Cars define turbos, turbos don't define cars

[crankbender]

Over the many years I have been around turbo cars I have had to explain this countless times. Maybe I will write up a faq for the site if the moderators are interested.

First off a good understanding of what a turbo is and how it works is needed to delve into this subject further.

Definitions
r - radius from the center line of the turbo.
Theta – angular position of the turbo
(anything)dot – the change is this position versus time commonly know as velocity either angular or linear (thetadot is commonly known as RPM).
Compressor (wheel) – intake side of the turbo
Turbine (wheel) – exhaust side of the turbo


A turbo is a centrifugal pump designed to move air (the fluid) through its own mass. The compressor wheel is designed to grab particles (molecules) of air. These particles are then spun around the center shaft at very fast velocities. Using the relation that acceleration in the outward direction is equal to r*thetadot^2 we can easily see that the air is forced out harder if the turbo is spinning faster (larger thetadot) or the wheel is larger (increasing the max r). This compressor movement takes energy to cause. This energy is taken from a pressure and temperature drop in the exhaust. The exhaust is caused to flow into the exhaust wheels by a nozzle. The nozzle causes the exhaust gasses to increase in velocity and when they impact the exhaust fins the momentum (kinetic energy) is transferred to the exhaust wheel. The air is then allowed to flow out of the exhaust.

What happens to the air after it exits the compressor housing? The answer to this question is really a fairly simple one and involves the intake and engine. After exiting the compressor housing the air flows up the intake piping to the intake valves. This flow is caused by a pressure gradient (drop). The amount of pressure drop experienced across the intake is a function of temperature, velocity, distance, and structure. It suffices to say at this point that it serves best to decrease this pressure drop as close as possibly to zero. At this point the air is at a given pressure in the intake plenum. The valves open at a given rpm and the air begins to enter. If we assume that the pressure in the plenum is held constant by the turbo the airflow into the engine becomes dependent on pressure only. At this point it becomes obvious that the compressor side of the turbo must be matched to the engine and intake!!!!!

Once ignition occurs inside the cylinder and the exhaust valves open the gas is free to escape. This air must enter the exhaust manifold and flow through the turbine. Because the energy in the air must not be conserved we can not power the turbo without taking energy out. This conversion is done in 2 steps. First the air is forced into a nozzle that transfers pressure into velocity. This transfer is done through bernoulli’s equation and an efficiency (feel free to do some research). Once this takes place the air in shot at the turbine wheel and some of its kinetic energy is transferred to the wheel spinning it. During this process the air cools and its pressure drops further. As the air then must flow out the exhaust (once again because of a pressure gradient), and the tip of the exhaust is at atmospheric pressure it can be seen that the pressure at the exhaust valves is much higher than atmospheric. Once again the it should be seen here that the exhaust side of the turbo must be matched to the engine and car!!!

So what does this all mean? Well first off it must be understood that a turbo is a controlled device. This is to say that the engine, intake, and exhaust set up will flow a given amount of air at a given pressure (small changes can be made by the selection of the correct exhaust wheel and housing). That said the turbo must be able to change in thetadot (angular velocity or RPM) to flow the correct amount of air to keep the intake pressure constant (as we made this assumption earlier). This is where the wastegate comes in. (IF ANYBODY WANTS ME TO EXPLAIN HOW THIS WORKS LET ME KNOW) we will suffice to say that the turbo matches perfectly.

Now saying a given engine (at a given intake pressure) flows X amount of air at 3000 RPM (typically the lowest rpm seen during a run) and Y at the highest shift point. This flow is the possible use of the engine and not what the turbo will flow…….

The job of anybody selecting the turbo is to look at the compressor maps (and turbine efficiencies with a given load and flow) to find the highest efficiency at X and Y. If you look around at some compressor maps you will see that a 2.0L does not need a HUGE turbo without a lot of mods. If you do happen to choose a turbo that is too small you will fall below the efficient range of the turbo and not flow well. If you have too small a turbo you will start adding more heat to the air and make up for the inability to flow mass by increasing volume per unit mass.

COMING NEXT….calculations on what your engine will flow assuming a perfect world.


