Let's get back on track here...

Back pressure from that tiny turbine wheel is probably tied to your boost issue.

 

at idle my BOV is closed, compressor wheel spins and map read 45 kPa

go figure

 

That makes sense. The spring is holding the bov closed, if you have a strong enough spring it can take more vacuum than that (you have about 8 psi of vacuum) to open it, may take 10 or 12 PSI of vacuum, depending on the spring in your bov. You will have some positive pressure between your turbo, and the throttle blade, probably less than a lb or two, but since your throttle is closed, separating the 2 chambers, you have vacuum in your manifold where your map is reading.

 

My dear doug, you are taking a vacuum reading from behind the throttle body.

The TB acts like a restriction. Imagine if there was no TB, what would happen? The restriction of the TB is what creates an engine vacuum behind it- that is where your map is installed.

To measure pressure between the compressor wheel and throttle body you would need to install a MAP sensor in this region of pipe. Furthermore, it would need to be sensitive to the 0.001psi range, or at least 0.01psi I would imagine, since the pressure created there is so slight.

If the bypass is closed it is simply too tight given the vacuum supplied, or perhaps it is not that kind of bypass. Some are not designed to hang open, like a factory unit, typically high boost pressure bypass valves are more "tight" so they are more likely to remain shut at idle. A factory bypass is traditionally a "push type" unit, where not only engine vacuum is allowed to pull it open, but the pressure from below the valve also helps a little bit. My favorite aftermarket, high boost bypass used to be the HKS "race", but they are old fashioned units and there is a much better style available to us these days (like a new TIAL style, seems to respond very nicely)

 

yes and I would expect its probably like 1.xx psi before the intercooler and 0.xx psi after the intercooler.

 

You are on the right track, lets discuss a couple ideas regarding this so called, pressure gradient you speak of.

First, the space between the compressor blade and the housing generally goes up with increasing compressor wheel size. Imagine an enormous compressor the size of a house, you might be able to almost fit a finger between the blades of the wheel and the housing. This operating clearance is determined by the intended application of the compressor and it's construction materials, like anything else on a vehicle. The reason this example is important is because it shows us that as a compressor becomes smaller and smaller the spaces between it's parts also tends to diminish, thus less and less air is able to squeeze back out as the wheel spins, which is one way that air gets out of the plumbing when the bypass is shut at idle (by flowing backwards through the compressor somehow), when the compressor is flowing more than the engine can accept, and this action slows down the compressor wheel, thus it is an anti-performance activity. ideally we would have the highest wheel speed, as close to the desired full-boost flow-rate speed as we could get for instant boost-performance. As wheel size decreases, the wheel also becomes lighter and will spin faster for a given a quantity/temp of exhaust gas. The size of the engine and the turbine also plays a role in the compressor wheel speed, and since we are specifically discussing idle situations, the idle speed itself will play a role since that plays a part in exhaust gas flow total.

We see there are many factors which influence compressor wheel speed at idle, let us now imagine a very tiny turbo, one the size of a cell phone perhaps, or even smaller, on a large V8 engine. For this example, you must imagine that it would spin very fast -> fast enough to provide boost pressure to the plumbing, even at idle, lets guess 4-5psi of pressure or more. The flow rate of such a tiny turbo would be high enough at idle situation to provide more than enough air to idle the engine, plus extra, and still be on it's compressor map (surely 10 to 20 horsepower is not too much to ask from such a turbo, and it would be enough air to idle a V8 of almost any make). These examples help remind us that we can create any situation we wish for depending on the parts we choose, and overall assumptions about pressure gradients should be avoided.

Now let us look at the pressure gradient itself in the plumbing, from the compressor to the throttle body. If we could measure along the way, we would indeed see that it falls off as we move away from the compressor wheel. If the pipe leading from the compressor wheel could stretch out to infinity it would wind up at whatever the pressure is at the end of the pipe (at infinity) which for our purposes, must be atmospheric pressure. The intercooler itself is simply a pipe with a given volume; it helps to imagine the intercooler as a large diameter pipe. In other words, if we take a 10'(feet) long 3" pipe, and cut 8'(feet) of that pipe out to replace it with a 1'(foot) long section instead, but that 1' long section has the exact same volume as 8'(feet) of our removed 3" diameter pipe, the pressure gradient would be nearly identical, even though we now have 7'(feet) less pipe. So when we measure a significant drop in pressure after any intercooler, what we are really doing is measuring a drop in pressure due to a given volume, and in some cases, restriction (depending upon the core design) of a section of pipe. At idle speeds, the airflow volume total is very low, compared to the ultimate capacity of an intercooler, even for small engines- so restriction factors are negligible, it is mainly the volume we consider for idle scenarios when discussing pressure gradients.

