How exactly does the LLY arrive at it's boost level?

There is no wastegate. We have a baro sensor, a boost sensor, and a turbo vane position sensor. Anyone know how those three are used in the boost game?

What determines the boost and does the computer verify its results? Does it just set the vanes at a certain spot versus rpm, or does it vary the vanes according to feedback and calculations from the baro and boost sensors?

Surely the aftermarket programmer guys or a tech or some other whiz on here knows how the boost is increased, measured, and handled. If someone could help out with an understanding of this it sure would be appreciated! :)


Thanks! :ro)

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I have been wondering this myself. It might be nice to be able to control the vanes in certain 1320' situations. Overspeeding the turbo could become an issue though:D

It is of interest that you can make more than 30PSI, but not with an ECM tuner.

That leads me to believe it's programmed to make XX PSI and adjusts the blades to get to the target. Also some tunes surge when they hit programmed boost. You can feel/hear the turbo opening and closing the vanes.

Do you think they made it as simple as just positioning the vanes at a certain spot versus calculated exhaust flow at a given rpm equals x boost?

The tuners are controlling the vanes for sure to get the boost they want. I am thinking there are only two ways they must be figuring boost.

1) Boost just may be figured off a compressor table in the computer, where x position along with x rpm equals the desired output.

2) The fancy way where the computer moves the vanes open and see's its results via a calculation off the baro and boost sensors.

Come on DP, somebody knows for sure!

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To maintain even boost, you'd have to vary the blades as RPM goes up.

Either all the programmers are really sharp, or they just plugged in a boost value and a subroutine with it's own variables adjusts the vanes to get there.

I got to sit in the truck and watch things at the most recent dyno day. On my N2O run (Predator/VA stack), the boost built steadily until the N2O hit at 18psi. The boost instantly jumped to 33psi and then fell off steadily until it was about 25 psi at 3500 rpm. I believe that the ecm was opening the vanes to control boost. I wish we could "fool" the ECM or control the vanes in order to maintain boost at higher rpms. Any thoughts?

Mike

what about a resistor in the boost sensor wires?

You would if you were controlling the boost via sensor feedback loop, but if the vanes are controlled via a compressor map then all you would do is position the vanes according to rpm since you know the exhaust flow and the compressor flow characteristics.

I would like to think they are doing it the fancy way via sensor feedback but it seems the boost would move around a lot and there would be big swings as it tried to get it right. I say that because it gets it right in a hurry, it would be a tedious task getting sensor feedback then opening and closing and opening and closing etc etc etc.

to the helms manual i go, unless someone already knows?

Acts closed-looped. I'll know for a fact this weekend. :D

Any luck with the helms? I keep hoping a vendor or tech will come in here with the definitive answer on how boost is controlled in the LLY.

The helms manual states that boost pressure is controlled independent of the engine rpm. The boost must be controlled via closed loop system. I wonder what the voltage ranges of the boost sensor are? It would be nice to be able to keep the vanes from opening under WOT. This could be the LLY equivalent of Juicegrips:)

The helms manual states that boost pressure is controlled independent of the engine rpm. The boost must be controlled via closed loop system. I wonder what the voltage ranges of the boost sensor are? It would be nice to be able to keep the vanes from opening under WOT. This could be the LLY equivalent of Juicegrips:)

I've been told both the Baro and Boost are 3 Bar sensors, so they should have exactly the same resistance with the engine off. Additionally, I would expect ~166 mV engine not running, ignition on. The higher the mV the higher the indicated pressure.


How deeply does the helms cover it? Does it specify feedback from the baro and boost sensors? I had assumed that the boost is controlled via feedback from the baro and boost sensors. If so, then you should be able to get in line with the boost sensor and jack the boost.

How about boost versus throttle position? for example, ramps the boost up via vane position to what the throttle is calling for and then fine tunes it via the baro and boost sensors.

Why would you want to keep the vanes from opening under WOT?

Come on DP members, someone has the inside info on this......where are ya info guy?????

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It is not covered in any useful detail.

It does state that at idle the boost sensor is used to verify the baro sensor reading.

There are 4 DTC's concerning the boost sensor (brief descriptions):

1) P0234- The measured boost pressure is above the expected range by 35kPa (~4psi) or more for more than 12 seconds.

2) P0237- The boost pressure is less than 37kPa (5psi) for 2 seconds.

3) P0238- The ECM detects that the boost pressure is more than 254kPa (36psi) for 2 seconds.

4) P0299- The ECM detects that the boost pressure input is 35kPa (~4psi) below the calibrated value. OR To obtain the desired boost, the ECM must command the turbocharger vanes closed further than expected. One of these conditions must exist for more than 12 seconds.

It would seem that the boost sensor has an accuracy in the range of +/- 35kPa (4psi). Also the ECM is measuring and calculating boost pressure at the same time. While this is useful to the ECM for detecting faulty sensors and other components, it will successfully detect our efforts. Only P0237 limits fuel delivery when set. The other 3 illuminate the MIL but have no other effect on engine performance.

I can't imagine the hell it would be to try and write tunes for these things. I took a few computer science courses and learned a couple programming languages in school so I can respect the time and energy that goes in to this sort of thing.

The ECM expects to see less than 200 mV from the boost sensor at idle, and a max of 5 Volts at ~36 psi, according to the helms manual. 166 mV at STP would probably be right, I haven't measured anything yet.

Tx Christopher: I want the vanes closed at WOT and high rpm to increase boost. I realize this may not be desirable for certain towing situations, but I think we are all thinking about controlling vane position ourselves.