Hope this all helped
Please feel free to let me know if this was at all helpful, if I failed to explain anything correctly, if I made a mistake, or if you just want to have a good intelligent conversation on the topic.

 

[DCJ98GST]

Originally posted by crankbender

Now saying a given engine (at a given intake pressure) flows X amount of air at 3000 RPM (typically the lowest rpm seen during a run) and Y at the highest shift point. This flow is the possible use of the engine and not what the turbo will flow…….

I think you may be missing an important point. The VE of the engine is related to the turbo.

There are two things that change when bolting on a bigger turbo.

1. A lot of the gain of bolting on a bigger turbo is the increased Volumetric Efficiency (VE) of the engine due to the less restrictive turbine housing and more efficient turbine wheel at the same compressor Pressure ratio (psi). This lets the engine flow more at the same psi therefore more cfm and more power.

Small turbines and turbine housings choke off the engine's exhaust flow. Try pushing 550 cfm out the little T-25's exhaust housing and you will get like 100 psi of exhaust manifold pressure. This will choke off the engine because it can't keep up with the pumping losses to create the pressure.

Otherwise we would all be matching up 50 trim wheels to really small turbines for quick spool.

For every set up there is a linear relationship between the intake manifold pressure and the exhaust manifold pressure. For a T-25 say it may be 15 psi (intake) will give you 45 psi exhaust mani pressure. Move to a 50 trim with a big TD06H turbine wheel and it may be 15 psi (intake) will give 13 psi of exhaust pressure. This lets the engine flow a lot more air. And more power. This is the trick to big pump gas power. Flow more air at a lower psi.

Big power will be made at crossover, which is when the intake pressures exceed the exhaust pressures.

The other less important factor is:

2. The charged air is heated less creating more air per volume as you have described. This is not as much of a gain especially if you have an efficient intercooler.

Originally posted by crankbender


The job of anybody selecting the turbo is to look at the compressor maps (and turbine efficiencies with a given load and flow) to find the highest efficiency at X and Y. If you look around at some compressor maps you will see that a 2.0L does not need a HUGE turbo without a lot of mods. If you do happen to choose a turbo that is too small you will fall below the efficient range of the turbo and not flow well. If you have too small a turbo you will start adding more heat to the air and make up for the inability to flow mass by increasing volume per unit mass.

Matching up a good hot side with the cold side of a turbo is FAR MORE IMPORTANT than a few efficiency points a turbo map.

 

[crankbender]

This is to say that the engine, intake, and exhaust set up will flow a given amount of air at a given pressure (small changes can be made by the selection of the correct exhaust wheel and housing)

1. A lot of the gain of bolting on a bigger turbo is the increased Volumetric Efficiency (VE) of the engine due to the less restrictive turbine housing and more efficient turbine wheel at the same compressor Pressure ratio (psi). This lets the engine flow more at the same psi therefore more cfm and more power.

This is what the previous line was saying. My wording may not have been great.

Big power will be made at crossover, which is when the intake pressures exceed the exhaust pressures.

I personally have never seen crossover. Do you have any proof that this ever happens. quick analysis (not taking all factors into account and therefore not infallable) would dictate that the exhaust pressure has to be greater than the intake pressure. If this does not happen you are basically creating energy which can not be done. again this does not take into account the increased mass of the fuel being added to the air.

For every set up there is a linear relationship between the intake manifold pressure and the exhaust manifold pressure. For a T-25 say it may be 15 psi (intake) will give you 45 psi exhaust mani pressure. Move to a 50 trim with a big TD06H turbine wheel and it may be 15 psi (intake) will give 13 psi of exhaust pressure.

You have exaggerated a bit here imho. are these numbers from experience or just something you are making up to demonstrate a point?
once again tho i agree that you do get some power gains from this if the car can handle the bigger turbo. This said 15 psi on a 50 trim may take more exhaust pressure than a 14b because of the impingment of the large exhaust wheel spinning so slowly. Too big can rob you of power too...not to mention lag.

2. The charged air is heated less creating more air per volume as you have described. This is not as much of a gain especially if you have an efficient intercooler.

I completely skipped this didn't i....hmmm. Also the intake temp of the turbo has a huge difference on how much of a bite the turbo can get.

