Actually the standard cut off rpm for steel valves is usually considered to be 7500 - 8000 rpm's in automotive applications (bigger valves(more mass) than most motorcyles and atv's), and it is recommended that titanium only be used in aluminum heads for cushioning the valve seats which get beat up from using ti valves at sustained rpm's higher than this. Many have gone back to using steel or sodium again on the exhaust side. Some of the newest stainless alloys are smaller stemmed(lower weight) but very strong. Using ti springs can help reduce the mass as well. Interestingly in the 2002 CRF 450 motor Honda specified stainless on the exhaust side for puposes of reliability. Many of the cheaper titanium valves on the market are actually used custom reground race valves changed out from competition motors where the valves are automatically changed after every use - or at least on every tear down - which is often the same thing. They would appear to be perfect to the human eye, but internal stresses will have taken their toll.

A lot of development is still being done in diamond like coatings which are used to impregnate the valve surfaces to prevent galling under heat, and which do not require undercutting the valve stem(other thicker coatings were then used to build up the stems to original o.d.) - and which many feel caused a weak spot at the radius of the undercut finish causing shear failures. Galling actually takes place both on the stem as well as on the valve seat as titanium has a tendency to like to "swap" metal with adjacent mating surfaces. So work has been done to coat the valve contact face(s) as well.

It's true that titanium valves can augment performance and are even a necessity in certain high rpm applications, but their longevity remains an issue still.....

 

Good info. If 7,500-8,000 rpm's is the limit for steel in auto apps, then why give a nascar valve as an example when these engines turn in excess of 9,000 rpms?

 

I used that example to show that there is a limit where steel valves can be used that relates to function not just durability. Above that limit in automotive performance applications, titanium or formerly sodium valves are used because higher rpm's are not achieveable without the use of a lightweight valve train. Everyone switched to titanium when they first came out and now some are switching back to sodium filled valves for purposes of durablity.

Motorcycle engines in this size (450) typically use smaller (lighter) valves than these engines (automotive engines) so even higher rpm's are achievable(maybe 10,500) while still using steel (stainless) valves and a bit more with titanium springs and retainers which helps further reduce the reciprocating mass. Where 14,000 rpm's are being chased then only the ti valve train will allow it. How many of us are running this kind of rpm?

If the Nascar engines could achieve the same rpm's with steel they probably would use them, it is NOT primarily for durability that titanium is used. The point was where you need to use titanium valves to achieve maximum performance, be prepared to pay the price.

 

I'll bite. I want to know everything you know. Could you please tell us. thanks