Jerry Feather

I have had some inquiry lately about me making replacement Plactic Intake Air Tubes for the 85-86 Porsche 928 cars. Fortunately I have an 85, so I took one of the fairly good tubes out to study. By looking it over a bit I could tell why the ones for those two years are unique and why they were provided in left and right versions.

I have decided to try to fabricate an induction tube to replace "these ones" and to do it by designing one tube that will function on either side of the engine. I also hope that my design will look a little more "purposeful" than the originals and that it will function better and perhaps with slightly less restriction than the originals might have. I will also incorporate some of the mild taper section for whatever good that might be.

The development and fabrication process I am working up is fairly complex and will involve everything necessary to go from a flat sheet of 1/16th inch thick ABS plastic to the final shape of the tubes.

I had initially thought briefly about some process whereby I would put some kind of ABS tubing into a female form which has the negative shape of the final piece formed inside. Then I realized that making that kind of form is far beyond my capabilities. Besides that, I am not aware of any availibility of the kind of plastic tubing that I might use.

Then I reversed my thinking and started my development with the use of a positive form with the shape of the inside of the new tube turned and formed on it, but the first problem there, aside, again, from the tubing to start with, is how to get the form out of the formed tube once it is formed on the outside of the form.

So I have been working on the design of such a positive form which will be made in about 9 seperate longitudinal segments. That way they can be taken out of the formed tube one at a time starting with the one in the center then the rest of them in sequence after that.

The center segment of the form will have a hole through it longways into which vacuum will be introduced. The form will have multiple tiny radial holes from the core outward to the surface of the form which will introduce the vacuum to the area between the plastic being formed and the form.

Oh, yeh, the tubing. What I intend to do is make my own tubing out of flat 1/16" ABS plastic sheet cut to the exact circumfrence of the gross form size and which will then be beveled on the opposing sides of the long edges to be joined and then it will be glued into a tube. I'll need to make some tooling for a trim router that will help me make a nice clean bevel on each edge, and then some kind of tooling to use to clamp the joint to cure.

There is a bunch of other stuff I will need to make to get this to work, and I have most of it pretty well figured out. The only thing I am not sure about at this point is my ability or any ability to get the sheet plastic tube to shrink down smoothly around the smaller segment of the form, from about 3 1/4 inch diameter to about 2 3/8 inch diameter, without wrinkling up. I am thinking that it might work with some careful application of massage with heavy cotton welder's gloves combined with artful application of the vacuum.

Here are a couple of pictures of my sketches of the wood form and two hardwood boards I'm going to start the form with.

More later about the additional things needed, such as the heat and vacuum sources. More later also about the turning process which will require some special tooling on my wood lathe.

syoo8

I will build an ABS idol, to which I will pray every morning, noon, and night, if you make an 86.5 intake tube.

(Well, maybe I exaggerate, but not by much.)

Thank you for taking this on, Jerry.

Avar928

Awesome stuff Jerry. Looking forward to it.

BIMMERMIKE

Go Jerry, Go!
Good Stuff.

WallyP

Just wondering - would this work?
1) Turn the shape of an intake tube from wood, 1/32" oversize.
2) Paint and polish the shape, coat with wax.
3) Gelcoat shape, lay up fiberglass, let cure.
4) Cut plywood stiffeners, bond to fiberglass on shape.
5) Bandsaw longitudinally (thinner blade).
6) Remove wood.
7) Insert ABS pipe, heat, inflate.

Glen McCartney

Awesome, I'm in for a set when you get this done.

SMTCapeCod

Bonus points for a trimmable extension (ie can be removed) that covers the are of occasional breakage on the elbows to the upper lid...or is that just me..

If the price is good, I'd be happy to put a set into daily use and shelve my originals, improvements welcome.

dr bob

That's kind of the principle around blowmolding, if you add heating and rapid cooling capability to the mold. Plastic beads are melted and extruded into the heated mold as a soggy pipe shape, a little differential pressure draws the soggy pipe shape out to the mold, which is then cooled to firm up the plastic inside. I suspect you could heat up some thin-walled ABS tubing with hot air inside, place it in the mold and blow it large inside before cooling the mold. Remembering that ABS used in pipe isn't exactly flexible unless it's thin (and weak).

I don't have an 85/86 car, so maybe a dumb question: can later S4+ tubes be used with a fab'd adapter at one end? I'm not clear on what makes the 85/86 tubes different I guess.

M. Requin

Has anyone queried the industry? If this was normal practice in '85 the solutions used then might be applicable to a smaller scale solution now.

M. Requin

I could be wrong but I think the minor diameter, where the tube fits underneath the cross brace, is smaller than the later S4 parts, based on what I've seen (but not worked with, so this observation could be full of it...).

Jerry Feather

Yes, Wally, I think it would, or at least something developed along those lines. The problem is that I think it is much more complex than the route I have chosen; and I have the feeling that the end result would not appear as well finished as doing it the positive form way.

In particular, I can't seem to get my mind around how to heat the plastic and inflate it at the same time.

Nevertheless, thanks for giving me some more food for thought.

SMTCapeCod

When I had my GTO, (LS type), an intake elbow was provided by the aftermarket- it was a fiberglass form covered with a (real) carbon fibre veneer. The run was shorter but it did have to contend with a 90' bend and being between a relatively fixed union at the fresh air intake, and a rocking engine at the other. I don't know what substrate they molded the fiberglass over, unfortunately.

Jerry Feather

Here is a picture of the tube off the passenger side of my 85. The right end in the picture is the small end and the one that connects to the intake box back of the engine. I wonder where the "elbow" is" to the upper lid." It look fairly straight there to me.

OTR18WHEELER

Interesting, as they are NLA. I have also pondered ideas of a revision. The originals seem to be the cheapest looking part under the hood. IMO there is no need for all the accordian type ridges. The overall form is the key, only two silicone boots at either end are needed. keep in mind $100 is the going rate for a used pair of originals, if you can find them.

snoz

What is the inside diameter of the intake box side of your example pictured above?