Matching up a good hot side with the cold side of a turbo is FAR MORE IMPORTANT than a few efficiency points a turbo map.

Absolutely true. I didn't talk about matching the hot and cold side because most of the people building or designing hybrid turbos know all this....or atleast should. If you pick one off the shelf it should be a fairly decent match (assuming you go with something tried and true as you should before you have more experience).

 

[DCJ98GST]

It appears that your 1st post implies that the engine flow does not change with a bigger turbo, and only volumetric efficiency changes due to the bolt on parts like exhaust, intake, cams ect.

Which I think is fundimentally wrong because the hot side of the turbo is such a restriction on the engine that moving to a larger turbine wheel and less restricive exhaust housing enhances engine flow and reduces potential knocking.

Originally posted by crankbender
small changes can be made by the selection of the correct exhaust wheel and housing)"

This is by no means a small change.

Originally posted by crankbender

I personally have never seen crossover. Do you have any proof that this ever happens. quick analysis (not taking all factors into account and therefore not infallable) would dictate that the exhaust pressure has to be greater than the intake pressure. If this does not happen you are basically creating energy which can not be done. again this does not take into account the increased mass of the fuel being added to the air.

I dont know why you would think that energy would be created just because there is a pressure differential. Remember the exhaust manifold air is hot and flow is less restricted therefore psi does not equal flow.

If you read Maximum Boost by Corky Bell he talks about crossover so it can be done.

Originally posted by crankbender

You have exaggerated a bit here imho. are these numbers from experience or just something you are making up to demonstrate a point?
once again tho i agree that you do get some power gains from this if the car can handle the bigger turbo.

The numbers are totally made up to discuss a hypothetical. I would not be supprised if they are somewhat accurate though. I would love for somebody to log the exhaust manifold pressure with respect to intake pressure for different turbos.

Originally posted by crankbender
This said 15 psi on a 50 trim may take more exhaust pressure than a 14b because of the impingment of the large exhaust wheel spinning so slowly. Too big can rob you of power too...not to mention lag.

I think you have it backwards. A larger turbine would flow more because of the same reason a larger compressor wheel flows more. Just reversed. Small turbine wheels have to achieve high tip speeds to flow more. In the world of rotating inertia's, the higher rpm takes alot more energy to achieve than the slow moving big turbines. Try to imagine forcing 500 cfm through a 1.5" diameter fan instead of a 2.5" diameter fan. The small one will have to rotate alot faster to flow the same amount.

And yes you will get lag with the big turbine, but more flow.

 

[14.5 drift]

your both right, the bigger turbo will flow more air, and the bigger turbo requires more force from exhaust to turn, correct? While the bigger turbo will be able to net the most hp, the smaller turbo is less restrictive, in the sense that minimul force is required to turn such a small wheel

 

[DCJ98GST]

Originally posted by 14.5 drift
your both right, the bigger turbo will flow more air, and the bigger turbo requires more force from exhaust to turn, correct? While the bigger turbo will be able to net the most hp, the smaller turbo is less restrictive, in the sense that minimul force is required to turn such a small wheel

No, the smaller turbo is MORE restrictive. You have to think in terms of flow. Once the bigger turbo is spooled up it will take less exhaust manifold pressure to create the same intake psi.

 

[DCJ98GST]

Maybe if I explain it this way it will help:

Say you have a 16G turbo

If you look at the turbo map you will see that it takes a shaft speed of about 115,000 rpms maintain a PR of 2.0. And say at 4500 rpm (engine speed) you are moving 250 cfm.

http://www.stealth316.com/images/td05h-16gsmall-cfm.gif

Now say you move to a bigger turbine, at 115,000 rpms (still needed to maintain the PR of 2.0) with a bigger turbine you will flow more air per rpm (turbo shaft). This will move the flow curve outward on the map. Now at 4500 rpm (engine speed) you will be flowing say 300 cfm. Same psi, same engine, same compressor, same turbo map, same everything except turbine. More power.

 

[14.5 drift]

I hear you on the housing, but if you take a t25 and hold it up to you mouth and blow it will probly spin, if you take a t88 up to you moth and blow probly get nothing. It is not the most impoortant aspects in turbo efficiency but none the less the larger turbo requires more effort from the exhaust fumes to spin.

