Pop up valve

seymore · 05-30-2008, 12:54 PM

My car has about 90,000 miles on it now and I have never put a pop up valve in it. I do have a high flow air box that is supposed to help with back fires. So I was just wondering if anyone has advice on what I should do. My car runs perfect and has never back fired. Oh yeah its a 1982 911SC. Thanks Jake

Hetmann · 05-30-2008, 01:02 PM

Wow, you are really lucky.

tcar · 05-30-2008, 01:42 PM

You should put one in. You can't believe how expensive a backfire will be.

It's not that expensive, even if you have someone else do it for you.

lindbhr · 05-30-2008, 02:28 PM

Jake,

If need one, I have a new pop-off valve I'll sell you for $30 plus shipping.

Thanks,

seymore · 05-30-2008, 05:19 PM

Does everyone have one on there car? And is there anyone that doesnt?

billybek · billybek's Garage

Mine came with one!

Peter Zimmermann · 05-30-2008, 06:22 PM

Quote:

Originally Posted by seymore

Does everyone have one on there car? And is there anyone that doesnt?

My '82 SC made it to 192K miles before I replaced the airbox - without a pop-off valve. It's all about tuning, cold control pressure, and correct starting procedure. My feelings regarding the pop-off valve are well known - I don't sell them, install them, or endorse them. There are some exceptions, but they don't apply to an '82 SC, which was delivered with the late-style airbox complete with cold start fuel diffuser. Enjoy your car, if the airbox blows tomorrow it probably would have blown with a pop-off valve anyway. In the years leading up to me selling my shop in 1999, we noticed a definite up-tick in the amount of CIS cars towed in with blown airboxes - and pop-off valves. Leave your car alone and have fun with it!

Formerly Steve Wilkinson · 05-30-2008, 06:25 PM

It's a little plastic miniature toilet seat that you epoxy in place. Cut a hole, use a jacknife or whatever, it's so easy it doesn't matter--obviously there are more sophisticated ways to do it--but it's so easy and basic that it's like minus one on the one-to-ten degree of difficulty scale.

Formerly Steve Wilkinson · 05-30-2008, 06:30 PM

Okay, Peter Zimmermann is in the building, and I defer to him, because he knows a million times more than I do about 911s.

Thank you for posting, Peter, an honor to have you on the site!

Zeke · 05-30-2008, 08:02 PM

It is my belief that for the average driver who gets his car serviced now and then, or when it needs it, the PO valve is a good thing. As the intake valves get weaker with age, all it takes is that one cold, damp morning when the car is starting to sneeze its way to life, and boom! Ask me how I know.

Peter Zimmermann · 05-31-2008, 09:02 AM

Quote:

Originally Posted by Formerly Steve Wilkinson

Okay, Peter Zimmermann is in the building, and I defer to him, because he knows a million times more than I do about 911s.Thank you for posting, Peter, an honor to have you on the site!

Thank you for the nice words, Stephan! At one point I wrote a fairly extensive piece regarding pop-off valve use on '73.5 - '75 cars, based on; (1) non-availability of quality service help, and (2) required adjustments to the hand throttle system that are rarely done correctly, if at all. I conceded that maybe in those instances a pop-off valve might have some benefit. However, any airbox, in a correctly tuned car with the cold control pressure set properly, that is fitted with an internal cold start fuel diffuser definitely does not need a pop-off valve. I was truly shocked at how many blown airbox tow-ins to my shop were fitted with pop-off valves, which told me that they might help in some cases, but usually short term, and that there must be a problem with the reaction time of the part itself being insufficient to prevent most explosions. If an owner has had his airbox "saved" by a pop-off valve, then the cause of the problem should be investigated and corrected ASAP.

dagriff · 05-31-2008, 11:21 AM

Quote:

Originally Posted by Peter Zimmermann

pop-off valve use on '73.5 - '75 cars,

Excuse my ignorance, but what changed in '76? (I ask because mine is a '77!).

Peter Zimmermann · 02:07 PM

Quote:

Originally Posted by dagriff

Excuse my ignorance, but what changed in '76? (I ask because mine is a '77!).

No ignorance at all - good question, actually. In 1976 Porsche introduced the automatic cold start system, and deleted the hand throttle that had become unnecessary. The new system went a long way toward controlling cold start mixture, which dramatically extended airbox life.

dagriff · 05-31-2008, 02:12 PM

Quote:

Originally Posted by Peter Zimmermann

No ignorance at all - good question, actually. In 1976 Porsche introduced the automatic cold start system, and deleted the hand throttle that had become no longer necessary. The new system went a long way toward controlling cold start mixture, which dramatically extended airbox life.

