SP Autobahn’s tuned 470bhp 1992 Porsche 911 Turbo 3.3-litre 964

2020 Antony Fraser and Drive-My EN/UK

Stuart Paterson fettles Porsches at his Stirling-based SP Autobahn workshop: we head for the Highlands in his showcase 964 Turbo. Words: Johnny Tipler. Photography: Antony Fraser.


Careful build and modern tuning techniques have taken SP Autobahn’s 3.3-litre 964 Turbo to an impressive 470bhp, with no loss of driveability. We drive it on some great Scottish roads.


I’m right at home, here in the driving seat of a 964; takes me back to the heady days of the Peppermint Pig – though that was a left-hooker. But here I am, helming Stuart Paterson’s pride and joy around the hairpins in Scotland’s achingly picturesque Trossachs region. Sure, it’s a 3.3 Turbo, which places its full-on performance on a different level to my old C2, but honestly, the handling around these serpentine Scottish hill roads is not that dissimilar.

SP Autobahn’s tuned 470bhp 1992 Porsche 911 Turbo 3.3-litre 964

SP Autobahn’s tuned 470bhp 1992 Porsche 911 Turbo 3.3-litre 964

So, how did I get here? Our Highland fling brings us to Stirling, where I was once a student – Bonnie Prince Charlie was in the year above, which tells you roughly how long ago it was – and now my son Alfie is currently bivouacked here – living in a Ford van – so it was an opportunity to have a beer with him. We’ve also an appointment the following day to visit Stuart Paterson at his SP Autobahn workshops on the outskirts of Stirling, and he proudly shows off his newly restored 911SC, a very wonderful thing, and invites us to take his 964 Turbo for a run.

“If you know what you’re doing, there’s a lot you can do to reduce turbo lag”

First, the back story. Trained as a precision engineering toolmaker working within the aerospace and motor industries, Stuart set up SP Autobahn 12 years ago. Leading up to that, he spent five years with Porsche Inspections, the operation run by Drive-My’s erstwhile contributor, Peter Morgan. ‘I’d worked with him part time since my 20s,’ remembers Stuart. ‘I’m now 44, so since my 20s I’ve been handling Porsches. In fact, I started working on cars aged 14, because my friend’s father had a garage and he bought cars from the auction and sold them on the forecourt. I got my first Porsche in my early 20s, a 911SC and, turning the clock back, that’s why I bought this one which I’ve just finished restoring.’ And very nice it looks, too, especially with its pert intake trumpets gracing the engine bay. ‘Working in the automotive and aerospace sectors for 10 years meant quite delicate, sophisticated actions, as well as building engines and assembling chassis.’

SP Autobahn’s tuned 470bhp 1992 Porsche 911 Turbo 3.3-litre 964

SP Autobahn’s tuned 470bhp 1992 Porsche 911 Turbo 3.3-litre 964

Meanwhile, he fettled and traded Volkswagen Golfs, amongst others, fulfilling an ambition to acquire a 911 before the age of 25 – a 1981 Guards Red SC. That was quickly followed by a succession of similar cars that he improved and presented at car shows before passing them on. A dozen Porsches later, he’d gained a reputation as a 911 expert, and became Peter Morgan’s sole Scottish representative, carrying out pre-purchase inspections for potential buyers. As Stuart says, ‘I was inspecting Porsches for clients from all over the world, including Australia and Hong Kong, as well as UK-based clients. Inspection reports were very thorough, and most came up with some issues which needed to be addressed.

When clients were still keen to proceed they would always ask, “can you do the work for us?” I couldn’t think of anyone else to recommend, and that’s basically how SP Autobahn came into being as a full-time business.’ After the first year, the service and repair side of the business superseded inspections and restorations, and now, 20 years on, that’s where his business is today. Stuart reckons he handles an average of 500 Porsches a year, and has around 600 cars on his database – plus 200 or so VWs.

He cites air-cooled and pre-purchase inspections as now accounting for just 10 per cent of his work, with 90 per cent of his time taken up with water-cooled servicing and repairs. ‘The turnover is mainly water-cooled cars nowadays,’ he says, ‘because they tend to be used more often, and I find them to be more fragile than the older built-to-last stuff.’

