cfm rating for turbos

[FastGSXauto]

Can someone explain what it means when they say a turbo is rated at 550cfm at 15 psi? I'm pretty sure it has nothing to do with the boost at 15psi, because that depends on engine flow characteristics. Does it mean that's the amount of air that can be pushed through the compressor housing with 15psi of back pressure. Whatever the answer is, i think this is a widely misunderstood topic with turbos.

[BatmanGSX]

Yes.

[FastGSXauto]

Yes what, 15psi of backpressure?

[Nick 92 TSi AWD]

Huh? It's pressuriving ambient air to 15psi, and at that pressure it is pulling through 550 cubic feet per minute of air mass.

[FastGSXauto]

pressuring intake air where? In an engine's intake? That cannot be repeated from engine to engine because of flow characteristics.

[Toast]

Pressuring air going anywhere. Where the air goes after the turbo isn't important in a test rating...

What matters is the pressure gradient the turbo is working with. 550CFM @15psi means that at a pressure gradient of 15psi between the compressor inlet and outlet (air going in and air going out), the turbo can move a maximum of 550 cubic feet per minute. That doesn't mean the turbo is always moving 550CFM at 15psi. It just means that is the maximum volume of air it can flow under those conditions.

Of course, that brings up a question I have. At what arbitrary condition is the turbo deemed to be at it's maximum flow capacity? Is it when the turbo finally self destructs? Is it when the efficiency drops below a certain level? Is it when the blades go supersonic? I haven't got a straight answer about that one yet. Anyone know for sure?

[Van]

Originally posted by Toast
Pressuring air going anywhere. Where the air goes after the turbo isn't important in a test rating...
What matters is the pressure gradient the turbo is working with. 550CFM @15psi means that at a pressure gradient of 15psi between the compressor inlet and outlet (air going in and air going out), the turbo can move a maximum of 550 cubic feet per minute. That doesn't mean the turbo is always moving 550CFM at 15psi. It just means that is the maximum volume of air it can flow under those conditions.


Well said...

Originally posted by Toast
Of course, that brings up a question I have. At what arbitrary condition is the turbo deemed to be at it's maximum flow capacity? Is it when the turbo finally self destructs? Is it when the efficiency drops below a certain level? Is it when the blades go supersonic? I haven't got a straight answer about that one yet. Anyone know for sure?

Most flow charts have wheel rpm on them. IMHO though, efficiency is what matters most. It is fairly easy to figure out what the 4g63 can breathe and at what psi of boost and each rpm. So is the turbo efficient when all of those are figured out and plotted on the flow chart? That's what matters most in my book... (Based on my calculations this makes a TO4E-50 trim a very attractive turbo from 20 to 25 psi of boost.)

[FastGSXauto]

>>What matters is the pressure gradient the turbo is working with. 550CFM @15psi means that at a pressure gradient of 15psi between the compressor inlet and outlet (air going in and air going out), the turbo can move a maximum of 550 cubic feet per minute. <<

there is no gradient between the inlet and outlet if the air isn't being resisted (inside a pipe). I think the gradient you mean is the backpressure inside the compressor housing itself. They flow 550cfm through the housing, and 15psi of backpressure results. So I don't think the 15psi has anything to do with the amount of boost you run. 550cfm is the maximum air the turbo can flow, as toast said. It's a common misconception that people think they are flowing 550cfm when their boost is at 15psi, and if they turn up the boost they will run more.
Van, how do you figure out how much the 4g63 can breathe at each rpm?

[Van]

Originally posted by FastGSXauto
there is no gradient between the inlet and outlet if the air isn't being resisted (inside a pipe). I think the gradient you mean is the backpressure inside the compressor housing itself. They flow 550cfm through the housing, and 15psi of backpressure results. So I don't think the 15psi has anything to do with the amount of boost you run. 550cfm is the maximum air the turbo can flow, as toast said. It's a common misconception that people think they are flowing 550cfm when their boost is at 15psi, and if they turn up the boost they will run more.
Van, how do you figure out how much the 4g63 can breathe at each rpm?

IMHO, a gradient is: "a rate of change in variable factors, like pressure or temperature." If you have atmospheric pressure at the inlet and the turbo is spinning at 100,000 rpm, it is creating a gradient in pressure, or creating more pressure by compressing the air. Just the way I see it.
It may flow 550 cfm at 15 psi of backpressure, but I do not think that the compressor housing creates the backpressure. The turbo is actually creating more flow than the engine can actually breathe and this is what results in the boost psi you see. I also don't think CFM is a very good way to rate turbos, I think lbs/min is much better, because that is related directly to compressor efficiency at a given wheel rpm.
I agree with the first part of the misconception you stated above about 15 psi and 550 cfm, but IF you turn UP the boost you WILL be running MORE air, period. (e.g. at 3500 rpm at 15 psi and 50F, let's say the engine will move 123 cfm, and then turn up the boost to 20 psi it will now move 130 cfm. But is the turbo more efficient at that pressure ratio? That is the most important part, IMO.
Here are a couple of links that give you info on doing the calculations...

http://www.gnttype.org/techarea/turbo/turboflow.html
http://www.turbofast.com.au/welcome.html

[Toast]

Originally posted by FastGSXauto
there is no gradient between the inlet and outlet if the air isn't being resisted (inside a pipe).

No kidding. But in the case of a turbo that's not entirely true. Because unlike a Roots type supercharger, a turbo compresses the air inside it's housing before it is expelled. Which is one of the reasons why turbos are much more efficient than conventional superchargers.

I think the gradient you mean is the backpressure inside the compressor housing itself. They flow 550cfm through the housing, and 15psi of backpressure results.

I certainly DON'T mean anything like that. If that were the case, then why wouldn't everyone be sticking gigantic compressor housings on their T25s or 16Gs? By your proprosed method we could increase flow just by swapping over to huge compressor housings. Not likely...

