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Mine had an O-ring that sealed the flap to the seat.
Old 07-13-2018, 11:30 AM
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Awesome. Thanks Harold!
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Jose - 1983 911SC Coupe
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Old 07-13-2018, 12:44 PM
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My previous plastic airbox had a pop-off valve with the O-ring; it was necessary to keep the o-ring lubricated with grease to prevent vacuum leaks. I didn't bother installing the pop-off valve when I replaced the airbox after dropping/shattering it.
Old 07-13-2018, 08:37 PM
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Hi guys. So I have wrapped up the aluminum airbox and wanted to loop back and let you all know what I did on this R&D project.

As you know the lower airbox had a couple of issues:
1. No cold start spider
2. No mounting provisions for cold start spider
3. Port IDs were too small.

I grabbed my cold start spider from my old airbox and ground off a small amount of material off the center legs. Just enough to be able to maneuver it into and out of the lower chamber but still have the ends stick into the ports to spray fuel into the runners and not in the chamber itself. Now I could mock it up in position and figure out how the heck to mount it.

I then fabbed a small wedge of wood to create a flat mounting surface for the lower end with the bolt hole. I then gave the wedge to an old machinist I know to make me an aluminum version to tack into the airbox.



I then measured the port IDs and using a drill and carbide burrs, opened up the IDs on the lower airbox chamber to match.



Even though it was a super slow process, I used a drill instead of a die grinder because I wanted to slowly sneak up on the correct ID and reduce the risk of screwing anything up. I then ground and filed radii on the inner corners as best I could in that cramped space.



A few days later my machinist came through with a little aluminum wedge that fit the box perfectly. It contains a threaded hole for a hex bolt that matches the hole in the cold start manifold. I cant remember if its an M2 or M4 but its tiny. That pocket is to clear the center part of the manifold.

I double-sided taped it to the inside of the box and tried installing and removing the manifold as if the wedge was welded in place.





With a bit of practice it was easy to install and remove the manifold with the wedge in place...



...so I went ahead and had a local weldor tack it in. The vertical wall has a small gap between the wedge and the wall which is a perfect place to locate a lock tab.



Here is the manifold installed with the single bolt in place. With the nose of the manifold inserted into the airbox wall right behind the cold start valve, and with the hex bolt on the other end, torqued, threadlocked, and lock tabbed into place, the manifold should never come out without my permission.

The plastic airbox has 3 simple screws holding the manifold in. I have read a few threads here where they occasionally like to come out and take road trips to the nearest combustion chamber.





Here is the cold start manifold threatening to spray fuel at me.

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Old 12-05-2018, 06:32 PM
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Next step was to figure out how to get the pop-off valve to seal as best as possible. Since this is just a big valve I started with some valve grinding compound to lap the valve and valve housing seats together.



I then cleaned and assembled them and leak tested with some alcohol. I dont know why this stuff is green but it made it easy to see leaks.

I bolted the pop-off valve assembly together to have the spring exert some pressure on the seat. I then filled the top of the valve housing with alcohol and it held for about 1 minute before it began collecting the drops you see here.



I then lapped the seats with some metal polishing compound and repeated the test. It then held a leak free seal for about 10 minutes! To me that was good enough to now assemble the box and proceed to a pressure and vacuum test. The box needs to hold vacuum for normal engine operation and we need to verify that the pop-off valve pops at some reasonable pressure in case of a backfire.

I then cleaned the valve housing and airbox mating surfaces...



...and sealed with Threebond 1184 for a leak-free seal at this joint.



I then torqued the assembly to the airbox with some NAS spec cap screws. The reason for this is that the screws themselves are in the intake path and I dont want them to ever come off and get sucked into a cylinder, so since they come with drilled heads I can safety wire them together.



Next I made a block-off gasket for the cold start valve with some silicone sheet I had lying around. I made a similar gasket to block off the throttle body.







Next I sourced an EGR block off plug (shown on the left) with Belmetric, then re-used another hex plug from my plastic airbox for the hole on the right. Some rubber stoppers in the port holes will help seal those.

