2.0 boost - when & what controls boost

[jlrusoin]

Sorry if this question has been asked but I cannot find it on here or google.
Have the Tazer JL installed and I’ve been watching the boost readout.

Notice when in 4 lo it is not in boost until over 3000 rpm’s and even then it’s a very low psi. When cruising down the highway I at times see 10+ psi in 8th gear when at 2000 rpm’s prior to down shifting. In 2 hi it is in and out of boost all over the rpm range.

This makes me curious about how the turbo is controlled. Why is boost produced at different rpm level and how? Know it’s by the ECU but what is monitored to control the level of boost produced and when it is being produced? Seeing this I assume they are trying to keep out of boost while in 4 lo so when rock crawling there is not sudden power surges. Is this correct?

Will be interesting to see if the turbo boost does the same in 4 hi. Does anyone know?

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[carlhenry]

their is a blow off valve in the intake at the manifold they sell a few with dif pressure they aint cheap

 

[jlrusoin]

I understand that and maybe I’m making it too complicated. It seems if it was just the blow off valve there would be the same boost psi at the same place in the rpm range and shift patterns.

 

[oldcjguy]

It's not just the ecu. Turbos work off of load and cylinder/exhaust pressure. When the engine is under load exhaust pressures go up and the turbo spins.
In 4 lo the gearing is so low and torque multiplication is so high that the engine doesn't have to work very hard to make the torque needed at the wheels. The engine rarely gets loaded heavily. If you were ever in 4 lo and the engine was getting bogged down, cylinder and exhaust pressures would go up and the turbo would spin up making boost. In 2 hi in 8th gear it's the exact opposite. Gearing is so high that torque multiplication is actually negative at the wheels and the engine can be loaded up very easily. Ok not negative but... When t-case is 1:1 (2hi) and auto trans 6th gear is 1:1 torque multiplication is 1. 300 ft lbs of toque at the flywheel results in 300 ft lbs at the output shaft. If 8th gear overdrive is 0.5:1 (just an example) 300ft lbs at the engine results in 150 ft lbs at the output shaft. When in first gear and say a 3:1 first gear 300 ft lbs results in 900 ft lbs at the output shaft.

So, you can imagine how much torque can be transmitted to the wheels with a 4:1 transfer case and a 3:1 first gear (12:1) So to deliver 600 ft lbs the engine only needs to generate 50 ft lbs. No going to generate much cylinder pressure there.

 

[jlrusoin]

It's not just the ecu. Turbos work off of load and cylinder/exhaust pressure. When the engine is under load exhaust pressures go up and the turbo spins.
In 4 lo the gearing is so low and torque multiplication is so high that the engine doesn't have to work very hard to make the torque needed at the wheels. The engine rarely gets loaded heavily. If you were ever in 4 lo and the engine was getting bogged down, cylinder and exhaust pressures would go up and the turbo would spin up making boost. In 2 hi in 8th gear it's the exact opposite. Gearing is so high that torque multiplication is actually negative at the wheels and the engine can be loaded up very easily. Ok not negative but... When t-case is 1:1 (2hi) and auto trans 6th gear is 1:1 torque multiplication is 1. 300 ft lbs of toque at the flywheel results in 300 ft lbs at the output shaft. If 8th gear overdrive is 0.5:1 (just an example) 300ft lbs at the engine results in 150 ft lbs at the output shaft. When in first gear and say a 3:1 first gear 300 ft lbs results in 900 ft lbs at the output shaft.

So, you can imagine how much torque can be transmitted to the wheels with a 4:1 transfer case and a 3:1 first gear (12:1) So to deliver 600 ft lbs the engine only needs to generate 50 ft lbs. No going to generate much cylinder pressure there.

Great information thanks

 

[jjandascog]

So, you can imagine how much torque can be transmitted to the wheels with a 4:1 transfer case and a 3:1 first gear (12:1)

You forgot the rear end gearing, even more torque to the rear wheels that out of the transfer case.

 

[jlrusoin]

This torque information is great and true but I'm thick. What tells the ECU boost is not required and for the ECU to open the waste gate to not provide boost? How does the engine know its under stress and more power is required (the turbo needs to provide more boost)?
The ECU appears to be "telling" the engine to stay out of boost while in 4 low until the rpm's are high.
On the highway boost will increase prior to down shifting, and then at the higher rpm less boost is produced, indicating the wastegate went from closed to open.
In 4 low the waste gate has to be being commanded to be open by the ECU. I figure this is because of the great gear reduction (and torque as a result) and large power, boost, is not required. Just trying to verify this is or is not true.
Why does this matter? It doesn't but would be interesting to know. It also helps make the argument the little 2.0T is good enough for rock crawling even without the turbo and V8 swaps are not necessary.
Would be a better conversation in person.
The below is basic but gets a little bit to my question.

___________________________
The ECU can vary this duty cycle signal to the wastegate solenoid to vary the boost higher or lower, to account for temp changes, atmospheric pressure changes, provide a steadier boost curve and even cut boost in situations when a failure has occurred. The ECU has many special maps/tables that determine the DC to use to control boost. This includes target boost tables, min and max DC tables, Boost error trim tables, air temp trims, speed trims gear trims and many more. The ECU uses these with a complicated algorithm to calculate duty cycle and then adjust it to reach its target boost. Here is an example of how this works and an example of the diffent typs of maps used to control boost.

