To Pop Off or Not to Pop Off
 

I am about to become the proud owner of an '82 911 SC. The car does not currently have a pop off valve and I am considering installing one. BUT, in Peter Zimmerman's The Used 911 Story, on p.125 regarding airbox pop off valves he says, among other things:

"...With the improvements made to the airboxes by Porsche, there is absolutely no need for an add-on device such as this."

The section talks about how pop off valves have been the cause of a number of "emergencies" at his shop.

Everything I've read elsewhere says install one or you'll wind up having a backfire blow up your airbox.

Also, do you have to drop the engine to replace the airbox on an '82 SC?

Thanks in advance for your help.

Try posting this at forums.rennlist.com . Peter Zimmerman is a regular there and "get it straight from the horse's mouth".

The later CIS systems do have fuel plumbing within the airbox to prevent a backfire. But, I have seen these later airboxes backfire and crack. Do install a pop off valve and gain some piece of mind.

BTW: You can replace the airbox with engine in the car, but its a real PITA. Air boxes are $$, and its not a fun project to R&R.

Listen to the man. An ounce of prevention..... not a fun job to replace an airbox. You will kick yourself for not installing a $50 pop-off valve and the 1/2 hr to install, especially after you pay plenty of bucks for an airbox and installation.

One way to look at the pop off valve is as an extra layer of protection if something on your engine isn't working exactly right.

When I purchased my 79 it didn't have a pop off and it was fine. After about 8 months the car developed an injector problem, it backfired on start up and blew up it's airbox.

It cost me about $1,000 to fix it.

AFTER that happened I installed a pop off valve and 14 years later that valve has never caused me a problem and it's backfired five or six times over that period.

If I had it to do over again I would install the valve after I purchased my car and spend the $1,000 on something fun.

The first thing after blowing a crack in my airbox was to install a pop-off valve while removing the CIS to repair the airbox. Before that I followed the same advice. That advice is assuming that the CIS is working properly and always will. Nobody ever has problems with CIS do they?

Install the pop-off valve, it's the cheapest insurance you'll ever buy.

Jerry M
'78 SC

Easily done. Try to borrow a right angle drill. I imagine that would make it a cinch. I would loan you one if I had one.. Welcome to the wonderful world of SC ownership.

Definitely install the pop-off valve. My '79 backfired about two days after I installed it and it worked like a charm - no damage to the air box at all. Periodically check the valve to be sure that it hasn't come loose. Best $50 of insurance you can spend.

You'll know if it's loose. The engine wont' start.

That is the only downside; sometimes they come off. They are made of a material that doesn't glue that well.

Do put one in, but be sure to clean the surface with a degreaser, then scuff it with sandpaper to give the glue some tooth.

It's a good excuse to buy a right angle drill!

I actually used a small right angle die grinder with the hole saw bit. It barely fit in there due to the height restriction but worked great.

I posted this same question on Rennlist and Peter Zimmerman posted this reply. Thought you guys may find it useful. Not sure what to do now...

"I have mentioned in the past that pop-off valves can be a bandaid for a car that is not cared for in a technically correct manner (regular CO% checks, proper cold control pressure maintenance, visual inspection for air leaks at the injectors, adjustment of the hand throttle on pre-'76 cars, etc.), especially those cars that live far away from a professional service facility. I'm not going to beat a dead horse here, but I have seen a large number of failed (blown) airboxes towed into our facility, that WERE fitted with pop-off valves. My '82 airbox lasted 192K miles, with NO pop-off valve (it did not blow, I found a crack during a routine service), and its replacement does NOT have a pop-off valve. I have owned 5 SCs over the years, none with pop-off valves, and have never gotten even a single cold start issue that might lead to a blown airbox. I use nothing but the best gasoline (Chevron Premium here in CA), and perform all maintenance to factory recommendations. My cars "hunt" for a few seconds on cold start (because the cold control pressure is set toward rich running) and the cars exhibit no starting/running problems.

Our shop has never had a car towed in, that we maintained for at least three years, with a blown airbox. Sure, there were times when we replaced airboxes; (1) while the engine was out for synchro repairs, clutch work, or head stud replacement, or (2) while performing a routine major (15K) maintenance we found the beginning of cracks in the airbox, and sold a replacement to our client. Those cracks are caused by tiny explosions inside the central (plenum) chamber of the airbox. A pop-off valve cannot relieve all the stress from those explosions, therefore the same cracks occur, with or without the valve. On '81-83 cars still fitted with the original box those tiny explosions are all but eliminated with the factory's addition of a fuel diffuser. It's a simple series of metal tubes that extend from the cold start injector into the 6 intake runners, which prevents atomized fuel from gathering inside the box itself. The diffuser has been added to all replacement airboxes, except for the '73.5 T, and if in doubt regarding what airbox you have check its perimeter screws. Boxes with diffusers are assembled with Phillips-head screws, early boxes have slot-head screws. The exception are a handful of the first diffuser cars, which were also put together with slot-head screws.

Some airbox failures occur because of the use of valet service (where the valet uses the foot throttle to "aid" in starting the car), and cars that have more than one driver, often with lack of training or understanding that the gas pedal must NEVER be touched during starting. I have seen SCs driving around with a not-working frequency valve, I've seen CIS cars with the ignition timing off by 10 or more degrees, I've seen fuel mixture so lean the needle wouldn't move on our machine, I've seen cold control pressure so far outside acceptable range that the car would lean-pop on cold start. A pop-off valve might serve a purpose on cars like those, but only as a bandaid. It won't prevent a blow for long, sooner or later the box will fail unless the car's root problem is corrected first.

For me, the problems that a pop-off valve bring to the table (lid stuck open, lean conditions, epoxy failure, etc.) outweigh any of its possible positives, therefore I can't recommend them, and I've never used or sold them. And Porsche never incorporated one into replacement airboxes, which I've always felt leant credibility to my point of view.

About replacement, the airbox can be replaced on hand throttle cars ('73.5-75) without engine removal. Complete removal, due to the organized replacement process required (not to mention cleanliness) of the engine on '76-83 models is highly recommended. As time has passed removal has become even more important because the cars are all more than 20 years old, and additional parts (oil pressure switch, breather hoses, thermostat o-ring, etc.) should all be attended to. Also, new injector seals/sleeves must be installed, along with the 6 sleeves that connect the intake runners to the airbox. Setting the CO% following replacement is mandatory. On pre-'76 cars hand throttle adjustment is also mandatory, because that might have been the reason that airbox blew! Speaking of hand throttle cars, cold and warm starts are not as carefully regulated as in the non-hand throttle cars, so maybe a pop-off valve might help, but I still wouldn't put one on my car.
Pete"

This is a good point!
I have also heard that a pop off valve can´t help if there is an airbox-cracking-backfire.

One big hole in his argument is that all of his cars that he used as an example are closely monitored and maintained by an admited expert in CIS systems. Even the customer cars that he has seen all are from a group that are maintained by that same expert.

Unless he collects information about the cars that aren't maintained in such an expert fashion it removes some weight from his argument.