02sierraz71_5.3

iTrader: (2)

I dont fully understand pressure drop values and how it relates to head flow numbers. Can someone enlighten me.

Also the standard for flowing heads seems to be 28" of water what is the pressure drop and the airflow rate in cfm that is the according standard?

Adrenaline_Z

When you state pressure drop, what are you referring to? The Manometer
scale? The pressure in the ports? The pressure loss through the port?

I don't follow the question. Are you asking if there is a typical rate of
flow for the standard test depression?

racer7088

28" of water equates to 1 PSI pressure drop and this number seems to come close to mimicking airflow near peak power on the dyno.

02sierraz71_5.3

iTrader: (2)

Im wondering about when a head is flowed at 28" what does the pressure drop mean?

02sierraz71_5.3

iTrader: (2)

Is the pressure independant of the amount of water? Another words can you set the machine up to keep no pressure loss increasing head flow numbers or am I thinking about this wrong?

Also how can I figure out when the air becomes turbulent @ lift x?

racer7088

The water pressure is just another unit of pressure mainly used because of the ease of using water in the manometer. We also have the units of HG or mercury or the good old PSI or pounds per square inch.

When the airflow in a head goes turbulent on the flowbench you will see flow drop as well and usually the port is noisy as well.

treyZ28

I think what you're missing is that air flows are a result of pressure differential. Fluids flow from high to low pressure.

For example. air flows out of an inflated balloon because its higher pressure inside than outside. soda flows up the straw because you create negative pressure with your mouth. Your mouth created a "pressure drop"

Sometimes pressure drop is used to describe a restriction. For example, pressure drop across an heat exchanger. Under normal conditions, a fluid flowing in something like a tube will have equal pressure through it. If you place a restrictive valve in the middle, it will cause a "pressure drop." The higher the pressure drop, the higher the restriction.

Adrenaline_Z

Let's see if I can help you out with your question. Please excuse my Photoshop
abilities!

The numbers you are questioning are likely the manometer values, which
have already been converted by Erik. 1 PSI = ~ 28 in./H20.

The manometer is taking the reading somewhere between the valve and
fan motor. The bench operator will calibrate the flow bench to achieve
a 28 " depression at all valve lifts tested.

For example, at 0.100" lift, the flow bench is adjusted to read 28" on the
manometer. A flow meter will record the air flow at that lift and depression.

If you can imagine how the bench functions, you will get an appreciation of
the numbers you are viewing.

If the valve lifts higher, what do you think the scale of the manometer SHOULD do? Should the reading drop lower than 28 in./H20, or should it
increase above 28 in./H20?

When you compare your cylinder head data, you will get CFM/Valve lift.
Common practice says these numbers are golden. I have since learned the
opposite.

The flow numbers mean very little in the real world. Do not select your cylinder
head based on flow numbers alone. Flow will change with:
- intake manifold, induction system
- cylinder bore size
- heat and fuel
- piston speed
- pulse mixing, sonic energy
- etc., etc.

With the flow numbers, you are only getting one piece of the pie and a very
little one at that.

02sierraz71_5.3

iTrader: (2)

Ok I think I see thanks alot for the diagram Adrenaline that really helped me.

So the manometer is simply measuring pressure and any increase or decrease will result in the water going up or down.

Im assuming you open the valve more pressure drops along with the water level so you turn up the fan increasing the pressure back to 28" and then read the cfm for that valve lift?
And 28" is equal to 1psi of air pressure going throught the orifice(open valve area)?
Actuall operating engine, I thought there was alot more pressure inside the cylinder pulling air through the head?
Am I thinking about this right?

Adrenaline_Z

Yes, that's pretty much the idea.
Exactly, except the fan is decreasing the pressure within the flow bench.
There are several different pressures occuring throughout the cycle. The piston
changes velocity during the stroke, and therefore creates a sinusoidal pressure
wave as opposed to a static pressure of just 28 in./hg.

02sierraz71_5.3

iTrader: (2)

Why not pulse the fan and get an average reading across defferent water levels , wouldnt that be more accurate? Obviously the fan wont move as fast(change pressure) as a piston but still seems like it would be closer to real world events.

Adrenaline_Z

That's a cool thought, but you would have to consider:

The fan would have to be pulsed to re-create speeds of different stroke lengths, combined with rates of piston acceleration over RPM.

If that's not tough enough, the fan pulses would have to be phased with
valve movement that would follow a specific cam lobe profile.

There would have to be some sort of mechanical/electronic intervention to
mimic valve position and piston position.

Each flow test would have to be performed for one specific engine combo.

The more you think about how a flow bench works, the less important the
flow numbers become in the real world.

02sierraz71_5.3

iTrader: (2)

I see what your saying so how am I supposed to pick a head if the standard flow numbers just dont do it. I dont have the money or time to sit my truck on a dyno and start swapping H/C/I combos.

Adrenaline_Z

From the info I've picked up in the last two weeks alone, I would say choose
the smallest runner volume for your application that flows the best average
low and mid lift values.

IE:

A 180 cc runner that flows 300CFM @ 0.500" lift, will likely perform better than
a 200 cc runner with similar values.

It's all about the quality of air flow and shape of the port which will reduce
turbulence that kills horsepower.

The valve and chamber sizes are fairly standard for aftermarket heads; you'll
find there isn't much variance for a similarly classed cylinder head.

If you're spending a good buck on cylinder heads, the tuner will have no problems
explaining which is best for your combo.

Old SStroker

Which 23 degree SBC heads did you have in mind that flow 300 @ .500 lift with 180cc ports?

You might get some disagreement on low-mid lift flow being the most important.

Just my $.02

02sierraz71_5.3

iTrader: (2)

Ive wandered about this "quality" of the air flow. smaller ports and more cfm =lots of velocity Ive always understood heads like this to work better with smaller durations.
Is there any way to actually measure "quality" or maybe velocity would be a better term?

Adrenaline_Z

None, it was just an example to state that a smaller port can outperform a
larger port with the correct displacement sitting under the head.

It's an open debate, so I wont disagree with you. However, I didn't intended to imply that low and mid are most important. I wanted to convey that more
average mid lift flow could work better than a head which is biased to high lift.

In my particular case, with only 0.513" at the valve, I wouldn't look much toward 0.500 to 0.700" lift flow if the valve is spending more time in the 0.100-0.500" lift range.

I would give up some flow at .500" to get more flow at 0.300 and 0.400".

It is my understanding according to Darin Morgan's, "Porting by Numbers" interview
that a port is shaped to give the best flow with the least amount of turbulence.

If the port is shaped to get better flow at .600 and .700 lift, then the air
speed might suffer at lower lifts which would hurt the average

gun5l1ng3r

I am sure with the speed of advancing technology, a fan with variable pitch blades might be programmable to simulate a piston of different diameters as well as different strokes.

As for the valve movement, you could use F1 similar technology with pneumatics and have infinitly variable valve timing and lift.

I know it won't be cheap, or easy to attain, but it is a step towards the future and eventually the perfect (GM) internal combustion motor.

treyZ28

what about variable restriction

Adrenaline_Z

I would prefer a dyno tested kit (heads, cam, intake) that is known to work well.

I think these companies that test and tune these kits are well worth a second
look, even if the wallet seems a touch lighter in the end.