I have to disagree with your disagreeing.
Backcountry had it right. If you were to decrease cylinder displacment you would be compressing less cc's into the same amount in the head.
More compression means less volume in the head or more in the cylinder.
In 4-strokes you get a higher domed piston, in 2-strokes you can shave the head, get a smaller dome, or use a thinner head gasket.

Think of where TDC is in the engine. Basically you are compressing everything below it into the small space above it.

To answer trx250104's question,
A higher compression piston has a little more material added to the top of the piston to make the volume above the piston at TDC smaller.
Like I said, Backcountry had it right.
ERIC

 

you would have to loose CCs because you are making the chamber smaller by increasing the dome on the piston.

But what you would lose wouldn't be worth being concerned with.

 

Ok, when you change the shape of the piston and leave the bore and stroke the same what has changed? Your saying nothing but is that true? When the piston is on its down stroke it does not take in the same amount of air and fuel with a dome top piston as opposed to the flat top piston. You've decreased the size of the cylinder (combustion chamber has stayed the same) with the dome top piston because the piston itself is displacing mass now. So now it takes in less air/fuel than it did before, right? The decrease in volume increases the pressure. If you made a dish top (concave as opposed to convex) piston the pressure (or compression) would go down and the volume would go up.

I understand what your saying about the TDC but it still doesnt seem possible that its taking in the exact same amount of air/fuel with the different sized/shape piston.

 

Displacement is calculated using the bore, stroke, and # of cylinders. There is no equation that adds/subtracts area for piston displacement.

Piston displacement is only calculated in doing static/dynamic compression numbers. Static compression equation would be displacement, cylinder head ccs, head gasket thickness (compressed), deck, and any volume subtracted or added for piston dome or dish.

 

OK I'll use my 250R engine as an example. Stock is 246 cc's and lets say say there is 22 cc's in the head or above TDC. To keep everything simple forget any other numbers like cc's in spark plug, squish band...
So 246/22=11.18:1 compression ratio

Now add to the top of the piston, lets say 1 cc worth so the cylinder volume is now 245 cc's and 21 cc's above TDC.
245/21= 11.66:1

Higher domed piston=higher compression ratio!
ERIC