To Pop Off or not to Pop Off?

[997at]

That is the question.
82 SC. Going to replace my air box. I could not really tell if I have a split or not but the two screws closest to the 4,5,6 intake runners are stripped and won't bite...tells me that somethings up.
Pete Zimmerman recommends to not put in a pop off. I'm inclined to not do it either. My only fear is the risk of a backfire while getting the car back up and running/calibrating. Any thoughts?

[gomezoneill]

If you want to take a chance on blowing up your new box don't. I would 100% recommend you install one. Pete's just a transmission guy what's he know.

[theiceman]

I blew my airbox after 25 years coz the Bentley was misprinted and I put the wires on the wrong way , blew the damn thing to smithereens with screws and plastic everywhere , no pop off in the world would have saved it.
Put the new one in with no pop off and don't regret it for a second ( 12 years ago or so ) . car never backfires even when it struggles to start on occasion and the idle drops right off. New airbox had plenum in it though.
I truly believe if it backfires something else is wrong, others I am sure disagree. I am in Canada and swing for 32C in the summer to -15C in the winter and still it never backfires so im not going to do surgery on a healthy patient.

just my 2c.

[Bob Kontak]

Originally Posted by gomezoneill

Pete's just a transmission guy what's he know.

Well said. He's got nuttin'.

I did read Peter's writeup on not using the pop-off valve. I remember him saying the air box suffers stress with backfires even with the pop-off valve so there is no guaranteed silver bullet to kill the cracked airbox issue.

However, I am the one doing maintenance on my car and I can screw up. I like having the pop off valve as a little bit of insurance.

[Bob Kontak]

Originally Posted by theiceman

New airbox had plenum in it though.

I have had a couple of those sonic boom backfires that should have taken it out. The (my) 81's have the plenum. Wonder if that's what saved mine?

[T77911S]

I have NEVER heard anyone say not to one in

[theiceman]

Originally Posted by Bob Kontak

I have had a couple of those sonic boom backfires that should have taken it out. The (my) 81's have the plenum. Wonder if that's what saved mine?

I think the general idea is the plenum delivers the fuel from the CSV right to the cylinders so it doesn't pool in the airbox and ignite causing a back fire. But I never had any before either so who knows.

[boyt911sc]

Originally Posted by theiceman

I think the general idea is the plenum delivers the fuel from the CSV right to the cylinders so it doesn't pool in the airbox and ignite causing a back fire. But I never had any before either so who knows.

The new airboxes with ‘spider’ plumbing have helped reduce the occurrence of backfire due to unwanted fuel accumulation inside the box but not totally eliminated it. As long as you have a CIS engine in your car, you will experience a backfire now or later. It is a matter of when it will happen. Specially if you are doing a start up after an engine rebuild or a newly assembled CIS unit.



The CIS airboxes in my inventory. All of these are cracked and some have the spider plumbing in it and other with and without POV. And I had disposed a lot of these because they were unusable and could not be fixed. I prefer airboxes with POV.

Tony

[theiceman]

Tony what product do you use to seal the cracks ? I have an old one I used marine adhesive sealant on it , just wondering if you have a product you use . ?

[Jonny H]

The 'toilet seat' is a win-win for the marketing men. If it ever 'saves' your airbox, then I'll wager your airbox is already split. You can easily check by attempting to tighten the screws round the edge or inside the box (particularly the ones near the ridges). If any screw is loose and fails to tighten, your box is split. End of story.

Here's my take on it:

The original airboxes are made from ABS plastic which is notoriously difficult to glue and if you've seen the bond made between top and bottom, you'll see a melted bond, probably done with acetone or similar solvent in the factory. The screws also indicate that the bond is not sufficient on its own.

After years of service, the seam starts to fail due to heat cycles and vibration and minor leaks cause the occasional backfire. Of course, the owner then thinks the car needs a tune and sets the mixture richer but he/she is never convinced it is running exactly right. CIS gets branded as 'temperamental' and difficult to setup. The owner 'drives around' the issue. Sound familiar?

