AirBox, Pop-off Valve ?

[nicfranc]

I have a 1980 SC without a Pop-off Valve installed.
It appears to have the latter air box with the philips screw
heads on the left side of the air box as opposed to the earlier air boxes with the slotted head screws.

The engine has 250K miles on it.
Should I install a Pop-off valve ?
Or leave well enough alone ?

[porsche930dude]

its never too late. i ground off the ribs around the valve so that it can set down in tight and get better adhesion. alot of guys didnt do that and they break loose eventually

[tazzieman]

Some say a properly tuned engine won't backfire.

And that the newer style box will greatly reduce the risk .

That said I did the mod when I bought the car ; 18mths later a bondo-like plug (PO) shot out of a disused port hidden underneath the airbox , massive backfire on a cold winter's morning. Maybe the valve saved my airbox?

Cheap mod , do it well and keep the seal slippery (bit of silicone spray). Check the valve each time you change the filter or once a year. Carry some quick set epoxy in case it goes whilst out on a journey.

Peter Z's Used 911 Story book said whatever you do, the airbox is gonna blow one day. Those words stuck in my head!

[neighbour]

Disegrented an airbox in 85.

Installed a a flapper.

No problems since..

You're way away from the light at the end of Daisy's dock, but it's your call.

Wait, WAIT.

[nicfranc]

In Peter Zimmerman's book: "The used 911 Story"

He states: "With the improvements made to the air boxes by porsche, there is absolutely no need for an add-on device such as this."

So is it just the earlier air boxes that are at risk ?

How many people have had blown air boxes and what year were the engines.

[tirwin]

Zimmerman has posted on here before and given quite lengthy explanations on why the pop-off valve isn't necessary. And while I tend to agree with his analysis that you don't need one IF everything is operating properly, it only takes one backfire to blow out the air box. So consider it cheap insurance. But like anything, there are potential disadvantages too. There is the possibility that the seal around the valve will eventually fail and then you'll be sucking false air. My car came with it installed.

[E Sully]

I would install one. Here is a previuos thread.
BANG! There goes my SC airbox, but why?

[boyt911sc]

Originally Posted by nicfranc

In Peter Zimmerman's book: "The used 911 Story"

He states: "With the improvements made to the air boxes by porsche, there is absolutely no need for an add-on device such as this."

So is it just the earlier air boxes that are at risk ?

How many people have had blown air boxes and what year were the engines.

nicfranc,

Well if you are Peter Zimmerman, you don't need any of these aftermarket pop-off valve. He's a professional mechanic and spent time with CIS cars. But for most car owners with CIS, the level of knowledge about this fuel injection system is quite insufficient. While I agree on most of his suggestions in the forum, this is one thing I beg to disagree.

A well tuned CIS car will eventually have an air leak some where, some day specially for those that oftenly dropped their engines for some work. This is like telling people not to bring a spare tire and jack because it will never happen to you.

Tony

[psalt]

The engine has 250K miles on it.
Should I install a Pop-off valve ?
Or leave well enough alone ?

nicfranc,

I agree with PZ, a Pop-off valve is a bodge on a late SC, don't hack it.
250K speaks for itself and is better than opinion.

[ASD]

mine is a 78.. no pop off valve 110,000 on it. Never has backfired. So far

[mikespitzer]

I can only say that I have a 1973.5T that had a stock airbox.
One day I had a massive backfire that triggered an engine fire.

Luckily I had 3 full size Fire Extinguishers on board AND the Fire Dept got there quickly, but I still ended up with $15,000 damage.
Fire was contained to engine compartment.

So I don't want to comment on the later models, but EVERY Porsche expert I have spoken with says on the very early CIS engines, the pop off valve is almost a "must do" since even when tuned well, these 73-75 era 911 CIS cars still seem to backfire quite a bit on start ups from all I have spoken with.

And if you do some Google research, you will see engine fires are more common than some of us may have known

[RFP]

The airbox on my [ex]77 blew back in the early 80s... I did the high-dollar fix: Webers!

Gee, I wish I had that car back!

Rob

[Dave Colangelo]

I have the valve installed (was there when I bought the car). I have had some backfires through out my ownership. There may be no correlation here but I have always found it worse when the weather is hot and humid. I have also found that it occurs more if the engine catches, fails to get going, then I try to start it again right away.

Id say do it if you are thinking about it, it may save you one day.

Regards
Dave

[Monkfish]

Seeing as this old thread's been resurrected I might as we'll ask - would disconnecting the cold-start injector save the air box if a CIS motor was prone to backfiring? I ditched the cold-start feature on my MFI car after it set the air filter alight!

[gazzerr]

Your car would be really hard to start when cold without the cold start valve.

[HarryD]

Originally Posted by mikespitzer

I can only say that I have a 1973.5T that had a stock airbox.
One day I had a massive backfire that triggered an engine fire.

Luckily I had 3 full size Fire Extinguishers on board AND the Fire Dept got there quickly, but I still ended up with $15,000 damage.
Fire was contained to engine compartment.

So I don't want to comment on the later models, but EVERY Porsche expert I have spoken with says on the very early CIS engines, the pop off valve is almost a "must do" since even when tuned well, these 73-75 era 911 CIS cars still seem to backfire quite a bit on start ups from all I have spoken with.

And if you do some Google research, you will see engine fires are more common than some of us may have known

At 160k I has a massive backfire (my PO valve was not up to the task). Replaced the cracked box and discovered it dropped a screw into the a cylinder as well. $10k+ later, back on the road with a new air box and a top end.

[HarryD]

Originally Posted by gazzerr

Your car would be really hard to start when cold without the cold start valve.

For a while my cold start valve was disconnected. It was a b***h to get my car started. Reconnected it and starts on first or second crank every time.

[Orange911S]

I didn't think I needed one, and was pretty sure I would mess up the install.
A week ago a big backfire blew the box.
'77 2.7 with 48K miles.
The good news is that this is the first time I have had the engine dropped and my mechanic will take care of the hard to reach oil leaks.
From what I am reading it could have been worse.
Jeff

[stlrj]

Originally Posted by Orange911S

I didn't think I needed one, and was pretty sure I would mess up the install.
A week ago a big backfire blew the box.
'77 2.7 with 48K miles.
The good news is that this is the first time I have had the engine dropped and my mechanic will take care of the hard to reach oil leaks.
From what I am reading it could have been worse.
Jeff

But the 77 2.7 did not have the new style air box with the internal plumbing. If it had, it never would have happened and you would never have needed a flapper installed.

Cheers,

Joe

[donlan]

Hello, I have a 2/'73 911T with 84,000 MILES and it has no pop off valve. The Bentley book mentions an inbuilt blow off valve. (The "air regulator valve blows back past it's centre position in the event of a backfire"?).Michael.