Bosch CDI box vs Classic Retrofit CDI+

We are about to deliver the 250th Classic Retrofit CDI+ unit for classic Porsche 911 models. We're delighted that these revolutionary products, proven to deliver power improvements right across the rev range, have become such an integral part of the upgrade process amongst 911 engine builders and Porsche aficionados. 

One of the shortfalls of the Bosch CDI is that it has no internal voltage regulation. The coil voltage decreases both with battery (alternator) voltage and also with RPM. 

We overcame this issue in the game-changing Classic Retrofit CDI+ ignition unit by employing a regulated power supply that offers a constant voltage to the coil under all RPM conditions and with battery voltages as low as 8 volts. This means you get the same spark energy under all conditions.

In fact, you get twice the spark energy of a Bosch CDI as our CDI+ units produce two sparks back-to-back, typically at 400us intervals, although it can be adjusted right down as far as the coil will allow: around 200us. This is a significant factor behind the instant 'butt dyno' improvements reported by all of our customers.

Here's a video we made of a Bosch CDI and a Classic Retrofit CDI+ under the same test conditions, demonstrating the consistent ignition control that all of our CDI+ owners enjoy. There are many other benefits come with our modern technology, but this is a very visible improvement that is easily felt when driving a classic 911 or any other classic car fitted with our technology. 

We are constantly developing the CDI+ firmware and continue to support the global community that has invested in our UK-built ignition units and all of our other products. Contact us with any questions!

About the author

John Glynn is a classic Porsche enthusiast, journalist and community leader. Founder of the group and online forum, John owns three classic Porsche models as part of a larger collection of classic cars and motorcycles. 

John has worked as a classic Porsche journalist since the purchase of his first 911 in 2003. John's blog at is one of the internet's most popular Porsche blogs. Read his monthly column in GT Porsche magazine or visit his site at for help with classic Porsche agreed insurance valuations.

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