Discharge Coefficient's

[shawn]

Hi,
thougth i would bring this up and see what i could learn. After doing some reading and following Darin's info. on this board I'm really starting to doubt some "truths" that have been perpetuated for years on valve sizing vs. flow. So i'm going to throw out a common motor and some different ideas and see what comments "the people in the know" have.
Ok, here's my example-
B/B Chev
14.5-1 comp.
Good flowing Conventional head (Dart, ect..)
4.350 bore
4.00 stroke
Let's split the difference and give it a .38 L/D cam of a typical Super Stock grind

Now on to the numbers. I realize that the average discharge coefficent is more important, but for that sake of numbers we'll use .500 lift.
A given head flows 350 cfm@.500 lift with a 2.19 valve which translates to a .739 d/c. In order for a 2.25 valve to have the same d/c numbers, it would have to flow 360 cfm, and a 2.30 valve would have to flow 368 cfm.This is roughly a 5% increase in flow.
Now the questions.
Given all things are the same, and the flow increase kept the d/c numbers the same, and the valve/bore ratio doesn't excede 53%, will there be any change in power by increasing the valve size?
Is it possible to have the d/c #'s to high for a "short turn" type head?
I realize that runner volume,min. choke velocities all have an effect in these numbers, but am looking more for a generalization, if there is one.
Is there a "target" number for exhaust d/c?
Hope this opens up a good subject for conversation!
thanks,
shawn

[shawn]

Larry?, Darin?, thought one of you would have some brilliant insight into this for us!
Shawn

[Darin Morgan]

Larry?, Darin?, thought one of you would have some brilliant insight into this for us!
Shawn

The " Discharge Coefficient " is the measure of how efficient a given area is in regards to volume flow verses area, divided by a theoretical maximum. I use the 146 cfm/SQin and not the 137 cfm/SQin that the SAE dictated years ago just because that's what all my data has been accumulated with from day one.

Window Area = Valve diameter * Pi * lift

window area * 146 = theoretical maximum flow for that area

Take your flow and divide it by your theoretical maximum. This is the ratio of effective flow area to actual flow area

This is your discharge coefficient.

Go to my web site and down load the spread sheet to make it simple. I got tired of doing all the calcs by hand years ago. I am sure most of you are familiar with it but the spread sheet comes in handy.

http://webpages.charter.net/dsda/

[maxracesoftware]

the 146 Constant comes from

Air_Velocity_FPS = (Test_Pressure ^ .5) * 66.2

where:
FPS = feet per second
.5 = Square Root of Number
66.2 = Constant

Test Pressure in Inches of Water ....so for any different Test Pressure
other than 28" Inches you can solve for by substituting in Formula above


350.2974736 FPS = (28^.5) * 66.2

to convert to CFM_per_Square_Inch multiply by .416666667


CFM_per_Square_Inch = 350.2974736 * .416666667

145.9572806 CFM/SqInch = 350.2974736 * .416666667

Then calculate just like Darin posted

The " Discharge Coefficient " is the measure of how efficient a given area is in regards to mass flow verses area, divided by a theoretical maximum. I use the 146 cfm/SQin and not the 137 cfm/SQin that the SAE dictated years ago just because that's what all my data has been accumulated with from day one.

Window Area = Valve diameter * Pi * lift

window area * 146 = theoretical maximum flow for that area

Take your flow and divide it by your theoretical maximum. This is the ratio of effective flow area to actual flow area

This is your discharge coefficient.

i haven't yet searched for reason SAE uses 137 CFM/SqInch
but its probably a "Correction" of .9384 % percent to account somewhat
for Valve Head shape and Seat angle widths, and maybe Stem.

[Darin Morgan]

My FTP server is on the fritz so I will post that .XLS sheet when I get home.

[shawn]

thanks for the response guys.The numbers that i got
2.19 valve .739 d/c @350cfm
2.25 valve .739 d/c @360cfm
2.30 valve .739 d/c @368cfm
all at .500 lift
I plugged the valve diameter into your spreed sheet (darin) and adjusted the flow numbers until i achieved the same d/c.Your right, that spreed sheet saves a bunch of time!
The question i was trying to get at was, will all of these valve combonations make about the same power,because the d/c numbers are the same? Or will the 2.30 valve that flows 18cfm more than the 2.19 valve trump the d/c #'s?
Hope this makes sense.
thanks again,
Shawn

[maxracesoftware]

thanks for the response guys.The numbers that i got
2.19 valve .739 d/c @350cfm
2.25 valve .739 d/c @360cfm
2.30 valve .739 d/c @368cfm
all at .500 lift
I plugged the valve diameter into your spreed sheet (darin) and adjusted the flow numbers until i achieved the same d/c.Your right, that spreed sheet saves a bunch of time!
The question i was trying to get at was, will all of these valve combonations make about the same power,because the d/c numbers are the same? Or will the 2.30 valve that flows 18cfm more than the 2.19 valve trump the d/c #'s?
Hope this makes sense. --thanks again, --Shawn

"Theoretically" apply a Flow_Factor to CFM numbers

2.19 valve .739 d/c @350cfm * .257 * 8 = 719.6 hp
2.25 valve .739 d/c @360cfm * .257 * 8 = 740.2
2.30 valve .739 d/c @368cfm * .257 * 8 = 756.6

Flow_Factors can range from .200 to .320
also the Flow_Factors have been found to be directly proportional to Piston Speed.....also change according to Combustion Eff %, Ve %, wave-tuning lengths, Compression Ratio, etc....