You seem to have this mistaken notion that when they came up with these ratings, they didn't use any instruments... Where do you think compressor maps come from? It comes from hooking up a turbo to TEST EQUIPMENT and measuring the result. Where do you get the idea that in order to test the turbo you have to let it blow air out into nowhere? It's a simple matter of hooking up a turbo to blow into a closed container with a controlled bleed. The chamber would receive the air (and act as a buffer) and the pressure level at the compressor outlet would rise. The controlled bleed would maintain the pressure in the container to 15psi (or any desired pressure). A flowmeter at the compressor inlet would measure the CFM of the turbo. Simple matter.


So I don't think the 15psi has anything to do with the amount of boost you run.

That statement made no sense at all.

It's a common misconception that people think they are flowing 550cfm when their boost is at 15psi, and if they turn up the boost they will run more.


Actually that's not entirely a misconception. Odds are they aren't moving 550CFM into a 2.0L motor at 15psi. But if you turn up the boost, then there will be more airflow as long as the turbo isn't at it's flow limit. And aftermarket turbos for our cars can handle more than 15psi with our engines. Simple physics. If you want more air to get into the engine you have to increase either the volumetric efficiency, RPM, or intake pressure.





[Edited by Toast on 07-04-2001 at 02:38 PM]

[FATBASTARD]

"Boost" or "PSI" in an engine is simply the engines *inability* to ingest the incoming air charge.

Backpressure is a good term for it. For example, if you're exhasut manifold is too restrictive, the pressure will increase... thus the reason for larger turbine housings, heavy clips on turbine wheels, and huge downpipes.

Ultimately, you'd want an engine that would provide ZERO pressure out of a turbo rated for 550 cfm @ 15psi. Why is that? It would mean that said engine could actually *use* all of the air that is being thrown at it, not simply pile it up in the intake tract.

There was a guy up here that run a 13.8 (not sure of MPH) running 10 psi of pressure. Now, he had a fully built engine, headwork, valves, pistons, huge t3/t4, etc.

Now, if you were to take that exact same turbo, with the exact sae boost controller at the EXACT same setting, and throw it on a stock engines car, there would be much more pressure... say 20psi. Why is that?

The 1st car can physically flow that much more air through it without causing it to pile up. The 2nd car CAN'T.

Hope I made some sense here.

[davejcb]

Originally posted by FATBASTARD
"Boost" or "PSI" in an engine is simply the engines *inability* to ingest the incoming air charge.

Backpressure is a good term for it. For example, if you're exhasut manifold is too restrictive, the pressure will increase... thus the reason for larger turbine housings, heavy clips on turbine wheels, and huge downpipes.

Ultimately, you'd want an engine that would provide ZERO pressure out of a turbo rated for 550 cfm @ 15psi. Why is that? It would mean that said engine could actually *use* all of the air that is being thrown at it, not simply pile it up in the intake tract.

There was a guy up here that run a 13.8 (not sure of MPH) running 10 psi of pressure. Now, he had a fully built engine, headwork, valves, pistons, huge t3/t4, etc.

Now, if you were to take that exact same turbo, with the exact sae boost controller at the EXACT same setting, and throw it on a stock engines car, there would be much more pressure... say 20psi. Why is that?

The 1st car can physically flow that much more air through it without causing it to pile up. The 2nd car CAN'T.

Hope I made some sense here.

Bingo. I remember reading in my engine theories book that boost was the amount of pressure building up in the intake manifold... So correct me if I'm wrong here, but wouldn't the amount of cfm flow be a lot more important than how many psi of boost you run? I like Fatbastard's idea of 0 psi if you flow it all in... Right?

[99gst]

Wait a minute. The amount of air entering the engine depends on the size of the engine. The total volume of each piston chamber is fixed. So, only so much air (a finite amount) can fit in a 2.0-liter engine at atmospheric pressure. If you want more air in there then it has to be under pressure (squeezed in there). Its not under pressure because the engine can't flow it, its under pressure because the turbo compressed it.

Think of a scuba tank. How do you think an hour or two worth of air is available from a little tank you wear on your back. Its under pressure (squeezed in there) at least a couple of thousand psi.

Take a 2 liter soda bottle that if full of air, how would you squeeze in twice as much air? Answer, increase pressure.
This is simple physics, Boyle's Law. A have provided a link explaining Boyle's Law for your convenience.
http://www.aquaholic.com/gasses/boyle1.htm

[Nick 92 TSi AWD]

Wow you guys have some very interesting ideas as to how a turbo works. From that theory it sounds like a turbo is just pushing air at the intake manifold, but due to some restriction it can't all get into the combustion chambers and backs up, building pressure. That is so wrong.

As toast already said a turbo compresses air INSIDE it's housing, the air leaving the turbo outlet is compressed already. It will remain compressed while it runs through the intake manifold and runners, then into the engine. If the engine is using all the available air that does not mean that suddenly you will no longer have a pressurived intake charge.


The pressure of air coming out of a turbo is the air pressure that will get to your intake runners no matter how much your engine can use <well ok so a little less do to pressure drop in bends of ic pipes, ic core, and the very slight amount of pressure drop going through straight pipe, but I'm trying to keep this simple>.

[dyezak]

Nick 92 TSI AWD was correct in his last post...although it was megerly explained. But he was correct. This whole thread should be renamed "Roots Blower Dynamics", cause that's exactly how a roots supercharger works (not the twin screw). The roots supercharger doesnt compress air at all, all it does is stack air in the intake manifold and intake runners in the head. Where as a turbo and various other superchargers actually COMPRESS the air. Anyone want to explain the compression further...I'm to tired right now. Although if the misconception continues I'll come back and explain turbo dynamics. Yawn. Good night.

[FATBASTARD]

Actually, I think both Nick and Dyezak are right, but I'm having a hard time seeing the logic.

