Many find the appearance of Porsche's first water cooled 911 a bit bland. One way of addressing this is by fitting an aftermarket ducktail instead of the retracting wing. This also sheds some weight from behind the rear axle - a win-win situation especially in a car with a rear-weight bias. Unfortunately the grille supplied with this part is typically nothing more than a flat mesh which may be sufficient for a track car but lacks the aesthetic refinement one would expect for an upmarket road car. How do we solve this?

The first step is to 3d scan the rear lid and import it into CAD. This will provide the basis upon which a new design can be modeled. We only need the outside shape so in this case it was done with the lid fitted to the car.

One of the interesting aspects of 3d scanning is how it highlights imperfections which can't be seen with the naked eye. In the harsh artificial light of cyberspace every small imperfection, dent and undulation is exaggerated, as can be seen below. In real life this part was sanded down professionally, painted and polished to a high gloss, and none of this is visible on the car. This makes 3d scanning a valuable tool for car restorers and a nightmare for body shops.

The second step is to design a grille which will fit within the confines of the scanned area while keeping in mind that this part will have to be 3d printed. This opens up design possibilities which would have been impractical with injection molding or casting. However 3d printing comes with its own limitations which have to be navigated around throughout the design phase. 

The car has virtually no flat surfaces so we decided to use curved ribs as the principle design element with thin, angled vanes running between them. The vanes were angled to be as non-restrictive as possible to the air path into the engine bay. This means that the engine and air inlet are essentially open when viewed from the front but closed when viewed from the rear, which is good to keep falling leaves out.

We were initially concerned about engine temperatures especially over 50mph at which speed the rear spoiler automatically pops up to increase downforce and to let in more fresh air. We couldn't detect any adverse effect on temperatures even in extreme ambient heat. If anything the car ran a bit cooler on a hot day than with the stock lid.

If cool air flows so easily into the engine bay then it stands to reason that hot air should escape equally well. So we removed the hot air extractor fan which blocks the right-hand side grille. We closely monitored engine soak temperatures (after hot runs on the road and dyno) and didn't see any difference. Presumably the hot air escapes so fast through the non-restrictive grille that an extractor fan is no longer needed. We tested it in ambient temperatures of up to 40C.

The material we chose for the grille was ASA (Acrylonitrile-styrene-acrylate) which prints neatly and can withstand prolonged temperatures of up to 100C. This is more than any engine grille should ever see unless the engine is on fire. In terms of toughness it's on par with ABS but has the added benefit of being UV resistant. Since this is not a stressed part there was no need for a stronger material although we have since produced it in carbon-fibre nylon to some customers.

The end result looks aesthetically pleasing and fits in with the rest of the car. It turned out to be surprisingly durable and has survived many adventures in harsh conditions as can be seen below. The car (which is now living a quieter life in the UK) spent years as test bed for various technologies in South Africa.

© 2020 by Martin Wiesner