How to measure displacement?

Am presupposing that displacement has to do with size of the combustion chamber. Multiplied by number of cylinders. And dependent on bore and stroke.

But am uncertain with regard to most commonly accepted method of measuring displacement.

Is displacement measured as the difference between combustion chamber volume at bottom dead center and at top dead center?

Or measured as the cylindrical volume from top of the piston at bottom dead center to top of piston at top dead center (or top of cylinder where domed piston protrudes above cylinder)? And ignoring volume of concave area in head around the valves?

Or some other means, such as volume of air intake during the intake stroke from tdc to bottom dead center?

Regardless of measurement method, it seems with all else equal that changing to a higher compression piston would decrease the measured displacement because the higher piston dome now occupies some of the displacement volume.

Thanks for some insight on this.

Good Luck!

Have a gander...

www.unlimitedallstars.org/calculatedisplacement.html

I was always under the impression it's simply
cylinder area x stroke. (by cylinder area, I mean cross sectional area)

This is because it's the volume of flow the cylinder will produce that's being measured. Therefore, the volume of the combustion chamber at TDC is not included. That same volume is also not included at BDC.

If the piston were to move very slowly, and the valves open and close fully at TDC and BDC, the volume of air the cylinder will draw until BDC, and therefore displace as it moves up to TDC, will be
CylinderArea x Stroke, regardless of chamber shape or piston shape.

Therefore, combustion chamber changes do not alter displacement. As long as bore and stroke stay the same, displacement stays the same.

EDIT: to confirm this with Sandy's link...

cylinder area =
= pi x (CylinderRadius squared)
= pi x (CylinderDiameter squared) / 4
= pi / 4 (CylinderDiameter squared)
this is approximately
= .7854 x (CylinderDiameter squared)
Bore is Cylinder Diameter so
= .7854 x bore x bore

so approximate displacement
= .7854 x bore x bore x stroke

without looking it up memory tells me its bore x bore x stroke x pie x number of cyls

BSKZ650 wrote:

without looking it up memory tells me its bore x bore x stroke x pie x number of cyls

have to divide that pie by 4 in order for the bore area to be correct.


mmmmmmmm, pie

Engine displacement is the volume swept by all the pistons of an engine in a single movement from top dead center to bottom dead center.

Wikipedia has plenty of information on Engine Displacement .

loudhvx wrote:

...volume of the combustion chamber at TDC is not included. That same volume is also not included at BDC.

If the piston were to move very slowly, and the valves open and close fully at TDC and BDC, the volume of air the cylinder will draw until BDC, and therefore displace as it moves up to TDC, will be Cylinder Area x Stroke, regardless of chamber shape or piston shape.

Therefore, combustion chamber changes do not alter displacement. As long as bore and stroke stay the same, displacement stays the same....

Thanks!

BSKZ650 wrote:

without looking it up memory tells me its bore x bore x stroke x pie x number of cyls

try radius X radius X pie X stroke

BSKZ650 wrote:

without looking it up memory tells me its bore x bore x stroke x pie x number of cyls

I believe it is piston top area times stroke. Take 1/2 piston diameter X 1/2 piston diameter X 3.1416 X stroke to get displacement of single cylinder. To get total displacement, multiply times number of cylinders.

easy,look on the end of the box your pistons came in! :woohoo: