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Design an engine

From conception to completion in just 600 words, here’s how to design a new high performance engine from scratch. Handy

Step 1: Set your targets

The first job is pinning down the basic specification, based on what kind of bike we’re building. Since we’re putting together a hypothetical new Triumph sportsbike engine, we’re going for a 1100cc triple. In the past, Triumph have been successful when they’ve gone their our own way – the Daytona 675 is a prime example – so we’re steering a path between the Japanese and European opposition. And when you consider the superbike capacity limit is 1200cc for twins and 1000cc for fours, an 1100cc triple seems logical.

Next we need to decide on the kind of power we want to make. An engine like the Thunderbird’s is all about torque, refinement and character – high revs and peak power figures just aren’t important. A performance engine is a completely different proposition. Ultimately it’s about good cylinder pressure – a product of getting plenty of air and fuel in and burning it efficiently – and engine speed, and how fast we can spin the engine to make power. Everything gets more difficult the higher you want the engine to rev, and if you want power, you really need to rev it.

Step 2: Laying down the basics

Once you’ve decided what kind of power and revs you need, you can work out your basic engine architecture; its bore and stroke dimensions. Performance engines are becoming increasingly over-square in their bore and stroke measurements as they chase revs and power. That is, the diameter of the piston is greater than length of the stroke, the distance between the top and bottom of the piston’s movement. This is because the limiting factor on revs is average piston speed. Pistons are pretty significant pieces of metal, yet with every revolution of the crank each one must fly from one end of its stroke to the other and back again, decelerating, stopping and accelerating back up to speed each time as it does so. Once you start asking the pistons to average much more than 24 metres per second, you get into difficulties. At such speeds it’s difficult to maintain a seal between the pistons and the bores, so oil starts to blow-by.

On our purely hypothetical triple, we’d like to rev it to about 12,000rpm. Working back from our 24 metres per second limit, a 12,000rpm redline gives us a 60mm stroke. In order to achieve our 1100cc capacity with three cylinders, that gives us a bore figure of 88.2mm. Our engine has a longer stroke and bigger pistons than the 1000cc competition (76mm x 55.1mm on Honda’s Fireblade) but can get away with it because it doesn’t rev as high. Just as the 675 offers an alternative to the 600cc Japanese competition, so our 1100 will be unique in its class, a torquier alternative to the high-revving inline fours.

Step 3: Getting stuff in and out; the cylinder head

On a high performance engine you want a very narrow (close to vertical) valve angle on the inlets, to give the charge coming in as direct a route as possible into the combustion chambers. Head design is all about getting stuff in; getting stuff out is easier because the piston pushes it out. On a bike like the new Thunderbird, where peak power isn’t a priority, the valve angle can be wide and inlet tracts long and meandering, but on this engine we want them as close to vertical as possible, though this does make for a deeper cylinder head and a taller engine overall. We’re going to go for four valves per cylinder too. Five doesn’t seem to yield any particular advantage, and even Yamaha, for whom it was a trademark for some years, have abandoned it. We’ll also plump for just the one spark plug per cylinder – you only really need two when you’ve got big bore sizes and vast combustion chambers, as found on motors like the Thunderbird and the mighty Rocket III.

As for the profile of the cams, we need to set targets for valve lift – that’s the main factor determining power. Naturally there are limits. Make the valves big (to flow gas quickly) and lift them a long way (for the same reason) and we’ll have trouble controlling them. They’ll generate a lot of momentum and demand really firm valve springs to control them, hence the lighter titanium valves now being adopted in sportsbike engines. As for valve timing, it’s a compromise between efficient low and high-rev running. Minimise valve overlap – having both valves open at once – and you’ll get strong low-rpm running but, when the engine really gets spinning, you’ll miss out on top-end power.

Specification (Entirely hypothetical, of course)

Engine: 1100cc, liquid-cooled, 12-valve, dohc, in-line triple
Power: 158bhp @ 10,800rpm
Torque: 83lb.ft @ 8100rpm

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