Pitot Tubes for velocity measurement - Has anyone made their own?

[Greg]

I was thinking of making up some pitot tubes for port velocity testing. Has anyone made their own, and what is involved?
Iwant to make a straight and U shaped one but am not sure of the design or the math involved.
I have a chart here somewhere I think from the Superflow manual that gives a conversion between pressure and air speed but are those conversions relative to the diameter of the pitot tube?
After looking at the price to buy them I'm now wondering if there is more to them than meets the eye as they are pretty expensive.

[maxracesoftware]

use a Straight or long 90 degree Pitot for Exhaust Ports

use a "J" or "U" 180 degree Pitot for Intakes

Pitot_Velocity_FPS = (Pitot_Pressure ^ .5) * 66.2

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Pitot_Velocity_FPS= velocity of air in feet per second

Pitot_Pressure = in inches of water

^.5 = Square Root of a number

66.2 = Constant

in the long run it will be cheaper and easier just buying a Pitot Tube from Dwyer

[SWB]

IT depends on what you are doing with them.

The type Larry describes are true pitot-static types which measure the difference between the static pressure and the impact pressure when used correctly. They work the same way a pitot-style bench works essentially. One side of a manometer is hooked to static and the other to the impact (actually total pressure) side of the pitot tube.

I have made probes to test static pressure and total pressure only. You could also use a standard type of pitot tube to measure both these things independently, providing you hook them to your manometer properly.

Static pressure is simply the "atmospheric" pressure anywhere inside the port. This type of probe would ideally have it's openings 90 degrees to the movement of airflow, but most of mine were simply an open tube facing away from the direction of flow. The tube is hooked to one side of a manometer and the other side is left open to the atmosphere.

I find these very useful, they won't tell you too much about actual velocity, but they will help you find the localized high and low pressure areas. As a general rule for modifications, you'd want to increase area in the low pressure areas and decrease area in the high pressure areas. I.E. low pressure = moving air, high pressure = stagnant air and making modifications to areas with stagnant air will not likely produce an increase in performance.

Total pressure does not work quite the same way, total pressure is when you have a tube with an opening which faces the flow of air. It will measure both static and impact pressure at the same time because you only have one source. The tube is hooked up to a manometer much the same way as above, leaving one side open to the atmosphere.

I don't really care for this type of probe, but it could be that I don't fully understand it either. I once tested an exhaust port using this type of probe and found that the pressure on the manometer would reverse just after the short side turn where presumably, the flow was reversing and forming a back-eddy. Otherwise I took higher pressure readings to mean greater flow and lower pressure readings to mean less flow. In this case, you should really be using a true pitot-static tube as described previously.

My best advice for you or anyone, is to make something quickly to play with. You cannot live without these things! If you are not using even the most simple type, I firmly believe you are wasting your time. Also be aware that bolting a manifold on the head will affect the flow patterns in your inlet ports and the same holds true for the exhaust side of the engine. That said though, you still need one!

SWB

[84-1074663779]

SWB's comments are extremely important with regard to pressure measurements in ANY type of flow bench.

Far too many people with orifice type benches think you can just run a manometer hose into any point before and after the orifice and then just read the pressure differential straight off the manometer. This also applies to measuring test pressure.

Measuring the true static pressure of moving air is never a simple thing to do. To convince yourself of this, cut off the end of a long straight probe at an angle, and introduce it into a fast air-stream. Rotate the probe and see the differences in pressure reading you get.

Building a proper pitot static tube is very easy and well worth the trouble for any static pressure measurements in moving air-streams.

[SBC]

SWB

Does a low pressure cause a bigger number on a well type manometer?

Thanks SBC

[SWB]

If you're testing an inlet port with a straight tube, one side hooked to one leg of a U-shaped manometer, and the other left open to atmosphere, you will get the greatest "number" out of the manometer at the lowest pressure area. That is, the water in the side connected to the straight tube, probe will rise the highest when placed in that area of low pressure.

So yes then, a well type manometer with the tube hooked to the scale side, will regester the highest at the lowest pressure area.

Hopefully that makes sense?

SWB

[bruce]

Ok to follow-up on this thread . . . I've had time the past day or two to actually sit down at the flowbench and do some of my own R&D work on my engine/head. I've been playing with my velocity probe (straight tube connected to a digital manometer (a u-tube)) Now I'm looking at flow over the valve seat measuring velocity to compare flow of a head and the width of the channel cut in the head (flathead engines remember). I see numbers that range from 8-12" so my higher number is an area of faster velocity, lower pressure due to the fast airflow over the tip of the probe which in turn creates the lower pressure at the tip? Whats interesting is I see this higher number right behind the valve stem area, is this due to the airflow coming around the stem and speeding up? Any discussion on my velocity probe setup and th enumbers I am seeing?

