Bonneville America Forums Forums Technical Forums Performance Mods 904 BB question

just tore my engine down to put in the 904 wisco kit
and my stock pistons are flattops so my question is
how can you get just 9.2to 1 compression from stock
flatops and a claimed 10.5to 1 from wiscos dished
pistons ever dished piston i ever saw was a low
compression piston are all of the stock pistons in this
bikes flatops.a flatop should run a higher compression
ratio then a dished piston right.

not necessarily dependent upon piston shape. The wrist pin location is lower on the piston (wiseco) relative to the top of the piston so it raises the compression. More dependent on volume of space the piston takes up vs the shape of the piston.

I second that.


mike

Remember also you're squeezing a little over 14 percent more cylinder volume into the same size combustion chamber, which increases compression ratio.

You'll notice the relief cuts for the valves in the piston- what you called "dished".