I too have heard this…The first time was AFTER I was running without a muffler for about 2 months…Never had a problem…I have no idea if it’s true or not. But the burnt valve when no back-pressure story has been around for at least 40 years.
The effect of back pressure on the mixture is true. That it doesn’t always result in burned valves isn’t evidence to the contrary. There are a number of factors that can allow you to get away with removing the mufflers and not burn the valves. In the pre-pollution, carburetor days when no one cared much about fuel economy and even less about unburned hydrocarbons manufacturers tended to adjust the fuel air mixture on the rich side. Even with the mufflers removed the mixture was still rich enough. In cars with EFI the O2 sensors will compensate (up to a point) for increased O2 in the exhaust. It is also depends on how hard the engine is run. Idling around town and short blasts on the highway may not heat the valves enough to burn them. For those who want to test the theory try a full throttle run up a deserted interstate for a while and report back.
MT, it wasn’t so much that manufacturers tended to adjust carbs on the rich side (although they did for more power), it was more a question of carburators produced much larger drops of fuel, and by the time it made its way to the cyliinder there was coalescing into bigger drops. Only the hydrocarbon molecules actually in contact with oxygen atoms burn, so the fuel droplets burned through in layers like an onion. Their time to do in the the power stroke was insufficient to allow complete combustion. Rich running was compensation for the fact that the technology at the time simply did not allow for complete combustion during the power stroke, so without backpressure still-burning fuel could blow past the exhaust valve.
This incomplete combustion condition also deposits carbon on the exhaust valves. Carbon retains heat. Great for bar-b-que brickettes, not so good for valves.
Injection systems are high pressure through the orafice and produce a much finer spray, much smaller droplets to burn through in the short time that is the power stroke. Putting the spray right behind the intake ports significantly reduces coalescing. The new direct injection systems go even farther, eliminating all opportunity for such. Thus, the drops are tiny and can be completely burned in the short time available to do so before te exhaust valve opens.
Think surface area. Only the surface of the fuel droplet burns. It must burn through from the outside in, one layer of HC molecules at a time. Large droplets equal low surface area per fuel volume, tiny droplets equal high surface area per volume. High surface area per volume means more burning going on at the top of the power stroke where it’ll do good, and more complete burning of the fuel.
But you are correct, they ran them rich, and that contributed to the possibility of burned valves without back pressure.
Still, many many engines were run with open pipes without problems. Only retentive people like me seemed to have problems. Maybe it was the lead deposition on the valves that protected them. Who knows.
By the time gasoline is inside the combustion chamber of a warmed-up engine, there is no liquid gasoline at all, no coalescing of droplets at all… The gasoline has COMPLETELY vaporized into a GASEOUS state, no different than propane, butane or N.G.
The older carbureted engines had an exhaust passage heating the intake manifold to insure complete vaporization of the fuel…Gasoline is VERY volatile and transforms itself from the liquid to a gas very readily…
I think that valve erosion happens when a valve fails to seal. A leaking valve letting high pressure high temperature gasses leak by will eventually get eroded by those gasses.
They don’t erode when they are open, they erode when they are closed and leaking.
I believe that the reason it is not so common with modern engines is that we have learned how to design heads with valve seats that stay round as the engine heats up.
Claude, an old friend, was an aircraft mechanic in the Pacific during WWII. His observations seemed to make sense to me. When operated at near maximum power at maximum RPM in a steep climb and then throttling back fully when turning into a dive from high altitude, i.e. cold, and then throttling up to climb again valves would often fail. Or so it was told to Claude. He was of the opinion that the problem might be the cause for many planes and the pilots failing to return. The planes and engines improved over the war years and later models were much less prone to burned valves.Of course, those were air cooled engines operating at their limits.
Caddyman, you’ve made a good point, but the fuel is sprayed into passing air (at significant volume, I might add) at ambient temp. While gasoline will evaporate at ambient, it is not instantaneous. Things happen very fast in the engine. At 3,000 rpm I dout that the gasoline has an opportunity to vaporize entering the chamber, and it’s then compressed by the compression stroke, which would encourage coalescing.
In short, I think your point about the aeromatic nature of gasoline as it affects coalescence is an excellent one, but in putting it into context with what the gasoline is being exposed to in its travels I can’t agree that it’s a factor. I still believe it’s an issue of there being excess gas in the chambers of the carbed engines due to the surface area issue and, secondarily, the manufacturers of the time having to run the engines rich due to the fact that they were unable to fully use the energy in the gas due to the low surface area per volume.
I’m glad you posted. I enjoy being challenged to justify my theories. It forces me to think through my statements.
It also forces me to acknowledge that my statement is more theory than proven.
With aircraft engines the mixture can also be adjusted manually. My feeling is that going too lean can damage the valves and this would be the root cause of any valve problems rather than a short exhaust system.
Hear-Hear. Loved that conversation. I was looking for an answer to a burning question when I came across this page. I didn’t get a direct answer from all the comments, but I got enough info to reason out the answer I was looking for. As a experienced mechanic and former racer and builder, I will agree with everybody, burned valve are the result of a badly tuned or built engine. The exhaust pipes have little effect if all other parts of the system are working properly. Thanks agin.
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The other common type of valve failure is valve burn , or burnt valves . Essentially this is caused by combustion gases escaping between the valve and valve seat when they are not sealing correctly. … Normally this type of failure affects the exhaust valves only, but it can also damage the intake valves .
Have this old 94 crown victoria…purchased for next to nothing to use as a daily driver it has around 160xxx original miles on everything from body to engine to trans. when I bought it, the car had about 5 foot of pipe including the cherry bombs on both pipes…(don’t ask I don’t understand why anyone would do that either).the car would almost seem to slip gears at low rpms and would bog down and make it to where you had to slam on the pedal and open her up to get her moving…I’ve since added another foot on each side and actual mufflers on both sides and it has made a major difference. I no longer putt along when I’m low in a gear and my acceleration and response time has noticeably increased. There is your example of backpressure being necessary in an old engine. That “theory” may not apply to all sizes of engines either. When you have a 4.7L v8 and it’s basically open because some idiot threw cherry bombs right off the down pipes from the manifold…driving through town at low speeds consistently can cause problems!