Shark Attack

I still ask the same question as an earlier post. WHY?

BarryW

Well I'll be danged! It sure is. Thanks.

fabric

dr. bob explained it in his post.

danglerb

It would be more desirable to have cool air from the front of the intake tube going down to the TB, than hot air from the TB going into the intake. It just doesn't seem like something Porsche would do, suck hot air into the intake without a reason.

fabric

Except that getting the air to go from the top of the covers down and out the bottom would probably have been a bit trickier than what they did. And it's not as if the ram/cool air setup doesn't have it's own set of compromises, as the air flowing from in front of the radiator is going through the hot engine compartment with no insulation already. Picking up a little extra hot air from inside the TB is probably not adding that much extra heat.

mark kibort

We are talking about very little volume flow here. VERY LITTLE. in fact, there is very little heat transfer for the flow going over the radiator and into the intake tubes, compared to what comes out of the radiator itself.

under WOT conditions the intake is cool to the touch. it doesnt get hot until you start strangling your engine during idle!

one way to get flow to the timing belt housing area would be to use the pressure of the intake , however, since the bottom of the engine is exposed to flow and pressure on the back side of the radiator, i suspect there would be no pressure differntial, thus no flow. ill be doing some pressure tests for diferent locations on the car and engine this weekend at 100mph.

so, the logical way to get flow through the timing belt area and out the timing belt air port, was to use this kind of venturi port . actually, its not the port in the air stream that makes the flow, as it in itself is not a venturi. for example, ports in a throttle body dont by themselves create vacuum, the neck down of the TB creats the venturi and thus the low pressure zone. put a small hole in this area and it draws air to be used as vacuum relative to the ambient pressure. (see vacuum advance, fuel reg pressure pull back, etc)
i bet the main reason for the flow is the large port shown in the posted picture where ram effect from the front of the car, pushes air in to the bottom of the engine and finds exit to a lower pressure zone in intake. There is no real volume flow, so heating the intake air is not going to be a concern.

mk

SharkSkin

The fact that air is moving across the opening of the vent tube will tend to pull air out the vent tube, unless there is more pressure inside the intake tube than the TB cover. Not a lot, but some. As you say it's not an ideal venturi, but OTOH it doesn't need to be to move some air.

funxman

""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?""

It would do both in theory up and down depending on speed and throttle position. High speed closed throttle-ram pressure would push. Open throttle would vacuum it out. Somewhere in between I would imagine a zero air movement momentarily. Either way ventilation is accuring. I know there is enough vacuum to draw out belt dust and particals when something is going bad. It saved my engine littering belt material all over my filter. Plus its a great place to monitor your belts tracking. All around good idea to put them there.

mark kibort

Here is the thing, its not going down, because of the chance of sand and dirt to be injected into the belt compartment. Think about it, when i race at laguna seca, i usually pull in a hand full of sand out of my filter after most race sessions. (if im not following anyone (rare) it can be clean. the point is, porshe wouldnt do a design prone to bringing in sand to the timing belt. next, the pressure of the intake (ram pressure) would be (and i will confirm this very easily) the same. front of the radiator will be near the same as the rear of the radiator, less a pressure drop across it, and then that increased pressurized air will vent under the car to meet slightly lower pressure air under the car. (or in my case with a hood vent in the appropriate position, be vented up to a lower pressure area on top of the hood. the point is, the air will be pretty equal on both sides of the vent . at full throttle, the air speed will increase accross the vent, when at part throttle, coasting, obviously no air will be moving across the vent hole. however, convection cooling could be possible due to the hot air trapped inside, and cool air enteriing the vent at the bottom of the engine.

as i said, ill get some pressure readings from the vent, inlet, front of the radiator and on top of the oil pan near the bottom vent, as well as top of the hood, nose and base of windshield of our cars at 100mph.

