On the Cappuccino it’s very easy to get a dump-valve-like “woosh” from the car. As standard the car recirculates the air “dumped” from the Air Bypass Valve back into the intake side of the turbo (after the airbox, before the blades) but by removing this recirculating pipe – or by simply disconnecting it from the ABV and plugging it (a plastic wine cork works a treat!) – the ABV now vents to atmosphere instead, creating this dump-valve-like noise. I’ve always been much keener on turbo chatter over dump valve noises however (in my head chatter = race, dump valve = street) so I decided to go one step further than the usual ABV modification to get the Cappuccino chattering.
But first, some instructions for the ABV modification (as some of this applies to the chatter mod too). Remove the recirc. pipe (33, in red) from the ABV (27, in yellow) and plug the end of the recirc. pipe with a plastic cork or similar. If you can tuck the end away slightly so it doesn’t interfere with the ABV exit – now dumping to atmosphere – all the better, and that’s it. Your car should now sound like this, though admittedly probably a lot less muffled than in the recording!
To create chatter we have to stop any venting of the dumped air, either by recirculation or to the atmosphere, so that the pressurised air builds up and tries and go back through the turbo. The chatter you hear is the air passing over the turbo blades and being chopped up as it does so*. To stop venting, we need to remove the ABV, blocking up the vacuum feed (32, in blue) as we do so, before sealing up the ABV mounting flange. I decided to draw up a custom blanking plate in CAD and get Waterjet engineers Accujet to run off a small batch for me – they did a fantastic job with the fabrication and the final article looks spot on. It sounds pretty good too!
*Some people say that this chopping and “stalling” will kill a turbo very quickly but it’s more about the shock loads – of which the choo-choo noise is an audible marker – reducing the life of the turbo over time. It’s basically up to you to decide whether the chatter is worth the risk! (More info)