II?
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      12-10-2008, 12:26 AM   #17

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Very Interesting

Last edited by PaulBailey@ltbmw; 12-10-2008 at 12:49 AM.
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      12-10-2008, 12:30 AM   #18
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This is the reason I said 3k and 4K rpm now feel like 7K rpm. The throttle response is AMAZING ! I can't wait for the full version of this map....
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      12-10-2008, 12:41 AM   #19

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Originally Posted by brusso View Post
This is the reason I said 3k and 4K rpm now feel like 7K rpm. The throttle response is AMAZING ! I can't wait for the full version of this map....

Bring on the new maps!!
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      12-10-2008, 12:46 AM   #20

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Just an FYI to the turbo newbs (and holy hell there's a bunch). Throttle angle once you reach about 50-60% of max has very little impact on power production...which is why all those turbo powerplants out there that feel like NA engines require TPS referenced boost control to maintain proper levels of increased torque per unit throttle angle change after approx 50-60% throttle opening. Nothing new. All the old Audis would open the throttle plate wide open at 40% pedal angle and use nothing but the boost control system of the car for "throttling" past that point.

Point being, 60% throttle opening at 10 psi (arbitrary number) does NOT equal 60% of the power of an otherwise wide open throttle run...more like 80%. As throttle opening approaches 75% opening angle, its also more like 95% that of "WOT."

The 2.3T in the MS3 and MS6 is the posterchild of this. Their throttle plates do not open past 75% for much of their powerband.
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      12-10-2008, 12:49 AM   #21
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subscribing for updates.
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      12-10-2008, 12:54 AM   #22
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nice write up shiv.can't wait for the second one

Last edited by veightkiller; 12-13-2008 at 10:56 AM.
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