Backpressure vs burnt valves myth

[POS carb]

Someone told me that a lack of backpressure in the exhaust system increases the chance of a burnt valve. My immediate response was "Why?" which was followed by a lack of facts.
I tried looking online but cannot find a reputable source, just a bunch of parrots in forums repeating what they hear...

Does anyone have any science to explain this? All I can think of is "magnets"

[Vanilla Sky]

I've heard the same, backed up with the same lack of facts.

[2oodoor]

I've always thought it fact that an exhaust manifold or header leak will create conditions that can damage valve seating, burn a valve, particularly in cold weather.
Now as for the backpressure "myth" I can stretch that to say a similar condition could occur but the odds are way way not in favor because two things, there isn't as much cold air and the modern engine metals more tolerant. Then too the particular engine being discussed.

[2oodoor]

I wouldn't advise open headers in 20 degree weather, I will say that!

[cygnus x-1]

Someone told me that a lack of backpressure in the exhaust system increases the chance of a burnt valve. My immediate response was "Why?" which was followed by a lack of facts.
I tried looking online but cannot find a reputable source, just a bunch of parrots in forums repeating what they hear...

Does anyone have any science to explain this? All I can think of is "magnets"

Total rubbish. The exhaust valves are cooled in two ways, one is by heat transfer to the valve guide, two is by heat transfer to the valve seat. Exhaust pressure has nothing to do with it.

The intake valves are cooled the same way, but they receive additional cooling from the intake charge. Port fuel injection systems actually take advantage of this by spraying the injected fuel directly onto the hot valves to help the fuel vaporize.


EDIT: I will say that I have heard the idea that running an engine without an exhaust manifold can lead to burned valves. If I had to speculate on how this could be possible; I would say that perhaps on some older cast iron block/head designs, where the cooling systems were marginal to begin with, perhaps the exhaust manifold served as an extra heat sink to transfer heat out of the head, thereby letting the valves stay cooler. Aluminum heads however (like on the A20 and most modern engines) conduct heat far better than cast iron heads, so the steel/iron exhaust manifold would not be needed to conduct away heat.


C|

[Hauntd ca3]

that old backpressure vs burnt valves rubbish comes from back in the day when people would put headers or a freer flowing exhaust on their car without tuning the mixtures to suit.
you free up the breathing on an old car with a holley,quad or what ever, all of a sudden the mixtures are a bit on the lean side so the valves run hotter than normal and are slightly more likely to burn.
not so much of an issue these days.
also incorrect ( slightly tight ) valve clearances can cause burnt valves due to the ex valve not seating completely and they again get to hot because they cant transfer heat to the seat

[AccordB20A]

good to know

[Ichiban]

A series of circumstances led me to have to drive my 20R/22R hybrid long distances at high speeds with a header (yes 4 equal length tubes) that dumped in the engine bay at the collector. That engine still runs today. I also can't imagine piston engine aircraft that commonly have NO exhaust off of the head having a reliability issue like burning up valves.

I imagine that there are other factors at work more involving the idiot doing the driving of the no exhaust engine, rather than the engine itself..

[Rendon LX-i]

I just put some guy in here on blast recently on the same deals rubbish is correct.

[2oodoor]

You can burn up that airplane engine pretty quick actually, the induction is totally different.

[carotman]

Total rubbish. The exhaust valves are cooled in two ways, one is by heat transfer to the valve guide, two is by heat transfer to the valve seat. Exhaust pressure has nothing to do with it.

The intake valves are cooled the same way, but they receive additional cooling from the intake charge. Port fuel injection systems actually take advantage of this by spraying the injected fuel directly onto the hot valves to help the fuel vaporize.


EDIT: I will say that I have heard the idea that running an engine without an exhaust manifold can lead to burned valves. If I had to speculate on how this could be possible; I would say that perhaps on some older cast iron block/head designs, where the cooling systems were marginal to begin with, perhaps the exhaust manifold served as an extra heat sink to transfer heat out of the head, thereby letting the valves stay cooler. Aluminum heads however (like on the A20 and most modern engines) conduct heat far better than cast iron heads, so the steel/iron exhaust manifold would not be needed to conduct away heat.


C|

I think you nailed it! Thinking of it, low back pressure might even make valves cooler since the hotter exhaust gases would flow farther before being cooled down.

Make sure the valve lash is properly adjusted and you're good to go.

I adjusted the valves on my high mileage Civic EG and only the exhaust valves needed adjusting. (they were tight). This makes me think that improper maintenance could be the cause of burnt valves more than a back pressure problem. Race cars would all suffer from burnt valves if back pressure was a significant factor in burnt valves.