 

[DCJ98GST]

Originally posted by 14.5 drift
I hear you on the housing, but if you take a t25 and hold it up to you mouth and blow it will probly spin, if you take a t88 up to you moth and blow probly get nothing. It is not the most impoortant aspects in turbo efficiency but none the less the larger turbo requires more effort from the exhaust fumes to spin.

You are thinking of psi and flow as the same thing. The exhaust manifold may have less psi but be flowing more, more energy to spin the turbine but less psi.

To continue your example, say you blow through the T88 it will take more breath because it is less of a restriction. Just imagine how much air you will have to push behind the T-88 to get the same psi behind it as the T-25. It just flows more. More top end. And more lag.

 

[14.5 drift]

Originally posted by 14.5 drift
I hear you on the housing, but if you take a t25 and hold it up to you mouth and blow it will probly spin, if you take a t88 up to you moth and blow probly get nothing. It is not the most impoortant aspects in turbo efficiency but none the less the larger turbo requires more effort from the exhaust fumes to spin.

I have never bee real good with under standing these types of stuff please bare with me. About the cfm, when you say more of it, that is very vauge. Do you mean that since the larger wheel is more efficient it has a denser air charge, hence the reference to cfm's. I could never under stand this expression. Some one tried telling me it was actually more air traveling through the pipe at the same psi, but that is impossible with out simply bigger pipes or a port of some type, only thing that made sense was not that more air was being pushed rather that denser air because less heat was being produced. Is any of this even close?

 

[DCJ98GST]

Originally posted by 14.5 drift
I have never bee real good with under standing these types of stuff please bare with me. About the cfm, when you say more of it, that is very vauge. Do you mean that since the larger wheel is more efficient it has a denser air charge, hence the reference to cfm's. I could never under stand this expression. Some one tried telling me it was actually more air traveling through the pipe at the same psi, but that is impossible with out simply bigger pipes or a port of some type, only thing that made sense was not that more air was being pushed rather that denser air because less heat was being produced. Is any of this even close?

Try thinking of it this way. If you blow through a straw (small turbine) it will be hard to flow a lot of air because it is small. Air molecules build up and create pressure. Your lungs will strain due to the psi in you mouth (exhaust manifold).

Now if you blow though a 3" pipe (big turbine), it flows more so air pressure will not back up. And you wont get red in the face trying to push the air through

 

[14.5 drift]

Originally posted by DCJ98GST
You are thinking of psi and flow as the same thing. The exhaust manifold may have less psi but be flowing more, more energy to spin the turbine but less psi.

To continue your example, say you blow through the T88 it will take more breath because it is less of a restriction. Just imagine how much air you will have to push behind the T-88 to get the same psi behind it as the T-25. It just flows more. More top end. And more lag.

So the turbine is limited by rpm? Like say if a t25 could spin at 29834982765492857rpm then it would be able to push alot of hp, right? So do all turbo's have a roof of 115,000 rpm's? Once you max one out, then you need to upgrade to a bigger housing, correct?

Lol, this was probably gonna be a reel informative thread, sorry for all the lame Q's.

 

[14.5 drift]

Originally posted by DCJ98GST
Try thinking of it this way. If you blow through a straw (small turbine) it will be hard to flow a lot of air because it is small. Air molecule build up and create pressure. Your lungs will strain due to the psi in you mouth (exhaust manifold).

Now if you blow though a 3" pipe (big turbine), it flows more so air pressure will not back up. And you wont get red in the face trying to push the air through

Agreed. And that would make sense to me if the air pressure was reading from the turbo housing some where, but it is not, the reading is taken from the intake pipe wich is 3 inches. A three inch intake filled with 10 psi should make the same hp no matter which compressor is boosting unless one provided cooler intake temps wich will cause denser air molecules. The big turbo will do 10 psi at 60k while the t25 is spinninbg at 115k and the higher speeds creates alots of heat. This is not right though is it?

 

[DCJ98GST]

Most turbines speeds are limited to the tip speed hitting the choke barrior which is about the speed of sound 1100 ft/sec. But the speed of sound changes with air density so it may be a little higher than that inside the compressor housing.

