"My 56reg (reg Nov 2006) 330d E90 with 100k was de-flapped by PMW 2 weeks ago. It was INSTANTLY noticeable, when I was driving out of their car park, that I'd lost some torque. I requested my old parts and after 2 weeks of driving, I'm going to re-install them. They are the cartridge type and look as good as new except for minor deposits which I'll remove before I put them back. I estimate a 50% reduction in torque at c1000rpm and possibly 25% at 1500rpm. I cannot understand people who say they do not feel any difference. They must never drive at under 1500rpm. Also, my fuel consumption has increased markedly, especially around town, where I estimate it to be c5>8mpg worse, and on the motorway, where I generally cruise at 80>85mph, to be 3>5mpg worse. At 80mph the engine is only turning at 2000rpm, so the flaps will have only just opened fully. My job is developing inlet and exhaust ports, and inlet and exhaust manifolds, and I know exactly what the swirl flaps (or port de-activation units as we call them) are for. At low RPM they close off one port which doubles the airflow through the open port which gives much improved air/fuel mixing at low RPM and thereby gives lower emissions, increased torque, and better fuel economy. This is done by changing the in-cylinder air motion from Tumble (with the flaps open) to Swirl - hence the name. I hope this adequately explains their purpose.
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