I was looking at the air intakes on another 928 that has the vents at the top of the timing belt covers, and It made me wonder which way the air flows through those vents, up and through the throttle or ram pressure from the front forces air down into the TB covers?

 

The 'venturi effect' draws air from the timing covers up through the nozzles to mix with the intake air passing to the engine. Airflow is UP through those little nozzles.

 

not quite, as the hole is before the "venturi". so, with ram pressure, which i have measured right to the level estimated by the formulas.(ie about .11psi at 100mph) the pressure in the tube would force air to an area of lower pressure. (that being in the tube or around the tube as it is not sealed and toward the intake air filter, where we can measure vacuum under WOT) next time i go out, i will measure pressure in the intake tube and then in the timing belt vent hole for confirmation. If i find some old venturi intake tubes, ill take a measurement at the nect of the venturi. the sensor i use is a Sunx sensor which produces very accurate measurements of vacumm or pressure.

If the air is moving out of the vents, the only reason would be that it is heated air that is less dense, so it rises out of the hot timing belt area and vents to the vent hole.

MK

 

My guess is that there is not much movement of the air through those holes and what does move, goes both directions depending on how fast you're going. Although the covers aren't sealed very well, there isn't much area for them to vent/draw air in other than the holes in the top. They just keep the TB area and distributor caps from condensing water and holding oil fumes to reduce corrosion and belt deterioration. Can you measure the DP across the vent or velocity through it?

Of course I could be all wet on this one. I'm curious as what you come up with on the pressure readings.

 

When I saw three replies I kind of chuckled to myself that there is no way it could be three different answers, sorry I underestimated you guys.

I am also real curious about the pressure readings. Temperature would also be sweet to know.

 

Dammit! I knew better than to post in this thread.

 

Mark, that will be an interesting experiment. Remember, the whole underhood area ought to be somewhat pressurized at speed, which means that in order to find flow direction through that tube it might be instructive to check the pressure inside the intake tube then check the pressure inside the TB cover(maybe at your TB tension-checking hole) under the same conditions. I expect that either measurement would be higher than say inside the passenger compartment with the windows up and the HVAC off. If, for example, the pressure is .11 psi inside the tube and it is .12 inside the TB cover, air would still move upward. Just measuring the pressure in one spot is of limited value.

In any case, I would expect air to move out the top vents in the TB covers at idle & low speed, which is when heat buildup would be more of an issue.

 

Hi Mark,
I am really interested in the result you get across the Venturi. I've only owned my 928 (4.5L '81) for 6 months now and have looked and looked and looked at these Venturis to try and figure out why they put them their.
Flow straightener?, Restrictor to engine airflow?, or just to muffle induction noise?
I would love to know at what point does the air pressure entering the Venturi become higher, than the air at the Air Filter side.
You mentioned 100MPH would be the point where the air travells down the Cam Belt area. Does that mean that @ 100 MPH they are a restriction to Engine Aiflow? Having still a little Diff Presure across the Venturi.
I do alot of Airflow testing here in Australia working for Honeywell Engines & Systems, so It would interest me greatly.
Cheers,
Glenn
'81 928

 

I don't care about the airflow direction as much as I do about the stoneflow direction.

 

Thats what they gave as reason for venturi in '81 yearly change doc. It also show picture where engine is sucking air from valve cover.

 

Why would the T Belt need to be vented? I think the "vents" are just holders for the intake tubes.

 

Mark--

The 'venturi effect' is caused by the intake airflow passing over the open end of the nozzle, and has nothing whatsoever to do with the venturi section located downstream in the intake duct. Don't get them confused. The effect probably isn't much, but it also doesn't take much. The intent is to make sure that the air in the covers is changed every once in a while. Just enough to keep the belt from getting cooked in there I'd speculate.

 

From the '81 InfoTechnik book-

 

Air is drawn in through a port between the crank and oil pump gears, which takes air from above the oil level sensor.

 

I totally overlooked that. thats why you're the "DR".

yes, that is correct. even with a moderate pressure differential from the belt area to this vent tube, it still will flow out due to this effect. i was thinking you were refering to the actual neck down of the tubes.

MK