forged rockers - friend or foe?

[Olivier]

Chris,

Did you gave up with the repop of the 906 forged rockers?

Olivier.

[chris_seven]

Olivier,

No - I have the forging tool made but just no time to make the machining jigs yet and also need some time to have all of the lash caps completed.

[Richardnew]

I've done a few articles on the valve train and rocker arms. This information is not Porsche specific but it's good basic information. We've seen some incredible development in valve train technology in the last few years. Most of it isn't necessary for a street car but if you race it's critical. Here's an excerpt from one of the articles.

"An interesting thing about rocker arms is that total mass is not all that critical. It’s the moment of inertia that counts. Moment of inertia refers to the rocker arm’s resistance to rotation. The higher this number is the greater the valve spring pressure needs to be. Manufacturers lower the moment of inertia by reducing the weight on both ends of the rocker arms. The center mass isn’t that critical since it doesn’t figure into the calculation. This means you want the ends of the rocker arm as light as possible. Low weight is especially important on the valve side of the rocker arm."

Here's the valve train article. Here's the rocker arm article.

If you really want to get silly we can think about valve springs. Not today though.

Richard Newton

[chris_seven]

We have forged the rocker to reduce the likelihood of the fatigue failures that occur at engine speeds of 8500rpm with an investment cast rocker that has very poor fatigue resistance due the notch sensitivity of the base material.

We didn't set out to reduce the polar moment of inertia as the FIA would be unhappy with this approach and the design changes needed hence we effectively used the original 1995-1966 design but with improved metallurgy. Our main customer base is for engines that are used in Historic Motorsport

The 906 Type rocker does have an effect on inertia but it is relatively small but another part of the issue is the acceleration of the valve due to the ramp angles used in high lift cam design.

Modern cams tend to be high lift with a relatively short duration and this increases accel rates significantly and does increase valve train loads and again a forged rocker would help to avoid failures.

It is always wise to ensure adequate valve to piston clearances so that missed gear changes don't always cause valve to piston contact.