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Last edited by 2jmotorsports; 12-05-2018 at 07:50 PM..
Old 12-05-2018, 07:06 PM
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I drilled a hole in one of the stoppers so I could insert a fitting for an air line and cut a pair of plates to clamp together and seal the ports with the rubber stoppers. One of the plates has a hole to clear the air line.



Assembled the airbox (without the cold start manifold) and was ready to test.

I tried to use a hand vacuum pump and quickly realized it didnt have big enough lungs to evacuate the large volume of air in the airbox chamber and draw a significant vacuum. Maybe a venturi style vacuum pump attached to an air compressor would have worked better. Regardless I was able to draw several inches of vacuum and it held it for about 20 seconds before slowly starting to drop off. To me this was good enough since the engine has much much bigger lungs and will be inhaling continuously so it should draw a healthy vacuum. Since I am replacing all the rubber components in the vacuum assembly there should be no other sources of unmetered air coming in.



I then conducted a pressure test with a pressure brake bleeder because it moves more air than the hand pump. I found that the pop-off valve cracks at about 3 PSI, according to the pressure bleeder gauge. Very low pressure and likely at a safe point before anything else can be damaged during a backfire.



Ultimately we wont know for sure until we run it. Time for final assembly.
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Last edited by 2jmotorsports; 12-05-2018 at 08:13 PM..
Old 12-05-2018, 08:09 PM
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I installed the cold start manifold, applied blue loctite and torqued the mounting bolt.



Underneath the bolt I had pre-bent a lock tab into the slot and locked 2 faces of the bolt as well. The manifold is mounted nice and solid.



I then assembled the lower and upper airbox halves with regular cap screws on the outer flange and more NAS cap screws on the interior and torqued everything down.

After a YouTube crash course in lock wiring and a few attempts...



...we ended up with this.





You can see that the manifold is located exactly where it should be, at the center of each port.





This concludes the remainder of my metal airbox blueprinting and I can now proceed with the rest of my CIS rebuild to test it out!

There are several reasons for attempting this rather than just going with the plastic airbox.

1. Extra factor of safety. While its true that properly tuned and maintained cars dont backfire, if you weren't lucky enough to own a well cared-for car (like me), this might help you get home after a backfire. Even if you rebuild your entire system and it is running perfect, future leaks arent impossible and this box might let you limp home to fix the root problem.
2. I read more than one thread on here that went "...I had to rebuild mine after the engine swallowed an airbox screw..." and leaving it alone wasnt good enough for me. All the screws in the intake path are locked in place.
3. This pop-off valve seems like a more robust alternative than the plastic one. Fortunately if it still ends up causing vacuum leaks, it can be unbolted with the airbox in position and I could try to re-develop a better one to bolt in its place.
4. Worst case I can simply drop the engine and swap out the boxes. Ill reseal my uncut plastic one and store it as a back up.
5. Curiosity. There are so many variations of these floating around I simply had to get my hands on one just to experiment.

Cheers.
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Last edited by 2jmotorsports; 12-05-2018 at 08:49 PM..
Old 12-05-2018, 08:27 PM
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Nice work Jose!
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Old 12-05-2018, 11:01 PM
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Quote:
Originally Posted by HaroldMHedge View Post
Mine had an O-ring that sealed the flap to the seat.
Exactly. But I had three POVs and when I shined a light into it (them) you could see light past the O-ring and that was enough to discourage me from using one.
Old 12-06-2018, 01:40 AM
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Nice work! One other thing I would do is to provide support at the intake end of the cold start manifold. I would be concerned that engine vibration would cause the aluminum CSM to crack at the restrained end. Since the two threaded holes for the CSV are right there, it should be easy to bend a steel support bracket that would fasten to the CSV mounting bolts. Just a thought!
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Old 12-06-2018, 04:59 AM
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Thanks Peter. Thanks Fred thats a great point.
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Old 12-06-2018, 07:30 AM
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As a nostalgic fan of CIS and other forms of Mechanical Injection, I am so impressed by this project, both the box itself and your approach to improving/making it fit for purpose. I'm also still marveling in my head at the tooling which must have been created to cast it. Thanks for posting. John
Old 12-06-2018, 12:15 PM
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