CFM formula

[pianoman]

OK, the turbo i wanna get flows 720cfm @15psi. It flows 50lbs/min. Now how would I figure out how many cfm and lbs/min this turbo would flow at 18, 20, 22, and 24psi? is there any type of mathmatical formula that anybody knows???
Thanks, Pianoman

 

[Cesar]

turbo's only flow air, they do not cause pressure (psi). Restriction cause pressure.

If that turbo flows 720cfm@15psi on this given application
then
it needs to flow 864cfm i order to make 18psi.

 

[pianoman]

Right, so if there was nothing to regulate flow, it would build boost till it blows. How did you come up with the number 864?

 

[myblack98gst]

ratios


720 cfm : 15psi is same as
48 cfm : 1 psi which would be
864 cfm : 18 psi
1152cfm: 24 psi

 

[pianoman]

I didn't think it was as easy as just dividing 720 by 15. but thanks.

 

[pboglio]

Pianoman,

Hmm, sounds like your talking about a T04B H-3. Looks like you'll get another 4 lbs/min out of it at 24 psi, maybe 54 lbs/min. Unfortunately, you need to get the compressor map for your turbo to get an accurate answer. I have the equation in front of me and although looking simple, a couple of the variables would need accurate values which would be tough to get. The equation would not come close to giving you a meaningful answer.

With your turbo flowing 50 lb/min at 15 psi, you should be asking yourself what does your ENGINE flow at 18, 20, 22, 24 psi. Now there IS a simple equation to figure that out. Lets just say your turbo will outflow your engine well past 24 psi on a 2.0 liter, 7000 rpm limit.

Cesar,

The compressor wheel accelerates the air to a certain angular velocity and then "dumps" it, rapidly decelerating it, trading most of the kinetic energy for a pressure rise. The compressor wheel, diffuser, and collector all contribute to the pressure rise. A turbo uncoupled from the engine with a source of exhaust gas to spin the turbine will still produce pressure inside its compressor housing. A restriction causes a pressure drop, NOT a pressure increase.

 

[Cesar]

A restriction causes a pressure drop, NOT a pressure increase

I'm not so sure about that...
a pressure drop would be measured after the restriction. pressure is formed before the restriction due to the inability to flow.

For example, sometimes boost is measured before the throttle plate (restriction), and vaccum is seen in the intake manifold when the throttle plate is closed during decel, or shifting. When this takes place the BOV opens to release excess pressure to prevent compressor surge, no?
I guess it's perspective really.

 

[pianoman]

http://www.bullseye-power.com/product_info.php?cPath=37&products_id=46 there is the link for the turbo, the compressor map is on there. This the turbo I wanna get. So im just trying to collect some info so I can get all of the supporting mods to match my needs. I don't like buying things twice, as im sure most of you don't, Just want to have a smooth operating car. I would be using a wide-band 02 and dsm link to tune. Im thinking a set of 680s would match this turbo? I can't read commpressor maps very well so help is needed. Thanks, Pianoman.

 

[pboglio]

Pianoman,

Sounds like a monster turbo, no harm in buying only once. You are going to need about every bolt-on known to man to fully utilize that turbo though. Gotta be a list as long as my arm. I'm including a link that shows you how to read compressor maps. I don't think 680 cc injectors are enough, no way. Anyhoo, good luck.

http://www.stealth316.com/2-3s-compflowmaps.htm#i


Cesar,

You stated:

"I'm not so sure about that...
a pressure drop would be measured after the restriction. Pressure is formed before the restriction due to the inability to flow."

Man, I wish I could send you my fluid dynamics book, I really hate arguing Seriously though, you can't generate a positive pressure increase upstream of a restriction that causes a loss, UNLESS you input POWER to do it.

Power = (specific weight)(pipe area)(velocity)(head loss)

A "head loss" is directly related to a "pressure drop", the bigger the drop the more Power needed to overcome it, no free lunch here. High pressure drops create high upstream pressure requirements to maintain the same flow rates. This has very little to do with turbo's anyway, or at least not to how they create boost.

