So after an .020" (.5mm) overbore and lowered compression ratio with supertech 8.6:1 pistons, I realized I need to update my REQ_FUEL on my megasquirt. I was thinking, oh, I'll just use the REQ_FUEL calculator in tunerstudio....

then I realized I don't know what it is anymore.

Stock BP 1.8 is 1839cc. Overbore to .020 with stock compression pistons is supposed to be 1862cc according to this thread
http://forum.miata.net/vb/showthread.php?t=323324

but then I have lower compression pistons. Is there a quick way to figure this out? or should I just leave it as stock displacement?

 

Changing the compression ratio will not change displacement. When the piston moves down in the cylinder, it will draw the same amount of volume as a high compression piston.

Only thing that will effect volume is bore and stroke.

 

ya... just a geometry problem. for one cylinder's volume: Pi / 4 * bore^2 * stroke / 1000. then multiply by 4 for 4 cylinders.

Pi / 4 * 83.5^2 * 85 / 1000 * 4 = 1862cc

But really there's no need to change Req_fuel for such a small change in displacement.

 

Thats why you have a WB O2.... what are the readings? That will be what you should look at before changing the fuel tables.

That little bit of boring is not enough to worry about...