Remind me of how the bi-mode exhaust works

[LIL QIK]

Shortly after I picked up my ZL1 I started modding and one of the first was putting on the complete Magnaflow 3" exhaust. Well the drone has finally gotten on my nerves enough to put my stock exhaust back on. So I get it on but don't think it's working any longer or working the way I remembered anymore.

So.... I was under the impression that it was loud when you first fire up and pretty much under wide open throttle only....

But brother is thinking that it's only quiet under steady state driving....

I'm wondering if my cam just isn't pulling enough vac to make them work correctly. Sorry....I know this is kind of a rookie question....but your help would be greatly appreciated......

[jsd]

Start the car, the exhaust is open till ~1000 rpm, then closed till ~ 3500 rpm

[LIL QIK]

so it's really only closed between 1000 and 3500 rpms?

[brian-nicola]

Yep.

[omegaman]

If you want more control over the exhaust bypass valve, consider buying the Forza Dual Mode Exhaust Controller. I have this device. It's excellent and easy DIY installation. $78.

[LIL QIK]

Thanks for all the input guys. Something ended up being wrong with mine....so I just bypassed it all and made them strictly vacuum operated. That way they work pretty much like I want. Its fairly quiet until I drive a little aggressively...and then it sounds a little aggressive...lol. Think I'll see how this route goes for a while...

[KaBoom1701]

First, a quick overview of how the system works

It is basically electrical control over vacuum actuation. The engine supplies vacuum to the system, the vacuum is stored in a tank to cover periods of low or no engine vacuum (WOT and start up). A check valve is upstream of the tank to keep the tank from dumping vacuum back to the intake manifold when there is no vacuum source. Electrical control comes from magic boxes sensing everything and deciding when to open and close Q15 (exhaust flow control valve).

The Camaro NPP factory system is programed to be open at idle, close about 1,500-2,000 rpm during normal driving and be open at > ~75-80% throttle position. I don’t have one to verify, but I believe the Corvette programing is different (closed at idle).

The bellow picture shows the vacuum lines, storage tank, control valve and actuator. The funny thing about my vacuum tank, it has “top” marked on it that is on the bottom. The tank is the black box on the right with a single vacuum connection, the control valve is protected by the Tetris shaped cover just to the left of the tank and the actuator is the round object attached to the output of the muffler. There are 2 actuators, one for each muffler. The control valve receives a 12v input to open vacuum to the actuator, when no 12v is present it dumps vacuum from the actuator but does not dump vacuum from the tank.