[Peter Zimmermann]

Pop-Off Valve - PART 2:

One more component of airbox life to consider is the spark plug wires. If you are looking at a properly maintained, CIS 911, you will immediately see that the plug wires appear to have a metal sheath covering each wire. Enter the aftermarket. Manufacturers of spark plug wires have claimed that those wires covered by a metal braid are dangerous and can shock an owner or mechanic. Really? Well, I’ve been repairing/maintaining CIS 911s since they were new, and I’ve never been shocked. Those braids, when correctly installed, are connected to ground terminals near the ignition coil. Aftermarket plug wire manufacturers also claim that original equipment plug wires are ridiculously expensive, and have short life expectancies.

Well, so that you know the rest of the story, those braided wires, including their connectors, have a typical life expectancy that exceeds 100,000 miles. The braiding also serves a purpose that somehow escapes those manufacturers. The wire braiding (sheathing) controls any stray sparks that might result from Porsche’s high energy ignition system, and diverts those sparks to ground. That spark control, according to Porsche, protects the CIS airbox by keeping sparks away from the intake manifold area where fuel and air combine to form a combustible mixture, and braided wires actually can reduce the chance of a mechanic or owner of being shocked.

So, instead of verifying cold control pressure, correctly adjusting the hand throttle, and making certain that the state of tune of the car, including the plug wires, is correct, the “fix” has become to throw a pop-off valve at the car. Instead of being a viable improvement, it’s nothing more than a bandage. Installed (glued into a hole drilled through the floor of the airbox) it sits downstream from all of the special apparatus, that Porsche and Bosch conspired to design, that could provide the perfect mixture to that wonderful six-cylinder engine.

Downstream means that when the pop-off valve’s seal leaks (they often do), or when the epoxy that holds the valve into the airbox doesn’t provide a total seal (which can easily happen), the engine will run too lean. A lean mixture in an internal combustion engine can cause everything from detonation, to burned pistons and valves. Not a pretty picture.

The theory behind the pop-off valve is simple. In the event of an explosion inside the airbox, severe enough to crack, or blow, the airbox, the pop-off valve is supposed to pop open, preventing the airbox from absorbing most or all of the internal force. After the event is over the spring-loaded valve is supposed to close, whereby the system will work as though nothing had happened.

That, of course, is a scenario from a perfect world, maybe even Hollywood. The reality is that the reaction time of the pop-off valve seems to be too slow, and if the box doesn’t blow the first time that a heavy backfire occurs, it probably will blow somewhere down the road, and not too far down that road. Every explosion impacts an airbox, and over time fine cracks appear where the upper and lower sections of the box are joined together. Introduce a lean mixture, and those explosions occur more frequently, and with more violence. Add a stray spark from an unshielded spark plug wire, and blammo! A blown airbox, in other words, is far too complex an event to be cured by a bandage. Analogize it as a bullet wound severe enough to kill its victim without surgery, a bandage won’t fix the damage or prevent loss of life. A pop-off valve will not prevent a blown airbox, delay it maybe, but not prevent it.

An additional strike against the use of a pop-off valve is a remarkable improvement made to the airbox sometime during the 1980 model year. No, Porsche didn’t invent its own pop-off valve; instead, it came up with a real fix. They installed an internal diffuser which mounted so that the cold start injector’s tip fit inside its end. The diffuser is a long, thin, metal section from which six skinny tubes protrude, each pointing directly into one intake runner. As the cold start injector squirts atomized fuel into the diffuser, at the moment that the engine starts to crank over, manifold vacuum pulls the fuel air mixture directly into each intake runner. This prevents fuel and air from collecting in the airbox’s central chamber, the deadly mix that just hangs around waiting for a stray ignition spark, or a spark from the carbon that’s formed on the backside of an intake valve.

For purposes of recognition, every airbox, when manufactured, is made of an upper and lower section. Those sections are joined with a layer of epoxy, and then receive additional strength from screws installed around the joint. Some of those screw heads are visible by simply peering into the engine compartment and looking at the left side of the airbox below the intake air snout of the air filter cover. If those screws have a slot for a flat-bladed screwdriver the airbox is the early, non-diffuser version (except for an unknown amount, maybe a handful, of boxes made when the update was first done), while late, diffuser equipped boxes are held together with Phillips head screws. All replacement airboxes, except the box for the ’73.5 T, have been fitted with internal diffusers for many years. To replace a blown airbox, and then add a pop-off valve to it, is nothing short of heresy.

The Petrol Blue SC discussed at the beginning of this chapter is not the only tow-in that I’ve seen with an airbox/pop-off valve related problem. Actually, many CIS 911s with blown airboxes, fitted with pop-off valves, have been towed in to my shop. They’ve also been towed in with a pop-off valve that had popped open, but refused to re-close, producing such a lean mixture that the engine had no chance to start. They’ve been towed in after the epoxy failed and the valve had come loose from the box, which is another no-start condition. They’ve been brought in by the owner with a whistling noise at higher rpm, caused by a leaking pop-off valve lid. They’ve been brought in with a complaint of poor top-end performance, a lean condition that caused detonation.

This article covers a subject ripe for heated debate. Here’s a thought to hold on to, Porsche never designed and then superseded the airbox with their version of a pop-off valve. I’ve said recently that maybe a pop-off valve would be OK for a ’73.5 - ’75 911 in an area of the country where little or no competent service is available, reluctantly, maybe. That said; I don’t like the pop-off valve.