 

[DCJ98GST]

Originally posted by 14.5 drift
Agreed. And that would make sense to me if the air pressure was reading from the turbo housing some where, but it is not, the reading is taken from the intake pipe wich is 3 inches. A three inch intake filled with 10 psi should make the same hp no matter which compressor is boosting unless one provided cooler intake temps wich will cause denser air molecules. The big turbo will do 10 psi at 60k while the t25 is spinninbg at 115k and the higher speeds creates alots of heat. This is not right though is it?

No its not re-read my first post. Remember we are talking about the hot side of the turbo.

 

[14.5 drift]

so the psi meter( probly not what it is called) is located inside the turbo?

 

[MNGSX]

No you drill the collector of your exhaust manifold and thread it for a NPT port. Attach atleast 10" of copper or aluminum tubing (I think that is better due to heat disipation) and place a pressure sensor there at the end of the tube. This way the sensor does'nt end up getting as baked as a professional athlete .

 

[crankbender]

I dont know why you would think that energy would be created just because there is a pressure differential. Remember the exhaust manifold air is hot and flow is less restricted therefore psi does not equal flow.

Your mass flow on the other hand is quite close. Knowing that an energy transfer must happen from individual control volumes of air in the exhaust side to the intake side it becomes easy to see that even at 100% efficiency you should not be able to get much more of an increase on the intake than the drop on the exhaust. In the real world we have huge losses and your pressure increase on the intake side is more than your intake pressure because of the pressure drop in front of the compressor wheel.

I have never bee real good with under standing these types of stuff please bare with me. About the cfm, when you say more of it, that is very vauge. Do you mean that since the larger wheel is more efficient it has a denser air charge, hence the reference to cfm's. I could never under stand this expression. Some one tried telling me it was actually more air traveling through the pipe at the same psi, but that is impossible with out simply bigger pipes or a port of some type, only thing that made sense was not that more air was being pushed rather that denser air because less heat was being produced. Is any of this even close?

When we refer to more air we are probably speaking of mass flow rates. If you only refer to density or volumetric flow you get errors. You really need to account for them both and that is done using mass flow. (more air is more individual molecules of oxygen in a given ammount of time).

so the psi meter( probly not what it is called) is located inside the turbo?

IMHO the correct place to tap fot intake pressure wastegate operation is just before the TB. This will take the resistance of the intake out of the equation and provide you with the pressure you want at the tb. This setup however is more prone to boost creep as you are increasing the lag time it takes for the wastegate to react. There are many opinions on where this should be done and varys based on what other systems/problems the car may have.

Try thinking of it this way. If you blow through a straw (small turbine) it will be hard to flow a lot of air because it is small. Air molecules build up and create pressure. Your lungs will strain due to the psi in you mouth (exhaust manifold).

You are neglecting the turbine in that housing. As the turbine itself does provide a restriction. First off the difference in nozzle cross sectional area does not differ nearly this much in turbos. Take some apart the nozzle cross sectional area is small even in the targer housings. Now the flow through the nozzle is interrupted at times (during the revolution) by the turbine wheel as it passes by. If the turbine wheel does not have sufficient speed as it passes the nozzle it backs it up significantly. This causes the pressures inside the exhaust manni to build for a small moment in time. Then the wheel keeps moving and the flow is released. As this is happening very fast you can't see this change by it does occur. The pressure you see is the average of this. In effect if the turbine wheel is moving too slowly it acts to add a restriction in the nozzle part of the time. This effects your mass flow rate significantly. To make up for it an increased flow must be realized when the wheel moves by. This increased flow must be caused by an increase in pressure. Using a smaller turbine wheel in some aplications will lower the pressure on the exhaust side.

You are also neglecting that as you switch to your 3" pipe you will never spool the turbo. This does not result in more power even tho you have decreased exhaust pressure.

It appears that your 1st post implies that the engine flow does not change with a bigger turbo, and only volumetric efficiency changes due to the bolt on parts like exhaust, intake, cams ect. Which I think is fundimentally wrong because the hot side of the turbo is such a restriction on the engine that moving to a larger turbine wheel and less restricive exhaust housing enhances engine flow and reduces potential knocking.

