valve guides question

[inushmiah]

why do people make such a fuss about bronz valve guides whats the diffrence between standerd and upgrading to bronz

[burgo]

google is your friend!!!!

taken from http://www.av-v.com/TechnicalInfo.htm

AV&V manufactures valve guides from two different materials; manganese bronze or micrograin cast iron.
Myth: Bronze guides don’t last.
Fact: Early silicon based bronze alloys used in valve guide applications quickly earned the reputation of having poor wear performance. It wasn’t until high nickel based bronze alloys (AMPCO 45) were introduced that the valve guide market was able to offer a superior bronze valve guide, suitable for high heat applications (Nitro-methane etc…).
But still, people realised that that material was not lasting as long as cast iron. One of the main reasons is because this material needs to be fit at a looser clearance to prevent seizing problems.
After extensive research, the majority of automotive and motorcycle racing head manufacturers and rebuilders are now installing manganese bronze alloy guides. The two principal advantages of this alloy is superior heat transfer and also that the engine builder is able to fit the guides at a tighter clearance for a better oil film control and longer life. We should call this alloy ‘’zinc bronze alloy’’ instead of manganese bronze because it contains nearly 30% zinc, 2-3% manganese. Manganese is the hard part of the recipe and the zinc is the bearing ingredient, a material which is also a self lubricant.
Myth: Why use manganese bronze alloy when I know that a stock Evo or Twin Cam guide can last 60,000 miles or more?
Fact: It’s true that for stock replacement, cast iron guides work fine. However, the problem starts with the high performance applications. In high lift applications, the guides need to be shortened. With more heat and side load from the valve involved, the oil film between valve and guide deteriorates more quickly. In addition to increased viscosity breakdown, cast iron guides are also an abrasive material which induces the premature wear that we’re used to see with this application. In cases where valves have collided with each other or with the piston, cast iron guides have been found to fracture and deposit damaging fragments into the cylinder and combustion chamber. This is not the case with manganese bronze guides.