Prologue: The double debut.

 

Two friends and one “triebwerk”. Our two F350 coupés – two of our backdating customer projects – were created in parallel flight.

 

The (development) history of a prototype that – apart from minor deviations – exists twice. Two friends from Rhineland set out on a long drive across the nation. One wanted to visit local garages specialising in the Porsche 911 with fan-type air cooling, the other wanted to accompany him. Both were taken in by the 964 model – the most beautiful nine-eleven for quite a large number of fans of the sports car brand based in Stuttgart-Zuffenhausen. Launched after the factory holidays in 1988, the third generation of the successful Porsche 911 model combined 88 percent newly developed components with traditional lines. Today, this continuity in design makes it possible to return to the look of the early seventies – the starting point par excellence for nine-eleven enthusiasts. However, this does not mean that the effort involved should be underestimated. In truth, it takes great technical expertise and passion, particularly for unusual, creative solutions, to dedicate oneself to one of the most popular tasks in the automotive aftermarket sector.

Chapter one: F350 to the power of two.

 

One placed a (large) order, another enthusiastically followed suit and ordered (almost) the same a second time.

 

Back to our two friends: when they finally reached dtw Germany in Schwelm “das triebwerk” on their journey around German workshops and garages, they had reached something special. From Christian Wilms and Stephan Rohleder, the two managers, they not only heard about the most diverse options and configurations – but more on that later. They also learned that they didn’t necessarily have to bring a base vehicle capable of being transformed, but could also order this in conjunction with all the work to be done as a full service option. So the next trip for Christian Wilms led south to the Italian border to see a coupé with sunroof and air conditioning, first registered in 1991. The mileage corresponded to the year of production and the original factory paintwork was to give way to a typical paint finish from the colour palette of the early seventies in the course of restoration. The decision was not difficult for one of them – and the other spontaneously went along with it. He had been carried away by the palpable enthusiasm for the transformation of the second black 964 – from the company’s collection – into the “F350” model by the Schwelm-based triebwerk. As a result: he too placed his order – and off they went – in parallel flight.

Chapter two: Water march!

 

Why sustainable investments in tangible assets made from sheet metal – from a professional point of view – do not belong in an acid bath.

 

Even though “amethyst” is a truly beautiful shade of red that originally adorned the coupé body of the 964, it was absolutely clear that in view of the extensive welding work necessary, the old paintwork would need to be stripped off. Background: before any welding work, the metal cloak has to be inspected for previous damage, possibly caused by accidents or corrosion. To do this, the metal should be completely bare and exposed. Only then can sustainable, long-term top quality be ensured. A gentle high-pressure water jet method is used for paint stripping. By dispensing with a chemical immersion bath process, no crystals can form and deposit in cavities or under the sheet metal seams that occur as a result of spot welding. In the long run, crystallisation can again lead to corrosion even on completely new sheet metal bodies. On the other hand, bare sheet metal without any chemicals is a guarantee for dependable, long-lasting work – and, incidentally, a real experience for project customers like Michael and Wolfgang. Both drop in once a week at dtw germany | das triebwerk in Schwelm to keep up to date and exchange their latest ideas.

Chapter three: An arduous 800 hours.

 

Before set-up in the rolling rack, an inventory needed to be made that would have revealed any previous damage – had there been any.

 

Back again to the preparation for the actual welding work, which took around 800 hours per vehicle. Cathode dip coating of the stripped sheet metal could not be used either, and this for a very special reason: air bubbles can form with this method – and where they do form, bare sheet metal is not protected against corrosion. “In addition,” explains Christian Wilms, “it has been shown time and again during restorations of the classic F model that sealing compounds have been applied most generously in the area of the partition between the passenger and engine compartments. This is removed by the high pressure water jet almost automatically.” The exposed sheet metal on the straightening jig allows a precise assessment of any previous damage. In the case of the two purchased base vehicles, the all-clear could be given – everything was in perfect order, all gap dimensions complied with the factory specifications prevailing in 1991. And – just for the sake of completeness: Michael also placed an order to find and buy a suitable used vehicle for him on the market. Which also happened – thus clearing the way for a core aspect of backdating, the (intricate) sheet metal construction.