Looking at your first post:

"turbo's only flow air, they do not cause pressure (psi). "

This ones got me scratching my head. Did you really mean what you said here, maybe you typed it fast and it came out wrong? Makes me wonder were you got this info, if it were true I would believe it. Every book that I read shows proof that this is a myth for Centrifugal compressors. Now, if your talking radiator fans than sure, they flow a high CFM but very little to any measurable pressure. I've read enough on the SPAL catalog to see this is true for fans. A fan 12" in diameter doesn't need or even could if it wanted to generate any appreciable pressure (.05 psi at best), but they can flow over 1200 dfm. For centrifugal compressors I'm afraid you've got it wrong, way wrong.

I'm looking at an enthalpy-entropy chart right now and the entire pressure increase on a turbocharger occurs within the confines of the compressor wheel, diffuser, & collector housing. I'd love to see proof that a turbo compressor needs to make boost outside the compressor housing or needs some kinda outside restriction to push against. I stand by my first post. Cheers.


Gene

 

[sweet97]

Gene in effect you're saying the compressor has a "built-in" restriction that reates the pressure. If there were a force turning the turbine wheel the compressor would "push" out compressed air from the outlet. I can see where the "restriction' thinking can come into ones thinking. Man I took Physics 101 in a local community college and that was enough to satisfy my "inquiring minds want to know" hunger!! Mark

 

[pboglio]

Gene in effect you're saying the compressor has a "built-in" restriction that reates the pressure. If there were a force turning the turbine wheel the compressor would "push" out compressed air from the outlet. I can see where the "restriction' thinking can come into ones thinking. Man I took Physics 101 in a local community college and that was enough to satisfy my "inquiring minds want to know" hunger!! Mark

Man I wished nobody ever mentioned restriction, gets ME all confused. O.K., I think were getting into the nitty gritty of compressor flow physics here so were treading on some assumptions and guesses. As far as I can verify out of my reference material, looks like a good portion of the compression occurs within the compressor wheel impeller passages, even as the air is still on the wheel rotating around and just as it falls off it. I'm assuming because of the expanding width radial passages, they kind of look like diffusers too. I'm still a little iffy about that assumption though. The rest of the compression occurs when it comes off the wheel and goes thru the actual "diffuser". I'm assuming the increase in cross sectional area causes a rapid decrease in velocity and an increase in pressure as well (i.e. Bernoulli's equation). I can verify this stuff but I am still having a hard time understanding myself the contribution & mechanics of each turbo part to the pressure rise. Pressure can only be increased by either a rapid deceleration of the air coming off a high speed rotating wheel or going thru a rapid deceleration due to a large area increase. My reference material is thin on detailed answers. Anyhoo, I made a bunch of oversimplifications but thats the jist of it. Books are wonderful thing I'll tell ya. Cheers.

Gene

 

[Cesar]

Pressure is formed before the restriction due to the inability to flow

you can't generate a positive pressure increase upstream of a restriction that causes a loss

I think we mean the same thing here.

It is true that there is pressure inside the compressor housing, once the turbo is installed on a running engine.. The turbo however is not the culprit causing the pressure though. It simply flows air, regardless of how much air. The reason why pressure can be measured in the compressor housing is because it is backed up from any intake valves or any other restrictions along the way that may be restricting the flow of the air into the motor. Mind you, if you were to measure the amount of pressure at any given point in the intake tract, you will get different readings depending on whether or not there were any temperature changes, changes in velocity, or a change in area that the air being flowed is allowed to take up.
That is why the wrong intercooler and the wrong intercooler pipes can make a turbo even more inefficient than it already is.

Pumps simply flow. regardless of whether it is liquid of gas. A restriction comes in many shapes and forms, but they are the cause of why there is pressure.

Just remember the cartoons where the guy steps on the water hose as the tap is running...

