Ideas: Flow Velocity, Flow Capacity, Flow Quality - Team Integra Forums - Team Integra
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Ideas: Flow Velocity, Flow Capacity, Flow Quality

Posted 02-14-2002 at 12:00 AM by MichaelDelaney

A lot of people wonder why an STR or Zex Intake manifold isn't good for an all motor integra. Similarly, many people wonder why I cringe when they say they want a 70 mm overbore TB on their all motor, street integra. Have you ever heard about the guy criticising that the headwork done on a teg as being crappy because the ports are too large?

These are examples of learning the difference between flow velocity, flow capacity or flow volume, and flow quality.

I. Definitions

Let's define these first:

1. flow velocity in ft/sec. is the speed at which the air flows through a tube (fancy term in aerodynamics is conduit flow velocity). simple enough but people often confuse flow capacity for flow velocity .Flow velocity is how fast bulk flow is travelling. Flow capacity is how much bulk flow is travelling. The Pittsburg Steelers running back Jerome Bettis is known as the bus because he is huge (flow capacity) and can run fast (flow velocity).

2. flow capacity is the bulk volume or how much air is delivered. The units of measurement are usually cubic feet per minute or cfm. When some head porting shops quote how good their work is, they quote flowbench numbers in cfm. If you took an AEM intake and measured it's flow capacity on the flowbench and then take a big ass sewer pipe and measured it on the flow bench, which would have more cfm? The bigger diameter sewer pipe of course! More volume of air is delivered. Does that mean you should put a sewer pipe on your engine?

3. flow quality needs a little explaining. When we talk flow, we usually mean dry air flow. However, in engines we are dealing with "wet" flow after a certain point. When fuel is injected into the oncoming air flow, we have to deal with wet flow. Why the distinction? If the flow quality is good, the fuel remains suspended in the air as a mist. People say that the fuel is "atomised". If the flow quality is poor, the fuel is no longer kept suspended and clumps into bigger droplets or rains out along the walls of the tubes.

Why is keeping the fuel atomised important? smaller droplets in a mist have greater contact area or surface area than clumped bigger drops. Smaller droplets burn easier, more completely, and faster when ignited. So when we ignite the wet air fuel mix, we get a bigger explosion because more of the air fuel mix is burned. A bigger explosion due to complete fast burn makes more power and has better emissions and fuel efficiency.

Ia. Rule of Flow Quality

The single biggest factor affecting flow quality (keeping the fuel in the form of a suspended mist) is flow velocity. Higher flow velocities keep the fuel atomised better than slower flow velocities.

II. What Determines Flow Velocity?

The simple answer to this is diameter and length of a tube. The more correct fancy technical answer is cross sectional area and length.

II a. What is cross-sectional area?

Imagine a salami tube. If you slice it and then take a look at the slice from the end view not the side of the salami view...calculate the area of the circle of that slice from the end view. For you algebra challenged folks, the area of a circle is pi x radius squared.

If your port is not circular in shape, here is a website that calculates cross sectional area for oval-shaped, D-shaped, ractangular shaped, etc. ports.

IIb. The Rule of Flow Velocity

Now the smaller the cross sectional area (or diameter) or the longer the tube, the higher the flow velocity. You have to memorise this rule of physics.

III. What Determines Flow Capacity (Bulk Flow Volume)

Great you say! Let's make all tubes long with small cross sections and we'll have plenty of flow velocity, right?

Well yes, to a point. Try sipping a long straw. Now take that sewer pipe and try to sip thru that. You can get more of your drink up th sewer pipe (more flow capacity or bulk flow volume) using the same amount of suction. The problem is the flow up the pipe is slow but you get more volume.

III a. The Rule of Flow Capacity

To get more flow volume you increase diameter and shorten the length. Another damn rule of physics to memorize.

The engine has to get a certain amount of air to make power. The amount is flow volume. This flow volume tells you nothing about how fast that volume is going down the tube.

