Some smart guy named Newton was hit on the head by an apple. It seems strange that some 300 odd years later, his physics laws would help us determine what happens in our engines. F=ma (Force=Mass*Acceleration). Now let's put this into perspective for a running engine.
Typical 250F stainless steel valve weighs 33g (.033kg)
Typical 250F titanium valve weighs 18g (.018kg)
A 250f engine sees 12,000 RPM quite frequently, even though peak power is 10,500 to 11,000. So using 12,000 as our known, the acceleration of a valve is 3.556 meters/second.
Stainless steel valve .033 kilograms * 3.556 meters/second = .117 kilonewtons or 26.3 lbs/f
Titanium valve .018 kilograms * 3.556 meters/second = .064 kilonewtons or 14.39 lbs/f
So there you have it, the dynamic running weight per valve at 12,000 RPM of each SS valve is 26.3 lbs/f or 105.2 lbs/f for a 4 valve head. Ti would equal out at 14.39 lbs/f per valve or 57.56 lbs/f for 4 valves at 12,000 RPM.
All of the 250F's come with titanium valves, with the exception being the Honda's steel exhaust valves. These engines are race engines, bar none. Adding stainless valves puts almost double the stress on the valve train that it was not designed for.
Am I saying under no circumstances should you ever put stainless valves in your engine? No, I'm not. Trail riders and beginners typically don't rev the bikes as high, so they are an exception to the rule. Stainless valves are more durable and they last much longer. Titanium valves have the downfall of the coating that lets them live in the constant 200 open and closed cycles per second. Once this coating is worn through, which is usually only .003" to .005" thick, the valves need to be replaced.
This article should not be taken as a bash on stainless steel valves. I mentioned above that the OEM's haven't designed the head to live with stainless components. It's true, but OEM's always "overbuild" in their design criteria for worst cases. Realize that dirt ingested by the engine is the worst scenario on valve life. Having professionally cut seats that are concentric to the valve stem with the correct seat angle and widths is the best scenario for valve life.
We at Williams Motowerx take precautions that help with the longevity of the titanium coatings. For optimum performance and reliability of titanium components, we have the customer run their bikes for an hour of break in, then have the seats cut (normally only done on Honda 250F's). This gives the seat a chance to settle in under varying heat cycles. Stock Honda intake valves will typically last 40 to 60 hours then.
So how do we decide what valves to put in our engines when it come time to replace? Decide by asking yourself honest questions based on what you learned from this article.
If you have more specific questions, feel free to drop me a line.
Jesse - Williams Motowerx
http://www.williamsmotowerx.net
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