Belly Pan Vents

[Zirconocene]

Because I'm a glutton for punishment, and can't seem to control my impulse to tinker and "design", I've been thinking lately about belly pan vents. I'm talking about the horns that are attached to the belly pan itself, which sit right behind the engine crossmember. Like many (most?) current 928 owners, the belly pan that I bought came without the air horns and used parts are rare as hen's teeth.

The originals are stamped metal parts and, I believe, that some of the design elements incorporated into the shape are a result of the fabrication process. However, because I am not this kind of an engineer, I'd like to open up the discussion on what's necessary, and what's not necessary for these air horns.

Now, just to be clear, I'm working from (not always fantastic) pictures I have from the internet, so it's not always exactly clear what I'm looking at. I'm going to try to make a CAD file for 3D printing to share, and would like some guidance/opinion about what's going on with the parts.

I'll add questions as they come up.

First up, on the larger of the horns, it looks like the interior volume expands up from the opening on the horn. Is there any reason why the interior volume/shape can't (or shouldn't) be extended down to the flanged base plate? (Hopefully, you can understand what I'm asking, but I can post pictures, maybe, to help explain).

That's the big question for now, as that's the part I'm working on designing.

Thanks

[dr bob]

From feeble memory, a guy in Colorado tasked himself with coming up with replacement pieces. I have no idea how that ended up but that might be an option unless you are determined to do it yourself.

For those playing along, Porsche decided to place a full belly tray under the engine to improve aerodynamics, starting with the S4 model in 1987. The original is reinforced plastic. The fluid-filled motor mounts sit above and next to the exhaust manifold collectors on either side. The mounts have dedicated heat shield panels, plus each has an air duct sitting just aft of a NACA duct to grab some cooling airflow from under the tray to pass over the mounts and protect them from exhaust heat. The ducts tuck up tight in front of the crossmember holding the engine mounts, and there's a a little hoop forward of each duct where screws secure the duct (and therefore the tray) up in place. Pictures will explain a lot more, and I need to sift through my library to see what I might have.

My '89 car still has the original tray and the original ducts riding underneath. It's still in place, but the car is in summer driving season. The tray will be off later this fall for a pre-hibernation service and prep. At that time I can take a slew of pictures and gather dimensions if you really want to make some. Eligible materials are pretty much limited to metal as the originals are formed. Not sure if there are any common 3D print materials suitable for the heat, but it would certainly worth checking/testing. I don't remember if the aftermarket trays have the little NACA section formed in the tray, or if they depend on having that formed as part of the little ducts.

From feeble memory, each duct will need at least two pieces, and forming them might best happen with a wood form and some hammer work. The factory pieces have rolled seams where the pieces fit together, and have pop rivets holding them in place. I'll guess that they are maybe 18ga or less steel. The ducts are not the same left and right, of course, so two sets of forms. Fun stuff!

[Zirconocene]

Thanks @dr bob , more pictures would always be welcome.

I have in mind the production of some CAD files for 3D printing. There are some materials available that I think would be suitable for these parts.

As you said, there is a lot of real estate that is consumed on these pieces to incorporate the rolled edges, so I'm trying to think through the consequences of making use of that "wasted" space. I believe that some of those elements are there due to manufacturing considerations, but neither do I want to discount the thought and compromises that I'm sure went on between the engineers and the bean counters.

I'm designing these in SketchUp, so the models won't be as nicely rounded and flowing as the stamped pieces. I know that that's generally important for air flow but I don't think that a little bit of added turbulence will matter much for cooling purposes.

I'll post more as I progress, thanks for chiming in.

Cheers

[Tom]

Thanks @dr bob , more pictures would always be welcome.

I have in mind the production of some CAD files for 3D printing. There are some materials available that I think would be suitable for these parts.

As you said, there is a lot of real estate that is consumed on these pieces to incorporate the rolled edges, so I'm trying to think through the consequences of making use of that "wasted" space. I believe that some of those elements are there due to manufacturing considerations, but neither do I want to discount the thought and compromises that I'm sure went on between the engineers and the bean counters.

I'm designing these in SketchUp, so the models won't be as nicely rounded and flowing as the stamped pieces. I know that that's generally important for air flow but I don't think that a little bit of added turbulence will matter much for cooling purposes.

I'll post more as I progress, thanks for chiming in.

Cheers

Have any pictures of what you are trying to make? If I can help, I'm happy to try. I use Fusion 360 -- not an expert by any means but can usually get'r done for car parts.

[dr bob]

Main Belly Pan - Metal.jpg (550.2 KiB) Viewed 2901 times

The two vents in question are the pieces standing up from the pan on either side in the picture. This is a pic of an aftermarket metal belly pan, with the original vents installed. Not enough detail in the picture to design, but maybe enough to share the concept.

The factory vents include mounting tabs that help hold the tray up in place, tabs highlighted in the second picture. Whether they are needed with a metal tray is questionable especially for cars limited to US highway speeds. The two square blocks allow a couple 6mm bolts to hold the tray up tight to the steering rack.

The centered duct at the bottom directs cooling air up to the starter motor, if any were wondering.

Main Belly Pan - Tabs Highlighted.jpg (96.71 KiB) Viewed 2901 times

I'll be able to get some better dimensions and profile details when I have the car up on the lift for hibernation prep later in the fall. It's serving DD duty now in the summer, although honestly it still isn't driven enough.