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4 Journal cam in 3 Journal Cam Tower?

I purchased a used 911sc motor (3.0) which apparently was put together from parts. I am having it torn down and rebuilt but my mechanic informed me that it has 3 journal cam towers from an earlier car. I have heard that it is possible to run a 4 bearing cam in the 3 journal cam tower but not sure if it is recommended. The alternative would be to source the proper 4 journal cam towers. What does everyone suggest I do? Thanks in advance
Old 11-17-2016, 05:53 PM
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Early journals are 47mm and later 49-you're going to have to check the carriers you have
but I'd opt for finding a set of later ones.
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Old 11-18-2016, 02:58 PM
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Quote:
Originally Posted by prschmn View Post
Early journals are 47mm and later 49-you're going to have to check the carriers you have
but I'd opt for finding a set of later ones.
Probably your best bet, especially if you are going to use stock SC or 964 cams.

Any idea what cams you have now? I'm guessing they are 2,7 CIS cams?

It is possible to have custom 4 bearing cams made to fit three bearing housings, but why bother. The larger bearings of the 4 bearing cams allow the possibility of more cam lift.
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Old 11-18-2016, 03:50 PM
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I'm guessing it would cost more to have the three journals bored out to accept 4 journal (larger diameter bearing) cams than it would to get a set of used 930 cam carriers. A pair of good used 3.0 cams ought not to be too expensive, given all the hot rodders who have put in 964 or other cams in these motors.

The switch was made when the 930 motors appeared, first as the Euro 3.0 Carreras and the early Turbo. Bizarre in the extreme that someone would do this.
Old 11-18-2016, 09:15 PM
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The 1977 parts let you go the other way. That was the transition year so they are basically 4 bearing castings but were run with the 3 bearing. Not sure what year your parts are but as I understand the '77 parts can be modified for 4 bearing.
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Old 11-19-2016, 05:50 PM
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Max - I don't think so. My '77 2.7 dropped a valve back when, and the engine parts became the donor for several iterations of a race motor. The Elgin 315 cam I ran in the '77 housings was a three bearing cam with the usual bearing diameter, not the larger 3.0 4 bearing diameter. I seem to remember feeling around where the 4th journal would be to see if an early 4 bearing race cam could make use of a 4th journal, and concluding it could not. I think there was not enough there to create a 4th journal if you rebored the whole thing to the larger size.

But this was an early '77 (rolled out of the factory in the summer of '76). Porsche sometimes made some changes prematurely at the end of a model run because they had run out of the old parts, or the dies were worn out, or whatnot. Or they kept using older parts until those ran out. Maybe something like this happened? Didn't the casting numbers change when they went to the 4 journal cams? And didn't the change from the 46mm big cam nut to the bolt system happen with the change to the 930 motor? So early on? The Euro Carrera 3.0 for '76/7? At least the SC starting in '78?
Old 11-19-2016, 09:15 PM
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Some 2.7 CIS motors had 4 bearing cam towers and the RSR ran a cam with 4 x 47mm journals.
Old 11-20-2016, 01:23 AM
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If you already know what the goal of this build is, then you will know what you have to do, and so should your builder. When I did my SC motor some years ago, I was able to source some parts from Patrick Motorsports for an upgrade, and it was less painful than you would think.

If this is a special engine, don't compromise.
Old 11-20-2016, 02:50 AM
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