Oil pump by-pass modifications
04-05-2003, 09:15 AM
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Oil pump by-pass modifications
I'm in the process of ordering new springs, pistons, etc for oil pressure relief valves. How can I tell if the by-pass mods have been done and what is the benefit of this?
Tom
Tom
04-06-2003, 05:20 PM
#3
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Tom,
The oil pump was upgraded in 1976 and the oil pressure relief valve was restructured to feed oil back into the pressure pump, instead of the crankcase. In 1978, there were loss of oil pressure problems with 911SC's on the track/autocross, and the first of several changes were made. One involving the pressure relief valve. The original bolt head introduced in 1976 is 25mm across with a slotted head. The first upgraded valve bolt, along with a new spring and guide, has a circular groove cut into the base of the bolt, and is 23mm wide. This was upgraded again to a new bolt, which has a 17mm hex head. A longer spring was introduced in 1979 (88mm vs 71mm), along with a spring guide. They all use the same pistons.
Looking over several Pano articles on this, in one of them Alan Caldwell says this retrofit isn't neccessary for 2.7L engines (PCA Panorama Dec 1999). There was also a change to the oil pump screen in 1980 to prevent cavitation and help improve oil pickup.
Looking over the parts catalog, your 77 2.7L would still use the same spring, however the 911SC bypass piston would be used after the existing piston inventory is used up. The only available base would be the later one with the 17mm hex bolt.
The oil pump was upgraded in 1976 and the oil pressure relief valve was restructured to feed oil back into the pressure pump, instead of the crankcase. In 1978, there were loss of oil pressure problems with 911SC's on the track/autocross, and the first of several changes were made. One involving the pressure relief valve. The original bolt head introduced in 1976 is 25mm across with a slotted head. The first upgraded valve bolt, along with a new spring and guide, has a circular groove cut into the base of the bolt, and is 23mm wide. This was upgraded again to a new bolt, which has a 17mm hex head. A longer spring was introduced in 1979 (88mm vs 71mm), along with a spring guide. They all use the same pistons.
Looking over several Pano articles on this, in one of them Alan Caldwell says this retrofit isn't neccessary for 2.7L engines (PCA Panorama Dec 1999). There was also a change to the oil pump screen in 1980 to prevent cavitation and help improve oil pickup.
Looking over the parts catalog, your 77 2.7L would still use the same spring, however the 911SC bypass piston would be used after the existing piston inventory is used up. The only available base would be the later one with the 17mm hex bolt.
