Corvette muffler exhaust valve

[maydog]

I have been doing some recreational reading about the engineering of the new corvette 7 liter. There have been several mentions of an actuated valve in the exhaust system. Some sites have said this is to increase backpressure and to improve low end torque and others have mentioned it is primarly for sound control.

I do not understand how restriction and backpressure can help build power at any engine speed. I am not well versed in the whole pumping mechanics of an IC engine, but to me it seems that that back pressure would be fighting engine rotation, pushing down on the pistons trying to force the exhaust out.

What is the deal, does backpressure actually help low end power (torque is just proportional)? Is this the reason it is on the corvette?

Finally, if this is true why wouldn't this exhaust system benefit other cars (say a sts)?

[32v_dohc]

If the pressure in the cylinder is more than 13-14 psi above the exhaust pipe then the flow is choked and exhaust back pressure doesn't matter. Once it gets below that then the mass rate slows down. The greater the pressure the greater the residual gases. My guess here is that they are using the valve to add EGR as well as sound control. This will allow them to modulate EGR with fixed overlap. It also allows them to open up the exhaust for low backpressure while making power and meet noise requirements when power is not needed.

HTH

John

[JasonA]

I think the idea of increasing backpressure to make more torque is an old wives tale.

I've always heard that if there isn't enough backpressure in the exhaust system, and if the intake and exhaust cam profiles have enough overlap, you can "lose" some intake charge as the very quickly outgoing exhaust gas "sucks" some of the intake charge right through the cylinder with it. The rumor was that some backpressure is needed to help "retain" the intake charge when it enters the cylinder. Any truth to any of that?

I understand that when folks put really big exhaust systems on the car, particularly with ricer 4-pots, they "lose" low-end power. Perhaps that's all in their imagination though?

[maydog]

Thanks for the useful info. I had seen some items on ebay a while back that were spring actuated exhaust tip valves. Claimed to improve low end torque. I think they packaged this along with a performance "chip" and electric bilge pump supercharger.

[an01sts]

I wonder whether or not they selected the wrong word, confusing back presssure with velocity: Once a pulse of gas sarts its travel in a tube, the inertia--momentium--pulls following gas along with it.

That's the science behing tuned exaust: the force of the traveling gas, scavenges the firing chamber, allowing more unburned mixture to take its place in the firing chamber.

An unmuffled, untuned manifold something that would have a certain amount of back pressure would make the engine run stronger than simply allowing the spent gas to exit a bare head port. Looking it from that perspective--the lack of exaust velocity--compared to the "back pressure" (no harness of the energy in the moving gas) of an untuned exaust manifold, yes, back pressure increases horsepower. But saying that exaust manfolds is restricting exaust gas isn't really a good way to paint the picture.