I actually do have a hand throttle (disconnected). Maybe it was "updated" at some time. I have never had cause to use it (even if it did work), and have started the car on the first try in -5c.

So I am still in a quandary as to if I should install the valve or not, especially as it ran for 30 years without one. Maybe it's time now things are getting old??
(I certainly don't function like I did 30 years ago, although I have always had a "Pop-off valve" & I seem to use it more often these days.....

Peter Zimmermann · 05-31-2008, 02:56 PM

If your car is a '76 with CIS there should not be a hand throttle (except if the car started life as a Turbo Carrera), but stranger things have happened. Please post your VIN so I can see how early a '76 your car is. Now, let's establish if you have an early, original-type airbox, or one with a diffuser. Loosen the two rubber straps for the air filter, and remove the filter and its cover. On the horizontal surface just under the filter (inside the box) there will be a few neatly-spaced screws. Are your screw heads slotted or Phillips-head type? If they are Phillips-head you have the late-style box with the cold start injector fuel diffuser. If the screws are slotted let us know and we'll cover some options.

dagriff · 05-31-2008, 10:35 PM

Peter,
They are slotted.
Vin is 9117300978
First reg 01/01/1977. It is a UK spec, so perhaps that makes a diff.

stlrj · 06-01-2008, 06:37 AM

Quote:

Originally Posted by seymore

My car has about 90,000 miles on it now and I have never put a pop up valve in it. I do have a high flow air box that is supposed to help with back fires. So I was just wondering if anyone has advice on what I should do. My car runs perfect and has never back fired. Oh yeah its a 1982 911SC. Thanks Jake

Leave it alone.

Your air box already comes from the factory with internal tubing that eliminates the dreaded explosions. I installed one of these boxes on my 78 3.0 and never experienced an explosion, period.

As far as I'm concerned, the factory solution works just fine.

Cheers,
Joe

K-Jetronic · 06-01-2008, 07:21 AM

..I have a handthrottle, 911SC, model 1978, vin 9118300147, jul 1977 and I am pretty sure it left the factory this way.

Porsche-O-Phile · Porsche-O-Phile's Garage

Put one on.

Trust me, changing the lower air box is a huge PITA that's best avoided if possible.

A new one is like 600 clams too.

Peter Zimmermann · 06-01-2008, 09:52 AM

Quote:

Originally Posted by dagriff

Peter,
They are slotted.
Vin is 9117300978
First reg 01/01/1977. It is a UK spec, so perhaps that makes a diff.

OK, your car is a RoW/Japan, '77 coupe. All well and good, I guess that I'm going to have to research that hand throttle - that system went away entirely for USA '76> 911s.

OK, slotted screws mean that you still have the early airbox with no internal cold start fuel diffuser. If it's not the original box, it's probably been on the car for many years - I can't remember when we pulled the first superseded unit out of a factory package, maybe early/mid-'80s. Because you have the original - style box it will blow, unfortunately when is the unknown. If you look closely at the perimeter of the box you will see where the upper and lower sections are attached - you'll see more of those slotted screws. Check that seam for hairline fractures in the plastic, that could indicate airbox age. Another thought, can you see remnants of a blue sticker on the side of the airbox, just below the fuel distributor on a vertical surface of the right facing side of the airbox? If the sticker, or a piece of it, is there, the box is still the original. In that case the box should be replaced as preventive maintenance. If the box is not original, and in good condition, it's a tough call. Cold control pressure can be set within a specific range that goes from slightly rich to slightly lean. If it's set for long airbox life when you start the car cold the engine will hunt slightly, for from a few seconds to as many as 30 seconds. That's a good thing, because the hunting indicates a slightly rich mixture. If the car starts great, but does not hunt, the pressure is probably set in the middle of the range. A lean cold start setting can increase the chance of a blown box - do you have a trusted technician who can check your cold control pressure?

Let me know if your engine hunts slightly before getting to a smooth cold idle. Also, can you photograph the hand throttle, maybe even what you can see through the oval hole under the carpet at the hand throttle side of the handbrake handle? Another photo that I would love to see is what injection apparatus that your car has that is located near the forward right corner of your airbox next to cylinder #6.