If you would like a picture I can make one, depending on how much you want to discuss this subject.

 

I appreciate that you edited out some of the belittling down speak. :thumbsup:

There's no doubt you have a pretty good grasp on how/why many things work the way they do. It's apparent to me, that our minds tick much the same way. However, I gotta say, I sometimes get lost in the way it's written, even when I'm in agreement with the root of what's being said. The analogies are often incomplete and/or misleading. Clouding the point they're meant to clarify.

It's a shame, because you have a lot to offer the community. Yet, your angle of attack seems to be a put off to most, and your point is rarely taken for the full value that it's truly worth. But, who am I to say? I could probably point that same finger at myself at times. Carry on. Do it however you do it.


Anyway... on to my main point of posting. The paragraph I quoted isn't accurate. Pressure drop-off through a length of pipe is a function of friction, not volume. Friction between the air (or any other fluid in a pipe) and the surface it's in contact with. And, the friction increases with the fluids velocity.

The intercooler statement Doug made is incorrect. It would be extremely unlikely that, at idle, there would be any significant pressure loss across the intercooler. There's just not enough velocity to incur friction losses. While, yes, technically speaking they exist. You just wouldn't be able to measure it.

It's a myth that pressure loss across an intercooler is due to the air being cooled. In a closed circuit system filled with air, yes, it would be true that cooling it would reduce pressure. However, we're talking about an open circuit system. All pressure loss across an intercooler is a function of friction. If you added a second intercooler, you'll likely have an even cooler charge. Yet, the loss will decrease due to cutting the velocity through each intercooler in half. It's friction, not volume or temperature, that causes pressure loss. There's a lot of surface area in an intercooler. Velocity... Surface area... Friction... Resistance to flow... Loss of pressure.





Side note to the Mod's: we've got at least three "**** you" imoji's, an "O" face bouncing on a pair of hairy *****, a steaming pile of ****, and even a thumbs down emoji... Is it too much to ask for a :thumbsup:?

 

I never said he was right or wrong, because I am not "down speaking" anybody, and I didn't edit anything of that sort out, I avoid it all together. Doug is a great friend and very helpful to me, I would never downspeak him or anyone on an internet forum no matter what they say or do.

Maybe YOU cannot measure such things, but I assure you, that in an age where we are measuring the speed of light accurately, that there is indeed an instrument sensitive enough to measure the pressure gradient across any such length of pipe.

If we are dealing with high flow rates, maximum output situations, there are many physical formulas which are developed include many more variables than simply friction. Temperature certainly plays a role!

However, in my examples, I am strictly dealing with an imaginary situation at idle speeds, low flow rates, where friction, although always present for us somehow, is not an ideal way to predict any outcome or pressure differential (gradient) of this particular example system (which is imaginary), and has no easily applicable formula for us to imagine so it is not to be considered. The actual pressure gradient (not "DROP" I never used the word pressure DROP because it is inaccurate in this example) is therefore best thought of as being due to the volume of any tube(s) which extends from the compressor outlet to infinity (I used such an example already). The end of the tube at infinity is at baseline (atmospheric) pressure, thus no flow exists there in that imaginary example. These imaginary examples are setup to allow people to visualize such things as spaces between parts which further their knowledge of how those parts work, and where air is "going" and how it is behaving.

 

You just have to know what to type

 

Dur..

 

I did. Well... not exactly. I said that what he wrote was inaccurate.

You're right and I agree. Perhaps I should have added that caveat. Or, perhaps not. Anyone here have such an instrument in the bottom drawer of their tool box collecting dust? Anyone???