 

[pneumo]

It is true that there is pressure inside the compressor housing, once the turbo is installed on a running engine.. The turbo however is not the culprit causing the pressure though. It simply flows air, regardless of how much air. The reason why pressure can be measured in the compressor housing is because it is backed up from any intake valves or any other restrictions along the way that may be restricting the flow of the air into the motor. Mind you, if you were to measure the amount of pressure at any given point in the intake tract, you will get different readings depending on whether or not there were any temperature changes, changes in velocity, or a change in area that the air being flowed is allowed to take up.
That is why the wrong intercooler and the wrong intercooler pipes can make a turbo even more inefficient than it already is.

Pumps simply flow. regardless of whether it is liquid of gas. A restriction comes in many shapes and forms, but they are the cause of why there is pressure.

Just remember the cartoons where the guy steps on the water hose as the tap is running...

This is a bad reference to base a scientific example, but if it helps you understand, I'll give it a try.

The act of stepping on a garden hose is not the cause of the pressure inside the hose. The Local Water Dept. sets the water pressure, and this pressure is created upstream of the cartoon guys foot. If anything, his foot, which is a restriction, causes a pressure drop. It doesn't create pressure. The pressure had to be there already in order to see a pressure difference from one side of his foot to the other.

The intake tract of a turbo car works the same way; any restriction along the path from turbo to intake manifold will show a drop in pressure. The pressure has to be there before the restriction in order to see a pressure drop, otherwise you'd be creating vacuum.

I had the chance to look inside a turbo compressor cover from a car that had an oily intake from the valve cover breather hose. The oil swirled around and stained the inside of the compressor cover, showing a unique pattern that followed the air path as it went through the scroll. At the shallow end the oil streaks came off the compressor exducer nearly at a tangent. The air was heading almost directly around the scroll with a slight angle to make way for the air coming off the exducer farther downstream. The angle of the air gradually changes as it moves farther along the scroll until at the deep end, just before the outlet, the air comes off the exducer at nearly a radial angle. Almost all the air velocity is spent just swirling around within the scroll in increasingly tighter and tighter spirals. Very little is directed towards the compressor outlet. It's like pboglio said, air builds extreme velocity in the compressor wheel. When it's dumped into the diffuser it folds in with more air which is also converting it's velocity into pressure.

The ammount of pressure built up in the compressor housing depends on the speed of the wheels and how many CFM or lbs/min are required to feed the engine.

 

[pboglio]

pneumo,

I like your actual observations of the oil swirl patterns left inside the comressor wheel.
Yes, the compressor wheel converts axial air velocity in the inlet of the turbo and produces radial velocity coming off the exducer, then whatever pressure rise it creates. Depending on the backsweep angle of the fins the velocity components may have different values, as well as counting on angular velocity of the spinning wheel itself, wheel rpms so to speak. I'm right now looking at the equation for work per mass flow and the blade back sweep angle and various gas velocities are all needed to figure work required to power the compressor wheel.

cesar,

You seriously need to pick up a book dealing with centrifugal pumps. I'll quote directly:

"For the simplest type of centrifugal pump, the fluid discharges directly into a volute-shaped casing. The casing shape is designed to reduce the velocity as the fluid leaves the impeller, and this decrease in kinetic energy is converted into an increase in pressure."

Now here they are talking about a water pump, like on your car. A turbocharger is basically a BERNOULLI machine. I mean, the only way to compress something is to push on it, applying a force. There can be no arguement there. There is no other way. Whether you push on it with a piston in a cylinder, spin it at high speed and then rapidly decelerate it, or use high speed resonant waves, its the same thing. Its the WAY that the force is generated that differs from turbocharging vs. fixed volume supercharging or something else.

High speed air generates a HUGE force just to be decelerated, F=ma. Force makes pressure, of course and nobody argues this. The turbocharger makes boost even without beeing connected, because fast air uses slow air to decelerate itself inside the housing. The high speed air that is still on the impeller wheel is COMPLETELY useless to the engine. An engine could never hope to rev high enough to accept this ultra high speed air. But all is not lost since one form of energy is traded for another. Now looking at the equation for calculating engine power, given a max engine rpm of 8000 rpm or so, you can use pressure to give you more power. Now this IS something that a turbo can give an engine, so it converts its useless high speed air to high pressure low speed air.

