"An airplane’s barometric altimeter operates by measuring the pressure at a static port on the surface of its fuselage."

The way i understan it the pressure comes in at the Pitot Tube and the Static Port is the vent.
So IMO no it's not the same.

 

Gene, I am not ignoring my own thread, but I'm in the middle of 12hr day work week and the wife's truck is acting up. I will have to fix that first, then get back to this. I'm sure everyone can appreciate my position here.

 

Here's 6 pics (3 in each post) that I've tried to make fairly self explanatory, but of course just to confuse things, I'll explain them anyway!

Pic #1 gives VAF=Volume Air Flow, ft3/min=cfm as a function of the CSA=Cross Section Area, ft2=sf of the tube, and the AFV=Air Flow Velocity, mph. You can multiply mph x 88 to get ft/min=fpm, which I've used before in other posts. Since VAF=CSA*AFV, the tubes with a larger CSA flow more CFM at a lower AFV.

The first red curve is the inlet tube from my new gapping Genedad hole to my AIS air box. The blue curve is for Tenn's 4" tube where he's taking static pressure measurements. The lime curve is the beginning of the tapered rubber boot, and the green curve is the end of the boot that attaches to the turbo inlet.

Pic #1 also gives the so called "velocity pressure" or RAM dynamic pressure, Pd as a function of air temp and air flow velocity in mph. These curves don't have anything to do with the different size tubes per se, and they give the stagnation pressure you'd feel if you stick the flat part of your hand out the window at a given speed.

Bernoulli's Principle states that for an ideal fluid, with no work being performed on the fluid, a decrease in pressure is consistent with an increase in velocity or a change in the fluid's gravitational potential energy. It turns out that air flows with velocities well less than the speed of sound are reasonable approximations for an ideal fluid. The part about no work being performed on the fluid is never really satisfied, but Bernoulli's Principle is frequently used anyway because it gives the easiest way to at least come up with some rough solutions for problems that would otherwise require complicated computer models.

One way of viewing Bernoulli's Principle is to consider the sum of the static pressure, Ps, and the dynamic pressure, Pd, which give the total pressure, Pt. Then the part about the total energy being constant as air flows down a tube with a varying CSA, and therefore varying AFV, is the same as saying that Pt is constant. So if Pd increases, then Ps decreases so as to keep Pt a constant.

Assume that the relatively still air under the hood has an initial Pd=0, and a Ps=14.7 psi, before the air starts moving toward the air filter. The following is somewhat a point of order, but while many might say the air is being sucked into the filter, it's actually being pushed into the filter by the outside ambient air pressure, to fill the relatively empty space at the turbo inlet after the compressor wheel slings the air molecules there into the compressor housing. The physics is that the ambient air pressure compresses and pushes the air molecules through the filter element, and then down the inlet tube to the compressor wheel, where they're slung into the compressor housing, as more follow behind them to keep trying to fill the void that's continuously being created by the compressor wheel.

It seems to me that the compressor wheel is the prime mover here, and it's definitely doing work on the air in direct violation of one the basic assumptions of Bernoulli's Principle! But ignoring that potential complication, here's how to use the 2 sets of curves in pic #1 to correct the raw data that Tenn is collecting. For example, start on the Y axis at 300 cfm, and move to the right to the blue line, and then down towards the X axis until you intersect the Pd curves near 40 mph. So for 300 cfm, a correction factor of about 0.5" H20 is pretty close independent of temp. At 600 cfm the AFV is 80 mph, and the correction is between about 2.3" for hot air, and about 3.3" for colder air.

If you look all the way over to the green line, you see why it's not a good idea to make the measurements right at the turbo inlet, because this just complicates the correction, and now knowing the exact temp at the turbo inlet is necessary. However, taking data there to check for a "vortex of doom" does have merit, it's just not a good place for the data I'm most interested in.