most Early to Late OEM engines have at least .200 Factor +

(Note=>these are for 600 RPM/SEC Dyno accel test rate)
normal Hi-Perf engines = .257
NHRA Super Stock engines = around .285
ProStock & Comp = .29 to .32
the better the total engine design and combination of parts , typically the higher the Flow_Factor will be

you will turn that 1 CFM into more HP

" How can i turn that 1 CFM into more HP/Torque ??? "

[Darin Morgan]

Ok, I managed to get the think uploaded. Its at

http://webpages.charter.net/dsda/

[shawn]

Thanks for posting that again Darin. Very cool spreed sheet. Ok, now another question. If you "go with the flow" then how much importance does the d/c #'s play?If you see a 10cfm gain by going to a larger valve, but have a drop in the d/c, will it make more power then?
thanks,
shawn

[Darin Morgan]

Thanks for posting that again Darin. Very cool spreed sheet. Ok, now another question. If you "go with the flow" then how much importance does the d/c #'s play?If you see a 10cfm gain by going to a larger valve, but have a drop in the d/c, will it make more power then?
thanks,
shawn

Of course this is another one of those questions that doesn't have a pat answer. If the engine is valved correctly and the induction system is sized properly then no it wont make more power it will kill it and in a big way. If its a an under valved engine to begin with, it will probably have little effect.

[Rick360]

Thanks for all the great info. Which has led me to more questions.

Is the "best" disch coef the highest that you can get it? Can it be too high causing something similar to the sonic choke at the ssr? Is the average over the range .200"- max lift most important or are some valve lift ranges more important?

What differences might be expected in other engine components (cam, manifold etc) between two engines one with a good D.C. head and one with a great D.C. head? I would think a better DC would like a little more int. duration since the air at the valve is moving faster. If the DC is not as good the intake charge should reverse directions sooner at the end of the intake cycle.

Thanks,
Rick

[Darin Morgan]

Thanks for all the great info. Which has led me to more questions.

Is the "best" disch coef the highest that you can get it? Can it be too high causing something similar to the sonic choke at the ssr? Is the average over the range .200"- max lift most important or are some valve lift ranges more important?

What differences might be expected in other engine components (cam, manifold etc) between two engines one with a good D.C. head and one with a great D.C. head? I would think a better DC would like a little more int. duration since the air at the valve is moving faster. If the DC is not as good the intake charge should reverse directions sooner at the end of the intake cycle.

Thanks,
Rick

(1) Is the "best" CD the highest that you can get it? YES

(2) Can it be too high causing something to sonic choke at the SSR? Yes it can choke but NO it cant Sonic choke. There is no such thing as Sonic choking in an intake tract. They operate at a maximum of about.5 to .55 mach 600+ft/sec, which is far below Mach. If the mean velocity in the intake tract where to exceed mach .55 the power would drop off dramatically. Sonic only occurs on the exhaust side and usually at lower lifts in super charged engines.

(3) The most important lift range is form start to finish. Attempting to place emphasis on one particular part of the over all lift curve is a mistake. Can you give up about little on the bottom of the curve to make it flow more up top? Yes but not much! You want the highest Discharge Coefficient you can get. Period. I think you are looking at this too much from a flow standpoint and not an over all efficiency standpoint. You want the window area exposed to the cylinder as efficient as possible. Speaking hypothetically, if your CD is higher, you could use a cam with less duration not more. Think about it.

As far as reverse flow and the reversion characteristics of a particular design during overlap or at the end of the intake cycle, well, that's a whole new thread within itself. I don't want to be vague about this but it's a pretty involved subject in its own right. I measure cross flow by pulling through the intake port and out the exhaust port on a chamber with a simulated dome and piston and see how much it can flow at each lift increment. I measure reverse flow (out the intake port) by pressurizing the exhaust port and reversing the procedure.

I would like the others to weigh in on this one.

[shawn]

I have a question, or a comment on reversion in a high speed engine. Is there really enough "time" for reversion to actually occur when running at speed?My impression was that intertia and wave/pulse tuning would all but eliminate almost anything but a "stall" at one point. I haven't investigated it enough, but what do you think?
Shawn

[Darin Morgan]

I have a question, or a comment on reversion in a high speed engine. Is there really enough "time" for reversion to actually occur when running at speed?My impression was that intertia and wave/pulse tuning would all but eliminate almost anything but a "stall" at one point. I haven't investigated it enough, but what do you think?
Shawn

The compression and rarefaction waves present in the intake and exhaust tracts during overlap and other parts of the 4 stroke cycle are tuned in such a way as too accent the gas exchange process and increase VE ( trap mixture) BUT they only do so over a given amount of time and over a given range of rpm. That range of rpm in which the positive ( compression) and the negative ( rarefaction) waves work together is about 1500 to 2000 rpm. The reason for minimizing reverse flows is to not only broaden the range of time the waves are usable but to also manipulate the tuning factors of camming and manifolding to increase VE. Usually when these Antireversion steps are taken an instant increase in both low end torque ( below peak TQ ) and top end power ( above peak power) can be seen even though no other steps have yet been taken to optimize the situation. Lets say for instance you minimize the reverse flow of the intake port by some means and not effect the inlet flow. You now have an engine that will hang on a little longer past peak power when the waves are decaying in amplitude and are decreasing in there ability to feed the cylinder because you will revert less mixture back up the intake tract.

You asked the question,"Is there really enough "time" for reversion to actually occur when running at speed?

Yes there is but Its not as much a TIME factor more so than a factor of multiple dynamic events starting to work against you instead of for you! By trying to prevent reverse flows, your just trying to delay the inevitable

[gas]

Window Area = Valve diameter * Pi * lift

window area * 146 = theoretical maximum flow for that area

In regards to (window) area, why would the circumference be used to calculate area, in lieu of area formula: Pi * Radius (squared) ?