To an extent, this is reflected in the contents of SP Autobahn’s premises: half of each, air-cooled and water-cooled. There are two hoists, occupied by 987.2 Cayman, 997 C2 S, 964 Turbo and 911SC. The 1982 SC is a totally fresh restoration, inside and out, that he’s had for over three years and, true to form, has modified the engine with individual throttle bodies. He acquired the SC from a local garage proprietor who’d begun a restoration, but Stuart made him an offer that he couldn’t refuse for the unfinished project. A stickler for perfection, Stuart elected to begin the entire restoration again, and when his regular workload permitted, he and his father gutted the SC shell, replacing any imperfect panels which had any signs of corrosion with new ones, culminating in a bare metal respray. Reassembly then took a few more months, integrating new hardware and seals throughout, while his retired father helped out with panel assembly. The original Berber seat upholstery was re-trimmed in cloth and half leather, plus new carpets and headlining, topped off with a period Momo steering wheel, re-trimmed with matching brown stitching and a 12mm off-centre steering-wheel spacer for extra clearance. Mechanically, the original 915 gearbox was rebuilt, incorporating replacement selector shaft bushes, and a fully adjustable RennShift gear linkage to enable a more precise gearchange, surmounted by a lathe-spun mahogany and oak knob. Stuart completely rebuilt the SC’s 3.0-litre flat-six, replacing any components that didn’t pass muster. In order to enhance its free-revving 204bhp he fitted individual throttle bodies to improve response, confident that they would retain their state of tune, unlike carburettors, for instance. A full stainless-steel exhaust system with a single outlet Dansk sport silencer completed the powertrain’s revised spec. The suspension and brakes were stripped, and the torsion bars wound down to lower the ride height by 20mm, with deep-dish Fuchs wheels and Yokohama tyres completing a very nice restoration.

It’s his 964 Turbo that holds our attention, however, especially with a drive in prospect. Stuart prefers the 3.3 Turbo to the 3.6 version. Launched at the 1990 Geneva show, the first 964 Turbo was equipped with the 930’s 3.3-litre flat-six, fitted with catalytic convertors that restrict power output to 320bhp, and a 20 per cent locking ZF differential. Like its predecessor, the 964 Turbo bodyshell is an exaggerated version of the standard 964, its bulging wheelarches cladding the three-piece Speedline wheels.

The 964 Turbo with the 3.6-litre engine was launched at the 1992 Paris Salon. Anyway, Stuart nailed his 1992 964 3.3 Turbo 13 years ago. ‘It was built in the best Porsche era, in my opinion,’ he declares. ‘Being a 911 Turbo, it didn’t lack performance; however, if something can be improved, I’ll always find a way to improve it.’ At first, he introduced a slight boost increase from 0.7 bar to 1.0 bar, which was all well and good, until one of the original head bolts snapped. This was the ideal opportunity for Stuart to get stuck in and strip and rebuild the engine, enhancing every component in the process.

He carried out most of the machining himself, including the cylinder barrels and head faces, which raised the compression slightly. Cylinder head gas-flow and valveseat angle work was outsourced to a specialist, achieving some gains, and during reassembly all bearings and seals were replaced, including stronger ARP conrod and head bolts. Camshafts with the 911SC profile were installed for extra performance, plus shortened exhaust manifolds with a cat bypass, in order to minimise turbo lag, while retaining the original K27 turbocharger, and a new Sachs clutch was fitted to cope with the extra performance. The 964 Turbo’s suspension was upgraded with Bilstein HD coil-over dampers, teamed with H&R springs, employing geometry settings slightly more aggressive than a factory 964 RS.