Remember, the slower wet air flows, the more chance for the fuel to separate out and become no longer suspended in that air flow. Nothing is simple in life is it?

Now with any rule, there are exceptions. When wet air is travelling fast in a straight line, it stays suspended. We all get that. However, when wet air reaches a bend in the tube, we have a problem with it travelling fast. Since fuel is heavier than air, it's harder to make it turn corners. It's like your chubby cousin and you (assuming you are lighter than your cousin) racing around a corner in your scooters. Because your cousin is heavier, he keeps on going straight instead of making the corner at the same speed. Fuel will separate out from air when it reaches a bend if the wet air is going too fast. The air will make the bend but the heavier fuel will want to go straight ahead and miss the bend. The solution? we must slow the wet air down at bends. How do we do that? we increase the diameter just before the bend. So you can have a smaller cross section on straight parts of a tube to get high flow velocity but a bigger cross section just before a bend to slow down flow velocity: all of this is done to maintain good flow quality.

IV. The Engine Builder's Dilemmma

So, this is the dilemma of the engine builder: you need enough flow capacity to make power and support the combustion for a given displacement but if you make the cross sectional area (or diameter) too big or shorten the tube too much, you get poor flow velocity and flow quality on straight parts of the tube.

At bends in the tube, the engine builder wants to slow the flow down so the fuel makes the corner and stays suspended but how does he make the wet air go fast again after the bend?

V. Reality Check: How Does All This Mumbo Jumbo Apply to Integra B18A/B and B18C Engines?

Where do we have tubes with air (dry or wet) travelling through them ?

intake, TB, intake manifold runner, cylinder head ports, exhaust manifold, exhaust.

So the apllication of the ideas of flow velocity vs flow capacity vs flow quality is pretty damn relevant.

An all motor integra will need more flow velocity than a forced induced integra. The turbo or SC will pump in tons of air (high flow capacity) at a high flow velocity.

So an all motor integra wanting more flow velocity into the engine will like smaller cross sectional area at the cylinder head intake port and long intake manifold runners compared to a turbo version.

This is why an STR or Zex intake manifold with huge diameter (big cross sectional area) and short runners will not work well in an all motor teg. The flow velocity will be killed. The flow quality will suck. The end result is the low rpm and midrange rpm power is killed.

This is why a huge diameter overbore 70 mm TB on an all motor teg will not work well. The same thing happens. You may "feel" a crisper throttle response but the low rpm and midrange power gets killed because although you get more flow capacity, the flow velocity and flow quality is sacrificed. The power gains are all seen in the peak rpms only and for a very narrow rpm range up top.

This why hogging out the intake ports during head porting is incorrect for all motor tegs...again you see a gain in power in a narrow powerband at the upper rpms only.

You want to increase diameter enough to have enough flow capacity to support combustion and make power but you don't want to go too big on diameter so that it kills velocity.

It's a fine line or tight rope engine builders walk. They have the experience to know how big the diameters can be pushed without sacrificing velocity and quality. How did they get this knowledge? by trying and making mistakes first and then trying again but using the lessons they learned from their previous mistake....

Hope you got something out of this. Ask any questions you like and I'll try to answer them if I can.

We'll get into solving some dilemmas of the engine builder later on in my subsequent posts and using these ideas for the exhaust side as well as the intake side...for now, I'm getting tired typing and will wet your appettite by saying as clues, have you heard of D-shaped ports and what they do and have you wondered what the Endyn/Wiseco Rollerwave piston top shape does? Ever wonder why they put dimples on golf balls instead of having a smooth surface?

The concepts on how dry and wet air flows through a tube involves the science (mostly physics which I hated in school ironically and it has come to back haunt me now) of aerodynamics.

We have seen that wet air likes to go fast in straight tubes to keep a) the fuel suspended and b) the mix in a state which is capable of burning quickly and completely.

Our problem is at the bends. We need to slow it down so that the heavier fuel can make the turn with it's partner (air) but after the bend, we must re-accelerate the mix in the straight portion to keep it mixed.