If you look at the green curve for about a 600 cfm, you see a Pd of about 10" H20, and if you consider that a vacuum (which it's really not) you might think that's not a good thing to put in front of a "compressor" wheel, and that having the tapered boot feeding the turbo isn't a good design. Remember that Pt is constant all along the tube, and a large Pd is exactly what you want because that means the random pushing pressure on the walls of the tube has been traded for increased RAM air pressure going straight (no TAG commercial intended) into the turbo inlet, and that's a good thing to have.

Pic #2 is the same as #1 except it's for a 5K ft altitude. Currently I'm at almost 7K ft, and as I've reported before, with my new Genedad mod I now measure a Pd of about 1.6" at 70 mph, just like the model in Pic #2 predicts. The reason I was touting the increase in Pd after the Genedad mod wasn't because the extra 0.5" or so increase in RAM pressure does all that much for performance (but every little bit helps, even if just a little bit), but it was because it showed that the grill strut I removed was definitely impeding the flow of the cooler ambient air into the inlet tube. As I'll show, the temp of the inlet air is much more important than the slight increase in RAM pressure.

Pic #3 is a summary table of the inputs to my PSD model that was used to generate some of the outputs shown in pics # 4 to #6.

 

Pic #4 gives CFM vs MAF for various turbo inlet temps, and also overlays some CFM and RWHP data from my early 99 PSD model. If you check the pic #3 table, you'll see that this run was for a 110F turbo inlet temp, and all the cfm data points from the model fall between the gold 80F and the blue 120F lines.

The shape of the HP curve is because the model runs from 20% throttle to 100% as the boost builds from 0 to 20 psi, and then stays at WOT from 20 to 30 psi. At the top end of the MAF scale for each RPM, you see that for the WOT part, the HP follows the MAF line, showing that it's MAF that determines and limits maximum HP.

Look at the region where the 100 CFM, 50 HP, and 7 lb/min MAF all come together. This represents a typical cruising around town empty situation, and as you see the cfm required varies from about 80 cfm for the coldest air to about 120 cfm for the hottest. So here, it would be hard to feel a SOP difference between hotter inlet air from an under the hood filter and the colder inlet air from and air box with a Zoodad. But even though that 40 cfm spread might not seem like much, that's a 50% increase.

Now look at 150 HP and 25 lb/min MAF (typical for me towing), now the cfm requirement increases from about 300 cfm to about 440 cfm as the inlet temp increases. Now that under the hood filter can make that poor turbo flow 100 cfm more air (a 33% increase) to make the required 150 HP. I say again, an under the hood filter is not a good option for towing hot and heavy for long periods of time.

Pics #5 & 6 are more data from the same run, and again that HP curve looks funny because the throttle is varying from 20% at BP=0 to 100% at BP=20 psi, and then held at 100% from BP=20 to 30 psi. When Tenn gets data, and I figure out how to correct it and convert it to cfm,I can then plot his CFM, BP, RPM points on this kind of display, and directly compare the air flow in the early and later engines.

 

The truth spoken so simply!

 

I will continue to insist that the jar analogy doesn't hold water in this case (no pun intended, honestly), unless your jar is perforated top to bottom and all around the circumferential surface area. The pool filter, on the other hand, that Joe was referring to is a comparable analogy due to the similarity in designed flow characteristics.

I do not disagree with your statements (or other previous statements by others) that pressure measurements anywhere inside a completely closed "jar" environment will all be the same (top, bottom, end, side, etc.). However, to compare pressure measurements on the other side of the jar's impervious wall material with the premeable air filter's wall is simply comparing apples to oranges. It is not just the shape of the two containers that must be compared, but also the materials of construction and overall flow characteristics.

 

The road to my enigmas all started here.... I looked on the internet for explanations, and the following quote says it in words as well as any...