The car still back fires occasionally so owner fits a pop off valve 'just in case'. One day, a backfire occurs and the pop off valve saves the day!! Wow, wasn't that amazing! Er, no. The airbox is still split.

Secondary to this, when a toilet seat is fitted, the installer is required to bond the device to the airbox with epoxy. This is not the correct glue for ABS and will eventually fail, causing an air leak. Boom! Wow, the pop off valve saved the airbox! Oh, wait, what caused the backfire? Er, the pop off valve.

Here's my own story. My car was on the original '82 airbox and hasn't got a pop off valve. It started to get a bit hesitant and I was having to tune the mixture a fair bit. This was about a year ago. It still started and ran ok and then one day, boom! Prior to this it had never backfired in my 7 years of ownership.

I ordered a new airbox from Porsche and the first thing I noticed is that it is made from PA6, not ABS. This is a glass fibre filled plastic which is far stiffer. It uses epoxy to bind the two sections together and the screws have been changed to self tappers with deep threads. This leads me to believe that the real problem is to do with poor bonding and the do longevity of the original ABS part.

Having fitted the new airbox, the car is a joy to drive, the idle is smooth and can be coaxed along on the throttle at walking speed with no jerkiness.

No toilet seat for me!

[theiceman]

Originally Posted by Jonny H

The 'toilet seat' is a win-win for the marketing men. If it ever 'saves' your airbox, then I'll wager your airbox is already split. You can easily check by attempting to tighten the screws round the edge or inside the box (particularly the ones near the ridges). If any screw is loose and fails to tighten, your box is split. End of story.

Here's my take on it:

The original airboxes are made from ABS plastic which is notoriously difficult to glue and if you've seen the bond made between top and bottom, you'll see a melted bond, probably done with acetone or similar solvent in the factory. The screws also indicate that the bond is not sufficient on its own.

After years of service, the seam starts to fail due to heat cycles and vibration and minor leaks cause the occasional backfire. Of course, the owner then thinks the car needs a tune and sets the mixture richer but he/she is never convinced it is running exactly right. CIS gets branded as 'temperamental' and difficult to setup. The owner 'drives around' the issue. Sound familiar?

The car still back fires occasionally so owner fits a pop off valve 'just in case'. One day, a backfire occurs and the pop off valve saves the day!! Wow, wasn't that amazing! Er, no. The airbox is still split.

Secondary to this, when a toilet seat is fitted, the installer is required to bond the device to the airbox with epoxy. This is not the correct glue for ABS and will eventually fail, causing an air leak. Boom! Wow, the pop off valve saved the airbox! Oh, wait, what caused the backfire? Er, the pop off valve.

Here's my own story. My car was on the original '82 airbox and hasn't got a pop off valve. It started to get a bit hesitant and I was having to tune the mixture a fair bit. This was about a year ago. It still started and ran ok and then one day, boom! Prior to this it had never backfired in my 7 years of ownership.

I ordered a new airbox from Porsche and the first thing I noticed is that it is made from PA6, not ABS. This is a glass fibre filled plastic which is far stiffer. It uses epoxy to bind the two sections together and the screws have been changed to self tappers with deep threads. This leads me to believe that the real problem is to do with poor bonding and the do longevity of the original ABS part.

Having fitted the new airbox, the car is a joy to drive, the idle is smooth and can be coaxed along on the throttle at walking speed with no jerkiness.

No toilet seat for me!

I agree with this 100%

my new airbox about 10-12 years ago looked like a pink adhesive bonding agent and it was hard as a freakin rock ! was yours the same ?

[Jameel]

Bought a new Porsche Genuine airbox last winter when I rebuilt my '83 3.0. Didn't put a pop-off in as per Zimmerman. But I also was very meticulous about the CIS initial setup (Dummies thread) and my engine popped right off (haha!) and hasn't backfired once since the rebuild, going on 8000 miles now. If it backfires and blows the box, I may just eat Ramen for a year and go EFI ITB.

[Jonny H]

Originally Posted by theiceman

I agree with this 100%

my new airbox about 10-12 years ago looked like a pink adhesive bonding agent and it was hard as a freakin rock ! was yours the same ?

