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Aluminum CIS Airbox Rebuild

Hello friends. So my ongoing 83 SC engine rebuild has finally reached the fuel system refurbishment stage.

I plan on rebuilding the entire 1983 spec CIS system, mainly for CA emissions compliance. I am going to try to keep the system as original as possible, provided I can find acceptable replacement parts. One exception though is the plastic airbox. I wanted to try to find a more robust solution in case of a backfire.

This is why I searched for one of these elusive metal airboxes. This one is a cast aluminum copy of the OE plastic one:









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 and 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.

In any case, I could not find a thread covering this topic in depth and figured you guys might want to nerd out on this contraption. I may need to pick your brains along the way too!
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Old 07-07-2018, 07:06 PM
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Yes, please!

You may be the first to document one of these here on Pelican.

Couple of things I recommend checking and have questions about:

- Verify port size matches your existing airbox.

- What does this aluminum airbox have present to replicate the cold start valve "injection spider" of the later plastic airboxes? This was supposedly the key to avoid airbox failure due to high fuel charge on cold start-up.

- What is the sealing method for the pop-off valve? O-ring?

I've been working (sporadically) on a design for a sheet-metal & composite airbox, but don't have that done yet. Am interested in anything you care to share about this one.
Old 07-07-2018, 07:19 PM
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First we tore the box down. "We" meaning my Porsche master tech and I:






Lower chamber separates from the upper box easily after removing all the screws. Nice to see this seam was gasketed. Any idea if there are replacement gaskets for this? I suspect not since the airbox is a single part number with no subcomponents.



Removed the big hex plug on the right side ports.



Found a flat washer shoved way in the bottom of the threaded hole. Not sure why.

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Old 07-07-2018, 07:21 PM
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If memory serves correctly, that port is for the EGR tube. Sealing of the EGR tube is accomplished with a couple silicone donuts compressed between that flat washer and the tube nut on the EGR tube.

Looks like no fuel distribution spider for the cold start valve...

PS - You've got the best Porsche tech in all the world helping you. Keep that going.
Old 07-07-2018, 07:25 PM
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Quote:
Originally Posted by fanaudical View Post
Yes, please!

You may be the first to document one of these here on Pelican.

Couple of things I recommend checking and have questions about:

- Verify port size matches your existing airbox.

- What does this aluminum airbox have present to replicate the cold start valve "injection spider" of the later plastic airboxes? This was supposedly the key to avoid airbox failure due to high fuel charge on cold start-up.

- What is the sealing method for the pop-off valve? O-ring?

I've been working (sporadically) on a design for a sheet-metal & composite airbox, but don't have that done yet. Am interested in anything you care to share about this one.
Yes, I will have to verify port sizes accurately; OD and ID. Last time I checked it was only a quick tape-measure check.

I noticed right away the injection spider/manifold was not present and I do not see any mounts for it. Actually I have no idea how it is mounted in the plastic OE unit either so we will have to tear that one down too and compare/contrast.

Pop-off valve has a chamfer or conical face on the piston and the mating seat.

We can deep dive into these later...
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Old 07-07-2018, 07:27 PM
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Pop-off valve details. Here is the top, 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. 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 shooting into orbit.




No such drama. Piston and spring stayed put.



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, if not pig-rich.



If you press from the bottom of the airbox upper chamber, you can pop out the bottom half of the valve (containing the seat). This part had no gasket and appears to have been siliconed to the flat airbox surface. You can see remnants of a blue silicone on the mating surface.



Hardware bagged and tagged.



Overall valve construction is 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.
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Last edited by 2jmotorsports; 07-09-2018 at 07:35 AM..
Old 07-07-2018, 07:53 PM
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Quote:
Originally Posted by fanaudical View Post

PS - You've got the best Porsche tech in all the world helping you. Keep that going.
Thank you! She is 6 and is starting to want to hang out in the garage more often.

Still waiting for my 4 year old to come around.
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Old 07-07-2018, 07:56 PM
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First time I've seen that airbox; I'd seen the built up / fabricated aluminum versions but that.....wow....it must be seriously rare. Did Bosch make it? Are these known to the elders/gurus/wisemen here on PP?

Amazed in Connecticut.....John
Old 07-08-2018, 05:39 PM
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Quote:
Originally Posted by jjeffries View Post
First time I've seen that airbox; I'd seen the built up / fabricated aluminum versions but that.....wow....it must be seriously rare. Did Bosch make it? Are these known to the elders/gurus/wisemen here on PP?