"Bernoulli's Principle states that as the speed of a moving fluid increases, the pressure within the fluid decreases. The fluid can be either a liquid or a gas. For Bernoulli's Principle to apply, the fluid is assumed to have these qualities: 1) the fluid flows smoothly, 2) the fluid flows without any swirls (which are called "eddies"), 3) the fluid flows everywhere through the pipe (which means there is no "flow separation"), 4) the fluid has the same density everywhere (it is "incompressible" like water).

As the fluid passes through a pipe that narrows or widens, the velocity and pressure of the fluid vary. As the pipe narrows, the fluid flows more quickly. Surprisingly, Bernoulli's Principle tells us that as the fluid flows more quickly through the narrow sections, the pressure actually decreases rather than increases!"

Trying to dig deeper than this I encountered mostly "flute music", which is what I call a bunch of fancy looking equations that are offered up with a few "gobble de gook" words sprinkled over them. If you've ever seen sheet music written for a flute, you know what I mean.

I never consider a problem to be truly understood unless I can put it into basic enough terms to state the solution in words using as few basic building block equations as possible. I came up with the calculation using the trade off between the random molecular velocities Vr, which are responsible for producing Ps, and their translational velocities, Vt, down the tube, which produce Pd, and based my calculation on keeping the sum of Ps and Pd a constant Pt. My answer seems to be correct, and I threw in the swarm of bees analogy to help visualize the molecules buzzing around aimlessly at about 950 mph vs their relatively slow 60 mph translational velocity down the tube. To recap, (60/950)^2(14.7)=0.0586 psi=1.6"H20, which I think is the correct value for the reduction in Ps under the assumed conditions.

In measuring Ps, it's important to eliminate the effect air movement by having the static port such that no air blows directly into it, and for measuring Pd, you need to determine the full effect of the velocity pressure by having the air stream blow directly into a probe. One way of doing this is to employ a "total pressure pick-up" probe that's aligned with the flow so that is receives the effects of both static pressure, Ps, and velocity pressure, Pd, and gives a reading of the total, Pt=Ps+Pd. Then if you hook the Pt output to one port of a differential gauge, and the other gauge port to a static port giving Ps, you get the difference Pd.

This is the basic setup I'm using now. My AIS air box inlet tube is the Pt probe which is hooked to one port using a hose, and the other port is vented in the cab to give static pressure. Therefore, when I'm at 75 mph and I shift to neutral and coast, the engine is flowing very little air at idle, and I'm basically measuring the velocity pressure, Pd, which is about 1.6" H20 in agreement with my bee swarm calculation.

In physics, if a method works for one problem, it should also work for all others of the same type, and if even one example can be found where the method doesn't work, it should be viewed as not being a correct method, even if it sometimes seems to give the correct answer. So, I started using the swarm of bees method for other problems that seem to involve Bernoulli's Principle, and am coming up with what seems to be enigmas!

Part of my confusion has to do with reference frames. If you place a total pick up probe in Tenn's tube you can sense the Pd of about 1.6" H20, and the Ps he measures should be reduced by this amount from the nominal Ps=14.7 psi in the engine compartment so as to keep Pt a constant. But what if you're traveling down the tube with the air, now Pd=0, which implies that Ps is still 14.7 psi inside the tube! Does this mean that an observer traveling in a tiny balloon with the air flow measures a Ps=14.7 psi in the tube, while Tenn is measuring a 1.6" H20 lower value for the same air mass in the tube?

I start getting a headache when I try to apply this kind of logic to some of the food for thought questions I've proposed, but here's one more to complete the set. You've still got this perfect 60 mph wind, but now Tenn turns around and goes 60 mph with the wind so that when he rolls down the window the outside air seems perfectly still even though his Speedo is reading 60 mph, so what is his Ps reading now?

It turns out that wind tunnel testing with colored dye shows that the air molecules that flow over the top vs the bottom of wings don't arrive at the trailing edge at the same time. The basic problem I'm having with air pushing up from underneath a wing to support it, is that the wing pushes back down against the air mass, which then pushes down on the air mass under it, etc... until the last air mass is finally supported by the ground. As an ex-pilot I'm familiar with this "ground effect", but it only applies for very low altitudes.

