Joe Engineer

911 Aluminum CIS Airbox Teardown

So I decided to go with an aluminum CIS airbox for my engine. Lets take a closer look at it.

If you are landing here for the first time instead of reading my whole CIS saga to date, here is a summary of why I decided to go with a metal CIS airbox instead of the OE plastic one:

1. I want additional factor of safety against backfires. While it’s true that properly tuned and maintained cars dont shouldn’t backfire, future vacuum leaks aren’t impossible and should a backfire occur, this box might let you limp home to fix the root problem.
2. I have read several engine rebuild threads that began with the engine swallowing an airbox screw. That was enough to convince me that all the screws in the intake path should be locked in place.
3. These metal airboxes usually have an integrated pressure relief valve (aka pop-off valve) that seems like a more robust alternative than the plastic one. This is just a guess at this point. If the metal valve causes issues in the future, it can be unbolted with the airbox in place and I could try to re-develop a better replacement.
4. Worst case, if this airbox has ongoing issues I can simply drop the engine and swap out the boxes. I will reseal my uncut plastic OE airbox and store it as a back up just in case.
5. Simple curiosity. There are so many variations of these floating around I simply had to get my hands on one just to experiment. I come from an R&D background so I love doing this stuff.

I chose the cast aluminum one over the other versions mainly because that was the first one I came across. After I studied it a bit more, I decided this was better for me than the stainless steel sheet one because it might be easier to modify using simple machining techniques rather than sheet metal fab and welding. Just a personal choice based on my fabrication strengths.

Cast Aluminum Airbox History (Maybe)

I was on a Porsche online group one day in a conversation about expandable oil drain tubes and other stuff people had come up with to overcome problems with factory parts and someone mentioned the aluminum airbox. I posted a few photos of mine and a gentleman said his dad developed them back in the late 70’s. He said the blown CIS airbox was the “IMS Bearing Issue” of those days. The gentleman said his grandfather made the airboxes and also invented the expandable oil drain tube. His uncle filed the drain tube patent. If that story is true then they were quite prolific Porsche aftermarket accessory inventors! Definitely something to aspire to. 😉

Airbox Overview

This airbox is literally a cast aluminum copy of the OE plastic one.

The best part is that it has an integrated pop-off valve!

I am not sure if it will be a direct swap of my plastic one but the ports appear to be the same size. I will need to verify if all the other connections are present. I am going to go through it to make sure it is airtight and verify the function of the pop-off valve.

Teardown

First we tore the box down. “We” meaning my 6-year old Porsche Master Tech and I:

The lower chamber separates from the upper box easily after removing all the screws. It was nice to see this seam was gasketed. In the future I may need to figure out how to make a replacement one since on the OE ones this is not a gasketed joint.

Next we removed the big hex plug on the right side ports.

We found a flat washer shoved way in the bottom of the threaded hole. Someone on the forums pointed out that this is where the EGR tube goes and there are some silicone gaskets that get compressed between a tube nut on the EGR tube and that washer. No EGR on my CIS so I wont be needing it.

I noticed right away the injection spider/manifold was not present and I do not see any mounts for it. At this time I had no idea how it was mounted in the plastic OE unit either so we tore that one down to compare/contrast.

Pop-Off Valve Details

Here is the top of the pressure relief valve, from the upper chamber side. It has big windows for the air to flow through, and a steel shaft that the piston rides on. The shaft rides in an aluminum bore with no bushing of any kind.

Here is the bottom from the lower chamber side. If I press on the bottom of the steel shaft with thumb pressure I can lift the piston off its seat. The spring rate is pretty low and I imagine cracking pressure should also be low.

Regardless, we very slowly loosened all 3 bolts evenly and kept my hand over the top to prevent parts from shooting into orbit.

No such drama. The piston and spring stayed put.

The top, spring, and piston come out in that order. You can barely see the shiny surface of the conical seat (still in the airbox) and the matching surface on the bottom edge of the piston. Interesting that the whole airbox was clean except for the interior of the valve. I guess the black soot means it ran at some point and combustion was taking place upstream of the cylinders (whilst going boom).

I pressed from the bottom of the airbox upper chamber and popped out the bottom half of the valve, containing the seat. This part had no gasket and appeared to have been siliconed to the flat airbox surface. Remnants of a blue silicone were visible on the mating surface.

All hardware was bagged and tagged.

Overall the valve construction appears quite good. There is very little noticeable play between the steel shaft and its bore. I suspect there is very little vacuum leakage through here, if any. I dont know about the valve seat fitment. I suppose you could true them up with some valve grinding compound and then pressure test the airbox.

Evaluation

I put each airbox on a scale and this is what I came up with:

• OE plastic box: 3.5 lb
• Aluminum box: 9 lb

I also ran into a couple of more problems in terms of applicability to my 83 CIS.

The cold start valve spider manifold mounts to the OE plastic box via 3 threaded holes in the lower chamber.

Which the aluminum one does not have.

Unfortunately you cannot maneuver the spider into the lower chamber of the aluminum box. The plastic airbox had just enough wall flex to allow you to slide it in/out.

So I will have to slightly grind down the end of one middle leg of the manifold to get it to clear the wall of the aluminum box. Then I will need to fab up some mounts to fasten it into place.

Lets see what the port sizes look like:

OE plastic box port ID: 1.360″ (34.5mm)

Aluminum box port ID: 1.285″ (32.6mm)

OE intake runner port ID: 1.340″ (34mm). Consistent with OE plastic box port size.

So it looks like I would be creating a very slight bottleneck if I used the aluminum box as-is with the slightly smaller ports. I might be able to match them up with a drum sander as the wall thickness of the airbox ports is pretty meaty. Lets double check what the correct port sizes are supposed to be.

Based on this thread, here are the different CIS airbox years and port internal diameters:

• 1973.5 (30mm)
• 1974-1977 (34mm)
• 1978-1979 US and 1978-1983 EU (38mm)
• 1980-1983 US (34mm)

Also, my 83 box did not have an EGR connection nor does it have an EGR at all. Lastly, the cold start manifold appeared in 1980. This supports that the box was likely developed when 911 CIS was still fairly new in the mid to late 70s.

Hmm…what to do? I mean, with a bit of work it should theoretically work like the OE one…

Next time Ill discuss the modifications necessary to make this airbox work for my application.

References:

Pelican Parts Forums – 911 Technical Forum