No lagging: ‘I’ve done a lot to try and minimise lag,’ he says, ‘and now the turbo kicks in around 1000rpm lower than it did before, and vastly reducing the lag gives a far wider power band. It’s a problem with older cars, that they do have a bit of lag. The CIS – continuous injection system, typically Bosch K-Jetronic – is quite complicated. CIS is different from pulsed injection systems like DME Motronic (introduced in 1986), in that the fuel flows continuously from all injectors, while the fuel pump pressurises the fuel up to approximately 5 bar (73.5 psi). If you know what you’re doing, there’s a lot you can do to help minimise turbo lag. You can design and build headers like mine, for example, and they are several feet shorter in length to the turbocharger, so when you put your foot down there’s less distance for the gases to get that large turbocharger spooling up.’ Of the two exhaust pipes at the back of the 964 Turbo, the left-hand one is for the waste gate.

Regulations imply that the 964 doesn’t need a catalytic converter due to its age, so removing this also reduces turbo lag and frees up more power. ‘The 964 Turbo runs to 0.7 bar as standard,’ Stuart explains, ‘so the boost isn’t hiked that much. Most people just turn the boost up, and then adjust the fuel screw to increase fuel input, and that’s the worst thing you can do, because it gives it fuel everywhere, and you can’t control it.

Eventually, the boost comes in and the fuel leans off because the boost pressure matches the fuel mixture. I’ve controlled that accurately, so when it’s not on boost, it’s not got added fuel. This car easily returns 25mpg, because it’s lean when it should be lean, and it’s running optimum when it should be optimum. It’s never running overly rich, whereas Porsche set them up at the factory to run mega-rich, to be on the safe side, but if you set something up too rich you are losing a lot of free power. I wanted to retain the original look of the engine bay, so that’s why I kept the original Bosch CIS fuel injection system, which is pretty reliable, albeit a bit crude with its fuelling control, especially when the fuel quantity has to be raised for running more boost pressure.’

He joined forces with a firm based in Flagstaff, Arizona, acting as guinea-pig for their stepper motor system that they were developing to work in conjunction with the Bosch CIS injection. Stuart established that it could be controlled more efficiently with programmable software, making gains in performance and economy. ‘After a year of testing and development work, I decided to develop my own system, which within a few months was producing the perfect air-fuel ratio curve. It’s similar to a modern EFI system, and it completely changes the way the car pulls, as well as providing a perfect fuel curve right through the rev band.’ The 964 has been on the rolling road a couple of times to compare air-fuel ratios and both times it has produced a conservative flywheel figure of 470bhp at 0.95 bar of boost. Stuart has test run it at 1.1 bar, but he says that although it breaks the 500bhp barrier it can induce some clutch slip. Indeed, his recorded data indicates that the standard K27 turbocharger is running out of breath near the top end. As he says, ‘most of the work was completed over 12 years ago now, and it’s performed faultlessly ever since. It was recently recorded aurally and visually over a few days to be featured in a leading car racing game, which will be exciting to see.’

Time for an outing in the 964. Much as we like the empty moorland bleakness of the adjacent Ochil Hills, we elect to do the photoshoot in the more wooded Trossachs, over by Loch Lomond. Out on the open road, it starts to come into its own. I can really feel what it’s about; it’s so smooth, cruising at 60– 70mph, a do-this-all-day thing, and then when I need to squirt past a slower vehicle it’s accomplished with a blink-and-you-miss-it dab on the throttle, making it truly a best-of-both- worlds car. The surge of power that comes in on take-off is astonishing; it’s instant, too, with no lag, which Stuart attributes to all of his complementary modifications. The soundtrack is as thrilling as the performance, a snarling crescendo rising to a shriek, with much popping and snuffling on the overrun at lift-off. ‘It’s backfiring because I’ve turned the fuel right back,’ Stuart explains, ‘but it’s off throttle now, so when you lift off, it doesn’t need fuel,’ which he verifies while pointing me to the discrete air-fuel ratio gauge. As I remarked, it has a firm, compliant ride, perfect for fast road work. ‘I’ve taken it on trackdays a few times,’ reports Stuart, ‘but for the most part it gets driven on the road, so you don’t want to go too stiff. You’ve got to have it usable if you want to enjoy driving it.’ It’s running slightly more camber than the 964 RS, to give it a better turn-in.

