Why Does The Exhaust Valve Have A Hard Life?

Cycle World Technical Editor Kevin Cameron demonstrates in this video why life for an exhaust valve in an air-cooled motorcycle engine can be very challenging. In a water-cooled engine, however, that same hot and busy existence is more of a vacation.
This Honda CBR600RR engine has eight exhaust valves and they are quite small-22mm in diameter. All of the valves are exposed to combustion heat, but exhaust valves are heated twice. When the valve opens and exhaust gas begins to flow out from under it, the valve is heated not only on the top of its head by combustion itself but the back side of it is heated as well by exhaust gas, whose temperature can be as high as 1,700 degrees Fahrenheit.
In the early days of motorcycle race tuning, engineers would run an engine without an exhaust pipe on it and look up the port to see if the valve was glowing red hot. The hotter the valve gets, the more likely it is to heat the incoming fresh charge for the next combustion cycle. After the spark lights the charge and it is compressed further by the expanding flame kernel, some part of the unburned gas around the outside of the combustion chamber, particularly that which is heated by the exhaust valve, will detonate or knock.
Because this particular valve is small, the distance from the hottest part of it to the seat-the part of the valve that seals against the cylinder head-is very short, so heat can flow from the center of the valve out to the water-cooled seat in the head. But the bigger you make an engine, the distance that heat has to flow out of the valve to the cooler seat becomes larger and the valve operates hotter.
We’re really fortunate in sportbike engines that their exhaust valves are such cute little guys because their short heat path means that their survival is pretty well guaranteed. In bigger engines, sometimes you’ll see chunks taken out of the head of an exhaust valve. A crumb of carbon holds the valve open slightly, leakage of combustion gas bypasses the valve, heats it up, and a flame channel develops. It looks like somebody went after the poor valve with a torch.
In an air-cooled engine, life is very hard for an exhaust valve. In a water-cooled engine, it is more of a vacation.
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[@blipco5]

Ive been working on engines for over forty years. I've rebuilt several. So as usual, as soon as I was notified of this video I turned it on but I didn't think I would learn anything new with so simple a topic. I was wrong...as usual.

[@upsidedowndog1256]

Absolutely Kevin! That is exactly why the aircraft engines require so much more maintenance. The owners rarely understand that difference. Air cooled motorcycle engines nowadays are on the edge too. As well as our liquid cooled high performance motorcycle engines. Especially the short lived motocross bike engines. Liquid cooling helps for sure. It comes down to what you expect out of a said engine. The more you ask of any of them the more maintenance you must do.

[@daveyt4802]

Needs the cool clown music. Cool fun fact with the radial valve!

[@jhernz86]

I had to replace a burnt valve on cylinder 3 of my 99 civic over the summer. Ended up rebuilding the entire head. Was it carbon that caused the burnt valve like Kevin says ? Possibly. I concluded that it was due to clearance being out of spec. (Was super tight prior to removal). The bad valve looked like a tiny bullet shot out of the combustion chamber. It was knarly.

[@zerosensai]

Must admit..those tiny exhaust valves are cute little guys for sure :)

[@franciscobrandao4832]

Why does the valve does not have a spring on it? I thought valves usually had springs on it to keep them closed.

[@ronaldredman8122]

As always awesome. Thanks!

[@naveen..873]

Hello can u please make a video on turbo compounding, with thermal efficiency and can it be fitted to a parallel twin bike engine

[@mohba01]

No..

[@mohba01]

The 2018 Honda Civic Type R has sodium exhaust valves!