BANG! There goes my SC airbox, but why?

[techweenie]

I've had cars with and without popoff valves. I've had the popoff valve work as designed and I've had one blown out of its seat. In both cases, the air box integrity was not compromised. It's a game of percentages, a big enough backfire will break the air box in any case.

[ArchHibb]

Thanks for the pointer to setting up the CIS.
As for the pop-off, it appears to be a wise choice, but like most things, is not completely fail safe.
Anyone else have any thoughts as to whether or not the slow cranking could have resulted in the backfire?

[HarryD]

The pop off valve will work depending on how big the backfire is. As Peter Zimmermann's link shows, any backfire can cause some damage, and a large enough one will still break the air box.

Also Porsche has made changes to the air box over the years to reduce the risk of a backfire. Those of us with an early CIS (like a 1973.5) lack these improvements. The pop off is more important to us as a result.

But then again, I am sure Pete has forgotten more than I will ever know.

[James Brown]

here is a picture of the manifold in the intake on a 81 sc. it shoots the fuel farther down the intake runners to help prevent air box detention.

[maui44]

Same thing happen to me. Install pop off with new box.

[theweaver]

Hi, I am interested in this, as I was only told about this few days ago, because I am changing my HT leads which have the braiding fitted. Obviously the new ones do not. 1980 SC. It doesnt look like I have any mods done already to the airbox.
Is there a link which shows the position of this valve, and also what does it look like.

I mentioned the braiding to the supplier of the replacement HT leads, he said it was because in Germany, owners complained of rodents biting the leads when parked up.....The mind boggles!

Thanks

[James Brown]

the blowing of air boxes is from intake valves being open and spark or other source of ignition igniting the fuel/air mixture in the intake manifold increasing internal pressure. nothing to do with the spark plug wires. more of a design failure and a quick fix. Backfires happen a lot in carb engines (the cis mimics carbs on cold start by injecting fuel into the air way upstream). pop-off valve is the best bet.

[aigel]

Originally Posted by theweaver

Hi, I am interested in this, as I was only told about this few days ago, because I am changing my HT leads which have the braiding fitted. Obviously the new ones do not. 1980 SC. It doesnt look like I have any mods done already to the airbox.
Is there a link which shows the position of this valve, and also what does it look like.

I mentioned the braiding to the supplier of the replacement HT leads, he said it was because in Germany, owners complained of rodents biting the leads when parked up.....The mind boggles!

Thanks

While the martens do chew wires in Germany, this certainly is not the reason they were braided with metal on the outside. The shields are carefully grounded towards the engine / chassis. Only electrical reasons would warrant such grounding: backfires and radio interference are the only two reasons you'd do that. I have heard both.

That said, you are okay with the non shielded wires, the wires are not the root cause for the backfiring. For saving yourself from backfires, you want the pop off valve. Check under your air filter, you may already have it: http://www.pelicanparts.com/techarticles/911_pop_off_valve/911_pop_off_valve.htm

[ArchHibb]

I have to get the patient into the OR before I can begin disassembly, so I haven't had any opportunity to handle the airbox. From the factory, are the top & bottom held together with just screws (permitting separation) or are they glued or sealed as well? If just screws, presumably there is some sort of gasket between the halves?

[timmy2]

Top and bottom are glued together with unobtanium glue....
Drop the engine and remove the entire cis system.

[diverdan]

Pop off valve is time honored solution. While I haven't been into my SCs lately there may be a simple engineering solution. My Alfa GTV 6s would backfire when I'd stop to get my key out for my mailbox on leaving my condo in the A.M. Engine not yet warm and restarted "Kaboom!" the intake would blow off at the rubber tube connectors. As I recall there are bolts that lock the SC intake system in place, while the Alfa merely had not so fixed clamps on the aluminum plennum. Admittantly the porsche system has a lot of plastic, but what if there was a way that it could allow tubes to slip off and prevent the destruction? Pop off valve works, but maybe there is another way.

