Here are some more results:
using a radioshack sound level meter positioned approximately 1' from the exhaust pipe aiming directly into the muffler:

'A weighting' - "responds mainly to frequencies in the 500-to-10,000Hz range, which is the human ear's most sensitive range" - the manual. it said to use this setting for measuring an area's noise level.
at 1650 rpm (what my car decided to idle at today) 97dB open 91dB closed
at 3000 rpm no load 103dB open 92 dB closed.

under 'c weighting': "cweighting has c-curve (flat) frequency characteristics. This setting causes the meter to respond mainly to frequencies in the 32 to 10,000 Hz range"
at 1650: 113dB open, 108 dB closed
at 3000: 114dB open, 103 dB closed

The meter was set to the slowest response setting. It averaged the sound levels over half second intervals. The values stated above fluctuated + or - 1 dB. I took the value that was the center value.

The test car is a 13bt half-bridgeport with a 500r turbo (gt35rs) and a 4" exhaust.

I will be making more of these if people would like one. PM me if you are interested.

 

Hooked up an electric motor to the little valve. The valve will trigger when power is applied to the trigger wire. I am going to hook this to a boost switch that will trigger at positive pressure. I am probably going to get a faster motor. Maybe something that can get it fully open in 0.5s-0.75s. Results to come, its going on the car this weekend/early next week.
a little video:
http://www.youtube.com/watch?v=cliHr85QO_4