Originally Posted by Peter Zimmermann (Post 6866332)

The pop-off valve article is too long, I think, for a single post, so I will split it into two parts. Enjoy!

The Pop-Off Valve - PART 1:

I watched the flatbed, with a Petrol Blue SC aboard, back into our parking lot until it was aligned with our service door. As the driver unloaded the car he said, “The owner told me that she heard a bang from the back, and then the car wouldn’t start.”

It took me about the same amount of time as an F1 car takes to change gears to know what was wrong, a blown airbox, of course. After the tow truck driver was gone I raised the SC’s deck lid, reached down and wiggled the air filter housing. I was right, the airbox was so thoroughly blown that it was in two pieces. Only later, when we started to repair the car, did we make an interesting discovery – the car was fitted with an anti-backfire, aka pop-off, valve. Obviously, it didn’t work. Much has been said and written about the pop-off valve, with dissenters in the clear minority. I’m in the minority.

Porsche forums, chat rooms, DIYers, even some mechanics, have joined hands and pledged to honor said pop-off valve. Why?, is what I wondered. This is a part designed by someone in his back yard, not the R&D center at Porsche. I don’t like, with a few exceptions, aftermarket parts. I don’t like the pop-off valve. Why has it had success is the compelling question.

CIS (Continuous Injection System) is a wonderful fuel injection system used on Porsche 911s from the 1973.5 T thru the end of SC production in 1983. The central component of the CIS system is a large, plastic part called an airbox. The airbox provides mounting points for other CIS components, such as the throttle body, the sensor plate housing, the fuel distributor, the cold start injector, six intake runners, the air filter with cover, and other parts that do specific jobs. When Porsche introduced the CIS system it wasn’t crude, like some designs, but it did prove to be basic.

1973 – 1975 911s with the system required the use of a hand throttle for starting the car. Instructions regarding its use, in the rarely read owner’s manual that comes as standard equipment with every new car, were clear. Pull the handle up to its stop and crank the engine. After the engine started ease the throttle down to a point where the car would idle smoothly. The items in italics were often ignored, or miss-understood. In addition to owners not operating the car correctly, the hand throttle required that its adjustment be checked, and corrected if necessary, at scheduled maintenance intervals. Adjustment, one more strike against the hand throttle system. Many mechanics don’t read that handbook either.

During cold starts, as an engine cranks, a very specific amount of extra fuel and air is needed for the car to start quickly and run smoothly. As the pollution control noose tightened around car manufacturers necks, cold start emissions were also measured, so intake systems like carburetors could no longer dump large amounts, at least more than needed, of fuel into the engine for that delicious, overly-rich cold start mixture. Into the fray enters the cold start injector, a part with its own electrical connector; that reacts to a signal received when the engine is cold enough to need a squirt of fuel to start properly. Air is the other component, and is supplied by an intake air system, with a butterfly valve similar to that in the throat of a carburetor, called a throttle body. The butterfly is operated by the driver pushing down on the throttle pedal, which (bear with me) in turn produces intake manifold vacuum that causes a sensor plate, mounted next to the throttle body, to move upward. That action, in turn, activates a counter-weighted arm that pushes up on a plunger inside the fuel distributor, which can then distribute fuel to the six injectors. So, the throttle body butterfly opens a measured amount, the sensor plate moves up a measured amount, the plunger moves up a measured amount, the fuel distributor distributes a measured amount of fuel – which is a perfect mixture for how far the butterfly opened. Hypothetically, let’s say that the perfect mixture is 14.8 parts air to 1 part fuel. Let’s say that the perfect cold start mixture, for a typical spring ambient temperature, is 14.8 parts air to 1.2 parts fuel. How can one regulate that?

Porsche engineers determined that by using a hand throttle, which would pull on the mechanical linkage at the throttle body, the driver could open the butterfly seven degrees, which was the perfect amount of air when combined with the amount of gasoline squirted into the airbox by the cold start injector. To adjust that seven degrees, a simple, mechanical device was used, which was accessed through a hole in the tunnel between the front seats. Adjustment was done by loosening a set screw on a small metal cylinder, which allowed the cylinder to be moved along the throttle rod inside the tunnel. When the warm engine would reach about 3800 rpm, with the hand throttle all the way up, the hand throttle was adjusted correctly. If the hand throttle opened the throttle body butterfly too far, the fuel air mixture was disturbed and the cold start mixture became too lean. A lean cold start mixture can cause backfires inside the airbox, eventually it will blow. Porsche took care of the hand throttle issue fairly quickly, and in 1976 introduced an updated system that only required the driver to turn the key to start the car. Newly developed CIS components took care of all the fuel/air mixing, insuring that the engine received the perfect mixture with which to start at all ambient and engine temperatures.

All of this discussion, to this point, has involved only the few moments that it takes for the car to actually start. Another important factor, and part, in the starting process is the Warm up Regulator (WUR). The WUR is an electrical device that controls something called Cold Control Pressure. Cold control pressure is crucial, and the factory manual shows a temperature chart for each CIS model year, and instructions regarding how to measure that pressure. For some reason those pages in the manual are probably the cleanest ones in the book. Anyway, the cold control pressure is the amount of pressure in the system, at the top of the fuel distributor, and it regulates how easy, or hard, it will be for the sensor plate arm to lift the distribution plunger up into the fuel distributor. Less pressure, richer mixture; more pressure, leaner mixture. Once again, a lean mixture is not the airbox’s friend.

Originally Posted by Peter Zimmermann (Post 6866364)

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.