Boosted Bonnie - What It Takes To Build A 230-Horsepower Twin

Triumph Motorcycles

By Alan Cathcart, Photography by Phil Hawkins, Andrew Cathcart, Steve Bohn

Though there's a massive amount of aftermarket support for Triumph's popular Bonneville model, it's almost completely focused on fashion and function. Performance parts for the new-generation Bonnie are practically non-existent, inspiring South Bay Triumph's Matt Capri-an avowed speed hound-to start a side business called Triumph Performance USA (, his own aftermarket company building hop-up parts exclusively for modern Triumph twins.

TPUSA offers everything from simple bolt-ons like exhausts and fuel-injection modules to full-on big-bore kits, but Capri had to get creative and pull out every trick in his book to coax 230 bhp from Triumph's pedestrian parallel-twin. Beginning with a standard Bonneville Thruxton 865cc motor, Capri fitted one of his own big-bore kits with three-ring, flat-top Arias pistons, lightweight wristpins and a special 360-degree stroker crank with Carrillo steel rods, increasing displacement to 984cc. Top-end modifications include a ported and gas-flowed cylinder head reconfigured for a low, boost-ready 8.5:1 compression ratio. Oversized stainless-steel valves (5mm larger on the intake side, 2mm larger on the exhaust) are opened by custom-ground cams and seated by Toyota valve springs. A total-loss electrical system sparks high-output Chevrolet LS1 V8 ignition coils.

The real key to making huge power, however, is the sophisticated turbo system that Capri created. An IHI turbocharger is located under the seat, while a dry-ice intercooler keeps the intake charge as cool as possible-key for turbo performance. Haltech provided the ECU that monitors the fueling and built the custom wiring harness, too. Highly modified 45mm Hilborn throttle bodies incorporate dual injectors. The primary injector provides fuel pre-boost, while the secondary injectors open when the programmable ECU's manifold air-pressure sensor detects turbo boost.

Water-injected and run on 110-octane race gas, the boosted Bonnie makes impressive rear-wheel figures of 230 bhp at 8500 rpm and 150 lb.-ft. of torque at 9000 rpm. A Barnett Kevlar clutch transmits all that power, though it takes the hand strength of a Viking to overcome the heavy-duty, dual-coil springs. "We always carry an extra clutch cable, just in case," Capri says.

This power-up package sits in a Triumph Speedmaster frame fitted with a Marzocchi inverted fork from an Aprilia RSV1000. Lightweight 17-inch forged-aluminum Carrozzeria wheels reduce rotational weight, as does deleting the front brake, which is allowed under Bonneville rules. (There's nothing to hit on the salt flats, really.) The remaining rear brake is a miniscule, 220mm EBC disc with a single-piston caliper operated by the front handlebar lever. The swingarm is from a Triumph America cruiser, and gives an extra 6 inches of wheelbase for improved stability. Some 60 lbs. of lead weight stuffed inside the swingarm enhances traction-a common trick for running on the salt. Pirelli Diablo block-tread roadracing rain tires increase traction in all conditions, even if the salt is squishy after it rains.

Believe it or not, Capri is entertaining notions of offering a street version of his record-setting turbo. "I really think we have something here that would make a great aftermarket performance package," he says. "The Bonneville engine is so robust, in best Triumph tradition, and I think it would make a great turbo street bike."

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