I am trying to figure out when my second turbo kicks in, as in driving down the motorway in auto mode and climbing the speed slowly up to say 100mph

I just cannot hear it, I hear the first turbo faintly with its weak high pitch whistle but when I say its faint, you really have to listen for it, but at what speed do you think the second turbo kicks in - I was expecting the turbo noise to be discernibly louder ?

I have a GAP IID, would it give me some data if the second turbo is working OK ?

Also is the primary turbo bigger than the secondary turbo ?

Second turbo doesn't activate untill about 2400rpm so even at 100mph it will still be at idle I would have thought.

It’s not speed, it’s rpm… / engine load / throttle position.

You will feel a good shove when it kicks in, keep your foot in it so it holds the gear. If you take your foot back it will change up and the revs will drop.

If you just drive on light throttle it won’t spin up..
 2011 Range Rover Vogue SE 4.4 TDV8

Previous cars :
2003 Range Rover Vogue TD6
1999 Discovery Td5 ES
1995 BMW M5 3.8 6 speed
1992 Range Rover 3.9 Efi Vogue
1992 BMW M5 3.8
1988 BMW 735i SE
1989 Ford Sierra XR4x4 2.9i
1981 Ford Fiesta Supersport

Use the flappy paddles if you want to keep revs up without being so foot heavy …