1973.5 911T Airbox Question

[Coloradocurt]

The attached photo of the inside of the airbox shows two possible issues (circled in yellow): The circle on the left I'm guessing is a missing screw; The other circle appears to be a chamfered hole a bit less than 3/8" in diameter at the surface. I have no idea of the purpose or if there's supposed to be something in that hole.
Anyone with a 73.5 CIS able to solve the mystery?
Thanks.

[Jonny H]

Box is probably split on the underside beneath the missing screw.

[pmax]

Time to R&R that airbox.

[Coloradocurt]

The other screws show no sign of looseness!

[HarryD]

Most likely missing a screw. My box was like that for years before it became an issue. A larger issue is that you do not have a pop off valve. Unlike later design CIS cars, the box does not have a diffuser to reduce the chance of a backfire occurring and blowing up your box.

Also, take care to keep your car in tip top tune. It really matters to prevent a backfire.

[Jonny H]

^ Screws don’t just go missing Turn the box over and look for a split. I’ll wager it is split along the seam where the two halves are bonded.


IMO a pop off valve is for folks who don’t want to believe they have a split air box. They split because they’re old and the material bonding fails. Buy a new one from Porsche and you’ll find it is made from a stiffer material (PA6) rather than ABS. The new ones also have stronger screws and bonding.

[pmax]

Read more about the "pop off" toilet seat valve here To Pop Off or not to Pop Off?

[Jonny H]

Originally Posted by Coloradocurt

The other screws show no sign of looseness!

Doesn't matter - check the box.

In this photo, you can see the boss for that screw and the split to the right of it.

You might need to lever it gently with a screwdriver to check for the split. Dont be shy, if it isn't split it wont break.

[Jonny H]

p.s. Another 'tell tale' is not being able to tighten that screw.

[E Sully]

I have 2 1973.5 CIS airboxes. Neither shows any sign of a screw ever having been installed. Your box looks normal.

[HaroldMHedge]

Your 3/8" hole should be the airbox drain.

From Oddity w/Air Box & CIS

Originally Posted by E Sully

This is on my 73.5T CIS air box. The hose attaches to the bottom of the air box. #11 attaches on the front of the engine to the right of the flywheel. The check valve is a rubber sleeve, #12 in the pic.

[Coloradocurt]

Thanks for the expertise, fellas.