How do we do that?

VI. D-shaped Ports

If you look at air flow it travels in layers of parallel lines called "laminar" flow. This is the kind of flow that keeps the fuel suspended. If we get whirls and eddys in the flow it is called "turbulence" or turbulent flow. We want to avoid turbulent flow which causes the fuel to separate.

In a cylinder head's intake port there are 2 main surfaces, if you at the side view of a port. The 2 surfaces are the roof and floor. There is a straight horizontal part, from the intake port entrance to the intake throat into the cylinder. At the throat, there is a downward bend into the cylinder and then a straight vertical part before the valve seat.

If you draw a line following the roof and measure the distance from the intake port opening or entrance to the valve seat, you will find that the length of the roof is longer than the length of the floor.

Air likes to travel the shortest distance. So the majority of air flow in a cylinder head intake port is at the floor. The air concentrates and travels the fastest when it hugs the floor and travels the shortest distance from point A to point B.

Why is all this long explanation of floor vs roof length important?

We know that bigger diameters or cross sectional area slow flow velocity and small diameters speed up flow velocity. If you don't believe me then turn on your garden hose to a trickle only. Now pinch the hose but not so much that it completely blocks the water flow. You'll see the pinched hose causes the water to squirt out farther and faster. This decreased diameter causes the venturi effect and speeds up flow velocity.

Headporters calculate how much flow capacity they need and then determine the smallest possible diameter to achieve this so that they maintain flow velocity in the straight parts of the intake port tube.

At the bends, since air flow hugs the floor and slows down with larger diameters, the headporters open up the floor's corners only. They keep a minimal overall cross sectional area but slow the flow down just at the bends by increasing only the floor diameter. The shape looks like a "D" when you look at the port from an end view where the flat part of the D is the floor and the curved part of the D is the roof.

from Jim McFarland Sept. 2002

Figure 1. Side view of an intake port showing the 2 fuel and air paths.

Explanation of Figure 1.

To maintain flow quality, you want to prevent mixture separation...fuel from raining out of the air and depositing on the intake port walls rather than travelling as a fine mist with the air down into the combustion chamber.

The main place mixture separation can occur is at the bend where the air and fuel paths can separate, since air has less inertial mass and can "make" the turn down the bend into the valve throat and valve seat. The air and fuel can take 2 separate paths. Air prefers the shortest distance possible between 2 points and the path of least resistance for the majority of air flow is at the floor rather than the roof or middle of the port.

Fuel prefers to take the path down the middle of the port. So they separate or take separate paths at the bend.

D-shaped or trapezoidal shaped ports slow the air at the floor and help bring to paths back together so they can coincide and travel at the same speed down the throat without losing mixture quality. The combustion quality as a result improves and gains are seen in the mid to low rpms.

The domestic hot rodders and motorcycle racers have used D-shaped ports for a long time now and it's catching on in the import Honda scene as well.

We solved the first part of the engine builder's dilemma on flow but how do we re-accelerate the wet air after it has completed it's turn?

VII. Speeding Up Wet Flow After the Bend in the Tube

To re-accelerate wet air flow after a bend and keep the fuel suspended, the engine builder uses the piston top shape.

The piston top is the floor of the combustion chamber. As the slowed wet air mix enters the combustion chamber or cylinder to fill it, the piston is coming up to squeeze the wet air mix before the spark is ignited.

You will notice in normally aspirated aftermarket Honda pistons that the piston top is domed and slanted such that the intake side is higher than the exhaust side on the side view of a piston top. The reason for this higher valve relief on the intake side is the fact that as the piston is squeezing the wet air, it reaccelerates the mix and pushes it to the exhaust side. We get the mix to speed up like when you squeeze your toothpaste tube. The fuel droplets are re-suspended and the hotter exhaust side ionizes the wet air just before the spark is ignited.