Here's another scenario.

take any given length of pipe (say 10 feet), straight 2", attach the turbo's compressor outlet to one side of it, and leave the other end open. According to your theory, if you were to tap a pressure gauge anywhere along that 10' length, you'd read 15 psi (or whatever the turbo's output is set at), correct?

Now, place a restriction on the end of the pipe. Neck it down to say, 1/2". Now what would the presure gauge read? I'd figure much higher than 15 psi, simply because of the restriction placed in it. Am I right, or have I been taking too many toots on the crack pipe?

I'm not trying to argue my point, I'm jsut having a hard time seeing the logic.

Ok, how's this sound?

I DO agree that my wording was incorrect... when a turbo comes into play, thing change... we aren't talking about a fixed RPM device here. Since the wastegate is controlled by pressure, then the turbo will do it's damndest to keep that pressure level, no matter how much air is being ingested by the engine. I should have said that a stock engine may take 50,000 rpms to create 15psi of pressure in the intake tract, while a engine such as Buschurs would require 100,000 rpms to keep the constant pressure of 15 psi.

Since Buschur's engine can actually ingest more air, the turbo (or centrifugal pump) must sustain a higher RPM rate to create the needed pressure. Am I correct here as well?

I must say that I'm actually impressed that there is some technical tlak on this board... I had previously avoided this place simply because of the majority of topics are "Can I remove my MAF sensor if I have an AFC" type of stuff. Way to go!

[BatmanGSX]

Will all of you just shut up. I liked my answer, it involved way less physics. I don't know how they come up with the rating of the turbo cfm but here are some other things to ponder:

A roots blower is still an air compressor. The only way you can see positive manifold pressure is by "stacking" air in the intake system. This is the way all forced induction works, turbo, roots, screw, etc.

Also I think someone was trying to say that boost is realized when the engine can no longer consume air. This is true and false. Because the air is physically taking up less volume at a higher pressure more air will always be able to enter the cylinders as the pressure increases. So the volume of air is fixed for a certain RPM but the mass is not. And that is what makes power, the mass of the air and fuel, not the volume. So boost is realized when the engine reaches the peak VOLUME of air at a given RPM. The mass of air an engine can consume is, in theory, infinite.

[Edited by BatmanGSX on 07-05-2001 at 02:35 PM]

[firepower]

Hey guys, you know we would all be laughed at if we went to a sports bar and talked about this shit. Which is why we talk about tits and touchdowns there. This is a great place for this discusion and I want to throw in my point of view.

For power what matters is the amount of oxygen injested per intake stroke. Nobody should disagree with that. The colder the air the more oxygen molecules per given volume at a given pressure. The idea behind a CFM rating of a turbo at a certain pressure is to describe this potential energy. The higher the CFM @ 15psi (what most turbos we are interested in are rated at) the colder the air at that pressure, therefore the more oxygen molcules and more potential energy.

Now for the intake restrictions. If you do all the head work; port, extrude hone, etc., and all the exhaust work; porting, clipping, exhaust, etc. then you have allowed the engine to injest more air per stroke. Everyone seems to understand that point so I won't harp.

What is important to us car guys is which turbo is going to do what we want. Some guys want a fast street car, some guys want a fast drag car, and some guys want a shinny engine compartment (others exist).

I think the best combo comes down to your pocketbook. Ideally we should get all the flow work done to the engine, and get the turbo to support your desired driving conditions. For drag racers they want the largest turbo that will be boosting full pressure after a shift point (say 5000 RPM). So they should get a high CFM @ max pressure. This will create the coldest charge of air and therefore have more molecules of O2 per combustion.

Once you get the turbo you want there are a lot of other things to make power, hence all of us being broke :)

One more thing I want to discuss has to do with the air being presurized in the compressor housing. If you engine is flowing more than your turbo (like it does when your not under boost) the air is being sucked through the compressor. Once the turbine starts to spin faster than the engine can injest the air the pressure starts to rise and what is happening is the air gets compressed, then expands after the compressor housing. Read this as: compress/heat the air, then expand cool the air. This happenes until there is so much air on the outlet of the compressor that it can not expand and cool. Thats when you have reached your boost setting (or if your not using a boost controller/wastegate) the maximum boost of your turbo for that given RPM.

For the record I have a 16G and I am no where near reaching its full potential so I see no need to upgrade to a larger turbo. A lot of guys choose larger turbos because of the ratings. Nothing wrong with that because at higher RPM's they will have more power than me with a similar setup. You just need to choose how you want your power curve to look. Small turbo's give less peak power and a flatter powerband over a longer RPM range. It really comes down to bang for your buck performance. If you are buying a turbo and all you care about is peak power get the biggest damn thing that will spool up by redline. A big turbo can make up for a poor flowing engine because it lets the engine injest colder air at max boost pressure.

Hope this helped somone, I know its a long winded post, but I am at work and since my job doesn't challenge me I rant about car stuff. :)

[Zazzster]

Turbo pulls air in.
Turbo pushes air out @ 15 psi.
Engine turning slow requires less air volume to mantain boost pressure.
Engine turning faster requires more air volume to maintain boost pressure.
Turbo will always try to maintain 15 PSI.
Engine that requires more air then turbo can supply will result in drop in manifold pressure.
Turbo pressure will remain at 15 PSI.
A turbo will still produce 15 PSI with nothing attached to the outlet.
Turbo will not presurize room to 15 PSI because it is too large. i.e. big ass engine.

I think I have it?????
This simple example is how I see it. With my stock 14b I have slightly less pressure at high RPMs. I also have boost creep untill the engine starts using all the air volume. (3" downpipe)
Please tell me where I'm mistaken.

[Nick 92 TSi AWD]

Originally posted by FATBASTARD
Now, place a restriction on the end of the pipe. Neck it down to say, 1/2". Now what would the presure gauge read? I'd figure much higher than 15 psi, simply because of the restriction placed in it. Am I right, or have I been taking too many toots on the crack pipe?



