# Comparison of commanded vs. actual boost pressure



## Snipesy (Dec 7, 2015)

kelaog said:


> I wouldn't think this turbo is capable of providing any boost pressure at idle but I'd be curious to see what everyone else is getting. I'm trying to track down a black smoke issue and I haven't been able to find a boost leak but perhaps this is one.


They make lots of boost at idle. It is needed for the EGR. That was basically the entire reason we switched to VGT.

Normal without EGR is like -1 to 0 psi. With EGR you can get up to 5 psi at idle.


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## pandrad61 (Jul 8, 2015)

VGT turbos can do a lot now a days.


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## kelaog (Aug 1, 2019)

Snipesy said:


> They make lots of boost at idle. It is needed for the EGR. That was basically the entire reason we switched to VGT.
> 
> Normal without EGR is like -1 to 0 psi. With EGR you can get up to 5 psi at idle.


Thanks! 

OK so what I'm seeing is normal then (no egr). Back to the drawing board. I will try unplugging EGR and see if the map is any different. I strongly suspect this is just a bad tune. I want to say my Lambda is around 4-5 at idle. I'll check again later.

Andrew


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## Snipesy (Dec 7, 2015)

kelaog said:


> Thanks!
> 
> OK so what I'm seeing is normal then (no egr). Back to the drawing board. I will try unplugging EGR and see if the map is any different. I strongly suspect this is just a bad tune. I want to say my Lambda is around 4-5 at idle. I'll check again later.
> 
> Andrew


Okay okay okay.
So if you unplug EGR it still thinks it has EGR.... And thus the boost will be higher.
Unless you eitheir explciitly tune that out OR are at very specific parts of a regen cycle (where tune OEM tune will intentionally run with no EGR).

This is also true if you increase the base flow tables (effectively same thing). The nonemissions example tune has roughly what the boost tables should be for a non egr setup.


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## Snipesy (Dec 7, 2015)

kelaog said:


> Thanks!
> 
> I want to say my Lambda is around 4-5 at idle.


I don't remember the exact MAF values but we nearly halved it for running with no egr. I want to say the stock 'no egr' setup was like 15 g/s and we changed that to like 8 g/s. At idle. Which you guessed it... Would roughly halve the lambda.

For the most part as long as the smoke limiter lambda value is lower than whatever the real lambda is.... No adjustment will be made. The 'desired' lambda value is going to be controlled by a calculation based on the MAF and the desired fuel. And this is how fuel adjustments are done. There is no strict 'lambda' table.

And how do you adjust MAF? You just adjust boost. The ECM does the rest for you.


"So if the ECM just bases it all on boost why is there a flow table for the EGR". Yeah idk. Ask GM.


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