If One Spinning Air Pump Is Good, Two Must Be Better. SPI's Twin-Charged MINI Cooper S Offers Proof
"The first thing I get rid of when I buy a new car is the warranty," says Hubie Fuh, of Cliffwood Beach, N.J. We like guys like Hubie.
That's because guys like him are brave enough to explore the unknown, to trade a perfectly fine factory warranty just to answer the question, What would happen if we do this? It's because of people like Hubie that the rest of us can buy off-the-shelf kits for our cars once our precious warranties expire. They're the rolling R&D. We're the benefactors.
As you'll recall, we named the MINI Cooper S to our Eight Great Rides list in 2004, and it was the winner of last year's burnout contest. So to begin with, Hubie started with some decent rolling stock. But Hubie's first impression mirrored our own".
The MINI was amusing at first. It handles great, but it lacked power," he says. Hmm. A refresher course in MINI 101 was in order. One trip to the nearest dealership and the Motoring Advisor's grip on the oh-shit bar confirmed completion of the course. Sure enough, Hubie's assessment was spot on.
After researching the existing OEM supercharger, Hubie came up with the idea of twin-charging the MINI-combining the OEM Roots-style supercharger and a Garrett GT28R turbocharger. He then went about developing a kit that's now for sale through his company, Sharp Precision Instruments.
Hubie wanted to keep the kit as simple as possible, so the two air pumps work in series with the supercharger, augmenting the turbo throughout the entire rev range. Inlet air is initially compressed by the turbo, then routed through the throttle body and into the supercharger, where it's compressed again.
It's then fed through SPI's custom top-mount intercooler and into the engine. The supercharger and the turbo work together; there's no complex sequential pumping and no bypassing of intake air. Full boost of 20 psi is achieved at 6000 rpm, but the kit makes 15 psi by 3500 rpm.
The stock Eaton supercharger bypass valve was removed since Hubie found it caused problems when used with this much boost. Two HKS racing blow-off valves, one for the Garrett and one for the supercharger bypass, rid the system of boost once the throttle is closed. As a result, the MINI now has noticeably better throttle response than its stock counterpart.
The supercharger remains stock, but the plumbing is not, with an upgraded SPI inlet pipe to the supercharger and custom 2.5-inch aluminum piping surrounding SPI's top-mount intercooler. Spent gases work their way through a custom-fabricated stainless-steel tubular manifold back to the Garrett.
In addition to the stainless-steel tubular manifold, the GT28R is held in place by a custom-fabricated stainless-steel downpipe. From there, exhaust gases are expelled through SPI's custom-built 3-inch stainless-steel exhaust and an A'PEXi N1 Turbo muffler.
With this kind of breathing ability, the MINI needed serious help in the fueling department. SPI's 440cc/min fuel injectors do just that, while an A'PEXi S-AFC II allows modified fuel mapping. According to Hubie, the addition of the AFC was by far the most difficult aspect of the project, and the first of its kind in a MINI.
"BMW isn't quick to share its wiring diagrams, so we had to trace every single wire in the harness." But the effort was well worth it, as Hubie tells us this arrangement is good for dyno pulls of 265-wheel hp on a Dynojet chassis dyno.