I did a set of custom valves before on a car that we had the flapper in the valve custom made with holes in it so closed it was quit and open it was straight through... I have done this on a couple NA cars that need big exhaust but wanted it to be quit on the street.
How fast are they to open? I knew a guy the did that to his Mustang and they didn't open fully till he reached the end of the track, but he insisted they helped?!
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They don't snap open but they do take 3-4 seconds to open fully. My next mod is a delay module to supposedly help torque in the short times at the track.