This way you get the wet air to keep the fuel atomised. In reality, the wet air fills the combustion chamber in layers with the richest air fuel ratio closest to the piston top (the floor of the combustion chamber) and the leanest air fuel ratio at the pentroof of the combustion chamber.Since the fuel remains atomised and ionized it will burn faster and more completely...making more power and better emissions.

The best example of using the piston top shape to reaccelerate wet air after the bend is the Endyn/Wiseco Rollerwave piston. The JE/SPR pistons also have a similar piston top that squeezes on the intake side and squirts or pushes the mix to the exhaust side to speed up the entering wet air mix again.

Now have you ever wondered why some headporters don't polish the intake port surface (especially the floor) to a smooth glass like finish? It sure looks better if you do and you would think a smooth surface helps keep laminar flow, right? Well, we have to go to another sport to answer this question and that sport is golf.

Which travels further a golf ball with dimples or a smooth surfaced golf ball?

Figure 1a. Actual photo of smooth-surfaced ball travelling (from right to left) through the air with air flow around it: notice the turbulent air that is revealed by the smoke streams behind the ball to the right. This is due to added aerodynamic drag (balll surface rubs against the air) on the smooth surface of the ball. This slows the ball down.

Figure 2a. Another view of a smooth-surfaced ball travelling from left to right with turbulence generated behind the ball to the left due to added stickiness of aerodynamic drag which slows the ball down.

Figure 2. Actual photo of a rough-surfaced ball travelling through the air. Notice the smoke streams used to reveal the flow pattern of air around and behind the ball is not a disorganized as the smooth ball. The flow is laminar and has less aerodynamic drag behind the ball to slow it down. The flow behind the ball to the right is laminar and in a cone shape. The rough surfaced ball actually travels faster and further. This is why golf balls have dimples.

In an intake port, a smooth surface wall of the tube would do the same thing. The layer of air flowing closest to the floor and roof surfaces on side view would stick to the surface and become turbulent disrupting the laminar parallel flow above it.

Fig. 3 Here is a photo of air flow sticking to the smooth floor surface and becoming turbulent. This layer closest to the smooth floor is called the boundary layer. Notice the nice parallel laminar flow above this boundary layer.


Figure 4. Air flow through the pipe.

Notice air flow if laminar has parallel lines and the front leading edge is in the shape of a cone. The boundary layers are the lines closest to the wall surfaces.

If we have a smooth wall surface, these boundary layer gets wider or thicker and obliterate the fast laminar flow in the centre of the tube.

Figure 5. Here is a series of photos of a tube with flow becoming turbulent (from smooth surfaces). The flow is travelling from left to right. Start at the top left corner and go ri end, then follow down going left to right. The last photo is on the bottom right. Notice the boundary layer closest to the smooth wall becomes turbulent and grows bigger and bigger until it occupies the tube width. This thickening turbulence kills flow velocity and we get fuel separation from the air and fuel rains out or forms big droplets which does not completely burn when ignited.

So for headporting not only is small cross sectional area important in maintaining high flow velocity. To keep the flow laminar away from the boundary layers and at the center of the tube, we do not want smooth surfaces. So the headporter should not polish the intake track to a glass smooth finish . Slightly rough surfaces support thin boundary layers and more laminar flow at the center of the tube. Laminar flow supports high flow velocities in the straight part of the tube. High flow veloci produce better flow quality.

Other surfaces where rough surfaces are used to reduce aerodynamic drag or stickiness from boundary layer turbulence growth include shark's skin in nature and the use of a 3M product called "rivlets" which are V shaped bumps in a skin layer that can be pasted onto surfaces of planes and sailboats. The America's Cup sailboats used this rivlet coating to go faster as well. So the humble dimples on a golfball tells us that to reduce turbulent drag and gain flow velocity, you don't want a smooth surface.
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  1. Old Comment
    phew...thanks for the head ache!;>
    so will I see an increase in linear air flow if I swap out my stock air filter box with a K&N type round intake tube ,and about how much gain if any in horse power on my 92 gsr.?
    Posted 05-26-2013 at 09:16 AM by mystock92gsr mystock92gsr is offline
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