One reply suggested engine makers would have to spend money retooling because they may make a motor out of a head from this, a barrel from that..
Maybe many years ago, but these days when the major players are the sze they are, I seriously doubt it. The makers of a motor are designing it for its given application right off the jump. They decide on a bore and stroke and thus the displacement is known right away. Why then do the designers, knowing that they are making say, a 300cc motor chooose a displacement of 280? It doesn't cost them anything more to make the displacement accurate or close to it, after all. If the bore needs to be 4 mm larger in order to be a true 300, why not make it so?

It's a question I have puzzled over and never had an answer for. Only the companies directorship knows for sure.
My best guess is that the buying public are percieved as naive enough to just read the label on the brochure and buy it regardless how close the motor is to the stated size. It probably has not been an issue of any moment so they don't care. Not enough customers have raised Cain over their money being spent on a trumped up product, and most never know anyway.
Their attitude is probably, "Ah just make it any old real displacement, those idiots with the chequebooks never know the difference anyway".

 

If it takes re-tooling, it costs as much to go up 1cc as it does to go up 50cc.

 

A small block Chevy can only bored/stroked to a max somewhere around 427cubic inches......and that requires a seriously expensive stroker crank and machining. The point is there are limitations. But you can also get a 350 block to 383 cubic inches by swapping in the standard 400 small block crank......for a whole lot less $$$ than the 427.

I think some of the displacement numbers in these machines also come from engine configuration limitations. I can't say this for fact for any of the engines because I'm not the designer and am not privy to such info.......but it wouldn't surprise me if it is the case for certain engines.

For example........the Polaris Sportsman "800" is only a true 760cc. That same basic engine is also configured to smaller displacement Sportsman "600" and Sprotsman "700" machines. I'm guessing that they only went to 760cc because they're up at about the max of what they can do for bore and stroke with that particular engine block. At the same time Polaris has obviously decided to go with the 100cc increments (Sportsman 400/500/600/700/800) in recent years from at marketing standpoint.

Another example........the Polaris Sportsman "400" is actually 425cc. That is the same basic engine as the Sportsman "500" H.O. engine. They came out with the "400" afterwards by using a smaller bore/shorter stroke version of the "500". They probably went this way because they determined it was a better way for them to get into the "400" than going the other way with a smaller displacement engine (probably didn't have a smaller engine capable of being bored/stroked enough to achive there goals). The question is..... when reducing the displacement of the "500" downward......why did they stop at 425cc if the goal is "400"??? Is it possible that the engine designers didn't want to go any smaller with the bore and/or stroke for some reason???.......I don't know but it wouldn't surprise me if that were the case considering it was originally designed for the larger displacement. (also makes me wonder how far it can safely be bored and stroked to on the upper end)

Another aspect is that sometimes they can swap out certain "standard" or "shelf" parts such as crank/connecting rod/piston and achive a displacement that is right in the ball park of what they want very cheaply and easily. Actually, when I think about it.......this is probably why they got to 425cc.

When it comes to the actual displacement sizes for some engines I think these technical criteria/parameters/limitations are the reason they end up at the displacement they do.

Again......I have no specific knowledge on any of these ATV engines.......but I know this has been common practice with auto engines for decades. The Chevy small block is probably the best example of many different displacements just by switching standard/shelf parts around. (Another example.....Chevy got the late 60's 302 trans-am racing engine configuration from a 327 block and a 283 crank)

The marketing people sure as ship don't want deal with any of this stuff at all........and that's why you end up with the 400/500/600/700/800.

Bob

 

I remember in the early days of dirt bikes we had

50's
60's
70's
80's
90's
100's
110's
125's


And back then the bike size was close to the actual displacement.

 

Probably the same Richard Heads that cut the 2X4 down to what it is now days!!!! Ever try to retro a house with old lumber? [img]i/expressions/face-icon-small-mad.gif[/img] [img]i/expressions/face-icon-small-disgusted.gif[/img]
I agree with the person that says we should have a 5cc rule. + or - 5cc's, fine, anything more than that and your scamming people.