As the boys, quite rightly say, this "ecotec" jobbie don't work.
As for the recomendations bit, it's all about money and the placibo effect.
if i get you to beleive it works then you'll tell others, it's a sales technique.
how it works it by using the vacum in the inlet manifold to open the valve.
At idle the engine has a vacum of arround -13 to -14psi (aprox) as you rev the engine by opening the throttle the vacum becomes less untill, at full throttle, the vacum in the manifold is 0 (neither vacum nor positive presure).
When you rev the engine then let off the throttle, the time it takes for the engine to return to idle is known as over run. In the over run condition the vacum in the manifold is more than idle vacum, thus, the extra vacum opens the "ecotec" valve.
Now, the reason the ecotec valve is no good is because in the factory ecu, it's programed in to cut off all the fuel going into the engine whilst in over run condition, untill the engine speed reaches somewhere arround 2000 to 1500rpm, then the fuel comes back in to stop the engine cutting out, and lets be honest, if there is no fuel going in then it can't save any more can it?
So, concidering the only time the valve and the fuel are active at the same time is when the engine is on over run bellow 2000rpm, just how much fuel can this save?
And finaly, with reguards to performance, if you use it on a turbo motor, like they say you can, then the boost presure in the manifold when you're on throttle will keep the valve closed.
Pete