CIS Airbox Clarification

[2jmotorsports]

I am doing some research on CIS airboxes since the one on my 83 SC recently blew. I am going to go through the entire system and replace the airbox and any seals/gaskets which may cause vacuum leaks, in addition to the proper fuel pressure tests.

From this post I have gathered that there is an original style airbox and a newer improved style which is less likely to accumulate fuel vapors leading to a backfire. I would like to buy a newer style one but wanted to clarify all the things to look for to tell them apart. Per the link above this is what I know so far:

Original style: slotted screws for flat blade screwdriver, no diffuser
Newer style: Phillips head screws, has diffuser

Is the diffuser a metal manifold with the small diameter tubes you can see in the center of each intake runner port here?



Are there any other ways to tell them apart? Is there a part number difference?

Right now I am looking at known good used airboxes but If I have to buy a new one to guarantee I have the improved one I am ok with that as well.

Thanks in advance,
Jose

[rwest]

Jose,

You're correct, the tubes that are showing in each tube are from the cold start valve and get the fuel to each cylinder rather than being able to accumulate in the main area.

If it has those tubes, it's the latest style.

[2jmotorsports]

Awesome! Other than making sure the tubes are there just make sure the intake runner OD is the correct size for my year?

[javadog]

Basically, yes. You'll be looking for a US 81-83 airbox.

JR

[pmax]

Originally Posted by javadog

Basically, yes. You'll be looking for a US 81-83 airbox.

JR

My 80's airbox has the diffuser.

Originally Posted by 2jmotorsports

Right now I am looking at known good used airboxes but If I have to buy a new one to guarantee I have the improved one I am ok with that as well.

Finding a perfect used airbox is a tall order. I bet many of these cars have leaks in their airboxes and still run to some extent.

[javadog]

Some '80 air boxes may have the distribution piping, some may not. Safer to say '81-up.

[boyt911sc]

Originally Posted by javadog

Some '80 air boxes may have the distribution piping, some may not. Safer to say '81-up.

Even the larger (38-mm ID) intake air boxes for '78-'79 SC USA and '78-'83 SC RoW/Euro are also available with diffusers. These are the later production airboxes which is an improvement from the previous older models (without diffuser). Air box 911-110-104-20 is the new production for (smaller) 34-mm ID intake airbox that has features used in very early CIS and late SC's. Original SC airboxes do not use any drain tube for the airboxes but the new production has it incorporated for wider application.

Tony

[fred cook]

Just for the record, I have an early SC large port airbox on my 3.3SS engine. It has the diffuser w/small tubes to each intake port. Here is a picture:



I think this airbox came from a 1979 engine.

[boyt911sc]

Originally Posted by fred cook

Just for the record, I have an early SC large port airbox on my 3.3SS engine. It has the diffuser w/small tubes to each intake port. Here is a picture:



I think this airbox came from a 1979 engine.

My '78 has the original air box and has the inside diffuser too. Not sure when this diffuser was introduced by Porsche but all the SC's I worked on and seen has the diffuser. How about the '77?

Would any '76 or '77 owners share this information with us? If you have a pop off valve, open it and inspect the interior of the air box. Thanks.

Tony

[Bob Kontak]

Originally Posted by boyt911sc

My '78 has the original air box and has the inside diffuser too. Not sure when this diffuser was introduced by Porsche but all the SC's I worked on and seen has the diffuser. How about the '77?

Would any '76 or '77 owners share this information with us? If you have a pop off valve, open it and inspect the interior of the air box. Thanks.

Tony

In the 81-83 airbox if you lift the pop off valve, you see nothing. A piece of plastic 1/2 inch down. It was redesigned from the early SC's and there is no viewing access to the inside from the pop off valve location.

Bummer is I cant state if the diffuser is the what the change was in 1981. A diffuser is one thing. What they did in 81 I think is another. Not sure.

[boyt911sc]

Originally Posted by Bob Kontak

In the 81-83 airbox if you lift the pop off valve, you see nothing. A piece of plastic 1/2 inch down. It was redesigned from the early SC's and there is no viewing access to the inside from the pop off valve location.

