As far as I'm aware, the noise limits are the same as last time, when 3 people got black flagged.
They were:
Bruce - high boosting RS500 engine
Martin - high boosting RS500 engine
Dave - he had a Vauxhall 2.0ltr powered Westfield on throttle bodies.

That's 3 cars out of 135.

Personally I think induction noise had a big part to play in all the above cars failing the drive by.

Scoobies are notoriously noisy, especially once they've got a modified exhaust, so it was no great surprise when a load of them failed.

Prokie's old Corsa had a sports exhaust and an induction kit and that passed fine - as did our Sierra - although it was only running about 350hp at the time...