Why do Three cylinder engines have unconventional cc's ?

[simonm]

All the triples seems to be 675cc rather than 600cc.  Why is there such a marked difference between a cylinder of 200cc and one of 225cc ?  I would have thought they could tune the engine, exhaust/weight to make any configuration work.  Obviously I'm wrong as Triumph and Yamaha have gone the 225cc route.


Does anyone know specifically what it is that makes the extra 25cc so important ?


cheers
Simon

[BBROWN1664]

To get the torque and bhp I guess.

My borrowed xs was a 750 triple. If you look at the Fazer 600 and took away a cylinder it wouldn't be as torquey and would only have 450cc or the thou would be a 750.

[stevierst]

Only an idea, but they'll be fairly standard piston sizes just to keep cost down, and then they'll vary the stroke length accordingly depending on what they want the engine to do (torquey or revvy) that would give you the oddball capacity.

[JoeRock]

Cylinder for cylinder, the more cylinders a bike has at a certain engine size, the more power it can create.
In terms of rough power, a 600cc inline 4 is pretty much the same peak power wise as a 675 triple and a 750 twin - they were also the size restrictions years back in racing classes at it meant they had roughly the same power output. Main difference between them is the torque delivery, you get a much more linear torque delivery with a triple, and then a "burp" of torque down low on a twin (although that is often accentuated through gearing).

It's also got to do with piston speeds, the pistons on a 600 IL4 can travel a lot quicker than those on a similar engine capacity size twin, so in order to get the power back the size capacity has to be boosted!

[ghostbiker]

Wasn't it to do with race classes?

[simonm]

I wondered purely as a 600 makes much more sense from a ved perspective.

[Dead Eye]

Bah, I write off the "tax" on bikes as little more than a slight niggle after the small fortune I pay out for my cars

£57 last week was a sheer joy after the £270 in May