The new Porsche airbox purchased a couple of months back was more of a dark brown colour when compared to the ABS one and is marked PA6. You can feel it is much stiffer and you can see a mottled effect on the surface due to the strengthening fibres.

The adhesive is thick and white. It has oozed out of the joint a little and set rock hard. Screws are BZP and bigger that the originals

[Reiver]

A 'for' viewpoint.....I've had a two backfires (before I put my AFR gauge in) due to lean conditions and the pop off saved the box from any major damage. The one big one dented the mesh on the filter (from the pop off lid).

It may be there is an issue somewhere but I can set the idle to 500 rpm and it spins smoothly...and when set at 900 rpm the CIS works perfectly (the AFR tells me so)....

Everything works as it should...so...?

[Flat6pac]

i like the inexpensive insurance policy vs. a new air box and engine R&R and install.
A lean mixture causes the backfire and even when it blows out the relief, got more JB Weld
Bruce

[Jonny H]

Originally Posted by Reiver

A 'for' viewpoint.....I've had a two backfires (before I put my AFR gauge in) due to lean conditions and the pop off saved the box from any major damage. The one big one dented the mesh on the filter (from the pop off lid).

It may be there is an issue somewhere but I can set the idle to 500 rpm and it spins smoothly...and when set at 900 rpm the CIS works perfectly (the AFR tells me so)....

Everything works as it should...so...?

But have you checked the screws, especially the ones next to the ridges inside? If you can't tighten them, the box is split.

[Jonny H]

Now I know there isn't much love for MM on this site but this is quite informative:

[Peter Zimmermann]

The original airbox on my '82 lasted 192,000 miles. I had, early on, set the Warm Up Regulator on the rich side of the scale, and simply dealt with a "hunting" idle during the first few seconds following a cold start. I discovered hairline cracks along the assembly seam, above cyl #3, when I had the engine removed to replace the cam shafts, and replaced the airbox at that time. I never even considered adding a pop-off valve, and it's been more than ten years since the replacement, without a single backfire. Jonny H and the iceman said it best, no toilet bowl for me! And, BTW, we NEVER used them at my shop either.

[Marc Bixen]

Originally Posted by gomezoneill

If you want to take a chance on blowing up your new box don't. I would 100% recommend you install one. Pete's just a transmission guy what's he know.

WOW! Just a trans guy? Maybe you should do some research before more stupidity comes out of your mouth gomezo..... Pete Zimmerman is a Porsche GOD when he comments, and is ALWAYS correct. If you don't know So.Cal Porsche history, here's a bit for you. Pete Z. opened Red Line Service, the oldest Porsche only repair shop in So. Cal. in 1975 after a stint at the dealer. Pete was a full fledged master of all things Porsche running Red Line until I purchased the shop at Pete's retirement in 1999. I had been working for Pete since 1995, after 13 years experience at other well known shops. One of those shops actually held the patent on, and manufactured the pop-off valves. I've built and boxed thousands of them, and installed hundreds. EVERY car that came in got one. Then I went to work for Pete at Red Line, and he explained that a properly tuned 911 didn't require this, and they cause numerous problems, so here's a short list of issues I've seen: 1) Pop-off installed backwards, first time it opens it gets stuck on the air filter and...giant vacuum leak, no start. 2) pop-off valve sticks to the o-ring, doesn't open and blows the box anyway. 3) Pop-off valve opens, the o-ring falls out, and again giant air leak, no start. 4) Epoxy gets old, pop-off falls out. and #5) MY FAVORITE, epoxy gets old, cracks, causes a lean mixture, and voila, the customer burns a piston and requires a complete engine rebuild. This actually occurred on a 74 Carrera.
Your choice, tune it right, or put on an unnecessary aftermarket part that most likely will create more AAA tows than you really want. In the 22 years plus I've been at Red Line I've seen less than 10 or 15 blown boxes, and probably only 5-10 of those were regular Red Line Clients.

[Reiver]

Originally Posted by Jonny H

But have you checked the screws, especially the ones next to the ridges inside? If you can't tighten them, the box is split.

No, but I will...the outside screws are tight tho.....when I get the car down tomorrow I'll check.