No. I think the statement is suffice, if the reader isn't inclined to vastly stretch the context in which the statement was made. For those that do (probably only you) I apologize.

Pretty sure that, even if someone did have such a device, even they wouldn't have taken it out of context. But, ya never know. There are some seriously nit-picky people in this world.


As far as I know, and I could be wrong... temperature wont make a difference in the open circuit system we're talking about. You don't suppose there's any chance that you're wrong, do you?

Actually, what do I care? If you want to think pressure across an intercooler is reduced because the air cools, go ahead and think it. Won't bother me a bit.

You're right. In this case it's not a good method. However, any method of predicting something that's essentially zero is a complete waste of time in my book. As you mentioned above... the pressure differential would be so low, it would take equipment that the Particle Colider scientists use to measure it. There is no good method for this case and our purposes.

It probably wouldn't even show any indication, not one fraction of an inch, on a manometer if the pipe were 100' long.

Do you not agree?

 

For imaginary situations it helps to use... your imagination. Ignoring facts or details because you cannot easily measure them is no way to progress.

The point here, in this place, on a non-scientific forum such as this one for this type of discussion is to find easily measurable attributes, such as volume, which are also easy to imagine for non-scientific folk who wish to learn more about performance. The minute you step away from easy to utilize ideals and easy to imagine, accountable situations, the words become useless to the majority of users. For example, the equation PV=nRT is extremely useful, introductory equation, yet it neglects physical atomic size, and so is skewed depending upon the actual mass of gas atoms in question. However, we can still use PV=nRT frequently for drawing up easy to imagine situations and scenarios for fairly accurate estimations which can be used to make intelligent decisions.


Imagine a length of tube stretching from the compressor outlet, to infinity. Now imagine what the temperature of an ideal gas is along that stretch, in an instantaneous snap shot at a steady state. There is no intercooler, it is just a single, straight, piece of pipe, and lets say the speed of the air at the time is less than the speed of sound.

Regardless what the air temperature starts off at when it leaves the compressor, eventually, due to the length of the tube, the air temp will reach a steady state with the atmospheric temperature surrounding the pipe, assuming the pipe has a thermal coefficient and transfers the energy of temperature back and forth between the atmosphere and it's contents (like aluminum would). Once a steady state is achieved, all variables may be held constant, and we may now look at an instantaneous snap-shot of the situation in the pipe, that is, consider the tube as an isolated and sealed compartment for examination to draw some conclusions.

The internal energy of an ideal gas depends on the temperature of the gas, and not it's pressure or density, and we can read directly from the physics book: "A change in the internal energy of a confined ideal gas depends on the change in the gas temperature only; it does not depend on what type of process produces the change in temperature". The first thing we notice then, is the gas losing internal energy as it cools off along the pipe. As it cools, due to the reduced energy, the air molecules are able to pack closer together, and have less severe elastic collisions.

This is why the pressure drops along the length of the tube, as volume remains constant, and temperature drops, internal energy falls off and the gas molecules pack closer together, yes become more dense, while losing internal energy.

The distinction is important because higher internal energies (hotter air) will yield greater power output- you will make the most power with the largest mass of hot as possible air, as long as it is reacted in the combustion chamber without damaging the engine. Long pipes, intercoolers, water injection, all reduce temperature and thus reduce the power output of an engine compared to the same engine without them, given the same flow-rate in terms of mass via the compressor. Those devices are generally used to make the reaction of fuel and air safer, not necessarily to increase power through air charge density. The compressor flow-rate in terms of mass/time supersedes any air charge density improvements made after the fact, unless those devices impact the actual mass/time rate at the compressor as well, which is possible. For example, methanol injection pre-compressor seems to have this exact effect, giving a greater density to all ingested air and increasing the mass/time flow rate of the compressor.


1. I believe you can measure a pressure differential inside intercooler plumbing using a gauge/logger which reads in inches of water above atmospheric pressure. This is how I measure crankcase pressure. No need for expensive elusive equipments. Remember that about 13" of water is 1" of Hg, or about one half of a PSI. So the gauge might read 3-4" of water which is less than 0.24PSI, the resolution seems well enough for the job, I would recommend a computer logger so you can try a myriad of idle situations and compare them at leisure.

