Not sure where put it, info from @STAVTECH facebook page about Ford Focus WRC (photo in a link): "After yesterday's post about cheating in motorsport got a massive response and countless replies with other stories (unfortunately, with no proof for 99% of them, just rumours and stories), I thought I'd post this awesome bit of rule twisting by the Ford WRC team with the Focus.
See that huge piece of welded metal there? That's a 45ltr, 2mm thick titanium tank that was normally hidden behind the rear bumper of the 2003 spec cars which were fitted with fatter US-spec rear bumpers, and you know what it contained?
BOOST!
Connected to the inlet manifold by 30mm diameter titanium tube and via an electronically controlled valve, the tank was filled with pressurised air from the turbo at part/off throttle situations where the engine airflow needs are low, but thanks to the anti-lag system the turbos compressor could still flow huge amounts of air which was otherwise surplus to requirements.
Thanks to the small 34mm inlet restrictor all WRC cars had to run in an attempt to keep power levels in check, the turbo, especially as the revs rose, couldn't supply all the air the engine would require to hit its true power potential, but with a 45ltr tank full of pressurised air, the valve would be opened by the ECU at full chat, releasing the air back to the engine with no inlet restrictor in the way, allowing a sudden performance increase, especially at higher rpm, over what an inlet restricted WRC engine could normally produce; albeit until the tank emptied!
As 100% of the air had originally still passed through the inlet restrictor, it was, technically at least, totally legal, but just like a lot of interesting inventions used in motorsport to bypass the regs, it was soon banned!" https://www.facebook.com/STAVTECH/posts/882622721914251