having read many debates regarding the Vectra 'c' mods i would like to pass on some facts surrounding this modification, after talking to a family friend who has worked for Vauxhall technical for 18 years.
Vauxhall spend millions on each vehicle they develop and were one of the first manufacturers to actually fit the air inlet pipes to the front of vehicles to ensure cold air feed. no matter how big the air scoop is, the AFM/ECU and cubic capacity of the engine will determine how much air is used and for ideal combustion this will be 14.7 parts(by weight) of air to 1 part fuel. The stock pipework is made such that it allows good linear/smooth airflow with good cold air feed from the front of the car to enter the inlet manifold quickly and without disruption. derestricting the airbox for more power is a myth and only serves to louden the induction noise and actually reduce the RAM effect that was there in the first place, it also upsets linear flow. The main thing is to have a cold dense air flow feeding the inlet manifold with as least disruption as possible. Which is exactly what Vauxhall have done with the stock tubing. So original airbox and pipework with a decent panel filter... job done and hard earned pennies stiil in your pocket.