Engine swept volume

[apexpredz]

Holding everything constant but only changing the bore size, but the same rod, same crank, flat top piston, same compression.
What effect would the swept volume play in deciding the necessary intake valve closing for cam timing if you increase the bore size? And if you decreased the bore size?
Meaning would you need to close the intake valve early for one and later for the other?

[MT Jeff]

It's not that simple. If you change the bore from say 3.5 to 4.0 inches with no other changes, same head, flat top, stroke etc. The compression goes up from say 8.2 to 10.1. That would require a later closing just to keep the dynamic compression the same. If your talking about going from std. to .030 over on a 4inch bore its only .2 pts comp. so not enough to matter much.

[MadBill]

<EDIT> Written before seeing preceding post. Here it is anyway.
Assuming the original configuration was optimized and the bore increase was substantial, the most direct effect would be the need for more cam lift and/or duration to 'band aid' the now-inadequate (for the greater displacement) flow. The bigger bore would also likely unshround the intake to some extent and so increase flow by a few percent, but not enough to make up the required increase.

The result would likely be a torque increase in proportion to the gain in displacement, but less than proportionally more power.

[apexpredz]

So in effect, if you go from a 4.000 bore to a 4.030 bore, holding everything constant, the increase in bore should not require a later intake valve closing and actually aid in unshrouding the valve?

I just heard something today and a guy was stating that if you decrease the bore you need to close the intake valve earlier since the engine swept volume was smaller. The example was going from a 4.000 bore to a 3.780 bore.

Is it more of the target dynamic compression issue or swept volume? Or does swept volume not really play a major role?

[bigfoot584]

So in effect, if you go from a 4.000 bore to a 4.030 bore, holding everything constant, the increase in bore should not require a later intake valve closing and actually aid in unshrouding the valve?

I just heard something today and a guy was stating that if you decrease the bore you need to close the intake valve earlier since the engine swept volume was smaller. The example was going from a 4.000 bore to a 3.780 bore.

Is it more of the target dynamic compression issue or swept volume? Or does swept volume not really play a major role?

Say you have a (X) cylinder bore configuration with compression of (Y) 150 PSI.
Shrink X you need to close intake valve sooner (which increased the DCR stroke) to retain the same Y.
Enlarge X you need to close intake valve later (which decreased the DCR stroke) to retain the same Y.
You changed both SV and DCR.