clearance volume

hello you said that if the piston crown is dished then we will add the dish volume to the clearance volume and this is a logic thing but what i can not understand that after that you said we'll represent a dished piston volume as negative here do you mean that it will decrease the CR or what ??? sorry for my poor language skills and thank you for your effort

Could it be some confusion where they're talking about the 'hump', or 'bump', of higher compression pistons where the dome is above the piston deck? That would be sticking into the combustion chamber and reduce the clearance volume, which is why it's a negative value?

Depending how precise you want to be it's common practice to check this anyway, as manufacturers may be a little incorrect - and if the piston is further modified...