Flapper valve
 

For the NA people, does anyone know how to control what RPM the valve on the Y pipe opens? Mine is opening around 4200 rpm which if I remember correctly it should be about 1000 less than that.

My '89 opens at about 4k, also. I researched the hell out of that AICS last year. 4k is the correct #. Always looking for cheap torque, I experimented with it by applying vacuum at different RPMs to see if it made any differences in HPs. None noticed, so I put it back to factory condition.

mine also opens right before 4k. at about 3700-3800. looks like its about normal.

How do you figure that out???