I just read the operation description in SI2000 for the turbo. (I'm a GMC tech) It looks like the ECM uses the APP, ECT, GEAR, LOAD, MAF, RPM and BARO to control boost. It uses the vane position sensor as feedback on what position the vanes are in. I think it uses the boost sensor just to make sure the boost is at a pre calculated # based on load rpm ect. I do not think you can change boost by fooling boost or baro sensor's. You might be able to change boost a little by fooling the vane position sensor. It is 1 volt when open and 3.5 volts when closed. If the pcm thinks the vanes are open a little more than they really are it should make more boost. Just don't fool it to much or the pressure sensor is going to tell the ecm that there is to much boost for this rpm/load. I don't know if this helps or not just my .02

I just read the operation description in SI2000 for the turbo. (I'm a GMC tech) It looks like the ECM uses the APP, ECT, GEAR, LOAD, MAF, RPM and BARO to control boost. It uses the vane position sensor as feedback on what position the vanes are in. I think it uses the boost sensor just to make sure the boost is at a pre calculated # based on load rpm ect. I do not think you can change boost by fooling boost or baro sensor's. You might be able to change boost a little by fooling the vane position sensor. It is 1 volt when open and 3.5 volts when closed. If the pcm thinks the vanes are open a little more than they really are it should make more boost. Just don't fool it to much or the pressure sensor is going to tell the ecm that there is to much boost for this rpm/load. I don't know if this helps or not just my .02


So how would you know if the boost or the baro were reporting falsely? If either of them were reporting lower than actual would the ECM place the vanes incorrectly creating too much boost?

I am looking for a scenario in which there is actually more boost present than the ECM believes there to be. You think the vane position sensor could cause it, but if the ECM is counting on the boost sensor to verify and it is off then what?

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...4) P0299- The ECM detects that the boost pressure input is 35kPa (~4psi) below the calibrated value. OR To obtain the desired boost, the ECM must command the turbocharger vanes closed further than expected. One of these conditions must exist for more than 12 seconds.
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WARNING: Wildass guess approaching!

Sure sounds closed loop to me. If the ECM is not relying on a subroutine to adjust vane position using measured boost, and is in fact using a table to dictate vane position, then how could the vane position be closed further than what it's expecting to achieve target boost? If it was table-based, it would just show low boost.

ECM's have these basic segments:

Tables: These are lists of values, also known as look-up tables. When it sees 1.24 lb/min and 5.27 PSI, it does 10.2 degrees. Think of them as a multiplication table when you were a kid. No calculations are involved. These are very easy to adjust for GM, are the most flexible, and the most common.

Fixed variables: These are numbers loaded to allow the computer to do it's calculations. 8 cylinders, mass of air at STP, flow of injector, gain rate of pump, 2000RPM when limping. They try not to "hard code" numbers into the program. It makes it confusing to alter the code.

Dynamic variables: Like when the Allison learns clutch pack pressures, the engine can adjust certain numbers on on the fly. Think of this as a scratch pad for temporary calculations.

Main code loop: Normally this not too large. It's the entry point for engine operation. It's a series of instructions that are completed in order, and most of it's instructions are jumping to a subroutine.

Subroutines: These are the real meat and potatoes of the program. It's where all the calculations and actions are done.

While they could make tables for vane positions, the problem would be the number of tables required. A whole bunch. Using a subroutine to adjust vane position using dynamic variables would be easier. Having tables for desired boost, then having the computer adjust the vanes to get there seems like the only reasonable what to get there.

Closed loop system sounds right to me. If you are going on a flat surface at 2000rpm's and your boost is say 5.0 on the Attitude then you start going up a hill and try keeping the engine at 2000rpms the boost will increase as does your foot pressure on the peddle.
I like what McRat said. Of course I don't know jack but it makes sense to me.

Just the words "expected" lean me towards boost control via certain vane positioning, more than likely based off of rpm. Otherwise it would be "calculated" as in read all the sensors then figured out where to land the vanes to get the results it wanted.

So if the ECM moves the vanes to where it expects them to be, and uses the vane position sensor to make sure it did what it wanted to do, and the vane sensor lies, then we could end up with more boost than "expected".

I still am looking for more info on its verification, somewhere along the way it checks to see if it got what it wanted. Those calculation sources are limited, they could only be off calculations via baro and boost sensors.

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It is not covered in any useful detail.

It does state that at idle the boost sensor is used to verify the baro sensor reading.

There are 4 DTC's concerning the boost sensor (brief descriptions):

1) P0234- The measured boost pressure is above the expected range by 35kPa (~4psi) or more for more than 12 seconds.

2) P0237- The boost pressure is less than 37kPa (5psi) for 2 seconds.

3) P0238- The ECM detects that the boost pressure is more than 254kPa (36psi) for 2 seconds.

4) P0299- The ECM detects that the boost pressure input is 35kPa (~4psi) below the calibrated value. OR To obtain the desired boost, the ECM must command the turbocharger vanes closed further than expected. One of these conditions must exist for more than 12 seconds.

It would seem that the boost sensor has an accuracy in the range of +/- 35kPa (4psi). Also the ECM is measuring and calculating boost pressure at the same time. While this is useful to the ECM for detecting faulty sensors and other components, it will successfully detect our efforts. Only P0237 limits fuel delivery when set. The other 3 illuminate the MIL but have no other effect on engine performance.

I can't imagine the hell it would be to try and write tunes for these things. I took a few computer science courses and learned a couple programming languages in school so I can respect the time and energy that goes in to this sort of thing.


Also it looks like the ECM is willing to live with as much as 4lbs boost too much without complaining. That isn't a very tight reign, thats 20% off at 20lbs expected and a healthy difference.

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