Originally Posted by Felix 808

They're not designed to work that way, search NACA Ducts. If you don't have a proper temp gauge for H20, it would be hard to see the difference. I installed my guage after the vents so I don't know, but temps while driving will be minimally effected. What I do know is that they help tremendously once the car is parked. I use to go out 3 hrs after a drive to plug my car in (Battery Tender) & all kinds of heat would still pour out of the hood when I opened it. I don't have that problem anymore. So for me it was both function & fashion.