I did not say it was 100% based on this. I have said (and requoted) where i said the exhaust housing and turbine have an effect here. This effect however is small between turbine housings and wheels that are appropriate for the application. Please don't compare a T25 to a t3/t4. If you are running 10 psi you will get zero gains across the power band by switching to a T3/t4 (this was the original point).

Agreed. And that would make sense to me if the air pressure was reading from the turbo housing some where, but it is not, the reading is taken from the intake pipe wich is 3 inches. A three inch intake filled with 10 psi should make the same hp no matter which compressor is boosting unless one provided cooler intake temps wich will cause denser air molecules. The big turbo will do 10 psi at 60k while the t25 is spinninbg at 115k and the higher speeds creates alots of heat. This is not right though is it?

The ammount of flow is based on the VE (volumetric efficiency or whatever else you want to use to describe it) of the engine. This is in some small part changed by the turbine housing and wheel.

Look at it this way. If you decrease the pressure at the exhaust ports and keep the intake ports the same you will flow more. The discussion has moved to how much a turbo change will effect the exhaust port pressure. This appears to be where we are differing.

Most turbines speeds are limited to the tip speed hitting the choke barrior which is about the speed of sound 1100 ft/sec. But the speed of sound changes with air density so it may be a little higher than that inside the compressor housing.

Speed of sound in air is a function of temperature only. the correct formula is c=sqrt(k*R*T) Temperature can be a function of pressure but is not soly based on that so you are almost correct

Everybody is agreeing that the exhaust side will change some when you change the turbine housing and wheel. Can anybody here help us out with some real world data on this.


Peter
I do love an intelligent conversation.

14.5 please do keep asking questions. This is here to help people understand the subject =D.

 

[DCJ98GST]

Originally posted by crankbender


You are also neglecting that as you switch to your 3" pipe you will never spool the turbo. This does not result in more power even tho you have decreased exhaust pressure.

My pipe/straw analogy was just to help 14.5 visualize the difference between flow with more and less restriction. I was not talking about just the turbine housing. Re-read what I said in context with his questions.

Originally posted by crankbender

Please don't compare a T25 to a t3/t4. If you are running 10 psi you will get zero gains across the power band by switching to a T3/t4 (this was the original point).

Using a smaller turbine wheel in some aplications will lower the pressure on the exhaust side.

I still dont see how you can say this. There is a significant difference between a t-25 and a t3/t4 even at 10 psi.

Yes lets all just put a Gt40 56 trim compressor on a t-25 turbine hot side because the hot side doesn't matter. THATS IT We have all been doing it wrong for all this time. We all can spool a huge turbo at 3000 rpms and have just as much top end because there should only be a small change. right?

All those people that choose the TD06H turbine wheels instead of the TD06 for a Green just dont know what they are doing, they can get the same hp out of a smaller wheel. right Garrett should stop making stage III and V wheels they are obsolete all the power can be had with a stage II wheel.

I dont mean to get sarcastic, but what you are saying dosent make sense. Sorry just trying to help.

 

[14.5 drift]

Okay, am good with the flow thing. Alot of missinformed sob's tried to explain this to me, the best they could come up with was, "I dont know it just flows more air" They tried to explain to me the blowing through the straw thing but, doesn't matter if there is a ten year old or a full grown man blowing on that straw because if they are both blowing only 10 lbs of pressure then there is no air ecaping the end of the straw any faster out of the adults straw. The only actual difference could be the cooler air charge would register as more Oxygen through the maf. You guys dont know how much that has bee bugging me

 

[14.5 drift]

As for the above topic, this is what I gather, sounds like you are both making points again, different points but argueing like one is disagreeing but you both agree but are talking about some thing different, sort of like who's on first. Sonds like DCJ is talking about efficiency, while crank is thinking about response, while the t25 at ten psi will be alot more responsive, still within efficiency range, versus a green super stage three would be more efficient as far as freely flowing exhaust through the turbine and making a denser air charge, all of the while just being more efficient. But what is more important, efficiency or response?


If I am lost then never mind me. Carry on amongst your selves. I will reread tomarrow cause I am pretty tired right now after work soo...

