Damage from holes in muffler???

[Matt_Jr]

This is kinda like the "should I tow in overdrive" thread. The real reason valves are damaged with leaking exhaust systems- If the exhaust leak is close to the maifold the exhaust valve can be cooled too fast when the engine is shut off. If the valve cools too fast it causes the edge of the valve to warp (same reason brake rotors warp). If the valve is warped it doesn't seal properly. This leak causes burning fuel to go past the valve while it is still at it's highest temperature. This causes the edge of the valve to burn. Thats how you get burned valves. This usually takes a long time to happen (many on/off cycles). If you never shut off the engine it wouldn't be a problem. It's possible no damage has occured to the engine at all on the tractor your looking at but the only way to know for sure is to do a compression test. If compression is low do a leak down test. This will tell you if damage to the valves has occured or not.

Just a note: It's not how hot valves/rotors get that causes them to warp. It's cooling them too fast.

 

[john_bud]

This is kinda like the "should I tow in overdrive" thread. The real reason valves are damaged with leaking exhaust systems- If the exhaust leak is close to the maifold the exhaust valve can be cooled too fast when the engine is shut off. If the valve cools too fast it causes the edge of the valve to warp (same reason brake rotors warp). If the valve is warped it doesn't seal properly. This leak causes burning fuel to go past the valve while it is still at it's highest temperature. This causes the edge of the valve to burn. Thats how you get burned valves. This usually takes a long time to happen (many on/off cycles). If you never shut off the engine it wouldn't be a problem. It's possible no damage has occured to the engine at all on the tractor your looking at but the only way to know for sure is to do a compression test. If compression is low do a leak down test. This will tell you if damage to the valves has occured or not.

Just a note: It's not how hot valves/rotors get that causes them to warp. It's cooling them too fast.

I'm not so sure about your valve idea there. Most are not warped and leak, they are burned from not sealing and hot gases going past there and no seat contact to cool it. Combustion gasses are in the 1200F range and the valve needs the seat contact to allow it to cool. Burned valves used to be fairly common from the big wads of gunk that built up on the back sides of the valves. With computer controlled injection that is pretty much a rare occurance.

I do agree that rotors and drums warp from differential cooling. Putting on the parking brake after some "spirited" driving is the usual one.

 

[RobJ]

If the valve cools too fast it causes the edge of the valve to warp (same reason brake rotors warp).

Hehe, your last comment is the topic of many discussions on some truck forums I read. Some believe (strongly I might add), that technically rotors don't warp, but the metal is built up in different areas, or moved around the surface. I read all the links a while back, don't recall the details was some of it made sense...but I still believe they warp. However!!! (read BUT), I usually have to have the rotors turned on my Ford SD every 20k, pads are fine I just have them turned to smooth them out, I'm currently at about 50k on 1 OEM turned rotor and a new replaced Autozone rotor(the truck has 125k on it). The other OEM rotor is ground and on the shelf for one more roll. The only difference is a different brake pad. I don't remember exactly what it is but it wasn't the cheap AZ part. But it also wasn't the most expensive. My driving patternes haven't changed either.

Rob

 

[RobJ]

I'm not so sure about your valve idea there. Most are not warped and leak, they are burned from not sealing and hot gases going past there and no seat contact to cool it. Combustion gasses are in the 1200F range and the valve needs the seat contact to allow it to cool. Burned valves used to be fairly common from the big wads of gunk that built up on the back sides of the valves. With computer controlled injection that is pretty much a rare occurance.

I do agree that rotors and drums warp from differential cooling. Putting on the parking brake after some "spirited" driving is the usual one.

Extra heat is also created by the air being forced through the small opening at high pressure. Like why your AC gets hot.

What a minute, are you saying if I drive around with my parking brake on that'll fix my warping problem? Nah...my back brakes (disk), never warp on me.

 

[orezok]

Back in the late 60 and early 70痴 when I was building 2 stroke and 4 stroke racing engines, we did a lot of experimentation with exhaust systems as at that time few people considered the exhaust as a source of increased horsepower.

What we found is that the pressure wave generated in the exhaust could in fact increase the volumetric efficiency. Long story short, as the exhaust valve (or port) opens a high pressure wave exits at the speed of sound. If the exhaust gases are in the 1,300 degree range as in a racing engine the wave will proceed to the end of the exhaust system at about 1,500 FPS (from memory so don稚 skewer me). When the pressure pulse reaches the end of the exhaust system or a change in pipe size such as a collector or expansion chamber, a wave will be propagated which returns back up the exhaust pipe. This is not a flow, but is a pressure wave. The benefit of this wave is that in order to get the maximum flow through the exhaust valve, you have to keep it open too long for best volumetric efficiency. The returning pressure wave if timed properly will result in a high pressure situation at the exhaust valve near the end of closing resulting in a sudden stop of scavenging of the cylinder. This allows the incoming charge to 菟ressurize the cylinder as if the exhaust valve was closed.

In an ideal system the pressure wave will reach the exhaust valve just as it is closing. In a racing engine with headers or expansion chamber the best design is to overscavenge the engine by way of the exhaust, that is to actually pull the fresh intake charge into the exhaust. This is a tricky proposition to achieve the optimum timing based upon exhaust valve duration, exhaust gas temperature and length of exhaust system.

It would seem to me, and I知 no combustion engineer, that the reason the Corvette does not have the AFM and other GM痴 do is that the Corvette uses an actual set of headers versus the log manifolds. It was probably design for high scavenging efficiency and the result of that is that fresh charge, high in oxygen is migrating past the exhaust valve (particularly at higher RPM痴) resulting in the burning of the valves.

I believe that zzvyb6 is correct about the oxygen migration, but I believe that the source is from the intake charge, not from the exhaust.

 

[Matt_Jr]

I'm not so sure about your valve idea there. Most are not warped and leak, they are burned from not sealing and hot gases going past there and no seat contact to cool it. Combustion gasses are in the 1200F range and the valve needs the seat contact to allow it to cool. Burned valves used to be fairly common from the big wads of gunk that built up on the back sides of the valves. With computer controlled injection that is pretty much a rare occurance.

I do agree that rotors and drums warp from differential cooling. Putting on the parking brake after some "spirited" driving is the usual one.

Yes, the valve needs to contact the valve seats to help keep the valve cool, but when the valve begins to warp that 1200degree air going through that opening doesn't help. Thats what causes the valve to burn. They warp and don't seal properly....then burn.

 

[Matt_Jr]

Hehe, your last comment is the topic of many discussions on some truck forums I read. Some believe (strongly I might add), that technically rotors don't warp, but the metal is built up in different areas, or moved around the surface. I read all the links a while back, don't recall the details was some of it made sense...but I still believe they warp.

Rob

You are correct in your beliefs. And some info, the technical term for warped rotors is "Out of parrallelism"...caused by rapid and uneven cooling of the rotor. Often happens when running through a puddle after hard braking. The hotter the rotor gets the faster the rotor cools because of the differential temps of ambient air and the rotor.