Dan

[timmy2]

Originally Posted by diverdan

Pop off valve is time honored solution. While I haven't been into my SCs lately there may be a simple engineering solution. My Alfa GTV 6s would backfire when I'd stop to get my key out for my mailbox on leaving my condo in the A.M. Engine not yet warm and restarted "Kaboom!" the intake would blow off at the rubber tube connectors. As I recall there are bolts that lock the SC intake system in place, while the Alfa merely had not so fixed clamps on the aluminum plennum. Admittantly the porsche system has a lot of plastic, but what if there was a way that it could allow tubes to slip off and prevent the destruction? Pop off valve works, but maybe there is another way.

Dan

This is what I went to after 2 cracked boxes, and then after the built in pop off leaked. Hard to find, but they are out there.

[theweaver]

Can anyone tell me where I can buy the pop off valve in the UK, along with price.

[Marc Bixen]

I've seen both sides of the story. In the 80's I worked for the company that held the patent and manufactured the Pop-off valve, we were also a Porsche repair shop. Every airbox car that came in either had one or got one. We shipped out 100's per week.
Then in 1995 I started working for Pete Zimmermann. Pete explained as he does in his book, that a properly tuned, well tuned, 911 doesn't need a pop-off valve. Well don't you know that 17 years later I haven't replaced one Red Line Serviced cars air box that was equipped with a late model air box, but I have had to rebuild a few pop-off installed motors for burnt cylinders obviously caused by air leaks/ vacum leaks, at the pop-off valve. Late model refers to SC and earlier replacement air boxes that have the cold start distribution manifold installed. Generally these air boxes have phillips head screws visible in the air filter chamber area. If you have slotted screws than you have an earlier airbox without the cold start distribution manifold.
If you have a pop-off valve, check the epoxy seal at regular intervals and also the condition of the rubber o-ring. I've also seen pop-off valve stick to the o-ring and basically be useless to there purpose. We've had a number of tow in's for no start with pop-off valve cars. These include valves not closing after, valves coming loose, or even valves being installed backwards and having the flip get stuck in the air filter screen.
Moral of the story, keep your car in proper tune and you don't need the gimmicks. Also as mentioned in "The Used 911 Story" Airboxes are a plastic/nylon material, they do get old, they do get brittle. If your rebuilding your motor, ALWAYS install a new air box, especially if you have no history, if it's over 60-70,000miles old,or it's an early style (slotted screws).

[ArchHibb]

Marc, Thanks for sharing your thoughts and experiences.

[jrolstin]

Be careful when installing your pop-off. I drilled into my fuel distribution manifold accidentally when doing mine. To fix it I just cut a small piece of sheet steel and epoxied it over the hole I drilled. Annoys me that I did that, but no one will ever know... hehe

[Paulporsche]

I'm not arguing w/ Marc or Pete. Like most of us here, I'm in awe of their accomplishments and credentials re these cars.

My car had just been serviced by an independent Porsche mechanic w/ a new CDI installed. During a stop somehow a wire from the box came loose and then, when restarting the car, the engine backfired massively. I'm certain that the popoff valve I had saved the airbox.

Now, maybe one could argue that the car wasn't "properly maintained" since if it were correct the connection wouldn't have come loose. I don't know. The thing was just installed a few days previously, and there appeared to be a bad connection, that wasn't readily visible @ the time of installation.

As Forrest Gump famously said, "It happens!"

In this instance I'm glad I had the valve.

[Marc Bixen]

I like the Forest Gump Quote.

[ArchHibb]

@ jrolstin - that's good advice! I can easily imagine doing that myself.

Thanks to everyone for sharing your knowledge and advice. I'm sure I'll be posting up lots more after the motor is out and I've waded in up to my neck! Till then...

[vintagecarlover]

does anyone know if a 911 with a 08 82 date of manufacture would come with factory pop off? or when were they delivered with a pop off? thx