the bike has a 449cc bore and the quad is 439

 

it's destroked to be under the 440cc ama racing limit

 

It's not sleeved down, it is destroked. That's where the .6 of compression is lost from the yz 450. They did this to make the 440cc rule.

 

you can put a motorcycle yz450f rod in there and make it 450cc if you want. But they put a shorter stoke to meet the 440cc race standards. So it will rev faster but loose more top end HP than the 450cc. I dont know if the loss in compression was due to the destroking though. The piston goes up just as high, it just doesnt go down as far as the 450cc. They probably changed the compression on purpose.

 

The crank has less stroke, The rod will actually be longer in this motor if the compression height of the piston is the same. The loss of compression is from it being destroked. Look up pistons for a chevy 350 and 383, 383 has more compression even though it is the same piston is used.

 

It is simple physics, the longer the rod is the closer the piston moves to the valves on compression strokes. This means that it smashes more air and fuel - basicly the definision of compression in an engine.

 

Are there any 450 stroke kits out? (Can you stroke a YZ450F today?)

Just thinking, if you were going to stroke the new quad. . . might as well stroke the $hit out of it.

Putting a 686 in my Wife's Raptor now so, wonder what these will do against it.

 

I am figuring that they used the same length rod. Therefore the one with the highest compression height on the piston ( the one with the longest stroke crankshaft) will have higher compression.

 

There is a company in Europe called TUMIC who do a 480cc aftermarket cylinder for the YZF450. You would have to use the longer stroke YZF crank, which bolts straight in by the way.. You get greater coolant capacity, 98mm bore, thicker sleeve and its able to be bored out to 100mm for 498cc.

 

In july's issue of dirtwheels they said that they changed the rules so that you can now race a 450 cc fourstroke. maybe yamaha found out to late.