words and photos by pablo deferrari

I had a problem.

the gas tank on our 928 developed cracks. I wasn't surprised to find a 35 year old "ostrich," as it's know by old school techs, with stress cracks in the tight curves where it met the metal cradle it was suspended on...I could've cursed Porsche's engineers, but I praised them instead.
this was Porsche's first standard production plastic gas tank, and one of the many such engineering breakthroughs the 928 would possess. but it wasn't their first crack at it.

you see, back in 1967, they teamed up with BASF and began development on a tank made of Lupolen for their 911R. lightness and safety were the benefits of such a design, and it proved itself in endurance races, rallies, and various world record runs. made in either 26.4 or 29 gallon sizes, so impressed were the engineers with their performance that Porsche was willing to offer them as an option in the 1970 911S.  
Picture
the 1967 911R
Picture
this would've been the first recipient of the Lupolen plastic fuel tank, the 1970 911S.
the problem was that a tank of this size was that there was barely enough room for luggage under the hood. so in 1972, this was solved by reducing the spare tire in size by making it an inflatable along with making the tank 22.4 gallons in size. keep in mind we're still talking about the 911 in case you spaced out a paragraph ago...the 928 was being developed around this time too.

what made plastic an attractive choice for the 928's design was the ability of it to be manufactured into complicated shapes, especially in the tight and contorted confines of the 928's rump. the other two points were safety and weight. 

it would've be too much to ask of steel tanks to perform all three criteria that the 928's design demanded, so Lupolen was the obvious choice.

plastic and steel fuel tanks, okay, you got that bit. 

but if you think it ends there, you'd be wrong...there's more to it than the external design. if you know your Porsche history, you'd be aware that the plastic tank that was offered to 911s had either carbs or Kugelfischer mechanical injection up until 1973 when the CIS, or K-Jetronic system was introduced in the 911T with Type 911/91 engine...that's when the plastic gas tank disappeared.

why? 

pay attention here...
Picture
the swish chamber that makes CIS and plastic gas tanks a marriage made in heaven...yes, my lens was dirty you anal retentive bastard.
Picture
the view from a little farther out...the hole at the top of the tank is where the fuel level sending unit screws into.
CIS stands for "Continuous Injection System," which means just that...the system operates on continuous fuel pressure that can't be interrupted. what this means is that with the current steel and plastic fuel tanks offered until 1973, fuel pick-up in the tank didn't affect the fuel delivery with carbs or mechanical injection systems when the car was subjected to heavy braking or driven aggressively around turns when the fuel level was low.

if the "suck" was interrupted for a moment or two, no problem. but you can't pull that shit with CIS...it depends on continuous pressure, interrupt it and she chokes and dies on you. so what's to be done? the image above is the answer.

a swish chamber.

this was what separated new technology from the old. that cup-like structure embedded in the bottom of your early 928's gas tank is what prevented you from being embarrassed in front of your peers while you pushed her hard around a bend, full bore, with your gas gauge reading vapors.

"what, this piece of German shit cost you fifty grand and it just sputters around turns like that?"

na, not a position you want to be in.

this swish chamber wouldn't be a problem to manufacture into a steel tank, but on a plastic one, it was a challenge...like building a Galleon in a bottle.

and because the 928 had to have a plastic tank, they needed to over come this hurdle...and they did. the solution was to hang the swirl chamber on a line inside in between the two halves of the form in which the tank was "baked" during its production. 

it remained there while the shaping procedure of the tank was going on all around it. after it was all said and done, the line holding the swirl chamber could be cut allowing it to be dropped in the middle of the tank and welded in place.

this design and manufacturing procedure solved a problem the CIS couldn't live with...

what was my problem?

finding a Lupolen welding rod to plastic weld my tank's cracks.

highball!


911 image sources: www.themotoringenthusiast.com, www.wired.com
 


Comments

09/18/2014 09:33

Nice post.You have provided great information in you post and some things I have not seen in other content I have read by others.

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