That would be correct, but the engine does not pose that much restricion. Remember the plenum of the intake manifold holds about 4l of air <somebody correct me if that's wrong>.

[dsmdojo]

Originally posted by Nick 92 TSi AWD
Originally posted by FATBASTARD
Now, place a restriction on the end of the pipe. Neck it down to say, 1/2". Now what would the presure gauge read? I'd figure much higher than 15 psi, simply because of the restriction placed in it. Am I right, or have I been taking too many toots on the crack pipe?



That would be correct, but the engine does not pose that much restricion. Remember the plenum of the intake manifold holds about 4l of air <somebody correct me if that's wrong>.



I do believe that is wrong. 15 psi is 15 psi. The turbo is only pumping at 15psi. Put a cap on that 1/2" pipe, the air in the pipe will only get to 15 psi, if it got any higher, it would back up into the turbo. air speed prob changes(with the 1/2" pipe open again)& cfm prob goes down, but it still would be at 15 psi max.
(I'm no engineer, thats just MHO, I could be wrong)

[FastGSXauto]

15psi does not come from the turbo itself. The turbo flows a certain amount of air based on how fast it is spinning (exhaust energy). If the engine can't breathe in all the air, the air starts getting compressed in the intake pipes after the turbo.

[Toast]

Originally posted by FATBASTARD
"Boost" or "PSI" in an engine is simply the engines *inability* to ingest the incoming air charge.

Nope, you're playing with words. By definition boost or psi is a pressure measurement. It's simply the measurement of the pressure of the intake air that reaches the combustion chamber. There is NO implication or reference as to where the pressure comes from. This seems to be the source of confusion here.

Backpressure is a good term for it. For example, if you're exhasut manifold is too restrictive, the pressure will increase... thus the reason for larger turbine housings, heavy clips on turbine wheels, and huge downpipes.

Backpressure is a very bad term for this. For the reasons I state above. Why? Because in the exhaust system, pressure is formed by resistance to airflow. But, this isn't the case in the intake system. In the intake system there is a pressure producing device. This device, the turbo, compresses the air inside itself before expelling it. Changes the entire situation.

Think of it this way. If your reasoning were true, then why do we have pressure drop from the turbo to the engine? If we made the most convoluted intake pathway and stuck in a monstrously restrictive intercooler, by your reasoning we would be gaining boost, not losing it.

Ultimately, you'd want an engine that would provide ZERO pressure out of a turbo rated for 550 cfm @ 15psi. Why is that? It would mean that said engine could actually *use* all of the air that is being thrown at it, not simply pile it up in the intake tract.

Once again, you are twisting the situation without even realizing it. The turbocharger will ALWAYS produce compressed air when brought up to operational speed. THIS IS A FACT. This is the way the compressor wheel of a turbo is designed. No way around it. What is happening in your example is that the turbo is producing XX amount of compressed air. Unfortunately, the engine by itself is demanding XX amount of air. Therefore, the compressed air is expanding in the intake tract and entering the engine at zero pressure.

Look at this again... please before you whip up some self proposed argument. Take a look at compressor map for ANY turbo. Tell me what the airflow is when the outlet pressure is atmospheric (ie 0psi boost, 1.0 pressure ratio, etc). The airflow is ZILCH. As in below any significant airflow. Think about this. Why is this true? Because by design a turbo cannot move air without compressing it (yes even so very slightly). It just doesn't happen.

The 1st car can physically flow that much more air through it without causing it to pile up.

Let me repeat.... again... THE AIR IS COMPRESSED WITHIN THE TURBO... BOOST PRESSURE IS NOT CREATED BY AIR "PILING" UP IN THE INTAKE TRACT. Come on people, this is basic physics. For those of you who still want to argure, please answer me this one very simple question.

If the air in the intake tract is already pressurized. How can air at a lower pressure even escape the turbo? Does air normally go from a low pressure zone to a high pressure zone? Are the laws of nature simply bypassed in a turbocharged car?

Oh... and for those of you still trying to twist the situation. It can't operate the same way as a Roots type supercharger. Because a Roots type supercharger isn't spinning even close to 50K when it begins to make boost.

This whole thread should be renamed "Roots Blower Dynamics", cause that's exactly how a roots supercharger works (not the twin screw)

You hit it right on the head. With a few exceptions, it seems that's the way people think turbochargers work.

[Toast]

Now, place a restriction on the end of the pipe. Neck it down to say, 1/2". Now what would the presure gauge read? I'd figure much higher than 15 psi, simply because of the restriction placed in it. Am I right, or have I been taking too many toots on the crack pipe?

You are twisting the situation. Nick gave a simple explanation. He even said he left out the issues of pressure drop to make an easy to understand example. So why are you trying to twsit the situation by adding pressure drop scenarios into your example?

A roots blower is still an air compressor. The only way you can see positive manifold pressure is by "stacking" air in the intake system. This is the way all forced induction works, turbo, roots, screw, etc.

Sorry bud, but you couldn't be more wrong. All these devices move air. But they do so in far far different manners. A turbo compresses air internally. A roots type supercharger stacks air. A screw type supercharger strikes a balance between the two. It compresses air internally but it also stacks the air into the intake. The way these devices go about their business makes a huge impact on their efficiency levels.


The higher the CFM @ 15psi (what most turbos we are interested in are rated at) the colder the air at that pressure, therefore the more oxygen molcules and more potential energy.

Nope, that's wrong. CFM is a volume measurement. It has no implicit correlation to temperature or pressure. A higher CFM of airflow does NOT mean the air is colder at that pressure and therefore always contains more O2. You could take X amount of air and heat it to Y temp and have Z volume(in our case cubic feet). OR, we could take 0.5X amount of air, heat it to 2Y temp, and have Z volume. Play with it however you want. But you cannot deduce a temp from just the volume rating.

