Workshop

Understanding Bore and Stroke

Your engine's bore and stroke dimensions play a vital part in the way it produces power. Here's how

It's all very well saying this, that or the other is slimmer, narrower, shorter or more shiny than before, but sometimes it helps to explain things in more detail to really understand the benefits.

Earlier this year we were treated to an all-new R1. 'It's got radial brakes so it stops better!' we said. 'It's got underseat exhausts so it looks better!'. 'It's got a shorter stroke and a wider bore so it revs higher!' Okay, so it has. But how? And why? We churn out bore and stroke figures in our tech spec panels month in, month out, but what relevance do they really have?

The relationship between an engine's bore and stroke determine, to an extent, how it makes its power. For a given capacity, 'long stroke' motors - ie those with a relatively long stroke in relation to the bore size - will tend to be relatively low revving but with strong low down power, while 'short stroke' or 'oversquare' motors - short stroke with a wide bore - will be able to rev higher. And, because more revs equal more horsepower (horsepower = torque x rpm divided by 5252, so increase the revs and the bhp increases too), manufacturers are always looking at ways of safely increasing the upper rev limit of their motors.

One of the major factors determining an engine's upper rev limit is piston speed. For every revolution of an engine, the piston moves up from the bottom of its stroke (bottom dead centre or BDC) to the top of its stroke (top dead centre or TDC) and back again. So in the case of the '04 R1, the 77mm wide piston goes from a standstill, travels 53.6mm up, stops, and comes back down again. At 10,000rpm it makes this journey just over 166 times each way every single second, at an average speed of 17.9 metres a second.

The stresses on a piston and conrod at high revs are massive. If the piston is forced to travel too quickly something's going to break. Put very simply, if you reduce the distance the piston has to travel - ie its stroke - it doesn't have to travel as fast and can make that journey more often. So that's what Yamaha chose to do with their '04 R1, reducing the stroke by 4.4mm and adding 3mm to the bore. Last year's R1 redlined at 11,750rpm. This year's redlines at 13,750, and makes its peak power 2000rpm further up the scale.

At that 13,750rpm redline, the R1's average piston speed over the 53.6mm stroke is 24.6m/s - that's just over 55mph in English. By way of comparison, the longer-stroke GSX-R1000 (bore 73mm, stroke 59mm), redlines at a lowly 12,250rpm, at which point its piston is travelling nearly as quickly as the R1's - 24.1m/s. Just 250rpm later, its piston has reached the same speed the R1's does1500rpm further round the dial.

But it isn't like bike manufacturers have only just worked all this out. It's only as time has moved on that production and material technology has allowed the manufacturing of components able to withstand such high engine speeds that engineers have been able to take better advantage of short stroke theory.

Of course it has disadvantages, one of which is that it makes the engine wider. See how the '04 R1's frame spars now loop over the motor rather than wrapping rond them? And by using all those revs to make
horsepower, power delivery, driveability and traction become serious issues, as does building a chassis to cope with it all. Then again, long strokes make engines taller. But look at the way that long stroke, 'low revving' Rizla Suzuki GSX-R1000 grunts out of slow corners in British Superbike racing...

Displacement

There ain't no replacement for displacement

An engine's bore and stroke dimensions
determine its 'size' or cubic capacity (cc). Properly referred to as displacement or swept volume, it's simply the difference between the volume of the cylinder(s) at BDC and TDC.

Cast your mind back to a maths lesson you may have had once. The formula for calculating the volume of a cylinder is the area of its cross section (pi x r squared, r being the radius of the piston, or half the bore width) multiplied by its length (the stroke). Multiply this figure by the number of cylinders to get the displacement. In other words:

Pi x (bore/2)2 x stroke x number of cylinders = displacement or swept volume

Using Yamaha's 2004 R1 as an example (with a bore of 77mm and a stroke of 53.6mm), we get:

Pi x (77/2)2 x 53.6 x 4 = 998.38cm3

"You can only make horsepower in two ways - by increasing displacement or using more revs," explains FW Development's Ian Park. "Fitting big pistons boosts the compression ratio, changes the bore and stroke relationship, it gives more power, more drive. Providing the piston's not too big there are no real downsides."

Kawasaki knew this when they gave their 599cc ZX-6R a 2mm big bore to make the 636cc ZX-6R - the torquiest of the current sports 600s (because it's not a 600...). We can apply Kawasaki's maths to Yamaha's R6. With a theoretical 2mm overbore, we get this:

Pi x (67.5/2)2 x 44.5 x 4 = 636.97cm3

There you go, an R636. Wonder if Yamaha have thought of that? But while boring out a motor is a quick and simple fix to more power, it's one that happens less often now and FW Developments rarely bore out customers' engines these days. Motors of old had liners inserted into the cast cylinder block. These liners could easily be bored out to whatever capacity the surrounding metal would allow. Modern engines tend to have their cylinder barrels/bores cast as one, with the bore itself coated with hard-wearing nickel silicon-carbide. Bore out the cylinder to a larger capacity and the barrel needs re-coating, which is an expensive, specialist job. Also, as engines become more compact, the metal between adjoining cylinders becomes too thin to machine away without compromising the motor's reliability.

STROKE OF GENIUS?

Elsewhere you can read our first ride report on Triumph's Daytona 650 (above), basically, no, actually, last year's Daytona 600 with a near-47cc capacity hike. But the Hinckley tech bods haven't bored the motor out to increase displacement, instead they've 'stroked' it - in other words, increased the stroke, in this case by 3.2mm. Previously, the Daytona 600's capacity was reached like this:

Pi x (68/2)2 x 41.3 x 4 = 599.95cm3

Now we've got this:

Pi x (68/2)2 x 44.5 x 4 = 646.44cm3

Longer stroke motors make their power in a slightly different way to higher revving short stroke motors, producing greater drive in the lower and mid-ranges because of the way the piston's downward force acts on the crankshaft.

This is evidenced in the remarkable wheelie-pulling abaility of the new Triumph, which lazily hoists its front wheel off the throttle when accelerating hard at lower revs - a trick you'd be hard pushed to pull on the old 600cc Daytona, which noticeably lacked in anything other than a screaming top end.

Of course the longer-stroked motor's downside is, as explained earlier, the limit on maximum rpm, as well as an increase in the engine's overall height - maybe only a few mm, but every one of those counts these days.

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