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Booming Bonnie


This article appeared in Motor Cycle News on 16/8/2006 and is reproduced with thanks.



When tuner Bob Farnham hopped up his own Triumph Bonneville he didn't bargain on the project turning into a profitable sideline. But that's just what happened - and for obvious reasons.

Simply put, Farnham transforms the 50bhp plodder into a 75bhp street racer that can clock 125mph and, according to him, run rings around Japanese 600 supersport bikes at track days!

His recipe for a snappier ride was to take a standard 790cc 2004-model Bonneville, gas flow the cylinder head, fit a Wiseco 904cc big-bore kit, skim 30 grams off the flywheels, tweak the carbs and fit heavier clutch springs to cope with the extra power.

This 983 hop-up package hikes the rear wheel horsepower by 50% to 75bhp at 7500rpm, and raises peak torque to 63ftlb at 4150rpm.

The stats don't lie. Compared to a stock Bonnie this bike has huge dollops of grunt and revs so freely you can barely hook gears fast enough to keep the rev limiter at bay.

I saw 120mph without trying, and this is just the "mild" tune!

Farnham reckons the Bonnie engine is way over-engineered and has more potential for tuning. A Thruxton Bonneville ECU would give an extra 500 revs, and even more power, while the "tiny" 36mm CV carbs could simply be replaced with 40mm items for even more top end, according to Farnham.

Even though the engine changes have unleashed so much get-up-and-go, they've been no detriment to the bike's all-round rideability. You can lazily poodle through towns at 30mph and no-one bats an eyelid, but out on the open road this Bonnie turns into a barking beast. I swear my mates had the same-spec Wassell reverse megaphones that Farnham has fitted to his bike on their T120 Bonnies back in the 1970s, but at only 105 a pair the price is as retro as the bike itself.

Other details that set Farnham's Bonnie apart are the handling and looks. One-inch longer Ikon shocks (262) -like a Thruxton's - mean you can point at the apex and this Bonnie is there, bang on, inch-perfect.

The stock Triumph front brakes have the same wooden feel as usual, even though the disc has been drilled and a CBR600 master cylinder with braided steel line fitted.

Bars from Triumph's own Thunderbird Sport look the part but they are fitted with neater Honda switchgear and a quick action throttle.

The rear mudguard is cut right down and there's a Wassell rear light and tiny indicators for the minimalist look.

And with the rich cherry red tank and the mirrors junked, Farnham's Bonnie looks not exactly cafe racer but more NYC bar hopper. I love it.

 The DYNO chart above shows a standard 790cc Bonneville (yes the lower trace!) and the Bob Farnham  version. This has a Wiseco 904cc kit and a flowed head with Megatron Exhaust Cans. Click on the chart above to get a scan of the original dyno chart.



COST (inc stock Bonnie): CALL FOR DETAILS
POWER (tested):         75bhp
TORQUE (tested):       63ftlb
WEIGHT (approx):      200kg



Air-cooled 904cc (92 x 68mm)
8v dohc four-stroke parallel twin.
Chassis: Steel cradle frame. 41mm forks,
Twin Ikon rear shocks adjustable for pre-load and rebound damping.
Brakes: 310mm front disc with twin-piston caliper.
255mm rear disc with 2-piston caliper.

Tyres: 100/90 x 19 front, 130/80 x 17 rear.


Gas flow cylinder head  call
904cc big bore kit    call
Bore Cylinders plus new liners  call
Exhaust Connecting pipes  call
Wassell Exhausts call



        Different specifications are available.

Please call to discuss.




This article is from August 2006

Wiseco piston


Click here for a pdf of the original article

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