So they should get a high CFM @ max pressure. This will create the coldest charge of air and therefore have more molecules of O2 per combustion.

Once again, this is wrong for the reasons I pointed out above. CFM is only important because in our situation, an increased CFM USUALLY results in a higher MASS flow rate of O2. It is the mass of O2 entering the engine that produces power, not the volume.

Turbo will always try to maintain 15 PSI.
Engine that requires more air then turbo can supply will result in drop in manifold pressure.
Turbo pressure will remain at 15 PSI.
A turbo will still produce 15 PSI with nothing attached to the outlet.
Turbo will not presurize room to 15 PSI because it is too large. i.e. big ass engine.

Where are you getting this from?!? Is there some mysterious law that states a turbo charger will always and only work at 15psi? Does a turbo possess some type of intelligence that drives it to work at 15psi only? I'm really confused on where you're getting this from :)

15psi is a nominal rating. It's the standard on which turbo flow rate is measured. Nothing more. 15psi means nothing special to a turbo.

Remember the plenum of the intake manifold holds about 4l of air

I don't know whether this is accurate. But I'll say that's a nice generous size for a factory intake manifold if it is true :)

[Toast]

I do believe that is wrong. 15 psi is 15 psi. The turbo is only pumping at 15psi. Put a cap on that 1/2" pipe, the air in the pipe will only get to 15 psi, if it got any higher, it would back up into the turbo. air speed prob changes(with the 1/2" pipe open again)& cfm prob goes down, but it still would be at 15 psi max.

Nope. Once again, there is this mysterious 15psi figure... :) You seem to be under the mistaken idea that a boost controller actually controls the boost... That is not the case. A boost controller controls the wastegate. The wastegate only controls the amount of energy a turbo receives (and therefore how fast and with how much energy it spins).

In your example, you would need a wastegate large enough to virtually stop the turbo to maintain 15psi. I don't recall seeing many 5" wastegates ;) So, the pressure in the pipe would skyrocket if the end was suddenly capped. Who knows what the pressure would spike to... Maybe 100+psi. The resulting pressure and reverse shock wave will act together and stall the compressor wheel. Causing some major damage in the process. But there's no way in hell the pressure would remain at 15psi max. Not the slightest chance.


15psi does not come from the turbo itself. The turbo flows a certain amount of air based on how fast it is spinning (exhaust energy). If the engine can't breathe in all the air, the air starts getting compressed in the intake pipes after the turbo.

Once again, this is incorrect. A turbocharger compresses air internally. Repeat after me...

A TURBOCHARGER COMPRESSES AIR INTERNALLY
A TURBOCHARGER COMPRESSES AIR INTERNALLY
A TURBOCHARGER COMPRESSES AIR INTERNALLY
A TURBOCHARGER COMPRESSES AIR INTERNALLY

Why is this so hard to understand? Or is it because no one believes the few people who understand (and have continually posted as such)??? Do we lack the credibility to convince you? Has Super Street finally muddled your minds??? ;)

If you don't believe this fact, then look for yourself. The answer is as simple as a turbocharger flow map. The answer is staring right at you. People still seem to believe that a turbocharger simply only moves air like a Roots supercharger instead of first compressing it. You ignore the fact that a turbo has a compressor wheel and compressor housing (do you think those names were chosen on a whim??). If this were the case, then the air leaving the turbo would be at atmospheric pressure and all air compression would take place in the intake tract. So, look at a compressor map and tell me what a turbo flows when the air leaving it is at 0psi of boost? Look at as many flow maps as you need until you are convinced... ;)

Just because there are no moving parts compressing air together (as the pistons do in an engine) does not mean the air is not being compressed. I think this is where people are having difficulty believing. A turbo is a centrifugal compression device. Incoming air is sucked in and trapped in pockets formed by the compressor blades and compressor housing (which is why the distance from the wheel to the housing is critical to overall efficiency). The rotating compressor wheel applies a huge force onto the incoming air. The spinning motion traps the air and then shoves it back in this pocket, toward the outer edge of this pocket. The forces act to compress the air. I'm sure everyone has experienced this at one point or another. An accelerating car or jet, going around a corner quickly, etc. This is magnified in a turbo since it's spinning at over 100K RPM. Since air is compressible, the forces act to compress the air. Then this compressed air is slung out the compressor outlet. Thus you get the general idea of how the compressor (gee there's that word again... compressor) side of a turbo actually works.

[BatmanGSX]

Originally posted by Toast
I do believe that is wrong. 15 psi is 15 psi. The turbo is only pumping at 15psi. Put a cap on that 1/2" pipe, the air in the pipe will only get to 15 psi, if it got any higher, it would back up into the turbo. air speed prob changes(with the 1/2" pipe open again)& cfm prob goes down, but it still would be at 15 psi max.

Nope. Once again, there is this mysterious 15psi figure... :) You seem to be under the mistaken idea that a boost controller actually controls the boost... That is not the case. A boost controller controls the wastegate. The wastegate only controls the amount of energy a turbo receives (and therefore how fast and with how much energy it spins).

In your example, you would need a wastegate large enough to virtually stop the turbo to maintain 15psi. I don't recall seeing many 5" wastegates ;) So, the pressure in the pipe would skyrocket if the end was suddenly capped. Who knows what the pressure would spike to... Maybe 100+psi. The resulting pressure and reverse shock wave will act together and stall the compressor wheel. Causing some major damage in the process. But there's no way in hell the pressure would remain at 15psi max. Not the slightest chance.


15psi does not come from the turbo itself. The turbo flows a certain amount of air based on how fast it is spinning (exhaust energy). If the engine can't breathe in all the air, the air starts getting compressed in the intake pipes after the turbo.

Once again, this is incorrect. A turbocharger compresses air internally. Repeat after me...

