jeyjey

You know, the hose that goes from the timing & balance shaft belt covers back to the airbox.

Cleaning up hydrocarbons slung off the belts? (That would presume the balance shaft front covers were leaking -- which they probably are -- but it still doesn't sound very Porsche-engineer-like.)

Airing out the timing chest? (If dryness was that critical seems like correcting it only when the engine was running wouldn't be a great idea.)

Something else?

Millermatic

I think it was blocked off in later cars. There's a part number for a cap.

So presumably it doesn't/didn't do much.

MAGK944

Millermatic

Of course... this still leaves open the question of what was it _supposed_ to have done...

Gage

My personal reasoning was that it was designed to draw cooler air through the enclosure and remove particulate from belt wear and perhaps even ozone from the cavity.
BTW mice prefer you not cap the port to the air cleaner housing
I have not however made any sense out of the rectangular port at the bottom of the later rear covers.??

Zirconocene

On the 928, these were never blocked off, as far as I can tell. I don't have any experience with the 928 GTS models about this, but those definitely have different intake runners than other 928s, and I know that Porsche was getting weird with some of the engine bits on those cars. Still, on the S4, those outlets are still "plumbed" directly to the intake tubes. I say plumbed because there is just a hole in the intake tubes that sits right on that little nub. Now, why Porsche would want to draw in some slightly warm air to the intake is another question, as I would assume that airflow over that opening would create a small vacuum to draw out any air in that area.

There's lots of speculation over on that board but I don't think that anything has ever been proven one way or another. It's not for oil, though. I think the consensus is that there needs to be some amount of cooling that is beneficial for the belts, given the overall tightly packed engine bay.

Cheers

Stephen03

I thought it was so timing belt teeth and debris would end up on the air filter and you would notice it more often

MAGK944

There is also this theory about why they removed the vent on later cars.



(apologies I don’t know how to link to a single post)

Gage

Yeah... I'm calling BS on that one. Any water would need to get past the filter and if there is that much water on the belts, the air intake snorkel in the fenderwell is going to pick it up also.

PetePorsche

Sounds like rubbish yeah. I've wondered about it too.
The only things we really know are: 1) warm air rises, 2) the hole is in the top part and 3) the hose ends in front of the filter.

It leads me to believe that this is some freak German overkill type of belt ventilation, to remove the very fine particulates, and possibly to remove excess heat (though nothing is gained by this because the engine itself is one big heat source, and there is no lower opening to pull in colder air)

Another interesting detail I noticed is that the hole is located in a narrow spot right above the upper balance shaft, maybe this is done on purpose to cause a higher pressure in the hose to the filter?

MAGK944

Absolutely my thought too.

Another thing is that at about the same time the TSB to remove the vent on earlier cars came out, another TSB instructed the removal of a big portion of the rear cover to accommodate the new water pump design.

Could it be that the vent, whatever it did, couldn’t do it’s job if the belt area wasn’t fully enclosed. So that’s why the vent wasn’t fitted anymore on the very late cars (ie: after they used up stock of the vented covers).

jeyjey

OK, here's another idea. Any water that does get in is going to get quickly aerosolized by the belts. Porsche engineers added the vent to apply a bit of negative pressure so the aerosols didn't get blown up into the dizzy cap. :shrug:

pfarah7

Here's my take (if anyone is asking): A few years ago, I took a nice three week, 5k mile trip up and down the eastern seaboard with my 84. I had performed the major service just prior to the voyage so I stopped in Myrtle Beach, NC at a buddy's house to crash and check the BB/TB tension (@1,000 miles). What I found was an abundant amount of rubber filings on the air filter as one of the brand new roller bearings had almost seized. An overnight order of a couple replacement parts from 944Online had me back on the road in a few hours but I was grateful for that back pressure or I may have discovered the problem a bit too late. While I understand Porsche's rational for the TSB, I prefer to keep that hose on both of my early cars...

BeerBurner

We had a conversation about that tube last week when a friend helped me change the belts. I have no idea what it's for but at least now I can offer up some suggestions...

Anyway, TSB or not, my car still has it at 195k miles it so I don't think it causes any harm. I'll just leave it in there if for no other reason than not having to get the parts to plug it.

BB.