No muffler = overheated/ruined valves

[redek]

I was listening to car talk episode #1124. They said that running an exhaust without enough back pressure could overheat the valves. Is this true?
http://www.cartalk.com/Radio/WeeklyShow/online.html
Segment 10, about 7 minutes in

[azruss]

most motors are designed to run most efficient with back pressure. this allows fuel mix to flow out the exhaust and then be pushed back into the cylinder before the exhaust valve closes for ignition thus giving a more concentrated gas mix for more power. not sure why it would create more heat unless the gas in the exhaust manifold is being ignited.

[So Cal Mark]

mixing exhaust with an incoming charge will dilute it, lowering combustion temps, but this NOT a way to make more power. Back pressure kills hp, the more overlap on your camshafts the worse the situation.

[azruss]

your are probably more up to date on this subject than i am, mark. i got mine from reading a old book i had on engine tuning. this phenomenom was very effective for single cylinder motorcycles. you werent sucking exhuast gases back into the combustion chamber but new fuel mix that made it out the exhaust while the x-valve was still open. back pressure produces a standing wave that kicks that new mix back into the head as the x-valve closes giving you more fuel to burn, thus more power. I agree that most motors have an excessive amount of back pressure that kills power. The old "extractor" exhaust they put on VW added 10% power just by dropping the back pressure. Isnt that what tuned exhaust do. they make the length of the exhaust runners identical in the x-manifold so that each cylinder experiences the same back pressure. I know this all relates to standing waves in both exhaust and intake plenums for the purpose of move gases in an out of the motor in a more efficient manner.

[Danno]

Every automotive forum I've ever been on talks about backpressure until someone comes on and schools them on scavenging. That's where it's at. Keep the gasses moving quickly and assist the flow of air. Too big of an exhaust system and you lose air speed and create weird turbulences in the system. Too small and the air can't flow through in the proper volume.

When you start taking about resonances, and exhaust tuning, then you are getting into the 2 stroke territory as well. We could go on forever with the topic, just know that whatever science you claim to be using, it's agreed that too small is bad and too big is bad.

[TronDD]

Yeah, I was going to bring up scavenging.

That's why headers of proper length work better than most manifolds. You want enough velocity in the header tube so that it flows past the end of the other tubes as the next cylinder's exhaust valve opens drawing the gasses out. It's like blowing over the top of your soda straw. The soda will get pulled up the straw.

Overlap in a camshaft benefits from the same thing. When both valves are open, lower pressure in the exhaust tube from scavenging will help start to draw the air and fuel into the cylinder.

Tim.