I understand how drag produces a lifting force, just like when you stick your hand out the window at a 45 deg angle and the air stream pushes your arm up. But this is not very efficient, because you have to provide the HP=F*V to overcome 1 lb of drag F for every 1 lb of lifting force. Wings have a L/D ratio of at least 3 or more.

The pic below shows wing vortices, which most consider to be a nuisance, but the guy on this site claims that these vortices are what's actually keeping the airplane aloft by directing a steady momentum toward the ground, kind of like a rocket motor. Here's a link to the site....
http://amasci.com:80/wing/whyhard.html

This is a quote from his site, I don't claim to understand it, but he does seem to address my original concern that the wing pushing back against the air only works if the ground is near enough to support the air so it can push back against the wing.

"The "exhaust" below a wing made visible:

What does this mean? It means that any downwards momentum stored within the vortex-pair must remain within the vortex-pair as it moves. In other words, the vortex-pair acts like a huge volume of downwards-moving mass, and it carries momentum as it goes. Yet as the vortex-pair moves downwards, it pushes the surrounding air upwards. Won't the total momentum be zero? No, because the vortex pair takes back all the momentum it has given to the surrounding air. It only makes a temporary loan, so that the surrounding air can be moved from the bottom of the vortex pair to the top (and so the vortex pair can move downwards to take its place.)

Forming a downwards-moving vortex-pair is like firing a bullet downwards: your gun will experience a "kick" of reaction force. Because the wings of an airplane form such a vortex, they must experience an upwards kick. Yet they fling no air downwards! They only fling PURE MOMENTUM downwards. The vortex-pair is a traveling pattern of pure downwards momentum which carries zero mass as it moves. Weeeeeeird!"

 

I stole this pic from DCSpecial to show where AFE mounts their restriction gauge, as it's the same location recommended by Donaldson, and the one Tenn is now using for his measurements.

 

Gene, how much air will the engine flow reving in park? I have a fool proof method to measure the air flow, but I won't be able to drive the truck. I know you probably have answered this question in one of your posts, but to be honest I don't always read all of them. I didn't know I had ADD until I started reading your posts.

 

Ask and you shall receive. However, I don't think it's a good idea to be honest by telling me you don't read all my posts! Either read them (they're like medicine, and good for you), or at least lie to make me feel better, and just say you read it but forgot, and could I please tell you again.

According to my shop manual, you can run the engine at WOT in park for a minute or so, which is the way they do a crankcase pressure test to measure blow by. You can even put it in any gear, and run at WOT for 5 seconds or so to measure stall RPM to test the tranny. I've never done either one of these to my engine, but if it's warmed up first it shouldn't hurt it.

The graph below should at least give you a ballpark estimate. This run is based on input parameters that occur when towing, and is for my early 99. Without loading the engine, you'll probably be between the bottom blue curve and the 6 psi green one. When you get time, post what you're up to, as I'm very interested. I've given up trying to figure out how airplanes fly, which is ok, as I never intend to get in one of those darn things again anyway!

 

I'm trying to figure out how all of this pertains to filter restriction.. If you stick a pressure guage in an open window as opposted to where it exits....All we seem to be talking about ,is the surface of the area ,of the intake...If you take a vacuum hose while on and put your hand over the end , do you think it makes a difference where you put the restriction sensor?....As far as filter collapse. there would have to be a reason for it to collapse ,,Structurally (wet) or from too much dirt...


The fact that air pressure readings change , depending on where you put the sender is a simple matter,,, that is the way it is,

BTW... where do they put the Pitot on an airplane( does it matter) ... the air flow past it will be the same...It is all comming from the same open window...No pressure until after it enters...

If you want to know what an air filter is doing(based on all new ones)..Measure what is coming out ..either through HP or air volume..Much easier than trying to decide where to put the guage...

