Jack wrote:Me too,that's why I'm just the rider...however,the opinion on my head came from C & W and I wasn't about to doubt it.
That only sounds complex...it's not. Here's the deal..We'll work with values at 10" of test pressure.
The situation is that a claim was made that in order to increase velocity, larger valves needed to be installed.
Ok, let's look at what's involved with that and the side effects that "could" result.
On a typical race motor [even a street motor] the throat is sized to 90% of the diameter of the valve. Some use 88% for that ratio...depends on the application of the motor..
Anyway, let's say you are installing 37.5 valves in a KZ head that had 36mm valves. You'd open the throat to 37.5 * .90 = 33.75mm
Now just because you cut the seats for the larger valve does not assure you that major gaines in airflow are guaranteed. What you can count on however is that if you don't open that throat up, you are not going to see any major gaines....
Velocity come in with this respect. Let's say the head flowed 88cfm with the 36mm valve and a throat size of 32.4mm [36 * .90]. After the installation of the 37.5 valves flow increased to 94CFM - .500" lift for both.
Let's say the MCSA Minimal Cross Sectional Area [smallest section of the port] is 33mm for both situations. Let's examine some flow data.
36mm 88CFM head @.500 lift:
Calculated MCSA Velocity = 159 FPS [feet per sec]
CFM per Sq.In = 58.0
Coefficient of discharge = .474
Curtain Area = 2.127 Sq.In.
Calculated throat velocity = 165 FPS
37.5mm 94CFM head @.500" lift:
Calculated MCSA Velocity = 170 FPS
CFM per Sq.In = 56.9
Coefficient of Discharge = .485
Curtain Area = 2.220 Sq. In.
Calculated throat velocity = 163 FPS
So what happend based on the numerical values?
Since the MCSA remained the same size but flow was increased, velocity at that point went up.
Since the throat CSA was increased but the flow gaines were not progressively on par with the increase, velocity in that area went down. More CFM would have raised that up. It would take 95.5CFM to equal what it was with the 36mm valve.
CFM per Square Inch dropped. It requires 95.8CFM to equal the 36mm valves rate.
Coefficient of discharge went up. Another way to look at efficiency of the valve.
Keep in mind that calculated velocities through a port do not always correspond to pitot measured values. To know the actual velocity measurements of local points in the port, you must measure with a J Bend Pitot tube.
IF you tested at 10" of test pressure on the flowbench and you had a pitot connected to a secondary manometer, the equalization point would be 209FPS. That's the point where test pressure and pitot presssure would be the same. Generally, you want localized air speeds to NOT exceed test pressure air speeds.
To make it very simple...if you make the inside of a pipe larger and whatever flows through it does not increase in volume...then the speed of whatever is moving through the pipe got slower...[throat size example]conversely if you increase the size of the pipe and the volume moving through it increases, it must exceed the original flow volume proportionally in order for the speed of the mass to either remain the same or to increase. [refer to the re calculated required flow numbers above]
If you increase the flow through a pipe without increasing the size at some given point in the pipe then that particular localized velocity will increase [see the MCSA velocity values above]