A TURBOCHARGER COMPRESSES AIR INTERNALLY
A TURBOCHARGER COMPRESSES AIR INTERNALLY
A TURBOCHARGER COMPRESSES AIR INTERNALLY
A TURBOCHARGER COMPRESSES AIR INTERNALLY

Why is this so hard to understand? Or is it because no one believes the few people who understand (and have continually posted as such)??? Do we lack the credibility to convince you? Has Super Street finally muddled your minds??? ;)

If you don't believe this fact, then look for yourself. The answer is as simple as a turbocharger flow map. The answer is staring right at you. People still seem to believe that a turbocharger simply only moves air like a Roots supercharger instead of first compressing it. You ignore the fact that a turbo has a compressor wheel and compressor housing (do you think those names were chosen on a whim??). If this were the case, then the air leaving the turbo would be at atmospheric pressure and all air compression would take place in the intake tract. So, look at a compressor map and tell me what a turbo flows when the air leaving it is at 0psi of boost? Look at as many flow maps as you need until you are convinced... ;)

Just because there are no moving parts compressing air together (as the pistons do in an engine) does not mean the air is not being compressed. I think this is where people are having difficulty believing. A turbo is a centrifugal compression device. Incoming air is sucked in and trapped in pockets formed by the compressor blades and compressor housing (which is why the distance from the wheel to the housing is critical to overall efficiency). The rotating compressor wheel applies a huge force onto the incoming air. The spinning motion traps the air and then shoves it back in this pocket, toward the outer edge of this pocket. The forces act to compress the air. I'm sure everyone has experienced this at one point or another. An accelerating car or jet, going around a corner quickly, etc. This is magnified in a turbo since it's spinning at over 100K RPM. Since air is compressible, the forces act to compress the air. Then this compressed air is slung out the compressor outlet. Thus you get the general idea of how the compressor (gee there's that word again... compressor) side of a turbo actually works.


That's cool and I totally agree with you, and that is why turbos are far more efficient than a roots blower. However, the principal remains the same, if the air was not "stacking" into the intake system then the air that the turbo compressed internally would decompress as soon as it left the compressor housing.

[FATBASTARD]

Actually, Toast is right.

I did some research into the fact... since I'm working at a pump place right now (both centrifugal and reciprocating)...
I actually forwarded this thread to one of the engineers here, and he said Toast's remarks were closest to the truth.

He said that everyone brought up interesting arguments... also stated the big difference between a centrifugal and a root's type pump was that a roots type is "positive displacement" which means for every revolution, there is XX amount of medium being moved... where as a centrifugal pump (turbo) doesn't. He also said that generally they use centrifugals to manintain or raise pressue...

This stuff is interesting... good thing I'm only a summer student. Looks like I have alot to learn :-)

[RabidDonkeyBoy]

hope this helps fan the flames.

there is a reason why it is called a compressor housing. cause the compressor wheel is compressing air in the housing.

if your engine is at idle the compressor wheel is not spining fast enough to make positive pressure in the ic ic pipes and intake so as soon as the air leaves the compressor housing it decompresses.

as you get on the throttle the compressor starts to spin faster & faster to where at some point the compressor is producing 0psi or the engine is ingesting all the air flow into it, and you have presurized the whole intake system to actual atmosphere pressure.

once you start building more than 1 psi you are pressurizing the whole intake system (not just staking air in the plenum), and the engine is having to breath in that amount of air because you are forcing it to.

this upward boost will continue until the wastegate starts to bleed off exhaust gas, or once the engine can start to breath in more air than the turbo can compress and the psi will drop hence why by 5000rpms on my t25 only makes 9 psi where at 3300rpms it does 14psi nicely.

i'm sure someone else can explain this better

[Zazzster]

Toast and BatmanGSX, FATBASTARD, RabidDonkeyBoy & anyone else I missed. Thanks...

I was basically trying to get to the same point. You guys did a much better job.

And YES I KNOW THAT 15PSI IS JUST FOR THE FLOW TEST.

Here is my original post:

Turbo pulls air in.
Turbo pushes air out @ 15 psi. (OR WHAT EVER THE WG ACTUATETORE IS SET FOR)
Engine turning slow requires less air volume to maintain boost pressure. (LOW RPM)
Engine turning faster requires more air volume to maintain boost pressure. (HIGH RPM)
Turbo will always try to maintain 15 PSI. (Via wastegate)
Engine that requires more air then turbo can supply (MAXED OUT 14B) will result in drop in manifold pressure (like RabidDonkeyBoy said about his t25).

Turbo pressure will remain at 15 PSI. (INSIDE THE COMPRESSOR HOUSING)
A turbo will still produce 15 PSI with nothing attached to the outlet. (INSIDE THE COMPRESSOR HOUSING)
Turbo will not pressurize room to 15 PSI because it is too large. i.e. big ass engine.(AT HIGH RPM)

I know I wasn't very clear. Just wanted to let you all know that I do understand.

Thanks again...

[FastGSXauto]

toast, if the turbo compresses the air internally, what happens when it shoots it out the outlet? It cannot posisbly stay at the same pressure because the volume has changed. Just like a tank of compressed air. It is compressed internally, but when you release it to the outside air, the air is no longer at 100+ psi. So the turbo has to fill the intake pipes before any compression will occur. The other factor is that the engine is sucking in air, so the turbo needs to be sending out air at a rate faster than the engine is breathing it in. Only then will the air in the intake pipes start to compress. Toast, you seem to think I am an idiot by telling me I am wrong, but I DO know how turbos work. You say a turbo doesn't just push air, but it does. Even if it compresses it internally, when it releases is it is nothing more than blowing air just like a tank of compressed air. Pressure changes with volume and the minute the air leaves the housing, it's pressure changes. The amount it changes depends on the amount of air getting sucked into the engine. Maybe you can provide some links to some tech info we can all read about.

