Originally Posted by RDY4WAR
I like his last comment where he mentioned that just because an engine has a large bore doesn't mean it can't be efficient, and just because an engine has a long stroke doesn't mean it can't make a lot of power.
For example, let's take an example I'm very familiar with. A simple 350ci Chevy small block with a 4.00" bore and 3.48" stroke. Let's take 2 of them, with the same heads, but one to be focused on a better horsepower curve and the other focused on a better torque curve. The prep of the cylinder heads, not just the flow, is important to efficiency and power. For maximum power, I'd want to recess the valve more in the seat, put it on a steeper 50-55* seat angle, and have more gradually steeper angles leading up to a rather large (>90% valve dia.) throat. This allows for less turbulent port flow at high lift and high rpm so you can maximize the peak power curve. On the other engine, I'd want to sink the valve as little as possible on a less steep 42-45* seat angle and less angles going to a narrower throat and preferably a more prominent short turn radius in the port itself. This forces the air to move around more at lower lift, creates more turbulence, and thus helps efficiency at low rpm.
The piston shape takes a factor as well. A piston with a flat top or D-shaped dish with a flat quench pad will create more turbulence and be more efficient than a piston with an open dish or dome top. The same applies to the chamber shape in that a shallower, more closed chamber will be more turbulent and more efficient than a big open chamber. Spark plug placement closer to center of the chamber means shorter distance of flame travel and thus more efficient. Pistons with a shorter top ring land (top ring is closer to the top of the piston) can be more efficient as it is exposed to more heat and can help with sealing at the expense of high rpm stability. You also wouldn't want a high top ring on a turbocharged/supercharged or nitrous engine.
I'm getting way off on a tangent. I like this topic a lot.
Great post! Really susses out a lot of the detail with respect to the topic.
Another issue is of course valve size and shrouding, which, within a given architecture (key) can put a significant cap on output potential. Comparing two engines of similar displacement and architectures:
1. The venerable SBC in 305 form
2. The SBF in 302 form
both engines are of very similar displacement, but the 302, having a 4" bore when compared to the 305, which had a 3.736" bore meant that the 302 benefited from a much more flexible selection of cylinder head designs and configurations, similar to the 327 and 350, both of which shared the same diameter bore. With the 305 there was a limit with respect to both valve diameter as well as the flow restriction as a result of valve shrouding from the smaller bore when working within the constraints afforded by it. This is a key reason as to why the 305 was never a popular performance build and was typically swapped out for a 350 so that heads with larger diameter valves could be employed.
Of course with multi-valve heads on an engine that doesn't use pushrods like a Modular, Coyote, Honda...etc, having to deal with side-by valves in a pairing isn't an issue, so bore size becomes far less of constraint with respect to potential power output.