The cabin interior with its sunroof and grey upholstery is mostly standard, although the steering wheel is from a 993 RS, and there are a few modern add-ons including the boost pressure and fuel monitor, which can be adjusted while in motion. And it is wonderful being in a regular 964 again, albeit right-hand drive; the controls feel so familiar from a decade of LHD 964 ownership, and the fact that it’s got wider wheels doesn’t really impinge at all at the sort of speeds that I’m going on the photoshoot. All the controls respond perfectly, it turns in accurately, it corners precisely, and it behaves like an accomplished 964 Turbo. It’s a very fine car, and obviously with all the engine work and tuning that’s been done, it’s extremely quick.

The exhilaration levels are really high as it accelerates – it really does kick in immediately coming out of the Dukes Pass hairpins. The more accustomed I get to the nature of the power delivery the better I know how much throttle pressure to apply, and things get more exciting the more familiar I become with it, and the more compliant and in-tune it seems with what I want to do, especially on the A821 between Aberfoyle and Brig o’ Turk (as seen in The 39 Steps), which must be the closest we’ve got in the UK to the Black Forest’s Schauinsland hillclimb. I whirl the 964 to-and-fro through the fabulous cambered S-bends for Lensman’s benefit, and then, once he’s happy, we decide to go for the big one for the static shots. That means a complete change of scenery, involving a run down south just short of Edinburgh, to the Forth Bridge – Stirling Castle standing proud on its escarpment and the formidable Wallace Monument notwithstanding. The M9 motorway is a relaxed cruise, and again, any necessary lane-changing moves are effortlessly accomplished in seconds. I just dip the throttle at 2000rpm and it takes off, and I can organise myself in traffic without any problem at all, so whilst it was inevitably in 2nd and 3rd gear earlier on in the mountainous twisty bits, I have only used 4th and 5th on the motorway. The arable Fife landscape is a marked contrast to the fabulous wildness of the Trossachs, and briefly I think we’ve been spoiled. However, the grandeur of the triangulated steel spaceframes of the original Forth railway bridge provide an awesome substitute.

There’s now a second Forth road bridge, a more dramatically suspended version, and we make the crossing to get to North Queensferry with its welcoming quayside. This 964 Turbo is such a smooth, easy car to drive in all circumstances, and that’s a credit to its owner and his tuning talents. It’s a great car, to look at and to handle in any given situation, though a fling in the Trossachs is most memorable.

CONTACT SP Autobahn Specialist Cars Ltd Unit 22 Borrowmeadow Road Springkerse Stirling FK7 7UW spautobahncars.com [email protected] Tel: 07973 385463


A magnificent machine deserves equally magnificent roads and there’s no shortage of premium Tarmac in Scotland. Left: Stuart’s careful tuning has liberated an impressive 470bhp from the 3.3-litre turbo engine. Below: Speedline is another Porsche design classic. Above and below: 911SC is another SP Autobahn project. Individual Jenvey throttle bodies should really get the 3-litre engine breathing. Period Momo wheel is a nice touch. Big rear Fuchs fill the arches. Grey interior contrasts purple exterior. Porsche’s ‘Sports’ seats of the era are best seat in the house With less turbo lag, SP Autobahn’s modded 964 Turbo feels altogether more sprightly than standard early 964. Two examples of outstanding engineering excellence. Porsche 964 Turbo is shadowed by the Forth Rail Bridge Typically, SP Autobahn’s Stirling premises sees mainly water-cooled cars for servicing these days. The 964 is proprietor, Stuart Paterson’s personal indulgence.


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Additional Info
  • Year: 1992
  • Body: Coupe
  • Cd/Cx: 0.33
  • Type: Petrol
  • Battery: 12 volt
  • Engine: 3.3-litre flat-6
  • Fuelling: Injection
  • Aspirate: Turbo
  • Power: 470bhp @ 7000rpm
  • Torque: 399lb ft @ 4000rpm
  • Drive: RWD
  • Trnsms: Manual 5-spd
  • Weight: 1399kg
  • Economy: 24mpg
  • Speed: 195mph
  • 0-60mph: 3.6sec
  • Price: £280,000
  • Club:

    {module Porsche 964}

  • Type: Petrol