I wish I had the smarts that some of you have , but I think this gotten a little off point..Testing an air filter should be a simple matter .Does it seal properly , does it filter out the dirt.. how long does it last, how much does it cost?....JMO....

 

Just speaking for myself, I could give a rat's hiney where a restrictor gauge goes. I set out to do this testing IOT provide Gene with some flow #s of my 99.5 and up model truck so he could hopefully refine his HP prediction model for my truck. That, and to see if I could possibly detect a vortex or any turbulence. I also wanted to look at the vacuum across the rpm range as I built boost getting on it really hard IOT evaluate my 6637.

Well, alas, I thought I had repaired the wife's truck but it is stumbling again. I got to listen to her drive off in my truck today to pick up our son. Man that thing sounds sweet. Brought a tear to my eye, then I set about to mowing the yard and weed eating. First time in 5 weeks. No rain.

So I am nowhere right now, and have to fix her truck still. Tested all 8 coils, replaced plugs, ran fine for 1/2 day and it is back. So that will take up some tomorrow for sure. Filter testing will be on hold until her truck is repaired. I CANNOT have her driving mine everyday.

 

The first chart is for the DONALDSON B085011 version of the 6637. The three data points from the DONALDSON web site for this filter are indicated, and I plotted a best fit curve through them. This is the curve I'll initially be using to get CFM estimates from Tenn's measurements of Inches H20 restriction vs BP & RPM.

Please note this isn't intended to be a WHICH FILTER IS BEST discussion! I'm giving this second chart with the AIS on it because I'm using this blue calibration curve for my CFM estimates for an early 99 to compare with Tenn's CFM estimates for a 99.5, and the intent is to compare CFM's of the two ENGINES at the same values of BP and RPM to see how much better the 99.5 engine flows, and to use the data in my engine model. It's my opinion that the major differences in the total air flow path due to the differences in turbo size, IC restriction, intake manifold size, 2" vs 3" boots, and the size of the plenums will definitely swamp out any small differences in CFM due to using different air filters. Yes, it would certainly simplify my life if Tenn would eventually wind up with an AIS to test so I can exactly compare apples with apples, but for now I'm forced to work the problem with two different air filters.

The second chart shows 3 filters and the theoretical curve for a reference pipe. The curve for the 3.5" x 30' length of straight pipe (green) demonstrates the theoretical curve for air flow through a straight pipe, where the CFM increases as the square root of the restriction, or said another way, the restriction increases as the square of the CFM. For the 7.3L Stock and 7.3L AIS filter, I used the data points I've been able to find so far, and plotted a best fit curve through them, which tends to follow the theoretical curve for air flow through a straight pipe.

One can see that the DONALDSON data points at 6" and 8" also tend to follow this straight pipe theoretical curve, and if that's all I had to go on for this filter, my red curve for this filter would've paralleled the theoretical curve from 6" on down to 0". But I'm stuck with the fact that the DONALDSON site gives only a CFM of 280 at 4" as opposed to the CFM of about 350 that's predicted by theory!

The above suggests two possibilities. 1) The B085011 filter flows a higher CFM in the range of 0" to 6" restriction than is indicated by the DONALDSON site data, that is, the DONALDSON data on their own filter is WRONG, at least concerning the data point at 4" restriction. 2) The DONALDSON site data is correct, and other filters like the AIS don't flow as well in this range as is predicted by theory. That's a possibility because I don't have any AIS data points in this range, but I'm trying to get some. However, the one data point on the stock filter suggests that it follows the theoretical curve in this region, and my limited CFM measurements using my restriction gauge with my AIS seem to indicate to me that my blue curve for the AIS is about right.

I hope after I get some data from Tenn, I can figure out from his BP, RPM, and previous dyno run, about how much HP he was producing when he measured various restrictions in the range of 0" to 8", and from that estimate his CFM, and then compare that to the red curve. I've also put out a feeler to get a ballpark cost estimate for flow testing a B085011 filter, which would provide the most accurate calibration curve.