Last year I swapped my tranny from A4 to M6.
Before the swap I already had a Vortech supercharger and shifting at high rpm was obviously not an issue.

Now, with the M6 I notice something strange: when I shift the engine speed doesn't drop as fast as it used to be without the supercharger.
I don't powershift, it's a normal shifting between 4000 and 5500 rpm.
It's like the blower was pushing some air into the intake manifold not allowing the engine to reduce the rpm in a proper manner.
There is about 1 second delay

Is my Bosch bypass valve too small? too slow?

Do I need an adjustable bypass valve in order to reduce the spring compression?

Do I have to increase the size of the rubber hose between intake and bypass valve?
In the original Vortech configuration the bypass valve is connected to the hose that goes from the intake to the brake boost, it's rubber hose: about 2 ft long and 0.15" inner diameter (5/32").

Or is this kind of shifting just normal?

Here a picture of the engine bay to have an idea of my setup...


Thanks for every hint!

Stefano

Hard to guess but could be popoff valve is either not opening at right time, taking too long to open, installed in wrong location or in some cases I have seen where 2 popoffs are needed.

Also could be too much timing in the areas where going into decel, still too much airflow and timing is set assuming there is less airmass or due to time during upshifts while lifting on gas pedal closing butterfly of TB but too much airflow due to boost

Original (supplied by Vortech) BOV was a Bosch # 0280 142 102. This valve was making a farting noise at a specific manifold pressure so I decided to install the updated version: a Bosch # 0280 142 108 (same as Audi # 034 145 710 C, also installed in some Porsche's models).
Theoretically it should open at the same pressure...

Timing: do you have an example of a boosted LS1 timing table? (for the shifting related cells). I'd really appreciate that!

Is it possible that I need to reinforce the butterfly spring?

What happens (or should happen) with the IAC pintle during shifts? (Idle Air Control).
Is it closing completely?
Is it a little open? If a little open is there a way to set it to closed during shift? (Using LS1Edit)

Thank you - Stefano

I compared the valves and find out that the updated Bosch valve needs 30 - 50% more force to open than the older one.
On top of this there is a restriction (!) in the connector for the vacuum hose.

The result is that the BOV closes too soon, building pressure in the manifold at lower RPM.
In that particular spot (when the BOV closes) the timing is still pretty high causing KR.
The reduction in the connector causes a delayed opening of the BOV resulting in high pressure between blower and TB, and also too much air entering the intake.

I reinstalled the older valve: no more KR and better shifting at high rpm

Nice work, this is an important find for others who also use bleed off valves like this with boost.

Anywhere on the internet people can look at the model numbers and see what their operating specs are so they know which to choose ?

I checked several US and European car links and forums... no way to know what those valves are rated. They just say which valve is closing harder than another, but no ratings.

There are some manufacturers of adjustable BOV's (starting from $120). I'd go with one of them and adjust it to better fit the specific setup.

Some can be adjusted by a screw, some other changing the spring (they have then different colors depending on the rating).

If I have time I'll open mine and try to make it adjustable.