BANG! There goes my SC airbox, but why?

[911SauCy]

Originally Posted by Gogar

Install a popoff valve into the new airbox, on the bench, before you put it in. Easy Squeezy!

Duh...
+1

[ClickClickBoom]

I have a large port Webb Industries listed in the for sale section. If you are interested.

[scarceller]

A poor mans solution for a pop off valve is to leave the rubber boot clamp loose at the throttle body. If the system backfires the boot pops off the throttle body. We did this in the early days before pop valve where popular.

[db_cooper]

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

No..not a rodent problem. Porsche had a 911 "recall"(I don't think it was called that) or "service campaign" somewhere around 1975/76. They installed the metal braided spark plug wires and some other bits (I don't remember what the other bits were), at no cost to the owners of 1975.5 and 1974 CIS cars that had been delivered with un-shielded wires. It was considered a remedy for blown air boxes. At the time the new car warranty for Porsche was 12 months 12000 miles and some owners were paying for the repairs. If you blew an air-box out of warranty with the original wires..Porsche took care of the repair cost.

Porsche at the time was confident that the un-shielded wires were the air-box problem. I think it helped..but they still blow-up from time to time for no particular reason.

[scarceller]

Thinking about why you get a back fire in the intake is most likely because some spark plug is igniting while the intake valve is opened and fule air mixture is being ingested into the problem cylinder. So what can cause this?
1- Spark jumping from rotor to the wrong wire in the cap
2- Erroneous trigger signal seen from the points at the wrong time
3- Or simply the CDI deciding to fire with out a trigger

Of these three possibilities I think #1 is most likely. These are CDI ignitions with plenty of voltage potential and I could see the possibility of having the rotor place it's arc to an adjacent post in the cap rather than the target post.

#3 could maybe happen if battery voltage is significantly low enough?

Just theory on my part.

[HarryD]

Originally Posted by scarceller

Thinking about why you get a back fire in the intake is most likely because some spark plug is igniting while the intake valve is opened and fule air mixture is being ingested into the problem cylinder. So what can cause this?
1- Spark jumping from rotor to the wrong wire in the cap
2- Erroneous trigger signal seen from the points at the wrong time
3- Or simply the CDI deciding to fire with out a trigger

Of these three possibilities I think #1 is most likely. These are CDI ignitions with plenty of voltage potential and I could see the possibility of having the rotor place it's arc to an adjacent post in the cap rather than the target post.

#3 could maybe happen if battery voltage is significantly low enough?

Just theory on my part.

Stand on the shoulders of giants and see Peter Zimmerman's posts (10 and 11) in this thread Blown airbox - replace, prevent and while I'm in there to understand about CIS operation and pop off valves.

[scarceller]

Originally Posted by HarryD

Stand on the shoulders of giants and see Peter Zimmerman's posts (10 and 11) in this thread Blown airbox - replace, prevent and while I'm in there to understand about CIS operation and pop off valves.

Very interesting explanation by Peter. Also keep in mind that any air fuel mixture outside the cylinder is highly volatile and does not take much to ignite. Peter really pointed out very well that during cold start the cold start valve pumps extra fuel all over the intake not just down near the backside of the valves like the injectors do. I think he hit the nail on the head. After reading his explanations I certainly tend to agree that most likely something just igniting this very rich mixture of fuel and air that's been so conveniently dumped into the entire intake system by the cold start valve. Here a faulty spark plug wire that simply arcs to any ground point anywhere near this nice fuel mixture could result in the backfire.

Air/fuel mixtures at ambient pressure are most volatile and easily lit off. In contrast, within a cylinder as compression progresses and pressure increases the mixture becomes more stable and harder to ignite. Ignition is most difficult in the peak torque area of any engine as VE increases. I'm just pointing out that air/fuel mixture just sitting within the entire intake is extremely volatile and easily ignited by any flame or spark source.

[ArchHibb]

scarcellar - Thanks for your comments. It's not likely I'll ever get a definitive answer that the low battery was the cause of the backfire, but the car has "only" 58k, has always been professionally serviced, and had never once backfired, stumbled, or failed to start.

Looking ahead to when I have everything back together, I will do my best to ensure I have not overlooked any potential causes of the original backfire. I will be replacing the rotor and cap (and spark plugs) as a precautionary measure. Hope to be back on the road in a few weeks. Spring has finally arrived here in Mass. hasn't it?

[everforward]

Think of a pop off valve like a fuse in an electrical circuit. You are correct in the fact that if the system is tuned properly this will not happen, but that is an assumption over the long run. Put in a pop off valve, if you have a back fire, thank yourself that you installed a pop off valve, and use the money you saved in buying a new air box to tune your engine back to spec. You've already learned a hard lesson, why re-live it?

[maui44]

A pop off valve is a must. Low cost good added insurance.

Just insure you install it properly.

[ClickClickBoom]

Originally Posted by timmy2

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

Then go to eBay and get a Webb Industries Stainless Steel beauty, I did and never worried again.

[theiceman]

Originally Posted by 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).

finally someone posts with some good sense.

[ArchHibb]

From the recent posts I realized that I haven't said which direction I am pursuing.
My replacment airbox has a pop-off and has been tested to ensure it is not leaking. I decided that the pop-off "might" be helpful if I make a terrible mis-step (being a newbie to CIS) but that potential benefit outweighed the responsibility of adding a new element to periodically inspect, maintain, and (let's hope not) troubleshoot.

And back on topic, when I began prepping the engine for removal, I discovered the O2 sensor was disconnected. Doesn't seem like a likely culprit, but comments are appreciated.