Imagine what would happen if an I.C. engine with no restrictions COULD rev to 50,000 rpm. This means a very very high average air velocity thru out the engine. This is bad news for a turbo because it lives on "velocity" differentials. The turbo is stuck at revving around 150,000 rpm due to sonic flow limitations. Weird thing in this case is that if you could spin to 50,000 rpm in the engine you wouldn't even need boost pressure. High Mass air flow needs either high intake pressure OR high engine rpms to make big horsepower or both if you can get it. Anyway, for lack of a better word, you would be STEALING the slow air away from the turbo that it needs to smack against to slow itself. I believe this happens to a SMALL degree in efficient engines at high rpm.

You have to think of a turbo uncoupled from an engine as neutral. An ultra high speed engine could literaly "suck" the boost right out of a turbo by making speed differentials smaller. On the other hand a super duper restrictive engine (high pressure drops from the turbo to the cylinder head) would force the turbo to work harder to beat its way past the restriction, just to maintain the same INTAKE MANIFOLD pressure. This is outlined in BERNOULLI's energy equation.

To summarize, a turbo compressor connected to a piston I.C. engine CAN have its pressure head affected by that engine. But the engine itself being connected to it does not CREATE the pressure head for that turbo. Boost is made just after the diffuser INSIDE the compressor housing, no connection to the engine needed. This statement excludes the engines contribution to turbine wheel shaft work creation to spin the compressor, just to simplify things. Cheers.

 

[sweet97]

Gene what is the diffuser in the turbo? I had a question but it's not relevant or maybe it is. My 50 trim has a compressor wheel with a 2.13" inducer and a 3" exducer. That's quite a diffeence for the mitsu wheels and I was wondering if this had any effect on this topic? At first I had exducer and diffuser as being the same in my mind.
Mark

 

[pboglio]

Ok,

Let me preface: I DO not design turbos, I DO not manufacture or build turbos for a living. My educational background I'd rather keep to myself I'm an enthusiast like the rest of you guys. Anything I say is paraphrased out of a book for accuracy. Everything else is theory or conjecture.

A diffuser is part of the compressor housing. A diffuser "contributes" a good portion to the pressure head increase inside a turbo. But my guess is that the radial passages on the exducer (exit portion) of the compressor wheel also look a heck of a lot like some kind of "rotating" diffuser. Thats a guess though. A diffuser is a "reverse funnel". Small area goes to big. The actual diffuser doesn't get big too fast or you have flow losses and heating. A fluid can be decelerated with minimal losses but it has to be done right. The air will always heat up when being compressed, even with zero flow losses. The one I'm looking at right now looks just like a thin passage between the big end of the compressor wheel and the collector portion of the collector housing. Looks kinda like a big washer you use on a bolt and nut, except its empty of course. The one I'm looking at even has turning vanes to help turn the flow from radial to tangential and out to the compressor outlet. Surprisingly they mention here in my I.C. text that the collector does nothing more than "collect" the air flow and redirect it to the engine. I thought it might also contribute to a bit of pressure rise but I guess not.

Now I see where the turbo experts claim that a big wheel in a small compressor housing with little diffuser size hurts. You can't increase the compressor wheel
size without eating up diffuser realestate. Using up most of the diffuser's radial "length" causes the airflow to decelerate too fast causing a nice decrease in diffuser efficiency and increase in heating. Anybody ever see the old 20g turbos with the 2" inlet and TD06 compressor housing? They look real funny on the outside because of the "large" diffuser size, a feature I think you'd want.

Actually, Mike Kojima's "Honda/Acura Engine Performance" book talks about the big compressor wheel and small diffuser problem. I also love this book because he even gets into compressor work and the power needed to drive them. He gets into more of an engineering analysis style that I can relate to, versus Corky Bell who I think kinda oversimplies for the non Engineer. Big compressors with small turbine wheels are talked about too.