Just because the air vent is in an air stream, doesnt mean it will pull vacuum. these kinds of ports are used in Venturi tube tests. they pull vacuum because the moving air stream is at a lower pressure than the outside of the tube air. its measured at the port. as you have a neck down, the speed of the air increases, and the pressure drops. this is how vacuum operated functions work on the throttle body.

mk

mark kibort

just did a battery of tests on the hyway today. The test were using a sunx sensor and remote pressure tube to find pressure values of the 928 at 60mph and 100mph.

the tests were conducted by placing the very accurate Sunx (+/- 15psi) sensor pressure pick up at 8 locations and then driving at 60 and 100mph and making a note of the pressure.

as some of you may know, i did cut a hole in the hood to vent air entering the engine through the radiator to allow it to vent to the top of the car, rather than creating lift under the car. some interesting results there. (100mph / 60mph)

in the intake, 8" from the opening , but in the tube.
.10psi, /.06psi. WOT dropped the .10psi to .6psi .

in front of the radiator
.12psi/.07psi

rear of the raditor, agains the engine around harmonic balancer area.
.10psi/.05 psi

Nose of the car, just below the nose, but above the inlet vents
.21psi/ .15psi

Base of the windshield, just around the wiper shaft area.
.12psi/ .08psi

right behind special hood vent on top of the hood (CORRECTION)
.00/ .00psi
with hood vent sealed
-.04/-.01psi ( VACUUM)
This was interesting showing that with the hood vent closed there is a vacuum area, in the exact area that is show on the wind tunnel test results. its relieved by venting air when the opening is not sealed, thus showing ambient pressure with no negative pressure generating on the hood at any speed, even 120mph

To specifcally answer the flow of air out of the timing belt vent, its pretty conclusive that the base ofthe engine at top speed is slightly lower pressure than the air flowing in the intake air tube. however, there is not a "venturi" effect with the vent sitting low in the air stream in the intake. It is an effect, if the air is flowing of the air having less pressure than ambient if moving in the intake. if its moving, its going to be at a lower pressure than what is built up on the backside of the radiator.so, air will flow upwards. most of the venting is probably due to convecction , as there is little air flow during cruise and techically, there could be a high pressure zone at the base of the engine that is lower than what is found in the vent area of the intake tube.

Ill post some pics of the set up if anyone is intersted. (or if anyone wants to see my hood cut out section. actually, im happy how it turned out. I have a feeling it may be a racing must, unless you have a real top notch spliter already.

MK

danglerb

A few questions;

".10psi, /.06psi. WOT dropped the .10psi to .6psi" Do some + or - signs go in front of some of those?

Does the meter measure absolute pressure, or relative (pressure inside the car at speed might not be the same as standing still)?

Does the orientation of the meter pickup tube matter, and what was it if it did?

Thanks for doing the experiment nothing like good raw data.

alex70

As funxman mentioned I thought an additional purpose was to evacuate belt dust and particles to the air filter which would prevent accelerated wear/performance of the TB parts, cam gears etc, thought I read that here quite a while back.

John Veninger

Gregg Brown mentioned it may be to remove the ozone created by cap and rotor. Keeps the T-belt from degrading.

mark kibort

The Sunx sensor measures relative changes. it would be interesting now to see what the cabin pressures are at 100mph. i actually disconnected the pressure tube in the cabin at high speed and it went to 0 . this is accurate and repeatable within .1%. the gauge is set to 0 and if the cabin pressure went up, it could detect pressure changes in the cabin.

the figures are positive pressure or negative pressure (vacuum) based on +/-.
however, the only vacuum you will see on our cars is going to be in that narrow range from just beyond the nose of the car, to just before the end of the hood toward the windshield. (just like the graph indicates)
The only negative pressure is the test on the middle of the hood, with my vent closed. Ive corrected it on the prior post. (your right i forgot the ("+/-")

It certainly shows the ram air effect at high speed for the base of the windshield and nose as well as the confirmation that the intake design is pretty solid. I didnt realize how good it was, considering it was sitting on top of the radiator. However, .12psi at 100mph and at the base of the windshield is only in the .5% gain area. Thats a whopping 1hp for a 200hp engine.

Mk

SharkSkin

Mark, it's not clear whether you checked the pressure near the balancer with or without your new hood vent open. If the former, that doesn't help us understand conditions in a stock car very well...