78 911 SC Airbox

[Guy Marshall]

Hi all,

I'm planning to install a pop off valve in the airbox of my 78 SC. Couple of questions:

It it ok to drill/install the valve with the box in place? It looks like a big job to remove it.

I notice the middle self tapper in the box is proud 2mm which makes me wonder if the box is blown already. The car runs fine so I'm tempted to screw it down and leave it:

Is there an easy way to test if the box has blown?

The car is a UK spec and has hoses missing from both ends of the airbox (for the carbon canister?). There is no canister in the car - did UK cars have them and is there any reason I should not simply block off the holes. Unfiltered air can't be good...

Thanks!

[dannobee]

https://www.pelicanparts.com/techarticles/911_pop_off_valve/911_pop_off_valve.htm

[mike sampsel]

I drilled mine with the box in place. This was 30+ years back.

[boyt911sc]

Originally Posted by Guy Marshall

Hi all,

I'm planning to install a pop off valve in the airbox of my 78 SC. Couple of questions:

It it ok to drill/install the valve with the box in place? It looks like a big job to remove it.

I notice the middle self tapper in the box is proud 2mm which makes me wonder if the box is blown already. The car runs fine so I'm tempted to screw it down and leave it:

Is there an easy way to test if the box has blown?

The car is a UK spec and has hoses missing from both ends of the airbox (for the carbon canister?). There is no canister in the car - did UK cars have them and is there any reason I should not simply block off the holes. Unfiltered air can't be good...

Thanks!

Guy,

It would take less than a minute to test your CIS air box using pressurize air or a smoke machine. But you need to isolate the airbox to do it. This subject has been well discussed and covered in many posts regarding CIS troubleshooting.

Secondly, if you do the drilling of the air box for the POV in situ, the debris from the drilling would be next to impossible to remove inside the airbox if you have the “spider” plumbing. I was drilling the 2” hole in situ for many years until I did one on the bench. And found my mistake. Many have done the drilling in situ with no known adverse effect on the motor but I will never do it again for my motor. To overcome this problem, remove intake runner #2 and install the vacuum hose of a vacuum cleaner while you are drilling the 2” in situ.

Lastly, do not install the new POV unless you have tested it for sealing. Don’t glue it and later find out that the POV is bad and defective. Fresh from the box, some of these POV’s are bad. So what do you do? A simple vacuum test for sealing will tell you if the POV is good or defective. Keep us posted.

Tony

[john walker's workshop]

If the screws are proud, the box has separated.

[Ian Comerford]

I blew my box after a misfire about 19 years ago and decided to see if it was fixable in situ. It did not run after it blew. I separated the top and bottom of the air box after removing the CIS stuff attached to the top of it to create some space. Some bigger self tapping screws and high temp liquid gasket did the job. Started first time and worked well until about 5 years later when it gave up the ghost and I bought a new one.
Acknowledging John's experience and comment above if your car runs and it has separated it hasn’t done so in an irretrievable way by the sound of it. I would have a go at repairing

Ian

[Guy Marshall]

Thanks everyone.

@dannobee I had seen your link but the example was installed out of the car - that's why I was asking if it could be done in place.

Based on John's advice the airbox is blown so the question is now moot. A new airbox (with popoff fitted outside the car!) is required.

Now, to drop the engine or not for installation...? And yes, I have read a number of threads on that too!

Thanks again.

[eastbay]

Removing CIS in car is completely doable, depends on your patience with the fiddly bits. I'd say it is much easier than replacing a heater core in a late model car.