Chapter four: Back to sheet metal.

 

Hand-made wings and rear side panel extensions, sheet steel bumpers, elaborately modified central tunnel.

 

Even if it is as intricate as it is exhausting: we love to process sheet metal professionally. This is demonstrated not least by our backdating projects. Firstly, we transform the front and rear of the nine-eleven generation 964 to the condition of the F model – including the corresponding supports. We also modify the original steel bumpers of the F model so that we can place enlarged oil and air-conditioning coolers behind the front air intakes. In addition, both units get powerful electric fans to support the thermal effect. This gives the whole thing the general impression of the aggressive exterior of a GT racing version when viewed from the front. The front wings and rear side panels, which are hand-crafted from sheet metal, underline this impression, as they take up the lines of the 911 S/T 2.3 created in 1970, which in 1971 was actually launched with sheet metal extensions due to a change in the technical requirements. The most prestigious challenge, however, is the replacement of the original central tunnel with the more authentic version in the look of the F model, in order to make the journey back in time perfect inside the car as well. This step is carried out on the frame straightening jig without adding a temporary sub-frame.

Chapter five: Let colours speak.

 

Better: give them a code number that everyone knows what is meant: “aquamarine blue 5707” comes from the Porsche classic par excellence, the 356.

 

After sheet metal construction and at least 800 hours of welding, a completely new paint job – on one of the two “F350s” in a historically relevant colour: “aquamarine blue 5707”, a colour that had already adorned the first Porsche that went into series production. The second Porsche enthusiast opted for a more discreet shade of grey, which, unlike the first specimen in “aquamarine blue 5707”, did not stand out with a block stripe between two finer outlines. Otherwise, both (special) vehicles have the finely crafted, lockable oil and petrol caps in common, which come from the Harley Davidson motorbike brand range and are labelled with the correct operating fluid in each case for safety reasons: oil at the rear right side – as was the case with Porsche’s legendary “oil flap” in the 1972 model year, petrol at the centre of the front bonnet. A special mechanism ensures that the lettering is always horizontal. In addition, the tank system is designed in such a way that nothing can miss the target or get lost in the 87-litre petrol system. Incidentally, the centrally located bonnet fuelling system is one of 16 equipment options available.

Chapter six: No compromises.

 

The thermally sophisticated exhaust system is part of a high-performance system in a confined space. It is non-negotiable.

 

Not among the equipment options is the exhaust system made of stainless steel tubing material, which fits in the confined space in the rear of the nine-eleven 964 model with simultaneous TÜV compatibility – yet still enables performance data on the level of related racing cars. With a measured 360.7 hp, the six-cylinder boxer engine of the F350, suitable for normal roads, is on the same level as the Porsche RS(R) 3.8 coupé, whose competition version generated 368 hp as an absolute top value. This is, of course, also made possible by craftsmanship and consistent engine design. The original power unit with “matching numbers”, which was brought from 3.6 to 3.8 litres capacity, is completely disassembled, revised and modified. One of the highlights of the sports engine, which is available in different displacement variants and whose (expected and often enough surpassed) horsepower figure is included in the name, is the in-house designed and CNC-manufactured throttle valve system. This increases efficiency to a maximum. “You can hardly achieve much more with a two-valve concept,” explains Christian Wilms assuredly. As the lead technician, he himself developed the throttle valve system. It is adaptive and operational in any 964. It also includes a special intake air collector, which ensures effective inflow.

Chapter seven: From very closed to very open.

 

The so-called worm-roller lock in the five-speed manual transmission offers extra traction and propulsion in all situations – and does not overstretch the driver.