 

[crankbender]

quote:
--------------------------------------------------------------------------------
I dont know why you would think that energy would be created just because there is a pressure differential. Remember the exhaust manifold air is hot and flow is less restricted therefore psi does not equal flow.
--------------------------------------------------------------------------------




Your mass flow on the other hand is quite close. Knowing that an energy transfer must happen from individual control volumes of air in the exhaust side to the intake side it becomes easy to see that even at 100% efficiency you should not be able to get much more of an increase on the intake than the drop on the exhaust. In the real world we have huge losses and your pressure increase on the intake side is more than your intake pressure because of the pressure drop in front of the compressor wheel.

Clarifying this. The pressure is the stagnation pressure not the absolute pressure. This is important because the runners are not long enough to maximize exhaust valve slap vacume so the air sitting outside the valves when they open is at higher pressure than the pressure at the turbine housing where you have constant flow. If you are talking absolute pressure crossover can occur...however the air in the cylinder that you are evacuating is not at the pressure in the exhaust but it has the same stagnation pressure....did that make sense? basically you have to use stagnation pressure for this stuff because the air moving is not being worked upon and velocities (or lack thereof) will effect flow substantially.

I still dont see how you can say this. There is a significant difference between a t-25 and a t3/t4 even at 10 psi.

Yes lets all just put a Gt40 56 trim compressor on a t-25 turbine hot side because the hot side doesn't matter. THATS IT We have all been doing it wrong for all this time. We all can spool a huge turbo at 3000 rpms and have just as much top end because there should only be a small change. right?

All those people that choose the TD06H turbine wheels instead of the TD06 for a Green just dont know what they are doing, they can get the same hp out of a smaller wheel. right Garrett should stop making stage III and V wheels they are obsolete all the power can be had with a stage II wheel.

I dont mean to get sarcastic, but what you are saying dosent make sense. Sorry just trying to help.

Did i ever say to do that. First off those are not balanced at all and you will have problems with the power conversion from exhaust to intake. it is all about balance. People who choose the TD06H arn't running 16 PSI with nothing but free mods either. Balance baby it's all about balance. Simple fact is most people know what goals they have for the car and not what turbo they want to use. This is ment to explain why they shouldn't just shoot big if the goals for the car don't warrant it.

...yeah ya got a little sarcastic there. its ok it just doesn't help to expalin it to people when you use obviously stupid combinations because they may not know they are stupid.


14.5
You are fairly close to the truth here. I believe the problem stems from i am speaking of car performance across the power band and DCJ is speaking of absolute maximums without reguard to how usable it is. I mean a torque of 300 at 8 psi is nice but if you don't get it till 7k rpms and until that point you are at 100 how usable is it? Allow me to elaborate and take into account the title probably should have been cars define turbos....

Assume the fuel mods are there to support the car

A bone stock car that you want to run 10 psi on and use on the street only
A 14b is going to be a better performer across the powerband of a street car than a 70 series

Bone stock 14 psi
probably want a 16 g here...once again you are too restricted to use the 70 series efficiently (you will make power wayyyyyy up on the rpm curve and street cars need it lower)

free mods + crappy exhaust/intake mods 14 psi maybe 18 on the occasional weekend
here you are in 16g 18g territory. you may be able to stretch to a 20g if you don't mind lag.

exhaust/intake (cheap) 14 psi daily 20 weekend
here you are starting to get into 20g territory at best. tho you can still use a 14b if you really hate turbo lag and don't want to use the rpms all the way up on an older engine. Probably the best turbo for this catagory is a 16-18g for a good compromise

etc etc

max pump gas street/track car (custom parts and everything)
into 50-60 trims here

full out track car (dedicated trailer) 9.5k redline
finally you are all the way into the 70 series....

the trailer is there to drag it home after you break the drive line.


Does this help define what i am speaking of. Although a bigger turbo is slightly more efficient at a given pressure you have to be moving enough volume to keep the compressor and turbine moving. The car then is less efficient if you shoot too big.

 

[boostnbuds]

I would like to thank you guys for the most informative thread i have ever read. Its people like you, who keep people like me rollin, and efficiently too...

 

[crankbender]

YW I am glad to see it is getting read.

 

[candela]

Good post and good to see you back Crankbender... I have not seen a ppst from you in like a year