4 bearing cam in 3 bearing housing?

[Alan Cottrill]

please don't flame me.

I'm just wondering...is there any reason you couldn't use a 4 bearing cam in a 3 bearing housing?

[Wayne 962]

No flames from me - it's a good question. Aside from the bearing surfaces not lining up with the bores in the housing, the actual outer bores are different diameters on the two types of camshafts...

I've tried to figure out why the factory went to the four bearing camshafts from the three bearing ones. Bearing wear and/or camshaft bending does not appear to be a problem on the older ones. The four bearing camshafts have a larger outer diameter, which means a larger "contact patch" for the bearing to ride on. That combined with the four bearings means significantly reduced loads. In addition, every one of the production engines that used the 4-bearing camshafts also used relatively mild camshafts (CIS / Motronic), which means that less load would be placed on them as well. The highest loads would be seen on the high-lift camshafts (like the early 'S' cams or 906 cams) when used with performance valve springs.

I have heard opinions that the change was made to correspond with camshafts and housings used in factory race engines, but I'm not 100% sure that this is accurate.

-Wayne

[Wayne 962]

Ahh, I just asked Steve Weiner who responded with the following:

"Torsional vibration problems from MFI drives and high RPM with big lift and duration profiles. The 4 bearing ones fixed that.

Race engines were always based on production parts and since everything had to be homologated by the FIA, Porsche usually
incorporated production parts into race motors with rare exceptions.

This never presented an issue for street engines as they never get wound to 8000+."

So, indeed it does appear to be a fix for the higher performance racing motors that got incorporated into the production cars...

-Wayne

[Alan Cottrill]

Thanks for the info. all I've had to work off of is pictures of 4 bearing cams so I couldn't really tell.

I have a chance to pick up a pair of sc cams in good condition pretty cheap but no housings. I've heard they're a decent upgrade for 2.7 cis. If I have to buy a pair of 4 bearing housings it's not worth it. At that point I might as well just get my cams reground.

[Wayne 962]

I might have a pair of housings in fair to good condition that I would let go pretty cheap...

-Wayne

[Bobboloo]

As Wayne has offered you might find out that they are cheaper than you might think. I just got a pair very cheap for use with my 4-bearing S cams that I'll be using in the near future.