2. There is no need to measure the pressure differential of an intercooler pipe for most enthusiasts. The point wasn't to go out and start measuring them; even I don't bother with that, and have not tried it. It was simply to reveal their existence, to uncover the hidden truth behind the plumbing, to get a feel for the invisible winds. The compressor creates a pressure differential, just like a piston sucking on a valve, or a fan churning in the ceiling. This action of a compressor imparts energy to gas molecules, which was extracted from hot exhaust gas by the turbine firstly. In other words, you spend $$ on fuel to move the turbine, so it can harness that $$ and turn it into kinetic energy on the compressor side, imparting additional energy to air molecules in the intercooler plumbing. Now it is OUR job as enthusiasts to HARNESS that additional energy; think of those air molecules as "special" now, and do something with them. throwing them away via an open atmospheric bypass, or forcing it back through the compressor due to a closed bypass is wasting $$ and possibly reducing performance. It should be recirculated back the compressor inlet where the energy can be turned over, re-used. In a cruising engine/vehicle it will also be used to reduce the losses incurred by a piston trying to pull on an intake valve for a fresh charge- another economy feature, as MPG gains.

 

 

I prefer the term "hypothesize", but I get it. And, I don't disagree at all.

Interesting that, you said pretty much what I said. But, presented it like you're saying something new.

It was you, wasn't it... that pointed out that nano pressure readings could be obtained?

You've flopped back and forth a couple times. So, which is it? What's your angle in this? Scientific absolute, laboratory grade "there is indeed an instrument sensitive enough to measure the pressure gradient across any such length of pipe" ... or "a non-scientific forum such as this one for this type of discussion is to find easily measurable attributes, "

Side conversation: Have you actually ever tried imagining infinity? Every time I do, I end up imagining and imagining and imagining... until I lose focus or get bored. This, BTW is a perfect place to use my "imagination" as it has little bearing on reality. :end side conversation

Okay, got it. Air flowing through really long tube.

Okay, temperature reaches equilibruim then we seal the ends of the pipe (the pipe that has no end )

Okay, we've turned an open loop system into a closed loop system. The air cools and the pressure in the closed loop system reduces.

That is true... for a closed loop (sealed) system.

I gotta stop you right there. That's 100% inaccurate. I'll allow you to prove it to yourself. Actually, you mentioned the basis for the truth above.

For a static mass flow rate? Hmmmm. Perhaps. There's a lot involved there.

Is that what you're basing the imaginary theory on? A constant volume process?

Tell ya what. I'll try imagining constant volume, and you try hypothesizing constant pressure. I'm sure we'll see eye to eye in about a week.

Okay, if the mass flow is static... Suppose I see the point.

However, if you want to use your intercooler to make a 5psi set up safer. Go right ahead. Pretty sure anyone other than you, OEM manufacturers included, use them with the primary objective of increasing the output while maintaining the same level of safety and reliability.

Just to clarify, there are two slightly different subjects we're discussing.

One, is Dougs statement that he would expect a pressure drop across the intercooler while his engine is at idle. You won't be able to measure that... unless you want bring up... again... that, "in an age where we are measuring the speed of light accurately, that there is indeed an instrument sensitive enough to measure the pressure gradient across any such length of pipe." Which was a silly thing to say then. I suspect it would still be silly if you said it again.

The other subject is my statement that pressure loss across an intercooler is a function of friction and has nothing to do with volume or temperature. In that case... well... duh, of coarse you can measure that.

Am I missing the point? Was there a point?

Not sure what that had to do with the price of rice. And FWIW, I'll be measuring mine.

So, after all this, I have a question. What do you think the result would be if you put in a second intercooler, plumbed parallel? Let's imagine the frontal flow of ambient air through them, or icy water also doubles, each one operating as it would have on it's own.

Is the post intercooler charge temperature the same or cooler?
Is the post intercooler charge pressure higher or lower?

 

You guys are making this much more complicated than it needs to be. All this square root of X plus infinity divided by absolute pressure multiplied by f~ is retarded.

 

Actually, applying numbers to it would simplify it quite a bit.

