Billet cams in 2.7 cam towers.

Henry Schmidt · Henry Schmidt's Garage

In 1975 (maybe mid-year) Porsche started producing cam towers for there 930 turbo engines. Those engines had four, 49mm cam journals, The cam towers were made for those and all subsequent engine until the 3.6 Carrera.
As Porsche so often did, they used the new castings to fill the need for earlier engines. They made 47mm cam towers with four journals but left out the oiling on one to work for the three journal cams.
Here's the issue. All the new billet cam blanks are 4 journal. In an early three journal housing there is no issue. The fourth journal is left floating free. With the late 47mm housings the fourth journal rides in a bearing that gets no oil. This can be remedied but if it is not, the chances of galling are pretty high which would ruin the cam and housing. If the cam seizes, the chain sprocket will shear the locator pin and explode the engine. If you're lucky, you only get bend valves and a chance to limp home on three cylinders.
The first pictures shows the un-drilled four bearing cam tower. The second is drilled for four bearing cams. If you look you'll notice that the casting numbers are the same.

Pio72porsche · 12-23-2022, 10:27 AM

So can the cam tower in the top picture be drilled to accommodate a 4 bearing cam? Do the bore holes have to opened up to 49 mm?
Thanks, Pio

Zuffenwerker · 12-23-2022, 10:50 AM

Wow. Great observation. The late 47mm 4 bearing housing was not drilled for oil on all 4 journals because they were using 3 journal cams…right? If they originally came with 4 journal cams they would all be drilled.

Pio, the bores are not ever enlarged. New billets are available in 47mm and 49mm.

Pio72porsche · 12-23-2022, 11:17 AM

Thanks for getting back to me so quickly. I was asking because I have an opportunity to buy a set of the 47mm, 4 journal housing like the ones in the top picture, but I have a set of 49mm cams that I want use. Also, this build will require higher valve spring pressure so I’d want to stick with 4 journal, 49 mm cams and towers. Thanks again!

Henry Schmidt · Henry Schmidt's Garage

Supertec is now offering to bore 47mm cam housings out to 49mm.
For those who have 4 bearings 49mm cams, the 3 bearing housings can be bored to 49mm. The 49mm cams are so stout that the 3 bearing housing offers more than enough support to function with issue.