Amazed in Connecticut.....John
I think it was a private venture by some very creative Porsche folks.

I was on the Aircooled 911/912 Facebook 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 this one and a gentleman said his dad developed them back in the late 70's. Apparently the blown airbox was the "IMS Bearing Issue" of the day back then.

I'll see what info I can dig up from that conversation.

Edit: guy was Marcel Dhont and he said his grandfather made them and also invented the expandable oil drain tube; his uncle filed the patent: http://www.freepatentsonline.com/4066281.html

Random factoid just for fun!
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Last edited by 2jmotorsports; 07-12-2018 at 06:45 AM.. Reason: Adding extra info
Old 07-08-2018, 05:55 PM
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Quote:
Originally Posted by 2jmotorsports View Post
Apparently the blown airbox was the "IMS Bearing Issue" of the day back then.
Well, at least the airbox issue can be solved with a drill, glue, and $20 worth of parts. Don't think it's as invasive as the IMS -

The later boxes with the individual tubes from the cold start pretty much solved the issue.
Old 07-08-2018, 06:10 PM
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I’d buy an aluminum airbox just for the cool factor!
Old 07-08-2018, 06:23 PM
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That is very cool, thanks for sharing.

Out of curiousity what does that thing weigh? Not a criticism just curiousity.
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Old 07-09-2018, 02:19 AM
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1. well tuned cars don't backfire. Fix the problem if it does
2. more issues from pop off valves causing no starts than they are worth.
3. worst case I would rather have my airbox blow ( relatively cheap ) than have something else more expensive take the blast. integrated pop off valve is kinda cool though and looks better than that plastic ridiculousness.

just my 2c ...

good to see you have a mechanic in the wings though .
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Old 07-09-2018, 07:07 AM
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I just think itís cool. The tooling to create/cast it must have been a huge investment. Just kind of amazes me.
Old 07-09-2018, 03:08 PM
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IIRC Webb Design out of Tampa Florida back in the 80's & 90's was a major fabricator of these aluminum air boxes
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Old 07-09-2018, 05:18 PM
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I think Webb Design did the welded sheet metal boxes. I've never seen one of these cast aluminum boxes.
Old 07-09-2018, 08:39 PM
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Quote:
Originally Posted by JohnJL View Post
That is very cool, thanks for sharing.

Out of curiousity what does that thing weigh? Not a criticism just curiousity.
Put each box on a scale and this is what I came up with:

OE plastic box: 3.5 lb
Aluminum box: 9 lb

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.



Also you cannot maneuver the spider into the lower chamber of the aluminum box. The plastic one has just enough flex to allow you to slide it in/out.



So one would 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 one would 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. Might be able to match them up with a drum sander as the wall thickness of the box ports is pretty meaty. Lets double check what the correct port sizes are supposed to be.

Based on this thread, here are the different airbox years and port sizes:

Quote:
The correct numbers for the port sizes of the airboxes (internal diameters) are 34 mm for the US '74 - '77 and US '80 - '83, and 38 mm for the US '78 - '79, and the
Euro '78 - '83. Just for completeness, the '73.5 airbox had 30 mm ports.
Also, my 83 box did not have an EGR connection nor does it have an EGR at all. This supports that the box was likely developed when the 2.7s were new.

Hmm...what to do? I mean, with a bit of work it should theoretically work like the OE one...
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Last edited by 2jmotorsports; 07-11-2018 at 09:54 PM..
Old 07-11-2018, 09:51 PM
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or buy a plastic one, throw it in and move on ....
you might eventually get to driving it
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Old 07-12-2018, 07:09 AM
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I do know a few guys with mid-year 911's and 2.7's still running CIS up here in the PDX area if that heavy box needs a home...
Old 07-12-2018, 10:09 PM
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Quote:
Originally Posted by fanaudical View Post
I do know a few guys with mid-year 911's and 2.7's still running CIS up here in the PDX area if that heavy box needs a home...
Thanks, Ill keep that in mind...

Quick question for anyone with a plastic pop-off valve installed: Is there a rubber gasket of some kind between the lid and valve body or is it a hard plastic to plastic sealing surface?
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Old 07-13-2018, 08:24 AM
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