 

There is a difference between what is possible, and what is imaginable. All imaginable things are to be considered as possible, yet sometimes the imaginable things are not possible yet, depending on the age/year. Sometimes we can leave an idea behind, and we die without knowing, and then hundreds or thousands of years later the idea becomes relevant, accepted, tested, theorized, and maybe even common knowledge. A good example would be the shape of the earth. 'They' were sure it was flat, then 'they' were sure it was round, then 'they' were sure it was ellipsoid, or whatever came next, and finally satellites and topography, computer age put accurate measurements onto paper as something else all together different.


my angle is to discuss anything as necessary for both entertainment and to pass useable information onto others in any form, even if it means being wrong or getting shot. Renegades of funk: "for renegades there is a time and age" -rage

Every week I calculate several, perhaps a hundred equations regarding solutions which utilize infinity as a beginning or end point. There are both summation notations and integrals which may begin or end at infinity, which either converge to some number, or diverge with some kind of proof. This is fundamentals of calculus. It is very real style of math and has many practical applications.


I never claimed the end of the tube was sealed, nor did we discuss what is happening on the compressor outlet, for a reason. If we assume no air can exit the compressor outlet back the way it came in, and it closely resembles an actual turbocharger during boost situation, where the [airflow volume and compressor wheel speed] is "on" the compressor's map, then there will be no surge behavior. This essentially "Seals" the compressor end of the tube, i.e. once the air passes through the compressor it can no longer exit that direction so long as the compressor does not surge. We can then visualize whatever air molecules are between the compressor and the engine in an instantaneous fashion. The system was never "closed" since energy transfer between the tube and the universe is still possible, even if only in the form of temperature.

Sorry, I have been through this time and time again, and I am 100% accurate. I will say it again so there is no confusion: You will make the most power with the largest mass, of highest temperature as possible air. Hotter air contains more internal energy and will always produce more power than the same mass of cooler temperature air. Chemical reactions, like combustion, rely on the temperature as part of the driving process, as they collide and exchange electrons and create products. What is the desired outcome of a combustion reaction in a gasoline engine? A temperature increase at constant volume that drives up pressure until a piston moves and volume begins to increase, released in a fashion gradual enough to continue to drive the piston without being so rapid that it begins breaking parts. What is the desired attribute of exhaust gas which drives a turbine? A high temperature, which increases space between exhaust gas molecules and facilitates a higher exhaust gas velocity given a fixed volume exhaust tube. Temperature is the key to understanding combustion reactions in general.


Take a good look at this equation. PV=nRT.
It really says this: If you hold volume constant, such as in a length of tube, and increase temperature, then pressure has to increase also. Likewise, if you hold volume constant, and decrease temp, then pressure ALSO must decrease. There is no trick, no fog, no magic. P and V are together on one side, and they are products, so if you hold V still, and raise ANY other variable on the other side of the equal sign, then P has to raise also.

In an instantaneous situation we must hold variables static in order to work a simplified equation, whether on paper or in our heads. Only a computer simulation (cough SIMULINK / MATLAB) can run multiple variables simultaneously, with ease, to report the kind of information we are grasping at. It is far too much work to run a multiple variable equation down on paper. Our single variable for the previously mentioned example I proposed was temperature. It varies along the length of a simple, straight tube, and as it does so, the internal energy of the gas decreases where Temp drops, as energy leaves the system to the universe, and according to PV=nRT the pressure must drop at the same time.


A compressor has a flow rate maximum. Lets look at a T-25/T-28 turbo, typical for 2.0L engines, for a moment. They come from the factory in many OEM applications with side mount intercoolers, for safety. If you remove the intercooler, the engine will make more power, but it might also detonate because the air enters the pump gas engine at higher temperature. The intercooler acts as a restriction, it is an extra volume of tube with high surface area and the compressor loses some of it's total flow to overcome that restriction. Since the compressor has a flow-rate max it wont matter what you do the air once the compressor has reached it's max; you can ice cold intercooler and it still has the same max flow. Since the flow is in units of VOLUME there is a chance a cooling effect can reach the compressor itself, i.e. the air entering the compressor could be colder, and have more mass. For example if the ambient air temp drops, the compressor flow rate is still capped at some CFM however now the number of moles of air (molecules of air) per unit volume has increase prior to entering the compressor, and the intercooler and pipes along the way to the engine are also exposed to that same temp drop so there is a chance the air can get back to ambient air temp before it reaches the engine as well. Since installing an intercooler before the compressor makes no sense (you would be trying to cool ambient air back to ambient air temp, there is no difference) you can see why achieving a colder compressor inlet temp, although desirable, is difficult to facilitate without some form of injectable ( like methanol) means.

