I have heard that straight/short/custom pipes can burn out valves on an engine. However, with a 4-cylinder Ford Model A engine, I have been told this is not the case because it has such low compression.
If there is any validity to straight/short/custom pipes burning valves, what is the cause and why does it happen?
Human Error
I have firm belief that the cause of burned valves with short pipes is due completely to human error, lack of understanding and poor judgement.
I believe this myth has been perpetuated by actual 'evidence' of having burned valves but the attribution of the cause is incorrect.
Guy with burned valves story
He has a hot rod he builds. He uses the engine out of an old Camaro that had glass packs on it. He orders short pipes, a Holly Quadrajet and an HEI. When it's all done he drives it around and it's loud. One day he tears his heads off cuz it's running crappy and discovers burned valves. He looks over at his friend and says, "Damn short pipes." His friend nods his head in affirmation and says, "Yup." The myth begins.
Poor attribution and assumption of cause
The builder had heard the story, was warned by his friends that his pipes should be longer, ordered them anyway and learned his lesson. Now he is preaching the gospel and telling all his car friends and everyone on the message board about how he put short pipes on his 'ride' and they burned up the valves. "You should have seen them. They looked like burned brussel sprouts on the end of black sticks."
His testimony is revered by his crowd because his rod is the baddest one and they go on and wash, rinse, repeat the story. Perpetual motion myth. It even generates it's own energy.
Two guys from Motor Vehicle Maintenance & Repair talk while dude builds his hot rod
"How does he know that the float level in the carburetor is correct, do think he knows that it will effect his AFR across the whole range?"
"I don't know, I don't see an exhaust gas analyzer around here at all."
"No, he doesn't have an EGA, what do you think he'll do to ensure his mixture is right."
"He said he can smell lean or rich, that he has a nose for it."
"HE DOES WHAT?"
"I know right, he's going to adjust his mixture by smelling his exhaust. What's he doing now?"
"He's eyeballing his timing. Says his eyes never lie."
"His advance is too retarded"
"I know, brutal. That thing is super lean. He's gonna burn his valves"
....and that's how it really happened. The guy with the burned valves jetted his carburetor incorrectly and set his float level too low. He didn't set his timing correctly either and had a late spark right at TDC, not before. The lean condition combined with a late spark left some of the lean over oxygenated intake charge still burning when his exhaust valves began to crack open. The still slightly burning superheated exhaust gasses from too much oxygen in the AFR from incorrect carburetor settings slowly overheated his valves and they deteriorated over time until they began to even crack a bit. At that point the performance drop off was noticeable enough where dude began to look into it. After months of troubleshooting he finally tore the heads off and discovered burned valves and gave attribution to the short pipes rather than his lean mixture and retarded timing.
Dudes valve. Bummer dude :-(
I agree with DucatiKiller that straight pipes by themselves are not to be blamed; if the valves are prone to getting burnt there is clearly another issue going on.
I don't have practical experience to support this, but fluid dynamics theory would suggest this is the case.
The flow through the valve is proportional¹ to the pressure drop between the combustion chamber and the back of the valves.
With straight/stubby/short/no pipes, the pressure drop will be greater, so the rate of exhaust gas flow past the valve will be higher.
↑ Flow rate → ↑ Wear rate
¹ - non-linearly proportional, mind
I am an ASE Certified Master machinist with 50 years' experience rebuilding cylinder heads. A much more likely cause for the valve burning problem is low seat pressure, worn guides, bad valve seals or a combination of them causing the exhaust valve to seat poorly.
Exhaust valves with less than about 40# on the seat do not have enough pressure to seat the valve well enough to transfer the heat from the valve to the seat. Some of the Japanese engines use only about #35 on the seat and are known to burn exhaust valves.
