Serious Discussion: Velocity vs. Amount of air available

[IndyBlk5.0]

I am some what confused. I realize that velocity is important in that it fills the cylinders faster. It also creates a vacume to keep more air coming. In doing so, I would guess that since the air is entering the cylinder faster, it is getting more air in the cylinder hense making more power. To keep velocity, you have to have some sort of mild restriction, therefore creating a vacume. I see it as sucking throw a small straw, and then sucking through a larger one.

On the other hand, you can make your port sizes larger therefore making the air easier to obtain, but with little restriction, the velocity would be down. Timing events happen in fractions of a second... By having low velocity (force pushing air into the cylinder) I don't understand why having a lot of available air would be beneficial having having high velocity.

I also have found (correct me if im wrong) that higher rpms create more velocity, therefore that is why bigger intake and head'ed combos make more power uptop. The air is getting forced into the cylinders faster by a decent vacume. This is when the high flow charitaristics of the heads and intake really kick some butt.

I understand that there are limits to both. You can starve a engine for air and have excelent velocity, but no power because the amount of air is not enough. Or, you could have so much air that the force sucking it in is not enough to actually suck the air in to be beneficial with huge heads and intake.

Keeping that in mind, would you rather have a motor with high velocity, somewhat starving the cylinders for air. Or would you rather have a motor with ample air, yet not enough vacume to take advantage of it.
The obvious answer is in between the two, but I want to see what sides you will take.

 

[L8 MUSL]

I would say that volume is more important then velocity.
The more air/fuel you can "process" through a motor, the more hp that motor (or should I say "engine") will make.
Obviously though, more volume will more then likely require more velocity to achieve the amount of air desired during the limited "event" of the opening of the intake valves.
I think I just confused my self

 

[90mustangGT]

I think you have it down IndyBlk5.0.

Volume is the amount of air. You want to get as much volume in the combustion chamber as possible. Air is mass, it takes energy to move mass, and energy cannot be created or destroyed, but it can be wasted. For example, lets say you have two tubes. One is 1" thick and the other is 2" thick. You blow air through the 2" tube at 25mph and 50mph though the 1" tube. The volume of air will be the same. However it will take more energy to push that air at twice the speed in half as much space. However this energy isn't nessasarly a loss because as moving mass of air leaves the tube it will create it's own vaccume, pulling more air through while the slower moving air won't near as much. Air also takes up space, so the capasity of the smaller tube would be half. The smaller tube would hit it peak capasity and then the air would have to compress to go through the tube, which won't happen in a N/A application, it just won't flow anymore while the larger tube will have lots more to go.

 

[maverick0716]

Large intake runners = higher rpm

smaller intake runners = lower rpm

An example are the AFR 165 heads and the AFR 185 heads. On a naturally aspirated combo the AFR 165 heads will make much more power lower in the rpm range and the 185's will make more power in the upper rpm range (6000+).....unless you factor in forced induction.

I think the original poster is right, unless you are going to be moving a ton of air through a large port head, there is no point in having them....they WOULD make less power (no velocity). I'd say for a N/A street application that doesn't see 6500 rpm, smaller valves (1.9) and intake runners would be beneficial......keep it 170 cc or less for the intake runners.

 

[IndyBlk5.0]

Forced induction takes all those theorys away. Instead of the engine using energy to suck air, it is getting it shoved down it's throat pretty much. The blower eliminates the problem with velocity, and gets better with what amount of boost you push through. Forced induction is easy.

What is better guys?

 

[IndyBlk5.0]

I would like to see everyones views on this subject. I realize there are many different views. So give them to me!

 

[proxses]

One thing about velocity is that too much will kill a motor.
Anything above 440 fps is way too fast and creates too much drag with the boundry layer at the port walls. So you get excessive tubulence, and hindered flow. (But too little turbulence will cause fuel to condense on the port walls) High velocity also has an impact on intake valve springs when the valve slams shut, and the air comes to a sudden stop.
This is where intake tuning comes into play. When the air hits a wall, it causes a pressure wave in the other direction. When this waves enters the plenum, it is reflected back. This is usually where torque peaks.
With large ports, the flow becomes lazy, and can't be tuned for low rpm's. Hence bad road manners.
With forced induction, velocity still matters, it's just that the air is denser, so more air will pass at the same velocity.
That's enough for now.

 

[9 Deuce GT]

Is there really a right or wrong answer? I think it depends on what is expected from a certain combo. Everyone says that you have to have a goal to build a setup that you will like. I think this is the case with just about everything in the world. In certain applications, each of the two, velocity and volume would be ideal. If you want a engine combo that sees street use or needs torque, like a dump truck or bull dozer or something industrial, you would want the velocity for pulling or pushing or getting something heavy to move quickly. On the other hand, if you have an application that needs to move at top speeds, like a drag car or wimpy import rice jockey, you would want something to have a lot of volume to really take off in the higher RPM's.

I think you really hit it on the head when you said that something in the middle would be ideal. I think that is what would be the perfect set up.