Bummer is I cant state if the diffuser is the what the change was in 1981. A diffuser is one thing. What they did in 81 I think is another. Not sure.

Bob K.,

Your SC has a pop valve. Open the cover of the POV and look closely. Use your eyes and you will see the inside (interior) of the airbox. If you don't have a diffuser, it would be a hollow cavity and you could stick a pencil 4"down. However, if you have a diffuser, the pencil would only go down less than 2". The black plastic thing you mentioned is not plastic???? It looks like plastic because of the dirt and carbon black deposited on the top surface. It is metallic but not magnetic? Get a piece of rag and wipe it off. I've been working of these airboxes for years and I know them like the back of my hand. I rest my case your honor.

Tony

[rwest]

Just changed out my '77 that I've owned for over 20 years with no record of it being replaced before and it did not have the diffuser.

[boyt911sc]

Originally Posted by rwest

Just changed out my '77 that I've owned for over 20 years with no record of it being replaced before and it did not have the diffuser.

Rutager,

Do you have a pop off valve installed on your airbox? If you do, could you see the bottom of the airbox through the pop off valve using a flashlight or some light source?Thanks.

Tony

[javadog]

You guys can quit looking in your airboxes. Porsche made the change for the 1981 models. You might find a few that made it into 1980 models, plus you'll find them in cars that have had their airboxes changed. Lots of those running around....

Here's page 9 from the 1981 service training booklet:

[javadog]

What would be useful is to document any changes between this airbox and the earlier ones, ie. any other changes that don't involve the cold start mixture distribution system. I have one of these newer boxes on my 77 Carrera 3.0 and I'm going to be making some changes to the CIS system on it and I'd like to know the original routing of all of the hoses, etc.

JR

[Bob Kontak]

Originally Posted by boyt911sc

I've been working of these airboxes for years and I know them like the back of my hand. I rest my case your honor.

Just call me Judge Judy.

[groovydude]

I just replaced the air box on my '82. The seam between the lower and upper boxes had failed. For what it's worth, mine was the newer style, AND it had a pop-off valve installed. The break happened in previous ownership, I have no idea how long ago or the circumstances.

[boyt911sc]

Originally Posted by javadog

You guys can quit looking in your airboxes. Porsche made the change for the 1981 models. You might find a few that made it into 1980 models, plus you'll find them in cars that have had their airboxes changed. Lots of those running around....

Here's page 9 from the 1981 service training booklet:

JR,

My '78 targa has a diffuser and it is the original airbox. Yes, it has not been changed because I am the second owner and I know personally the first owner. Bought the car with less than 60k miles and came all the service invoices for the car. I have been tinkering with CIS engines and found these diffusers in several early SC's. Plus I have bought several CIS airboxes with larger ID for '78 & '79 SC's as spares and they all have diffusers. I have them in the garage if you want to see them.

So I don't buy your claim that the diffuser was only available after 1981 (?). Not everything you read is true. Then how would explain the presence of diffuser in some early SC's?

Tony

[javadog]

Tony,

I tend to trust Porsche's service information. There is no way that Porsche wrote a bulletin about a new part that had already been in service for three years. They aren't that stupid. If you go back and look through the parts microfiche, you can find the change in part number that occurred in calendar year 1980, when they new one became available. There may also be a parts bulletin on the change; I don't have a collection of those.

My guess is that your '78 had an airbox replaced. You may know the owner, he may not remember it, but that doesn't mean anything. Thousands of these things got replaced and most of the owners had no idea what the part was in the first place. They certainly wouldn't have known what the differences were.

This was not the first change made to the airboxes. They did other things to strengthen them, so if a backfire occured, maybe the airbox didn't split. They also made other changes to minimize the risk of an explosion; they changed the ignition wires, added a capacitor, etc. This was discussed often in the Porsche community in the '70's and early '80's and I remember the discussions and have lots of notes from that era. I remember when the pop-off valve was the latest craze.

I've been tinkering with these cars a long time, too...

JR

[rwest]

Tony,

My old air box I took off is sitting by my deck in half and doesn't have the diffuser. The new box I bought has the diffuser in it and I decided after reading several different opinions, not to install the pop off valve.

Rutager