 

The drive system of a sports car is naturally of great importance. Porsche equipped the nine-eleven generation with a five-speed manual transmission – apart from the versions with Tiptronic. Today, almost 30 years later, these rare units are in any case in need of a technical overhaul. This offers a favourable opportunity to improve traction when cornering, and to do so noticeably, by retrofitting a limited-slip differential that is precisely tailored to the respective application. The worm-roller lock gives the F350 two faces: during slippage, it closes up to 100 percent – a rigid, locked system for extreme driving conditions. If the driver releases the accelerator, it opens again immediately to prevent spin. This intelligent solution forms a functional unit with the Bilstein “B16 PSS 10” threaded sports suspension, of which even the two-time former rally world champion Walter Röhrl was personally convinced on the demanding BILSTER BERG at the end of April 2021. The upgrade with Brembo brake calliper housings with perforated discs and Pirelli “Classic” tyres on 17-inch rims in Fuchs design do the rest to combine driving pleasure with the greatest possible degree of active safety.

Chapter eight: Walter. Approved.

 

Two-time rally world champion Walter Röhrl saw the performance of the two F350s for himself at the BILSTER BERG at the end of April 2021.

 

It is a special moment for a constructor, a partner and entrepreneur and their team when one of the world’s most respected volunteers takes to the cockpits of the two debutants – especially as he is also known for not mincing his words when need be. But Walter Röhrl, the best rally performer on the globe in 1980 and 1982, has surprisingly little to complain about during his first encounter with the two F350s from “das triebwerk”. Although he is astonished by the special characteristics of the previously described worm-roller lock, the statement so feared in the industry: “Go away with that rubbish!” remains absent – and indeed for the whole (Fri) day. Although the development and presentation driver in the service of the Zuffenhausen-based dream (car) factory gives the odd tip on fine-tuning the Bilstein sports suspension, he seems very satisfied with the workmanship of the two “restomods”, and this despite his generally blunt critical attitude. The same goes for their two proud owners, for whom the world champion’s blessing has something sacred about it – almost like Urbi et Orbi. One of them is already transferring his just ennobled new acquisition into his private collection, the other sees his choice confirmed – and is looking forward to one or two more active kilometres on the road. One would like to shout out to both: “Carry on!”

Final chapter: So many possibilities.

 

The list of options is not necessarily short with 16 positions. It offers a great deal of (choice) freedom, not least inside the car.

 

Do you remember the chapter “Back to sheet metal”? Among other things, it was about the modification of the central tunnel. Of course, an original gearshift knob from that era is part of the genuine, unadulterated feeling of the newly-dawning seventies. With the finely milled gearshift system, every gear change becomes an experience – and thanks to a specially manufactured gearshift gate based on a 964, a precisely guided (driving) pleasure. The driver, of course, also wants to be precisely guided – ideally in her or his own beautifully designed lightweight seats, finished in high-quality carbon fibre. The offset rev counter, which indicates up to 9,500 rpm, is to be understood as a reminiscence of the glory of the Carrera RSR – even if the F350 does not quite reach this limit. Chrome rings around this and the other circular instruments, a retro radio with hands-free facility and the air-conditioning control panel round off the range of options. End of story? Not quite – because from the original side vent windows at the rear to the specially shaped sills and future performance upgrades for the engine, there is still (infinite) scope. So stay tuned – and don’t leave this station!

Chapter ten: The technical documentation.

 

The comprehensive specifications of all the measures and options implemented on the “F350” so far can be found here:

 

Vehicle type: Porsche 911 Carrera 4 coupé (model 964), dtw backdate to performance version F350.

Model year of the base vehicle: 1991 (M programme).

Base vehicle: version with steel sunroof and air-conditioning system.

Body (standard equipment on delivery from the factory): 2-door, 2+2-seater, self-supporting, closed coupé body made of hot-dip galvanised sheet metal on both sides in lightweight construction; deformable plastic front trim with integrated light-alloy bumper, attached to impact tubes; plastic side sill panels; rear spoiler in the boot hatch extending automatically at 80 km/h; deformable plastic rear trim with integrated light-alloy bumper, attached to impact tubes.

