Porsche 928 S4, is that a turbo in your tailpipe?

July 13th, 2007

About three years ago, John Thomas purchased his first Porsche 928. It was a 1989 Guards Red automatic that came out of a collection in southern California with about 110,000 miles. John bought it on e-bay, flew down to San Diego and drove it back to Washington. He had intended to build a high horsepower engine, and the 928 was a good start. It was already one of the fastest cars of its time.


Not too long after the purchase, he started looking at ways to build power. Stroker, supercharging and turbocharging were the options. All were pretty expensive, and at the time, no one had been able to effectively build a Porsche 928 turbo car for a reasonable amount of money. Squires Turbo Systems (STS) hit FEMA about that time, winning an award for the rear mount turbo (RMT). John contacted them to see if this arrangement would work on his 928. Given the lack of room in the engine bay, the rear mounted turbo seemed like a potential solution. STS thought the idea was a good one, but that lacked enough vehicles nationwide to invest in the necessary R&D for a “kit”. They were willing to work with John with regards to tech support, and let him develop a Porsche 928 turbo kit.

They started with a “base kit”, a turbo, oiling pump and little else from STS that we could use. The 928 engine is pretty unique in its set-up and tuning was an issue as well with a separate LH and EZK for fuel management and timing. Everything else was custom for the Porsche 928.

Not too long into the project, they lost a very old headgasket and spent a year re-building the engine. They bored it out to 4″, just a little over stock, nikicelled the engine, and put in forged pistons and rods, lowering the compression to 9:1 from the stock 10:1. They could have rebuilt it in a month, but the pistons took a year to produce.

In that year however, JDSporsche developed a tuning capability for the Porsche 928, and was a real boon to the development of the RMT for the 928. It gave them the tuning ability which is essential on any forced induction engine. The car currently produces 400 horsepower and 438 pounds of torque to the rear wheels, and runs in the low twelve’s at 10lbs of boost with no intercooler needed. Based on a cost per horsepower basis, the kit is very competitive to both the supercharged and stroker approaches to big horsepower engines.

During the project, they have tested to failure, every component that was a weak link and worked through solutions that kept the installation and modification of the stock engine at a minimum.
The stage one kit is set up for 5 lbs of boost and will see about a 120hp gain from the stock engine at about 260 RWHP stock. The kit uses the stock injectors, mild modifications, custom stainless steel exhaust, boost pipe and a carbon fiber cross over. Future plans include 30# injectors, boost controller, fuel management, methanol injection and other features that should produce an 11 second car in the quarter mile with 460 RWHP and 500 RWTRQ.

Check out some pictures and timeslips for this unique Porsche 928 S4 turbo!

1989 Porsche 928 S4 Turbo

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