If the ECU sees the driver giving 70% throttle at 4000 RPM it reads a map that says it needs to hit 14psi. The ECU starts with a duty cycle of say 50%. Say it only hits 12psi at 50%, the ECU knows its off by 3psi and adds more duty cycle based on a boost error trim table. Say this equals 5% more duty cycle. Now its running 55% and the ECU is seeing 15psi of boost. Its still a bit high and using this boost error trim table it now pulls some duty cycle from the overall 55%. Lets say it removes 2% and now is at 53%. Now it hits its 14psi target and the ECU is happy until the boost starts to fall as the RPMS rise, or until the driver gives 100% throttle making for a new target boost. The calculations start all over again. This back and forth, and calculating happens many times a second to provide the engine with a steady boost curve. This is a very simple version of what actually happens. There are things like the time it waits to see if the calculated duty cycle actually worked, and start RPM's, gear ratio comps air temp trims, and many other things. This is a pretty complicated process but its what makes modern day turbo cars function so well with little turbo lag and all the safety features.

 

[Deleted member 53226]

Sorry, all you could not be more wrong. Everyone knows boost is controlled by a big red button on the dash panel. How do I know? Back in the day I had a bitchin Trans-Am that had the same boost option. Bought it from some effeminate idiot named Michael.

Decent ride, but had to put black duct tape over some awful aftermarket red led's on the hood. All kind of weird blinking lights and switches on the dash. Never did figure out what half of them actually did. Also, the damn car wouldn't shut up and had a snotty rich-guy accent, but it sure could move when I hit Turbo Boost!

The car aggravated me after a few years with all its whining how I never changed the oil, and telling me I couldn't eat my Burger King meals in the car, or have my dates with various "ladies of the night" while parked. Around 150K miles (and never having changed the oil) I ended up selling it to some deaf guy who didn't even know the car talked!

 

[oceanblue2019]

It's not just the ecu. Turbos work off of load and cylinder/exhaust pressure. When the engine is under load exhaust pressures go up and the turbo spins.
In 4 lo the gearing is so low and torque multiplication is so high that the engine doesn't have to work very hard to make the torque needed at the wheels. The engine rarely gets loaded heavily. If you were ever in 4 lo and the engine was getting bogged down, cylinder and exhaust pressures would go up and the turbo would spin up making boost. In 2 hi in 8th gear it's the exact opposite. Gearing is so high that torque multiplication is actually negative at the wheels and the engine can be loaded up very easily. Ok not negative but... When t-case is 1:1 (2hi) and auto trans 6th gear is 1:1 torque multiplication is 1. 300 ft lbs of toque at the flywheel results in 300 ft lbs at the output shaft. If 8th gear overdrive is 0.5:1 (just an example) 300ft lbs at the engine results in 150 ft lbs at the output shaft. When in first gear and say a 3:1 first gear 300 ft lbs results in 900 ft lbs at the output shaft.

So, you can imagine how much torque can be transmitted to the wheels with a 4:1 transfer case and a 3:1 first gear (12:1) So to deliver 600 ft lbs the engine only needs to generate 50 ft lbs. No going to generate much cylinder pressure there.

Good info, and remember you also have the axle ratio working to multiply again.

 

[Lapis]

As far as how the ECU reads boost pressure and controls it, that comes for the MAP (Manifold Absolute Pressure) Sensor.

But I think part of what you may be confusing is whether the turbo is making boost or not. A turbo is different from a supercharger in that the amount of pressure is not linear with the RPM band. A turbo is not always spinning are a certain amount when the engine is.

For simplicity, lets imagine 1:1. A supercharger is always spinning 2000rpms when the engine is 2000rpms. regardless of load, gear, etc. This is not the case with a turbocharger. The rate the turbo charger spins is based on the pressure in the exhaust, which is based on the load on the engine. So when your engine is turning 2000 rpms in first gear 4lo and easily gliding over rocks with little to no effort, it's not producing very much exhaust pressure, so the turbo may only be spinning 50 rpms, not enough to build any boost. It's not that the ECU is wasting all your boost through the bypass valve, it's just that the engine isn't under enough load to build boost.

 

[YellOhJL]

As far as how the ECU reads boost pressure and controls it, that comes for the MAP (Manifold Absolute Pressure) Sensor.

But I think part of what you may be confusing is whether the turbo is making boost or not. A turbo is different from a supercharger in that the amount of pressure is not linear with the RPM band. A turbo is not always spinning are a certain amount when the engine is.

For simplicity, lets imagine 1:1. A supercharger is always spinning 2000rpms when the engine is 2000rpms. regardless of load, gear, etc. This is not the case with a turbocharger. The rate the turbo charger spins is based on the pressure in the exhaust, which is based on the load on the engine. So when your engine is turning 2000 rpms in first gear 4lo and easily gliding over rocks with little to no effort, it's not producing very much exhaust pressure, so the turbo may only be spinning 50 rpms, not enough to build any boost. It's not that the ECU is wasting all your boost through the bypass valve, it's just that the engine isn't under enough load to build boost.

Yep. Same reason why you don't make boost when you rev the engine while not in gear; no load.

 

[oceanblue2019]

Wastegate sets base PSI max and an electronic boost controller (controlled by the ECU) will control boost over the wastegate setting.
Load builds boost, but even at full load, the ECU may be limiting boost to wastegate only pressures in 4lo.

I'm surprised this motor is only pushing 10psi, I'm looking forward to the tuned 4-500hp 2.0ts of the future.

The ECU controls the waste gate to regulate boost. ECU also controls the bypass valve to recirculate boost when the throttle closes to prevent compressor stall.

Long ago these were diaphragm operated off engine vacuum/manifold pressure but for many years now it's all solenoid controlled by the ECU. Better response, less plumbing, and ability for better diagnostics and limp modes.

With a JB4 I see closer to 22psi peak in tune-1. Tune-2 was a bit higher, but with winter gas I've reverted back to tune-1.

The 10 psi mentioned here has to be a mistake; it is hard to capture max boost events with the Tazer JL readout - you really need proper data logging at a fairly high sample rate.

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