[BatmanGSX]

I think the air inside the turbine housing is at a much higher psi than whatever you have boost set to. So a pump that is pumping (turbo), lets say, 45psi, should have no problem filling a pipe between a biger, slower pump (engine), with 15psi. So this is why turbochargers are so much more efficient than a roots blower. Does this make sense?

[99gst]

Originally posted by FastGSXauto
So the turbo has to fill the intake pipes before any compression will occur. The other factor is that the engine is sucking in air, so the turbo needs to be sending out air at a rate faster than the engine is breathing it in. Only then will the air in the intake pipes start to compress.

No, no ,no. As said before, the turbo is the one compressing the air. The intake pipes do not need to "fill" up with anything. They are full of air at atmospheric pressure, 14.7psi, (maybe at a slight vacuum) but there is always air in there. To say the intake pipes have to fill up with something implies that it is a total vacuum in there when it isn't.

Let’s analyze this logically. Before the turbo spools up the engine is taking in air at atmospheric pressure (or a little less if you’re in vacuum; your boost gauge will tell you this). Think of this as a single file line. The instant the turbo spools, compressed air leaves the housing but there is uncompressed air is in line ahead of it so boost hasn't hit the engine yet. Once the engine has consumed all of the uncompressed air then the compressed air that left the housing a second or two earlier hits and all of the air behind it is also compressed until the throttle is released.
This is part of the reason there is turbo lag. Even when the turbo has spooled, it takes a second or two for that compressed air to make it to the engine. Why do you think people fab up custom intercooler pipes trying to shorten the distance between the turbo housing and the throttle body. The less distance between these two, the less time it will take for that first wave of compressed air to hit.

Ask yourself, why is it that I always hear the turbo spool before I feel it kick in?

Lets go back to the single file line analogy.

1st in line at the throttle body is air at vacuum.
2nd in line at the intercooler is air at atmospheric pressure.
3rd in line just outside of the turbo compressor housing is air 2psi above atmospheric pressure.
4th in line inside the turbo compressor housing is air at 5psi above atmospheric pressure.
Now the line moves and the air that was 1st in line has been consumed. New order

1st in line at the throttle body is air at atmospheric pressure.
2nd in line at the intercooler is air at 2psi above atmospheric pressure.
3rd in line just outside of the turbo compressor housing is air 5psi above atmospheric pressure.
4th in line inside the turbo compressor housing is air at 7psi above atmospheric pressure.

This is the general idea. Air is not "stacking up". Hope this makes sense.

[dsmdojo]

*


[Edited by dsmdojo on 07-07-2001 at 04:24 AM]

[Nick 92 TSi AWD]

Two words:
TURBO LAG

maybe you've heard of it? exactly. As was stated above pressure in the intake pipes is already at outside air pressure, 14.7psi or 1bar. "but my intake manifold has a vacuum..." uh yeah that's because the throtle plate isn't fully open and won't let the intake valves suck all the available air in.

I have to wonder if some of you have ANY idea how an engine even works reading your posts.

Originally posted by FastGSXauto
toast, if the turbo compresses the air internally, what happens when it shoots it out the outlet? It cannot posisbly stay at the same pressure because the volume has changed. Just like a tank of compressed air. It is compressed internally, but when you release it to the outside air, the air is no longer at 100+ psi. So the turbo has to fill the intake pipes before any compression will occur. The other factor is that the engine is sucking in air, so the turbo needs to be sending out air at a rate faster than the engine is breathing it in. Only then will the air in the intake pipes start to compress. Toast, you seem to think I am an idiot by telling me I am wrong, but I DO know how turbos work. You say a turbo doesn't just push air, but it does. Even if it compresses it internally, when it releases is it is nothing more than blowing air just like a tank of compressed air. Pressure changes with volume and the minute the air leaves the housing, it's pressure changes. The amount it changes depends on the amount of air getting sucked into the engine. Maybe you can provide some links to some tech info we can all read about.




PS Toast, thanks for trying to explain as best as possible. I think it will take a T-25 upside the head to make it sink in though....:)

[BatmanGSX]

Originally posted by Nick 92 TSi AWD
I have to wonder if some of you have ANY idea how an engine even works reading your posts.

And, on top of it, you poo heads know nothing about fluid dynamics either... SO THERE!

Seriously though, I hit myself on the head with a T-25 and I want one last clarification, a "yes" or "no" will do:

A turbo just compresses air and does not "pressurize" the intake system (like we know that a blower does)?

If the answer is "yes" I am an idiot, if it is "no" then my posts were still accurate.

[Nick 92 TSi AWD]

Originally posted by BatmanGSX
Originally posted by Nick 92 TSi AWD
I have to wonder if some of you have ANY idea how an engine even works reading your posts.

And, on top of it, you poo heads know nothing about fluid dynamics either... SO THERE!

LOL

A turbo just compresses air and does not "pressurize" the intake system (like we know that a blower does)?



A turbo just compresses air, and sends that compressed air through the intake track. I hope you have a helmet, hitting too hard might cause permenant damage.....;)

[BatmanGSX]

Nick, on a side note (I know this isn't the place but... eh, oh well), your "TDO5 20G", is that a TDO6 compressor housing and 20G compressor wheel with a 7cm exhaust housing and TD05H turbine wheel? I want to turn my 18G into that. With the 20 degree clip your turbo is very similar to a BR20G, no? Thanks.

[Nick 92 TSi AWD]

Originally posted by BatmanGSX
Nick, on a side note (I know this isn't the place but... eh, oh well), your "TDO5 20G", is that a TDO6 compressor housing and 20G compressor wheel with a 7cm exhaust housing and TD05H turbine wheel? I want to turn my 18G into that. With the 20 degree clip your turbo is very similar to a BR20G, no? Thanks.

It's the smaller 20G compressor housing, not the TDO6. Yes it's a TDO5H turbine wheel <for now, might switch to a unclipped TDO6 turbine wheel>. It has a 7cm turbine housing.
I believe Buschur has a 25 degree clip and probably some other super secret buschur ingredients or something lol.