Body construction (dtw | das triebwerk): after complete dismantling, residue-free paint stripping using the chemical-free high-pressure water-jet process; checking of input dimensions on the frame straightening jig; set-up in the rolling rack; 800 welding hours including all modifications; widened front wings and rear side panels as well as side sill panelling made by hand from sheet steel panels in the proportions of the 911 S/T; replacement of the original central tunnel inside the car with a flatter version based on the F model of the early 1970s; F model conversion of front and rear including front and rear carriers; modified/widened front and rear bumpers made of sheet metal; front apron with additional front spoiler lip to enable a continuous baseline; enlarged engine oil and air-conditioning coolers in the cooling air intakes of the front apron; LED main headlights (Kaege).

Colour scheme (dtw | das triebwerk): new complete coating job in “aquamarine blue” (Porsche 356); block stripes in “savanna beige” with narrower outlines in “ruby red”.

Basic engine: reconditioned six-cylinder light-alloy boxer type M64/01 (MY 1991, ex works with cylinder head gaskets); two valves per cylinder; camshafts driven via cam chain; dry sump lubrication with separate oil reservoir; engine control via DME (Motronic).

Engine design (dtw | das triebwerk): individually manufactured, CNC-milled solid aluminium individual throttle valve system (adaptive, fits every six-cylinder boxer engine from 911 S 2.4); 993 generation valves with thinner 8-millimetre valve stems (compared to 9 mm on the M64/01 model); pistons with higher domes; new FVD cylinders; displacement expansion to 3,800 cc; Schrick camshafts; custom-made intake air manifold above the adapted intake spreader.

Exhaust system (dtw | das triebwerk): individually manufactured, CNC laser-cut exhaust system made of stainless steel tube material with two HJS 200 Zeller catalytic converters, one of the two exhaust manifolds is used as a heat exchanger for heating the vehicle interior.

Mixture setting: digital, freely programmable ECUMASTER engine control unit.

Compression: 11.8 : 1.

Engine output (standard on factory delivery): 250 hp at 6,100 rpm.

Engine output (dtw | das triebwerk): 362.7 hp at 7,800 rpm.

Power transmission: revised, manual shift five-speed gearbox type G50/03; original factory gear ratio; retrofitted worm-roller limited-slip differential (optional); single-mass flywheel; reinforced sports clutch; drive via double cardan shafts to the rear wheels.

Braking system: dual-circuit system with axle-wise split; four-piston aluminium fixed callipers front and rear (Brembo); internally ventilated and perforated discs (355 mm diameter front, 328 mm rear).

Wheel suspension (front): individually on light alloy wishbones and struts in McPherson design; Bilstein threaded shock absorbers “B16 PSS 10”; coil springs; anti-roll bar.

Wheel suspension (rear): individually on light alloy semi-trailing arms and struts; Bilstein threaded shock absorbers “B16 PSS 10”; coil springs; anti-roll bar.

Wheels: Bride alloy rims in Fuchs design (9J x 17-inch front and 11J x 17-inch rear).

Tyres: Pirelli “Classicʺ (235/45-17 front and 285/40-17 rear).

Interior: circular instruments with chrome rings; offset rev counter up to 9,000/min; individually manufactured and profiled bucket sports seats made of fibreglass-reinforced plastic with heating; gearshift gate with own components and others adapted from the G model G50; gearshift knob adapted from the F model with individually milled gearshift system; retro radio with hands-free facility; digital control panel for the air-conditioning system; rear-view camera monitor in the interior mirror.

In-car entertainment: largely invisible, individually manufactured “Dr. Boom” sound system with resonators under the interior door panels, correspondingly converted into a sound body.

Empty weight (according to DIN 70 020; standard scope of delivery from the factory): 1,350 kg.

Empty weight (new): 1,180 kg.

Total time required: 1,800 project hours.

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