 

It can be seen as hundreds of subjects, or just one: physics, which means "nature".

It does not matter if you or I have the equipment necessary. the statement is still accurate because I know for a fact such equipment does exist. I was not suggesting we actually try it; I only wanted to point out that you can "never say never". I can also assure you that such a gradient, or differential of pressure does indeed exist, at idle, at cruise, in boost, it doesn't matter; even if the difference is so slight as to be immeasurable, it surely does exist.


When we consider ideals such as friction, we must consider the space between gas molecules and their surroundings which may collide with each other, as well as other matter via their electron clouds, perfect elastic collisions do not transfer any kinetic energy to their surroundings and is what maintains gas molecule velocity in a tire for example, and why a tire does not deflate each time an air molecule bounces from the inside of it's surface, thus maintaining the shape of the tire. Air moving along a tube with a flow rate is different because I believe there is work involved, the compressor does WORK on the air in the tube, energy is exchanged in a series of events that starts in the combustion chamber (which creates hot exhaust gas to drive the turbine, etc) and is difficult to follow exactly, mathematically, without a complex simulation environment. This is why I provided the example with a pipe that leads to infinity: if you check the pressure at the pipe's end at infinity, what is the pressure there? It will be baseline (atmospheric) no matter how much pressure is produced by the compressor initially. That is probably because the amount of work required to provide a pressure increase in a pipe at infinity is also an infinite amount of work. I say probably because I have not actually calculated anything of this sort; I am more familiar with the events in a combustion chamber than in the air tube leading to it. It is far more important for us to realize that the intercooler, or even any length piece of plumbing, which provide a path for air, and those extra molecules inside them act as additional resistance to flow, i.e. additional mass of molecules to move. For example try drinking from a 1' straw, then a 10' straw. The 10' straw is going to be more difficult to drink from, because of it's additional volume, it will require more effort on your part to drink from, the pressure differential in the longer straw will be greater. This additional effort from an engine's point of view is lost power. The longer the "straw", the larger the intercooler/pipes, the more effort the engine will need to provide to drink from them, the harder it will be for the compressor to move the air inside them anywhere. If the straw were an infinite length you would need an infinite amount of effort to drink from it, no air would ever move at the other end without that infinite effort, which is obviously impossible. We can not blame "friction" for this phenomenon without an equation which relates it somehow to the events we are discussing; There is a work requirement, and how much work is needed to move those molecules around as you increase the volume of your straws (intercoolers and pipes) in a meaningful unit should be displayed.

For our purposes, we should stick to easily measurable, imaginable units, such as pressure, volume, temperature.



I do enjoy questions. The larger the volume of your straw (additional intercoolers/plumbing), the more energy will be lost trying to move the air inside them, the less energy will be leftover for the engine to make power (work) with. Additional plumbing, whether an intercooler or a simple piece of straight pipe, provides more area and time for the temperature to exchange with the environment, and it also increases the differential of pressure (like the 10' or infinite length straw example above) regardless of the temperature. As to the rest of the question, It depends what the temp of each item was initially, i.e. was the engine just started and run at WOT while the exhaust manifolds were still cool enough to touch? Or was it fully warmed up without any kind of blankets, such that all of the intercoolers and their plumbing were heated to some high temperature above ambient. There are too many variables you left out of the question. I will say this since I am supposed to be giving insights as I type: Blanket, wrap, and coat the exhaust of an engine such that as much temperature energy is conserved inside it, and as little as possible is allowed to escape to the environment. The Turbine should be kept as hot as possible at all times, up until it reaches a maximum temperature allowable at which point we "thermostat" the turbine somehow (often water injection is used) to keep it from going above that point. Similarly, the compressor should be kept as cool as possible, and allowed to relieve as much of it's temperature energy into the environment (do not wrap or coat the intercooler plumbing, for example).

