I think one down side was condensation. Causing headers to rust, but I heard this years ago. I think this applied to headers that had a much thinner wall thickness than what 928's have. Don't know, but I am thinking of doing this now since I have the engine out as it would be so much easier.

 

The pipes themselves get hotter which may promote cracking and get them wet you might get more rust. But the reduction of underhood heat is probably worth it.

 

the other solution might be to draw in cold air for the hvac from another place ie the side of the car. i think the idea of having a spoiler to divert the airflow after the vent is actualy very clever, on my car i can clearly see that water beads are already less disturbed in the wake of the vents. but i think the size of spolier needed would be impractical - i think this is what mark is saying as well.

 

the sides dont have a higher pressure zone to push air into the cabin, the base of the windshield is the second highest pressure zone next to the nose.

I forgot what your vents were used for. also downforce? where they are located is just where there is a transision from low pressure into the higher pressure zone. actually in the graph, it shows the high pressure starts about a foot back from the end of the hood before the windshield.

mk

 

mark, i was not so focused on downforce when thinking about this mod but was considering more the issue of heat evacuation - although any air evacuated out this way will improve downforce. i think your vent has a much larger surface area than mine and perhaps is more functional as a result - i was drawn to the idea of having a lip spoiler at the leading edge. youve got me curious now so i will have to do the wool test on the highway and see if it gets sucked back in through the vent!

i didnt measure exactly the position of my vent relative to that diagram of pressures over the bonnet but the opening is roughly where the strut brace is. i imagine there must be quite some pressure under the bonnet to lift wool at 45 degrees.

 

actually, based on your picture, its located perfectly. Nice job. (based on the factory pressure charts)

maybe the lip enhances the flow....

put the wool tufts on the vents and watch what happens. I did a test with the tufts and had sealed the opening, and what a difference.

mk

 

The leading edge lip will in theory create a localised lower pressure zone downstream - decreasing (ie even less pressure) the general low pressure at that point on the hood = good for extracting air through the vent. Dare I say venturi?

So long as the lip doesn't induce vortices behind it which could very locally create a high pressure at the surface of the vent. Only way to tell is to put woolies on the surface of the vent itself. If they point up and to the rear and at a steady angle (Angle itself doesnt matter depends on the combination of speed of air out of the vent and speed of air over the hood) what matters is the woolies being straight and steady.

If the woolies wiggle about randomly then you've got vortex shedding - same thing happens with the woolies on the leeward side of a sail when the sail is sheeted in to far - the air over the leading edge going to the leeward (low pressure side) can't follow the curvature of the sail and "falls off".

 

jon, im sailing this weekend with sunsail - 4 races over sat and sun so i will keep an eye on the tell-tales! but as usual the car is again in a workshop where the headers are being fitted and i dont think i will have it back before im off to visit the rellys in nz next month. i will definitely rig the car with some tell tales when i get back and take a look. interesting what you say about vortex formation - is there an advantage in forming these at some points to organise the turbulence?

 

downsides to header wrap that I've seen (had it on my motorhome w/a built 440 mopar), they soak up oil, look like hell after a while as they stretch/unravel. kinda hard to keep it intact. also the extra heat can crystalize the metal, causing it to crack eventually.

it did cut under hood temps quite a bit though. didn't notice much performance gain, but it was a 12k lb rig, so it didnt' run a lot of 1/4mile stuff. seemed to quiet the motor down a bit also. less reverberation from the tubes I suspect.

dont' think I'd bother w/the 928 after using the jet hot coating stuff. seems to work pretty well w/out all the downside headaches.

 

Nick where did you get those vents that you added to your hood? Are they off the shelf or did you custom make them yourself ? I like the look.

Cheers
Matt

 

matt, i spent an age looking through the internet and found these guys: http://www.carbontrix.com/vents.htm the vents im using are 13x10.