Hmm,All talk about flow and not weight on velocity.
What about initial velocity when the valve opens and the piston moving down?
You can have two water pipes, one really big and one really small. If you were to look at a constant volume to observe what is going on. Sure, big pipes move more water, but slower compared to the smaller pipe with same applied force. This can be a problem when you have a limited window of time. Also the effects of increased flow without accounting for air velocity shifts the power band to the higher rpms. So basically turns your lower to medium rpms very sluggish. But it will open up the higher rpms for more power.
That's because you increased the area. Remember air has mass, and with mass you have inertia effects. Means F=ma applies.
So to put things in conception, lets for now hold acceleration as constant for easy viewing, increased area means you increased mass flow rate which means it requires more force to move it. So in power band perspective, lower rpm less air being moved, and higher rpm's less restrictive and more air moved because there's enough force to move it quickly. It's like the concept of a water pump, if you want more water out of a hose, you need a bigger pump because it has more force to move it.
Now we understand what happens with increased area happens. Now hold area. Let acceleration vary over time. More acceleration means more force to move the air. Especially if you have a greater initial velocity which allows you have a better draw vacuum. Vacuum is pressure. Pressure is force over area, (F/A). For our example, constant area, means more force acting on the air. The effects in the engine? You will see increase in entire rpm range.
But in the real world, mass flow rate is not constant because it's inertia.
The real trick about the engine is to imagine the engine as a air pump. The more air you can get in and out. The more power you have.
Porting tricks:
It's about allowing the air move with less change in direction while maintaining the same cross-sectional areas. This allow you to maintain inertia and overall air velocity.
Unshouding the valves, again allowing air to move with less change of direction.
You see where this is going? If you can change the port to move air without changing direction as much and changing the area, you have more power in all bands of rpm.
Why flow # are useless to me?:
The setup is to put suction on combustion chamber of the head, and crack open the valve. A constant suction is a steady state flow problem.
My problem with it, it doesn't measure changing velocity. And it's steady state. It doesn't take account for VE or air reversion (where air bounces off the valve and heads out of the runners in reverse direction). By the time air get it's maximum flow rate is when the valve is closing.
I can carve open that port like no other, plug it into that bench and it will report a massive flow gain. In real applications, I will see little power in the low to middle band of rpm, I would constantly need to stay in the higher rpm to have any useful power.
Which brings me to my final point:
Does flow matter?
Yes if you are talking about getting the most air in the shortest amount of time. A big pipe will always flow more, but it's harder to move in such short time.
But if you are talking about increasing the area, especially for higher rpms...
You can shrink the intake port and shorten your intake runners to reduce high rpm restriction and you will make more power. This is because the combustion chamber makes only so much suction power, or pressure. Pressure is Force/Area. The smaller the area the larger the force is, therefore you are going to have a greater initial velocity, which yields a higher final velocity. Or greater suction force, better VE, more power. And hell'of'a responsive motor!
Modified by Rare_f8 at 7:08 AM 2/5/2010
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