jim dorociak

Hi everyone, going to rebuild my 95 993 engine before putting it in my 1993 RSR street project.

Question: I want to backdate to non hyd rockers. Do I have to used 964 cam towers? or can I just change the rocker arms to the old style and use what I have? thanks! Jim

Mike J

I know that, from timing my 993 cams the old fashioned way with solid rockers, that I had to grind the old-style rockers to fit. The cam towers have all the oil passages and the rockers are narrower on the 993. I would get a set of solid lifters made for the 993 and use those, or the other choice is to change out the cam towers. I don't think the old style rockers will just drop in (at least the ones that I used did not).

You don't have to worry about the SAI passages so that's good. I also assume you are going to dump the power steering pump as well? Mind you a 964 cam tower would have the correct mounting points.

Cheers,

Mike

NineMeister

I was also under the impression that the 964 rocker would not fit the 993 tower correctly until I investigated the problem in detail. All I can say is that on the last two engines I played with I found a minor machining fault which had closed up the gap in the tower that the rocker runs in, which was easily rectified using a small file - a lot easier an option than machining all the followers.
If you carefully inspect a the rocker shaft mounting face of a 993 cam tower, one side has the oil feed hole. In the case of the engines I worked on there was a slight dent next to the oil hole (presumeably caused by a clamp during machining), and this dent had swaged a small lump of material into the gap between the bosses. Obviously all I did was to remove the swage with a small file and voila, all the 964 rockers fitted perfectly.

The next problem to solve is excessive oil supply to the rocker bearing. The problem is that if you use a 964 rocker, the oil feed holes in the rocker bearings line up with the oil feed hole in the shaft, so you loose excessive oil pressure as the rocker moves with the cam. There are two solutions here:

1. Use a standard 964 rocker and run a restriction in the shaft.
2. Use a standard shaft and have the rocker rebushed without the oil holes.

I prefer to use method 1 and we simply have restrictors made and fitted into the oil gallery through the rocker shaft. From then on, adjustment and cam timing is pretty much as any early 911 engine.

Just one final thought though. The camshaft opening ramps are different between a mechanical cam and a hydraulic cam, so although you can use mechanical lifters on stock 993/993RS hydraulic cams you may experience wear or running issues later on due to the opening ramp profile, so it is always better to use a cam that is designed for a solid lifter.

Red rooster

Colin,
As the hydraulic cam ramps are much milder than a mechanical cam I dont see how there will be a wear issue with mechanical lifters + hydraulic cam.
Having said that , I dont see any point in just replacing the lifters either !!
The hydraulic lifters work fine up to some pretty dizzy rpm with hydraulic cam lead in ramps.
As there are some fierce hydraulic cams around , why change back to mechanical lifters ?

All the best

Geoff

NineMeister

From what limited observations I have made, the longer ramps are there to "pick up" the follower and apply pre-pressure to the hydraulic lifter and snap the nrv closed before beginning the lift profile for real. This longer ramp is at constant acceleration and could effectively "wipe" the lubrication away from a solid lifter, but I only mentioned it because I do not know for sure what the long term effect would be. That said I have fitted 993 cams to 930 engines before and not had issues, but I don't want to leave myself open by not mentioning the differences, if you know what I mean?

I have seen problems with hydraulic lifters dropping the lift whilst used with relatively mild sport camshafts, so for track applications I would always prefer to run solids and be 100% sure that we are getting full lift all of the time.

Mike J

Great info, thanks Colin for chiming in!

Cheers,

Mike