Our most recent trip to see Ferg’s 1970 Bonnie revealed something that looked tantalisingly close to a motorcycle. Well, with the engine out and the tank off. But the basic cycle parts are back together and that’s already allowed Ferg to do some thinking about tweaks that might help on the salt flats of Utah.
More power, Igor
I’ll leave it to James May to bore everyone to death with aerodynamic equations on thrust versus drag, but the basic concept is simple. The faster you go, the harder it gets to overcome the drag and increase your speed by any given amount. So, for example, the amount of extra horsepower you need to increase a bike’s top speed from 100mph to 105mph is quite a lot less than you need to increase the speed from 105mph to 110. What’s important about that is Ferg is seeking to break the world land speed record for pushrod 650cc production motorcycles, which currently stands at 106.4mph. The current record holder would have needed significantly less horsepower to overcome the previous record than Ferg is going to need to exceed 106.4.
Therefore, quite a lot of work has been put into the Bonnie’s powerplant. Pulling into his driveway, Ferg was retrieving his new billet crank from the van, looking very shiny but still in a raw state. It needs to be ground for bearings and balanced (by ‘Bob the Balancer’) but that will need the pistons, which is a project in itself. The pistons are going to be custom-made for the combustion chamber by CP Pistons in the USA, based on a mould made by the cylinder head maestros at Landon Motorsport.
Talking of which, the results are in for the gas flow work and it’s good news: the flow rate has increased by over 35%. There’s still some work to do though, to balance the intake efficiency with the exhaust. With a modern overhead-cam engine this would normally be fine-tuned by a series of changes to the cam profiles and timing. With the pushrod Triumph’s camshafts sitting in the bowels of the engine, that's not really an option. So Ferg has asked for the best set-up they can do via bench-testing and that will be that.
Other power tweaks will include boring out the Amal carbs from their standard 30mm to around 33mm, a single-coil ignition and possibly a total loss electrical system, thus avoiding the horsepower drain of a dynamo. A new arrangement for the crank breather is also under consideration.