[HighPsi91]

Oh god... this thread is funny! I think I have just lost some respect for a few posters here... that I once thought had a grasp on things. Some of these posts make me think of the Willie Coyote cartoons. He sticks his finger in the end of a hose... and you see a big round buldge of water come towards him. Good stuff...damn good stuff.

[16g-95GSX]

Okay, while we are on the subject of compression and combustion...

What if you were to hypothetically install an air tank that was filled with pure O2 or at least a mixture that contained more oxygen and rigged it up to the intake instead of pulling ambient air?

A "scuba tank" like mentioned before would be able to hold a ton of air for dragging and then you would never have to worry about ambient air temps, in fact the hotter the better, do to the fact that the hot air is less dense allowing for faster acceleration through it.

I know this sounds alot like a NOS system, but would Oxygen work better?

[99gst]

No. Simple reason is your car would have a much greater risk of BLOWING UP. Pure O2 is very volitale and flammable. Screw up with NOS and you blow your engine, screw up with pure O2 and you'll be waiting for everyone in the afterlife.

[dyezak]

The previous post is correct. If you injected pure O2 into your engine the results would be less than apealing. N2O is made up of 2 nitrogen attoms and 1 oxygen attom connected by an electromagnetic bond (some might say "chemical", but the properties of electrons denote the sharing of covalance electrons therefore the bond is electric/magnetic). Once N2O is subjected to 870C this compound breaks up into nitrogen and oxygen. The nitrogen acts as a buffer to the oxygens combustion process and makes it controlable. Pure oxygen injected into your engine would create an "uncontrolable burn"...otherwise known as deatonation. Now, this wouldn't be just any form of detonation, this would be severe, like the detonation that goes on with a brick of C4. So please, if you value you're ability to chew solid food, don't inject pure oxygen into your engine.

[16g-95GSX]

Yea I wasn't planning on trying it anytime soon =).

That's awesome, I never knew exactly how NOS worked. I love learning new stuff. Thanks

[HighPsi91]

I have seen a guy on the tractor pulling circuit(Big Rigs) use a huge air tank mounted on the back of his rig for forced induction. Instead of a turbo he just plumbed the engine to a huge tank of compressed air (normal air). It seemed to work pretty good. A bit impractical for a car though... as the tank had to have been around 250 gallons at least. But yeah pure o2 would be cool!! Someone do it and please have a digital camcorder handy!

[BatmanGSX]

Originally posted by dyezak
Once N2O is subjected to 870C this compound breaks up into nitrogen and oxygen.

It's 572F.

[tooueay]

I'll pitch in my $.02. The turbochargers you find on DSMs are centrifugal flow compressors. The housing acts as a diffuser duct. It's the act of the air getting stacked up in the diffuser duct that pressurizes the air above ambient. Very simple design that Pratt and Whitney uses in many of their engines.
Steve TeRonde
AWAC First Officer Dornier 328

[fourg63]

Originally posted by Nick 92 TSi AWD

A turbo just compresses air, and sends that compressed air through the intake track. I hope you have a helmet, hitting too hard might cause permenant damage.....;) [/B]

(Not being a smart ass by any means)
So why do I see all these Supra guys running 27psi blowing off their I/C pipes all the time?Wouldn't that mean that the intake track is being pressurized?Explain it to me like I'm a 2 year old:)

[Nick 92 TSi AWD]

I said I wans't gonna post here at all anymore, but just for you ;).

Think of it like this, you are flowing pressurized air THROUGH the intercooler pipes, but you are not actually PRESSURIZING said air inside of the intercooler pipes.

Another place you could think of is the cooling system. While the water flowing through your coolant hoses is pressurized, it was not pressurized IN THE COOLANT HOSES. It was pressurized inside of the water pump housing (guessing you've probably done a tb job or at least been able to see the inside of a water pump, it makes for a good visual). When your old radiator hose blows up is that because the water inside it was being pressurized? No.

Ok now we're going into two year old mode:
Imagine you're inside a elevator. More and more people keep on cramming inside it. Everyone is crushed into each other. Then the door opens, everyone is packed so tightly that people fall out of the door.
Or just imagine those clowns in the little car at the circus...
Now just think of the pressurived air in the ic pipes or the water in your coolant system as the people in the elevator or the clowns in the car.

Make sense?

[HighPsi91]

Originally posted by Nick 92 TSi AWD


Ok now we're going into two year old mode:
Imagine you're inside a elevator. More and more people keep on cramming inside it. Everyone is crushed into each other. Then the door opens, everyone is packed so tightly that people fall out of the door.
Or just imagine those clowns in the little car at the circus...
Now just think of the pressurived air in the ic pipes or the water in your coolant system as the people in the elevator or the clowns in the car.

Make sense?

I dont get it??? I think I need 1 year old mode.. I gots clowns in my engine mommy!!!

[99gst]

Refer back to my "single file line" analogy above.

[HighPsi91]

Originally posted by Nick 92 TSi AWD
Another place you could think of is the cooling system. While the water flowing through your coolant hoses is pressurized, it was not pressurized IN THE COOLANT HOSES. It was pressurized inside of the water pump housing (guessing you've probably done a tb job or at least been able to see the inside of a water pump, it makes for a good visual). When your old radiator hose blows up is that because the water inside it was being pressurized? No.



The water pump in an engines cooling system is not where the pressure is created. True, a pump of any kind will increase pressure of the pumped medium in some way. The pressure in the cooling system is from the heated water. Case and point. Start the car when its cold... take of the radiator cap. Does it seem pressurized?? NO. Now go drive around till its hot... Now ignore the warning right on the cap that says "do not remove when hot" and remove the cap... what happens??? Heck you can even shut the engine off and pump nothing... but the results will be the same.
((Disclaimer)) "If you do this and get burned dont blame me"