Mark,

I actually have no idea about the 50 trim inducer, exducer sizes. I trust 100% in the compressor maps in my Turbonetics catalog. Too many people are getting good results for me to question that wheel. Only downside I would think would be a bit of lag. I have no experience with it though. My guess is that a 50 trim wheel would have great efficiency. The map for it has "large" high efficiency islands, WHY its so efficient I can't say for certain. If I knew that I would be working for Garrett


Warning: Now I'm speaking purely theory, no books to back me up here. Think about this, without an exducer diameter being bigger than the inducer on the compressor wheel, how the heck would you ever turn the airflow from axial to radial. I would think for efficiency it would be preferable to actually have a nice size difference to get a big radius to turn the airflow. Your compressor wheel is basically a rotating 90* pipe elbow. With air speeds at least 400 ft/s at the beginning of the impeller, the flow losses will be big if they need to turn a real sharp corner. The bigger the turn radius the better. Now this requires more wheel mass right where you don't want it, so there is a balance between efficiency and turbo lag. This follows from what I hear that large TRIM wheels (inducer and exducers that are closer in size) are less efficient.

I tell you, I would love to talk to a turbo designer for like 6 hours straight. I think an Engineer at Garrett could answer every question I have. Later

 

[sweet97]

Thanks Gene. Did I understand correctly that the larger the trim the less the difference between the inducer and exducer? There must be a way to "calculate" the trim using the inducer and exducer sizes but the math escape me. I believe that the turbine wheel in my 50 is called a 76 trim. This is good info if I got your descripton correct. Now for the formula to calulate trim form the forementioned ex. and in. sizes. I believe the turbo i have has a 2.13" inducer and a 3" exducer so a 50% difference is not exactly there. I will try to find some wheel sizes where the stats for the wheels are given. mark
PS: the disclaimer is hopefully a diffuser or is that a de-fuser?

 

[pboglio]

Thanks Gene. Did I understand correctly that the larger the trim the less the difference between the inducer and exducer? There must be a way to "calculate" the trim using the inducer and exducer sizes but the math escape me. I believe that the turbine wheel in my 50 is called a 76 trim. This is good info if I got your descripton correct. Now for the formula to calulate trim form the forementioned ex. and in. sizes. I believe the turbo i have has a 2.13" inducer and a 3" exducer so a 50% difference is not exactly there. I will try to find some wheel sizes where the stats for the wheels are given. mark
PS: the disclaimer is hopefully a diffuser or is that a de-fuser?

Mark,

Use this simple forumla: 100*(inducer dia/exducer dia)^2
so in your case 100*(2.13/3.00)^2 = 50.4 or 50 trim

It works on both compressor and turbine wheels.

Cheers.

 

[sweet97]

Gene could you put that formula into words because I do not know what * and ^ mean to do. All i have is 100*(.71)^2=. Sorry,remember I use webTV and am computer illiterate! mark

 

[98spyderboost]

Wow. I didnt think that web tv stuff ever survived the broadband burst?!?!?!? Oh well.

 

[pboglio]

Sorry,

The * means multiplication, the ^ means to the power of, so for instance:
4*(3)^2 means 4 times 3 to the 2nd power. Of course you raise 3 to the 2nd power first, then mulitiply that answer by 4. Same thing applies to your inducer and exducer diameter terms. You first divide the inducer by the exducer diameter, then raise that to the 2nd power, then that whole mess gets multiplied by 100 to get your trim number. Sorry, I'm used to programming using math symbol abbreviations. Hope that helps.

Gene

 

[sweet97]

Wow. I didnt think that web tv stuff ever survived the broadband burst?!?!?!? Oh well.

What was the "broadband burst"? Heck my TV has "rabbit ears"! When I drive down the road I'm looking at the tops of green trees and the diffrent hues of the blue sky, white fluffy clouds. just amazing!
Thanks for the lesson Gene! I have seen turbo's offered with a choice of 69 or 76 trim turbine wheels. Anyone know the advantage or difference between the two? Mark

 

[Strm Trpr]

I didn't think it was as easy as just dividing 720 by 15. but thanks.

I came up with 864 by doing a proportion.
720cfm=_X cfm_
..15psi.....18psi

864cfm

I'm sure the compressor map is not linear though.....