Steve Bryant

iTrader: (1)

There are a couple of threads (at least) right now touting the advantages of broad band torque and drivability and how less is more regarding the size of a cam (duration and lift). These threads have been started by folks that have been on this forum for several years . . . long enough to honestly say "been there, done that" about a few topics.

The same "less is more" concept expressed in these threads applies to heads that feature higher tech, high velocity, exceptional flow for port volume. Guys like Joe Mondello figured this out years ago. Read this article by Joe published several years ago on LS1 heads http://www.babcox.com/editorial/ar/eb120118.htm.

I work as an engineer for an aircraft manufacturer and have a commercial pilot's license and have been a flight instructor and I'm pretty familiar with basic aerodynamics. If you multiply flow rate times velocity, the product is energy in the context of head performance. It's pretty easy to hog out the runners and bowls and get big flow numbers, but the velocity factor becomes a dog (to rhyme with hog). Unless you are racing only (most folks don't), the drivability and fun factor of a car (or truck) with more torque on tap right away causes the scale to tip toward the advantages of small runners optimized for velocity and flow and a moderate size cam.

Give it some thought.

Steve

airflowdevelop

hmm..can you explain this to me?

DavidNJ

Well, the units are off, however a faster moving column of air has more energy, since in is a function of the square of velocity.

Steve Bryant

iTrader: (1)

Yes, the goal of the whole induction and exhaust system (cylinder heads included) is to pack it in and pack it out. The "it" is molecules of oxygen that can combust with the fuel. Increased velocity has a ramming effect that will bring more oxygen into the cylinder before the intake valve closes. I could also say air, but the 21% of the air that we care about is oxygen. Additionally, the velocity improves the atomization of the fuel (even with port fuel injection) and extracts more energy from the fuel that is burned. This improved energy extraction boosts power, thus improving performance and efficiency for the amount of fuel burned.

Steve

airflowdevelop

where did we come up with the fact that increased velocity = "ramming effect"...if it were only that easy!


A couple of terms that due some research.
Fuel Seperation
Sonic Choke
Dis-laminar Flow or "dulisive flow" (sp)

welcome to my world...I have no problem making all the velocity you want...unfortunately..it no makey no power.

Oh...and I should note...I have a couple dozen instances, where the nice smooth port, which just happened to be 20 - 40 fps slower than the fast lumpy port, made more torque, longer, and more Hp, longer... Why is that?

learn me O' wise one.
Dennis

DavidNJ

How did you measure the velocity?

airflowdevelop

pitot...

Adrenaline_Z

Velocity means squat if the valve is closing.

Head flow wont necessarily get you more charge after IVC if the motor is
not tuned for that specific point in RPM.

I think pulse tuning, and valve timing are more responsible for the "ram effect"
than flow through the head, but that's just a guess based on a few books that
I've pained through.

airflowdevelop

ding ding ding ding...I think we got ourselves a weiner...

You cannot have trapted VE without proper valve timing... This is where the (although not correct by term) ramming effect takes place.

This is exactly how Stock and Super Stock engines work.

Dennis

airflowdevelop

oh..and I am not a mechanic, nor do I have a 10 second ride.

DavidNJ

Isn't the pitot giving you point velocities? How many samples are you taking? Where int the port are you taking them? Is a fixture holding the tube? How is alignment maintained?

airflowdevelop

David,
Point velocity, in reality, is what we are concerned about. The system I use is completely digital, and I can average or filter the input for any length of time you may desire. As best that I know, this is the best way currently to measure local air speed (it is the same method used by all F-1 teams and most pro-stock teams, some are still using a diff. manometer.

I can say one thing...it can hear a mouse fart from 10 feet away.

Dennis

Adrenaline_Z

That's OK, neither do I.

I could have used your flow bench a few months ago. I had to buy mouse
traps to get rid of my little visitor:

airflowdevelop

eww..secondary income!

Wet 1

iTrader: (4)

Okay guys, I want to throw a wrench in the works... Add FI.

Forced Induction, does everything above now change? I've found in the past that my FI engines tend to respond better to heads that would be considered too radical (large and slow ports) for the otherwise same NA engine. Many have argued that the small port heads are still superior with FI engines... I say BS since the FI engine doesn't have to work under vacume conditions. It just needs the means to get as much of that charge into the CC as possible which is why I believe I've always found the larger ports work better (although they do give up a little off boost, but more than make up for it while 'on').

Also, much of this focus has been aimed at the intake side, what about the exhaust side??? What happens when you hog out the exhaust ports (such as a C5R port)? What about when a turbo is thrown into the mix?


Interesting reading guys!

SStrokerAce

iTrader: (2)

FI doesn't change things. Filling a cylinder is all just a process of higher pressure moving to lower pressure. NA and FI both do that, with NA stuff you just have to manipulate everything to make that happen as long as possible when the valve is open.

Listen to what Dennis writes guys... he made a good comment the other day.

"Anyone can be a hero on a flow bench at 28", then you crank it up to 60" you start thinking, man I don't know what I'm doing. Now go over 60" and anyone will look like a dummy."

As for the ram effect... well it would be better to explain it has wave tuning, and both the intake and exhaust have that and both can help you increase the differential between the intake port pressure and the cylinder pressure. That pressure difference can be over 100" of H2O at high lifts, and over 200" at lower lifts.

This is something everyone should take note of....

http://www.du.edu/~jcalvert/tech/fluids/orifice.htm

Bret

P.S. thanks for that link Dennis

white2001s10

iTrader: (4)

Actually FI changes quite a bit. FI creates a higher pressure differential around the valve, which you should know will increase velocity in that area greatly. Other things being equal, the smaller port will start seeing losses earlier with FI due to the higher velocities.

The speed of pressure waves also changes in the port due to the increase in density.

These two factors alone logically suggest that a port with larger volume is better suited for a FI application where boost will be a significant factor.

SStrokerAce

iTrader: (2)

No they both work on the same concept... the difference is the amount of CFM flowing into a motor at the same RPM, you increase DENSITY of the air charge in a FI motor. The velocity stays the same, the issue with a FI motor is that the difference between the cylinder pressure and port pressure stays at a more constant difference. You can have higher pressure differences in a NA port relative to the cylinder pressure than you can in a FI motor, but this is only at certain times of the intake valve lift curve.

There is also virtually no help from pressure wave tuning in a FI motor, The boost pressure is a much higher pressure at work in the motor.

The FI port might want to be larger due to the high volume flowing thru it, but that's when you start sizing the port due to the HP level you are working with rather than the RPM and cubes below it.

Bret

white2001s10

iTrader: (4)

It sounds like you're saying the velocity isn't dependant on the pressure differential ?
That idea is completely alien to me and doesn't seem to follow the laws of physics.

Old SStroker

If you doubled the density of the air (2X as many molecules per cubic foot) but kept the cfm and therefore velocity the same, wouldn't you get twice as much air into the engine?

Thought experiment: Assume 3 Bar boost (~45 psi) without increasing density. You'd need to increase the velocity by about 3X over NA velocity, right? That would get intake velocity well over Mach 1. Do you think that's happening in a blown engine at the valve as well as at the supercharger outlet?