I'm going down this route after finding one stack cracked. The cost difference is crazy £50 each for the 964 and £15 for the 993!!
I don't agree with the comments on the internal size difference causing a performance issue as the stack diameter is smaller than the inlet port diameter with both 964 and 993 stacks. The rubber connectors are also the same for both.

 

Is the position for the injector the same? I think I've heard that the 993 injectors are positioned a little higher up on the stack, or something like that, compared to the 964 version. Is this this correct? If so, does it matter? Or are they identical?

 

The injector position is the same but the fuel rail mounting is slightly longer. I will see if they need shortening a few mm when the new parts arrive.
One thing that did become apparent while doing this was how badly degraded the rubber connectors were after so many years of heat, oil and fuel. The were all swollen so that the middle rib was not seated against the stack and I would presume not fully seated on the other side against the manifold.

 

Keep us updated. This is nice find if it works!

 

It has been working on my car for a couple of years.

Regarding the injector and fuel rail positioning I did not notice a difference however I wasn't explicitly looking for it. Perhaps it is slightly different however I experienced no complications from anything mounting related during reassembly.

Just thinking through what ras62 said "fuel rail mounting points are longer" than that has to mean the fuel injectors mount equally higher in the the 993 manifolds since (a) the 964 and 993 share the same size injectors and (b) through my own experience I encountered no fitment issues (i.e. I did not have to "shorten the fuel rail mounting points a few mn").

 

I just found some pictures on an old camera of the 964 and 993 stacks side-by-side. If you look closely, you can see that the mounting point for the fuel rail is slightly longer than the 964 piece and you can also see that the injector boss at the bottom also sits slightly more proud of the base of the manifold.

So I guess that answers that question. The fuel injector protrudes less into the intake tract on the 993 pieces than it does on the 964 pieces.

I'll try to upload those pictures sometime this weekend.

 

Here are some pics of my old and busted 964 manifolds. All six of my manifolds were in this condition.

20160214_221354_zpsolawwjtn.jpg
Slight cracks visible from the top side.

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Big cracks hiding in the bottom. Basically all 18 mounting points were like this.

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Base gasket totally shot and falling apart. This can't possibly provide a good seal.


I'm still unsure if the cracked mounting points = trashed manifold. It looks like the metal sleeves are bonded to the manifold so I don't know if the cracks affect the pieces integrity and ability to seal. I went ahead and replaced all of them with the 993 pieces and new gaskets. I kept all the cracked 964 pieces on the shelf in the garage.

 

IIRC the injector sits in the same place in the housing but the 993 fuel rail connectors are slightly longer. Not a problem as the 964 rail is still long enough for the upper O ring to seal. The only difference will be a few mm gap between the bottom of the fuel rail nozzle and the clip on the injector where before the clip clicked onto the fuel rail nozzle. Shouldn't be an issue as the inlets are under vacuum but if unsure I will shorten the housing to 964 height.

 

Rethinking my original post on this. The cracks around the mounting inserts wouldn't let any air leak directly into the ports. But, if the steel inserts aren't anchored to the plastic, then the two could slip and cause the plastic sealing surface and gasket to not sit flat and leak. I'll put poking around with a stethoscope on my to-do list.

Additionally, Pelican has terrible prices on Porsche factory parts. $90 vs $58 currently through Sonnen. For the price, when will someone start making these out of alloy?

 

Was just discussing that the other day...thinking to figure that out and make them from alluminum...

 

They all seem to crack like this at the mounts, doesn't affect the seal once you have a new rubber in there (the new ones are green) just clean them up and change the seals, including the top and lower injector o-rings, and bolt them back up.

The new seals will take up and tiny distortion just fine.

 

Ok, so the learning is that the 993 stacks is the way to go. Same function, less money.

I've also heard from engine builders I know that the stacks shrink of age. So the diameter can be up to 2 mm smaller than a new part. So this could be a reason to replace them, not just the cracks around the mounting points. One workshop removes some material inside the stacks and press in a steel lining to make keep the original diameter.

 

That's exactly what I was thinking - The splits / cracking outside at the mounting points well away from the seal aren't going to let air in so what does it matter.

 

These cracks will just get worse until the insert becomes suspect, see the pictures earlier, those inserts should be flush with the plastic not protruding. However just as bigger issue are the seals top and bottom and also the bottom lip of the stack which simply disintegrates again as per the pics posted earlier. BTW the lower seals can be green or black depending on Reinz or Elring supply.

 

I installed the 993 stacks today so a quick summary.

Stack inlet diameter is 39.5mm for both 964 and 993, hence the use of the same rubber connectors. Stack outlet diameter are 39.5mm for 964 and 42.5mm for 993. Port size on the 964 is 41.5mm but there is a chamfer edge to the port so the result is an almost perfect alignment.

Molding of body are similar but not identical. Both the injector and the fuel rail mount are about 4mm higher on the 993. This seems to have been done to move the tip of the injector away from the airflow in the stack.

It's amazing the detail Porsche went through to extract a little more performance from the engine, although given the inlet size of these stacks I seriously doubt there is any performance benefit between different types of inlet i.e alloy v plastic.