 

Magnehelic Guage, fitting each end of intercooler. - Pressure Drop measured. They will measure pressure difference across a gauze air filter. I've bought a couple over the years for < 45 bucks used, VERY common in the HVAC industrial plant stuff. Handy as heck for figuring out if a CAT is bad. OR if a Air filter is too small.

Usually Vac measurements are in inches of water column aren't they?

I always thought the Pressure Drop in the intercooler is a result of the sudden increase in the volume of space the air is flowing through (The intercooler) VS the size of the pipe. Sudden expansion of the air helps cool and after it goes through the intercooler tubes it gets recompressed back by the sudden drop in the space re-entering the pipe, if the change is abrupt enough you can get a wave in the transition zone that sort of makes the outlet look smaller .. If you exceed the speed of sound through a restriction in the tube it causes a mach bubble and makes a 2" pipe look like a 1" pipe (give or take) I used to work at a university where they ran a Formula SAE car, the students did lots of fancy flow checking on the restrictor plate provided by SAE and figured out that it was better to port match the intake path to the restrictor because pushing air at a hole in a plate causes the air to go sonic and restricts flow.. Was fun to watch them smoke test it in a clear chamber..

 

Thank you for the info! It is always good to have those with experience post into a theoretical discussion. I've no experience using such devices as the you described earlier, but I figured something had to exist like it, and many others most likely do as well.

As to your "pressure drop" question, let us create an example that simplifies the situation.

Consider that we remove all of the insides of an intercooler, so that is simply a large container. What would you call it, then? Well, to me, I would call it a pipe, as any other length of pipe. Lets say for example purposes the pipe has a volume of 4 liters. So now we have a hollow intercooler with a 4 liter capacity, or a pipe with a 4 liter capacity installed.

Now suppose we measure the pressure drop at some operating condition and find that it is 1psi of pressure drop.

If we remove that 4 liter hollow intercooler and replace it with a simple, straight 3" diameter pipe, that also happens to be long enough to provide 4 liters of volume, and re-measure the pressure drop at the exact same operating condition, the pressure drop should also be 1psi. In other words, the pressure drop for 4 liters worth of pipe is the same whether it occurs suddenly or over the course of a long distance, because 4 liters of air under the exact same conditions contains the exact same mass (number of moles of air molecules) and requires nearly the exact same amount of work to move, except that it might be a slight bit easier to push air through a short 4 liter tube than a long one, but that is just a guess and I wager it would have very little effect on the outcome of such an experiment.

Now, an intercooler is not a hollow pipe. It contains additional surface area that may effect the air's ability to navigate the pipe. This turbulence, or additional resistance to flow, factors into the pressure drop we will record, depending on how severe it is, which depends on the manufacturer and current flow rate of the intercooler. They are often rated at some pressure drop per unit PSI of flow rate, if you are shopping for one, but that is just a general idea/guess as each installation is unique. Tight bends in the plumbing also offer additional resistance and should be avoided; each restriction will go against the work done by the compressor on the airflow, exactly like having a restrictive air filter, and are to be avoided in general, unless necessary (as when needed to more fully move heat from the air in the tubes to the atmosphere).

now lets take the example one step further, and start adding a bunch of restrictions to the pipe. Eventually, the pressure will drop enough to become a vacuum, as if there were no longer any turbocharger. The same thing would happen if the pipe started getting longer and longer, between the engine and the turbocharger, as the engine trys to create a vacuum by pulling on the intake valve, the pipe is so long and there is so much mass of air in the tubes that all of the work done by the compressor is lost, and now the engine is essentially naturally aspirated again. If you kept increasing the length of the tube even more, the vacuum would increase more and more, exactly as if the air filter were too small on a naturally aspirated engine. The same exact thing happens to an N/A engine when the tube that feeds the throttle body becomes longer and longer, exactly as if the air filter were too small, again. These examples should tie together what it means to have a restriction, as it affects the pressure (creating vacuum) in the tube, both when there is a restriction air filter, or too long of an intake tract (pipe) which feeds the engine, or both.