This article is one in a series that have been released in conjunction with Wayne's book, 101 Projects for Your Porsche 911. The book contains 240 pages of full color projects detailing everything from performance mods to changing your brake pads. With more than 650+ full-color glossy photos accompanying extensive step-by-step procedures, this book is required reading in any Porsche 911 owner's collection. See The Official Book Website for more details.
Half-way through the 1973 year, Porsche introduced a new type of fuel injection to be used on its 911T model. The Continuous Injection System (CIS) was to be the system of choice on production 911s for the next 11 years, until Porsche introduced the Motronic System on the 911 Carreras in 1984. The CIS system is a pretty good and quite reliable system, however, there is at least one upgrade that should be performed on all CIS cars - the installation of a pop-off valve in the air box.
The CIS system is a vacuum controlled fuel injection system that depends on tight seals in order to accurately meter and distribute fuel to each of the cylinders. If a vacuum leak exists in the CIS system, then you will have problems with steady idling and other general performance issues. Take a look at Project 31 for more information on troubleshooting CIS systems.
One problem that sometimes results from having a tightly sealed vacuum system is that excess air pushed back into the system from an accidental backfire has almost nowhere to escape. It's quite common for the CIS cars to backfire: especially the early ones when started cold. If the backfire is large enough, it can send enough high compressed air back through the intake manifold to actually blow apart the black plastic CIS air box that is located in the engine compartment. Replacement air boxes from Porsche are hundreds of dollars, and good used ones are sometimes difficult to find. Even if the force doesn't destroy the air box, the occasional backfire can blow apart or weaken seals and increases the likelihood of developing a vacuum leak.
The solution is to install a pop-off valve in the air box. This proverbial trap door allows the explosive force from a backfire to escape through the air filter instead of destroying the air box. The spring loaded pop-off valve has a seal around its outer edge that maintains the normal operating vacuum pressure of the CIS system. When the car backfires, the force of the air pushes the spring-loaded valve open, and releases the air out of the system. This pressure relief saves the air box from damage.
The procedure for installation of the pop-off valve is straightforward. Simply drill a hole in the bottom floor of the air box, and epoxy the valve in place. The first step is to remove the air filter housing. This will gain you access to the inside of the air box. Using the template that comes with the pop-off valve kit, map out the location where you want to drill the hole. It is wise to drill a small pilot hole before using the hole saw. The hole must be drilled at exactly a right angle to the floor of the air box, and the pilot hole will aid significantly in getting the proper alignment. There will also be some ridges on the floor of the air box that may cause some difficulty when drilling. Start slowly, and work your way down with the drill until the hole is complete. If the clearance is too tight, then you might need to use a right-angled drill.
After you've drilled the hole, vacuum out any remaining plastic chips that have fallen down into the air box. If you miss a few, they will be sucked into the engine and harmlessly burned in the combustion chamber. Test fit the pop-off valve in the hole, and file the edges of the valve and/or hole if necessary. The valve should fit flush with the ribs in the bottom of the air box.
Before you epoxy the valve in place, make sure that you clean both the edges of the hole and the valve with some alcohol to remove any oil or gas from the surface. Mix the epoxy according to the instructions that came with the kit, and apply it liberally to the valve and also lightly around the edges of the hole. Install the valve in the box, and rotate it slightly to insure that the epoxy is spread on all surfaces. It's important to create an airtight connection here. Install the valve with the hinge parallel and closest to the rear of the car.
Let the epoxy dry at least 24 hours before starting the engine. Following that, your CIS system should be well protected against the threat of backfires.
Position the template on the air box and punch a guiding hole with a nail and hammer. Be careful not to tap too hard, as you don't want to crack your air box. The pictures that are shown in this project are of an air box that was removed from the engine for clarity in the photos. It is not necessary, nor recommended to remove the air box for this procedure.
Drill a pilot hole using a small drill, and then switch to the hole saw. Make sure that you hold the drill absolutely vertical when you are drilling the hole. Access may be a little tight if you are drilling from inside the engine compartment.
Test fit the pop-off valve prior to mixing up the epoxy. Make sure that you place epoxy around the entire circumference of the pop-off valve: you don't want any air leaking through the hole. Photos provided by Jim Jordan and Bob Tindel.