I plan on spending the better part of today playing with this head, I grind a little flow a little, grind a little, flow a little . . . been enlightening thats for sure . . . just wish I knew what it all meant . . . sure that will come in time.

I'd post pics but due to the sensative nature of my testing and the lack of flathead porters I'm leary of "sharing" my efforts.

[maxracesoftware]

I'd post pics but due to the sensative nature of my testing and the lack of flathead porters I'm leary of "sharing" my efforts.
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i know what you mean

Also you're not going to findout what you need to know with a straight probe in an intake port, you need a 180 deg Pitot velocity probe to measure port velocity !

Don't worry about the Pitot Probe's tube itself affecting or disturbing air flow !

Secret=> no matter what Test Pressure you are flowing at ...if your Pitot Probe velocity pressure is equal to or greater than your test pressure ....then that Cylinder Head will not in all probability make the HP you thought it was going to make according to the Flow numbers"

any Pitot Pressure much greater than your Test Pressure ..and HP can drop like a rock..you might not see much difference ... for example, if you are Flow testing at 28 inches and your Pitot Probe shows 28 inches...you will just start to see these HP Loss effects, or then again maybe as soon as 25 inches of Pitot press when Flow Testing at 28 inches.

if your Pitot Probe velocity is correct then the following Ratios will work ...if Pitot Pressure is higher than Test pressure then the following Ratio Factors will not seem to work properly ;

Peak_HP = Flow_CFM@28" * Factor * Number_of_Cylinders

257 Factor = for beginning engine builders and engines near 10.0:1 Comp Ratio

.285 Factor = would be for Professional engine builders with wet sump pans, lightweight rotating assemblies, low tension great sealing rings, deep oil pans, etc.
excellent use of inertia/wave tuning with 9.5 to 11.5:1 Comp Ratios or
11.5 to 13.0:1 CR ranges without fully utilizing inertia/wave tuning effects

.300 to .310 Factor = Current ProStock Technology with dry sump, unlimited carburetion, Hi Comp Ratio, ultra lightweight rotating assembly, etc, max use of inertia/wave tuning, etc, 14:1 to 17:1 Comp Ratios
(usually no better than .3200 efficiency or no worse than .2980 eff %)

One more Secret=> when checking Port Velocity with 180 deg Pitot Probe...you must maintain Test Pressure !!!
if Pitot tube itself cause test pressure changes, you MUST MAINTAIN and ADJUST test pressure back ...then take your Pitot Probe velocity pressure reading.

An intake port that has too much velocity can be as bad or worse than a port that has too low port velocity !

[sheppard00]

Ok I know this is kinda old but I just found it. I have a 180 U type pitot tube and a msd style bench... do I need to make a seperate U type manometer for the pitot tube or can I use the inclined manometer... also How do I read it if I can use it

[larrycavan]

[sheppard00]

so i make another manometer and then multiply the square root of the in of h2o reading by 66.2?

say it reads 25 in h2o

fps = (25^.5)66.2
fps = 5 x 66.2
fps= 331

so is this correct?

I read somwhere that I should be looking for between 280 and 300 is that correct?

[larrycavan]

[color=#000000]Here's another method that uses port area and flowbench readings without any pitot tube. This method came to me from Harrold Bettes at Superflow. Thank you Harrold.

Flowbench Data and Port Area Measurements Provide Local Velocity

If the port area was expressed in square feet (ft2), then dividing the flow value in CFM by the area in ft2 (square feet) of the port, the result is FPM (feet per minute). Dividing FPM by 60 yields FPS (feet per second). The specific advantage of this method is that it is not invasive (puts nothing in the flowpath) to the port and is a quick reference.

Port Velocity Using Flowbench Data and Port Area Flowbench data = at 25

[sheppard00]

thanks that should be helpfull also

[86rocco]

I made pitot tube type velocity probe earlier this years, you can see pictures and details in . I still haven't had a chance to do controlled rigorous testing of it but calculations, backed up by crude preliminary tests, indicate that a good sized vertical manometer will be required, the airflow from a simple shop vac is capable of generating velocity pressures in neighbourhood of 30" W.C.

[Unkl Ian]

How many inches of vacuum was your flow bench
pulling when you saw 30" with the velocity probe ?

Thanks

Ian