Backpressure...again...

C

CobraSix

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So, this weekend I was listening to Click and Clack on cartalk. One segment conclusion perked my interest and made me have some questions:

http://www.cartalk.com/Radio/player5/pl ... segment=10

Basically, they surmise that the engine valves overheated due to the fact that the muffler had been disconnected for 3 months, leading to insufficient backpressure causing exhaust gases to flow too fast causing the valve to overheat.

What say you guys?

Now to me, not sure if that makes sense. I understand how backpressure can help scavenge air, but for some reason this doesn't make much sense.
 
Hrm. PV=nRT, but the temp isn't stable at all. Assuming nR is constant, pressure dropping, volume expanding, and the whole shebang being pushed through an opening port by a piston.

I'm going to need a lot more time and coffee to even imagine exhaust gas velocity at the valve/port and how that might be affected by pipework. However, race cars often run open headers and I don't see them dropping valves on a regular basis.

FWIW, I can't hear the segment while I'm out here on the ship, I forgot my satellite radio* and we can't do streaming media. :( So I'm not sure of the details, but, if it was a speed-density EFI car, or some forms of carburetor, lack of backpressure would result in a lean mixture, raising cylinder temps and possibly burning the valves that way (and probably detonation as well...).



*Ironically, I got the satellite radio so I wouldn't miss Car Talk while I was on the ship.
 
lack of backpressure would result in a lean mixture, raising cylinder temps and possibly burning the valves that way (and probably detonation as well...).
Click to expand...

Makes sense if it affected the O2 sensor. Too much O2 would make the EFI read a rich burn and lean out the mix. Then you would have a chronic lean burn condition that would make it run hot.
 
Well it was a 92 Volvo 940, but from I can tell thats a mass flow. This occured when the car was around 6-7 years old. Basically the girl was trying to determine if it was her that killed the car, or if the repair shop was swindling them out of their volvo for parts. The exhaust only became mentioned because she said that she got the exhaust fixed prior to going home after 3 months and the car was so quiet that she forgot to shift and made it all the way to 70 MPH in 2nd gear before she noticed. The cartalk gurus made the leap to the exhaust system at the end.

If the velocity stays high with low resistance (no back pressure) you exhaust temps should drop through the system. Putting a restriction on a hot gas will just lead it to get hotter (or stay hotter longer) due to increased pressure.

I can't imagine that the temp drop would have been enough to force the car to enter open loop cycle on the EFI and back into choke.

But maybe you are on the right path, since O2s are designed to operate at a certain temperature, if it wasn't reaching the temperature (not sure if those volvos are 5 wire heated o2s or not). May have to learn more about O2s.
 
Oh, i don't know squat about O2 sensors. I just know that it is what checks and adjusts the f/a mix and a lean condition burns hotter. C&C generally have pretty well informed theories. However, like us here, they also do a fair amount of educated guesswork based on experience and a grasp of physics, coupled with a high degree of familiarity with the specific object(s) at hand.

By way of example, my wife's Sube had a bad vibration in the front when braking from high speed. I had it more or less fixed with a front end rebuild after 150 k. However it returned after another 20 K. I just heard C&C talking to a Sube owner who had a similar problem. I only heard the last part of the call. They warned her that the problem was the constant velocity joints and that the car was dangerous to operate until she got them fixed. I took our Sube for the 175 K service and asked them to check the CV joints. They said it was warped rotors and milled them for $30 or something instead of the $350 or more for the CV joints. Whatever. The boys are right a lot of the time too, especially about the "dead mouse in the heater or the vomit/broken egg/dog poo on the rear carpet that smells when it's humid" kind of stuff.
 
I agree, find them very informed guesses, especially considered a lot of the callers have no clue about cars.

I like listening to the callers and trying to guess for C&C chime in. I love their thought process.

Just this one had me puzzled.
 
I like listening to the callers and trying to guess before C&C chime in. I love their thought process.
Click to expand...

That's what I do. Plus, I am always impressed by the large number and long memory for older models that the boys have. And they know a lot of the tricks and wrinkles about how the newer cars work too.

As for the burned valve, I think that the answer was a substitute for, "Hmmmm. That's funny. I guess I really don't know."
 
There is no such thing as too little backpressure, and the idea that the exhaust gases would flow too fast and cause overheating of the valves is silly. In fact, gases cool when expanding, not the contrary. The reason a proper tuned header makes power is precisely because the timing of the returning low pressure pulse creates a vacuum situation on the exhaust side of the exhaust valve and helps to empty the combustion chamber.

A more likely scenario, already mentioned, is that the freed up exhaust created a lean tune situation and running lean does indeed burn valves.
 
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