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Old 06-22-2012, 04:42 AM   #1
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Default calculating new engine displacement

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?
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Old 06-22-2012, 08:05 AM   #2
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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.
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Old 06-22-2012, 08:50 AM   #3
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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.
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Old 06-22-2012, 11:45 AM   #4
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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...
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