Losing boost mid-pull

[Blue5.00]

Running a gen 1 coyote with a 2021 GT500 Predator Supercharger. I have a pressure sensor tapped into the lid to monitor boost pressures. I am also currently running a 4 inch pulley - supposed to make about 8# boost.
In normal driving I never get out of vacuum. However, when I put my foot in it, it builds boost like Id expect up to 5-6 psi and then I lose all boost. This car goes from 6psi about 2/3 through a gear to slight vacuum (about -7 psi) MID PULL. It really falls on its face.
Im lost on this one, too. The bypass valve appears to be set up correctly and the inlet/outlet match multiple sources online. There are also no sign of belt slip.
My thought process is maybe the bypass valve wasnt designed for this low level of boost - custom pilley making about 8psi vs the factory 12psi - and is acting up.
Any help is appreciated!

 

[DSG2003Mach1]

What ecm? Maybe something is triggering the bypass to dump boost?

 

[Blue5.00]

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[Blue5.00]

What ecm? Maybe something is triggering the bypass to dump boost?

ECM - engine control module? Stock ecm and ecu. Tuned by VMP. However this bypass is purely mechanical. The top port goes to a vacuum reference after the throttle body and the bottom port goes to the intake tube.

 

[biminiLX]

Got to be a bypass issue.
Why such low boost? Factory GT500 is 10-11psi (on my dash).
-J

 

[Blue5.00]

Got to be a bypass issue.
Why such low boost? Factory GT500 is 10-11psi (on my dash).
-J

I would like low boost to be safe with this unopened gen 1 coyote. Paired with the fact that I cant reliably get higher than 92 octane fuel and I was already experiencing knock with a 3.5 (~10 psi) pulley.

 

[biminiLX]

I assumed it was an octane issue but if you don’t drive it a bunch Boostane could be your friend. I’m an E85 guy but my tuner has really done well with 93/Boostane as well.
My younger brother is about to fire a single turbo Gen 1 build and I plan to lock his access to the wastegate after we get a 10# spring
Good luck man
-J

 

[Blue5.00]

I did some more testing today. First I moved the physical bypass valve down. Im thinking this will give it more "leverage" to seat and seal when boost is present. I also checked all hoses and everything looks to be in order - nothing is really standing out to me.

Then I went out for some datalogging. Some characteristics Ive noticed...
- It does this in every gear (maybe not first but thats kinda irrelevant)
- It does NOT do it at WOT. I was able to test out my custom 4 inch pulley and it indeed makes just at 8 psi WOT.
- It does not seem to do it went I gradually get into it - maybe. I was able to merge onto the highway in 3rd and more or less not encounter anything. However, Ive done very slow/stable pulls in the past with it still losing boost.
- I am not encountering any knock whatsoever.

The datalogs look fine (to my untrained eye) and Ive verified about 30 times that the bypass is routed correctly. To which it functions normally: vacuum occurs except when on throttle, no weird grinding noises either.

Genuinely at a loss for what this can be, unless its tune related, but its done this since the start of tuning and has stayed through 5 tune iterations. Ill send in the datalogs and cross my fingers that theyll find something...

 

[Blue5.00]

I assumed it was an octane issue but if you don’t drive it a bunch Boostane could be your friend. I’m an E85 guy but my tuner has really done well with 93/Boostane as well.
My younger brother is about to fire a single turbo Gen 1 build and I plan to lock his access to the wastegate after we get a 10# spring
Good luck man
-J

Its funny I just ran a tank of octane booster through (napkin math to about 96ish octane) it as a pursuit to find knock or false knock - thats why my pulley is the size of a dinner plate. But I did have some driving time with the stuff and the cutting out still occurred.

 

[biminiLX]

Replace the bypass and recheck unless something obvious from their datalog review would be my best advice.
Does it ever occurred at WOT?
-J

 

[robvas]

Have you had a PD supercharged car before?

 

[Blue5.00]

Replace the bypass and recheck unless something obvious from their datalog review would be my best advice.
Does it ever occurred at WOT?
-J

Sounds good, hopefully the tuner has something to say. It has not done it at WOT yet from my recollection.

 

[Blue5.00]

Have you had a PD supercharged car before?

Nope, first time.

 

[robvas]

Nope, first time.

I'm almost wondering if it's normal, you have to basically be WOT to get "all the boost"

Part throttle won't make boost after a certain RPM

Some people think it will always be there

 

[Blue5.00]

I'm almost wondering if it's normal, you have to basically be WOT to get "all the boost"

Part throttle won't make boost after a certain RPM

Some people think it will always be there

I can definitely tell when the bypass closes. But, from my memory, it loses boost without me changing pedal position - its just during a pull it almost "gives up" towards the top (5-6 psi) and loses all boost.
Maybe I need to beat on it harder.

 

[Blue5.00]

Ok I understand if this is the ghetto way to go about this, but not sure how to upload whole datalogs. Im noticing that my TB angle is dropping seemingly right when I lose boost pressure! Dark blue is TB angle actual and the light blue/green is rpm.

This log is a 2-3-4-5 shift and it lost boost each time. Can anyone make sense on why my throttle body would do this - or if its normal?

That could be why Im losing boost: the TB would close significantly, making the bypass valve open!

 

[Blue5.00]

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[biminiLX]

Tuner should be able to help that.
I’m assuming there’s a limit or ‘safety’ closing the throttle.
WOT is 82* and if your foot doesn’t change from WOT then the EUC is closing the throttle.
Here is my 1110rwhp pull. You’ll see the power graph where the ECU closes

the TB (due to some safety load limit) then it open back up.
Stock DBW TB tuning is a tricky devil.
-J

 

[Blue5.00]

Tuner should be able to help that.
I’m assuming there’s a limit or ‘safety’ closing the throttle.
WOT is 82* and if your foot doesn’t change from WOT then the EUC is closing the throttle.
Here is my 1110rwhp pull. You’ll see the power graph where the ECU closesView attachment 1888122 the TB (due to some safety load limit) then it open back up.
Stock DBW TB tuning is a tricky devil

A quick little flex on the 4 figure dynograph huh? Haha nice.
So youre thinking its a safety parameter concerning load values? Even when these weren't WOT pulls (max TB angle actual = max TB angle desired at ~45 degrees)?

Regardless, do you figure this is the culprit?

Thank you very much for your insight btw!

 

[biminiLX]

Yes a fun flex
But in building that monster (‘14 GT500 5.8 with a 2.65 TVS) learned more than I want to on TB DBW and factory tuning (SCT then HP Tuners).
You can see there’s a 6200-7200 range roughly where it exceeded factory load
Parameters and shut the throttle but it reopened at 7200-7500. Had us fighting that and other load limit tables with the TB.
Based on seeing your TB angle close I’d suspect a tuning parameter.
I had several we had to find but it could be it’s fine at WOT but not expecting boost at lower TB angles and shutting you down.
Let us know what VMP says on the tune. Good luck.
-J