Classic Retrofit – A Stitch in Time

21st Century technology for an old warrior.

Date Published: 03 Mar 2017
Retrofit Classic

The last few years have been witness to the updating, backdating, and tuning of classic Porsche 911s. But in a world suffused with derisory bank interest rates, unmolested classic 911s are now making top money while new limited edition 911s are literally being traded as commodities.

With classic 911 values sky high, even the lowly SC is finally making its way up the price ladder, and originality is becoming more critical than ever to retaining a cars value.

That said, sympathetic updates that edit out known weak points are welcomed, particularly so where they are not too obvious. Thus the fitting of Carrera 3.2 chain tensioners to 911SCs has always been a good move, as is the fitment of an aftermarket pop-off valve to prevent the notorious airbox blowout syndrome that afflicted early SCs.

Thanks to minimal on-board electronics, the 911SC and Turbo from this period are potentially less prone to the kind of failures that plague modern cars. But their Bosch type fuses and fuse holders suffer from oxidisation and corrosion, resulting in poor electrical contact. In addition, the ability of their CDI (Capacitive Discharge Ignition) packs to produce a strong and consistent spark tends to deteriorate over time.


As most enthusiasts are aware, there are lots of modern solutions to classic car electrical issues available in the form of electronic ignition systems and rotor-less distributor inserts, all of which aid starting and smooth running. But until recently there was no solution for the ageing fuse panels and CDI packs that are ticking time bombs in old 911s.

Even if you replace these parts with new old stock if you can find them, they too will eventually fall victim to the same problem since their claim to being state-of-the-art componentry is now four decades old. In addition, devices like the CDI packs need to be run occasionally or you will find half the new old stock can be DOA, or dead on arrival as they say in the electronics business.

Because of this, when we recently learned that substitute components that use 21st Century technology for significantly improved performance were now available, the news came like a breath of fresh air. Importantly, these new parts do not change the outward appearance of the original factory fuse panel and CDI pack.

The British company behind these products is Classic Retrofit, a recently established automotive offshoot of a company that makes electronic components for the likes of Airbus and the UK Ministry of Defence.

CDI Shipping

As a failure can be catastrophic aircraft systems are mission critical. Thanks to being used to designing and producing components to aerospace industry standards, Classic Retrofit left the starting blocks with the knowledge and ability to design and manufacture their electrical components to a standard way beyond that normally seen in the automotive aftermarket.

The whole adventure started when Classic Retrofit founder, Jonny Hart, bought a 1981 911SC and had it stripped back to bare metal and repainted in a rather vibrant shade of metallic yellowish green.

Going through the car with a fine tooth comb, Jonny also had the engine rebuilt and being the curious engineer that he is began to look for obvious flaws in the car that might compromise reliability.

His examination immediately flagged the fuse panel and CDI ignition pack as potential weak points, and he set about redesigning them to use modern components.

With the quality of the Classic Retrofit fuse box for early 911s speaking for itself, Jonny began to approach renowned classis car restoration specialists in the UK. The first to grasp the potential was Eagle E-Types, who immediately commissioned bespoke fuse boxes for their ultra high quality restored and improved Jaguar E-Type cars.


Alfaholics, who specialise in the restoration and updating of the 105 Series Alfa Romeo models, were the next company to take on board what Classic Retrofit had to offer, and Classic Retrofit are in talks with two major players in the Porsche world.

The Classic Retrofit fuse panel uses modern spade fuses in place of the old tubular Bosch style fuses. This is designed to be a direct replacement for the original in the luggage compartment, and uses the factory fuse panel cover so it looks totally original. Another plus point is the headlamp relays and diagnostic LEDs built into these new units.

Fitting takes under an hour and all you need is a suitable screwdriver to swap over all the wires from the old to new panel in exactly the same order. Ditto for the smaller exposed fuse panel on the left hand side of the engine bay. A fuse panel for pre-1973 911s had just been added to the range.


With the fuse panels in production, Jonny turned his attention to the Bosch CDI power pack that provides the high voltage to the spark plugs. Spark plugs usually require between 12 and 25,000 volts to fire properly although modern engines may require up to 45,000 volts, hence the switch from distributors to the more accurate and powerful coil pack technology that began in the early to mid ‘90s.

An electrical engineer with 35 years of real world experience, Jonny stripped the Bosch CDI unit down for examination and found that its internals comprised of two circuit boards, one above the other, connected by some wiring.

As with devices from the era the board components were surface mounted, and a failure on one of these boards required the unit to be stripped down for a possible repair. “The reliable service life of such a unit would be around 15 years, after which it is running on borrowed time,” he said, showing me the disassembled innards of the Bosch unit.

Using a 21st Century flush mounted circuit board with programmable parameters the Classic Retrofit ignition pack offers superior consistent and reliable performance over the long term. The improved engine power, response and smoother running are bonuses.

“Our componentry also incorporates programmable functions,” Jonny explained. “This gives us the ability to change parameters such as ignition timing for specific road and race applications.”

I was very curious how this would look on the test instruments and when Jonny called up the relative screens, I was very surprised in a positive way to see how much faster the Classic Retrofit unit built up and released its charge.

Where the original Bosch CDI unit discharges one spark per combustion cycle, exactly as it was designed to do, the Classic Retrofit replacement is able to fire twice in the same time with a recharge interval of 200 microseconds. Seeing this on the test instruments, I remarked to Jonny that his ignition pack was in effect a “poor man’s twin plug system” to which he smiled and agreed.

Car manufacturers use twin plug ignition systems to increase combustion efficiency. With two sparks firing sequentially per combustion cycle the fuel in the combustion chamber is burnt more completely, releasing more power and lowering raw emissions. It also makes the engine subjectively smoother. These are the reasons Porsche adopted a twin plug configuration for the 964 and 993 models.


Rather than using a 12-plug distributor as on the 2.8 RSR of 1973, the engineers designed a twin-head distributor, whose second head is driven by a rubber belt. However, the advent of high voltage coil packs by the time the 996 went into production resolved the issue anyway.

There is no substitute for testing under real world conditions, and preferably in motorsport as well. Jonny was well aware that his reputation was on the line, so he sought out partners whose names in the Porsche world were as strong as his is in the aircraft industry. The four parties approached in the UK were BS Motorsport, Historica, Neil Bainbridge, and Parr Porsche. In the US, Jonny works with Pelican Parts and Loren Beggs of 911 Design. His dealer in France is Station Compteurs.

The reaction was even better than expected. Historika said their test car ran noticeably better and showed 20hp more on the dyno! Neil Bainbridge even called him up on a Saturday and asked if he could come up right away to see the results for himself. Dyno testing showed that the 2.0 litre flat-six benefits the most from this pseudo ‘twin spark’ ignition pack, picking up power and running noticeably smoother, with improved fuel economy as well.

I can certainly testify to this myself as I went for a run in Jonny’s own 911SC. I have driven many of these cars over the years and own one myself, but I have never encountered such a smooth and torque rich response from a bog standard motor.

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