Is pop off valve really necessary

viper4490 · 04-24-2007, 06:53 AM

I have 76 911 and would like to know is the
pop off valve really necessary? I cant find the tech article anymore...I remember seeing it once...But they must of taken it down...anyone know why?

azasadny · azasadny's Garage

Yes! New airboxes are very expensive. BTW, I have 2 airboxes for a '74 CIS for sale, along with almost all of the other CIS components as I switched to Webers last December.

jwernquist · 04-24-2007, 06:57 AM

YES!!! Make sure to install a pop-off valve. Its really really easy to install and it will save your A$$!!!

azasadny · azasadny's Garage

When a CIS engibne backfires, the pressure produced is significant and can easily blow out the seams of the airbox, vacuum hoses, intake runner connections, etc...

LakeCleElum · 04-24-2007, 07:25 AM

I think a new air box is over $400.....I knew one guy that just drilled a one inch hole and put a cork in it. Wouldn't do it myself, but he thought it would do the trick?

Nostril Cheese · 04-24-2007, 07:44 AM

Friend of mine had a 76S without one. I helped him replace the airbox. It's not fun with the engine in the car. It's well worth the money.

Paulporsche · 04-24-2007, 07:55 AM

You don't indicate where you live, but if you live anywhere there is a fluctuation in ambient temps to where it gets "cool", sooner or later you will turn the key, your 30+ YO WUR, which has gradually been going out of adjustment, losing elasticity in its bimetal strip and having its screen fill w/ crud, will come together w/ your not-exactly-right mixture setting and your engine that now requires a richer mixture after years of carbon buildup and you will get a backfire. If your airbox straps are old, you may get "lucky" and just blow the top off the box. If not so lucky, you will probably break the box. With the popoff valve, you at least have a fighting chance to continue on and investigate why it happened.

azasadny · azasadny's Garage

Here's a picture of the airbox/popoff valve i was running on my car until I converted it over to Weber carbs. The airbox is a brushed aluminum box and the popoff valve is also aluminum, really very nice and substantial.

viper4490 · 04-24-2007, 09:44 AM

Im in Jacksonville Florida....so its always pretty hot...But I'll most likely do the valve...Thanks for the replies!!!

niall · niall's Garage

Ok I have a 2.7L CIS 1975, I've bought the pop off valve, must fit it now.

I also have a 3.0L CIS would this motor require a pop off valve also, as I have only heard them mentioned around 2.7's

EarlyPorsche · 04-24-2007, 11:17 AM

Haha I like the cork idea, but it could really hurt someone or the car if it flew off.

Eric-325I · 04-24-2007, 11:24 AM

Quote:

Originally posted by Paulporsche
You don't indicate where you live, but if you live anywhere there is a fluctuation in ambient temps to where it gets "cool", sooner or later you will turn the key... you will get a backfire. If your airbox straps are old, you may get "lucky" and just blow the top off the box.

Yes, has happened to me and was scared crapless when it did. Thank god for my pop-off valve. No damage to box at all.

Eric

Ron.G · 04-24-2007, 11:34 AM

Air Box BOOM!

Paulporsche · 04-24-2007, 11:37 AM

niall,
Put one on the 3.0. There probably should be one on every CIS 911.

safe · safe's Garage

I didn't think it was necessary until mine backfired last year. My airbox survived but the rubber boot was left in 2 pieces...
Actually this spring on one of my first rides the darn thing backfired when shifting during an overtake. It just shrugged it of and kept on going.

Kaliv · Kaliv's Garage

I have one of these in my 3.2L 1987 911. Should I investigate getting a pop-off valve? Or are they only needed for the stock box?

the · 04-24-2007, 01:54 PM

Quote:

Originally posted by EarlyPorsche
Haha I like the cork idea, but it could really hurt someone or the car if it flew off.

Only if you have people living in your airbox.

Kaliv, a 3.2 does not have CIS and does not use/need a popoff valve.

I'd definately run one on a '76. The very late SCs had revisions to the injection system that made intake backfires a lot less likely, but it's still cheap insurance, even on those cars. But a '76 is a prime backfire/blown box suspect.

1982911SCTarga · 04-24-2007, 02:46 PM

My post from an earlier post:

Quote:

The lack of science behind the airbox flapper upgrade has always bothered me, along with instances of the device failing in it's intended purpose. How quickly and at what pressure can the expanding gases escape without damaging the airbox? Why not just drill a hole and put a big rubber cork in the airbox?

And: Peter Zimmerman's perspective.

Note, however, that I have an '82 with the cold start plenum in the airbox. If I had an earlier-version airbox, I might view things differently. Even then, I'm not sure the pop-off valve would save you from the mother of all backfires.

Brian

the · 04-24-2007, 03:06 PM

You pay your money and take your chances.

I bought my '79 from the original owner, I saw the receipt for replacing the blown airbox and installing a popoff valve.

In the decade that I've owned the car, I've never had a problem with the popoff valve, but have had a couple of backfires on starting the car that I'm pretty sure would have damaged/destroyed the airbox.

It might not save you from a nuclear backfire, but it definately can save you from many.

Of course, it is true that if it is installed improperly, or someting on it fails, it could cause the car to not start. No device is perfect, everything has some chance of failure.

Gotta do the research and come to your own conclusion, but on a pre-80 or so CIS car, I'd use it.

Hawks911 · Hawks911's Garage

Viper,

If you go the do-it-yourself route on the pop-off valve, be very careful on the depth of your pilot drill versus the hole saw. There is an aluminum plenum underneath the cover that does not provide much room for error. Ask me how I know!

Mike

For sale: Slightly used tube of Permatex Blue RTV Gasket Sealant