Quote:
Originally Posted by Maestro666
I have a theory that the hissing is caused by vibration of the exhaust piping when you hit a certain frequency. The hissing gets louder as you change out exhaust components, not because you are freeing up exhaust flow, but because you are changing to piping that vibrates more freely.
You could test this theory by jacking the car up and clamping some heavy weight to the most exposed section of the exhaust.
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There is a couple of old threads on the hiss. The throttle blades go wide open just off idle. The speed is controlled by inlet valve lift and injector dwell time. The hiss comes from the throttle blades being open and no fuel being injected. Creating vacuum being pulled through the throttle bodies. That's what I read any way.