A big turbo can make 10 psi at, say, 8500 rpms, which will result in more horsepower, since making the same torque at higher rpms makes more power. A big turbo is also less of a restriction in the exhaust, so it saps less power. Combine that with an engine designed for better high rpm flow, tuned more aggressively than the stock ECU, and you can get lots more power at low boost levels than on a stock setup.

 

Turbo A, a 35mm tubro, can make 10 PSI. Turbo B, a 75mm turbo, can also make 10 PSI. Turbo B, with its much larger compressor, moves more air at 10 PSI than turbo A does. Not exactly sure how that works in great detail though. My theory goes something like this.

Maybe its because when the valves open and the pressure tries to equalize itself between the intake runner and the cylinder, the bigger turbo can keep flowing more air into the intake manifold and keep the same pressure even though the valve is open and causing pressure to be lost for a split second, where as maybe a smaller turbo wouldnt be able to push enough air out to keep the pressure in the runner the same without equalizing to a lower point for a split second while the valve is open adn air is moving into the cylinder.
Say the pressure in the intake runner is 10 PSI, and the pressure in the cylinder is at 0 PSI. Now the valve opens, and normally, so (if the runner and cylinder had the same volume) the pressure equalizes to say 5 PSI in the intake runner and 5 PSI in the cylinder. With a smaller turbo, something like this may happen. But with a larger turbo, my guess is that it moves enough air that when the intake valve opens and the pressure equalizes, it keeps the pressure in the runner at 10 PSI, and thus makes the pressure in the cylinder rise to 10 PSI, rather than them both dropping off for the split second that the valve is open.

*deep breath*

Id still like to see what RickyB says though.

 

PSI is not the same as airflow. A big turbo can move 500 cfm's at 15psi and a ct26 might move 300 cfm's at 15psi. CFM=Cubic Feet per Minute. Airflow is what makes power, not boost. Remember, boost is only air that isn't making it into the engine.

 

I believe it is because of the efficiency and airflow of the turbo you select. Compressing air creates adiabatic heat, and turbos add additional heat to the intake charge known as polytropic heat. The intake air temperature affects its density, which affects how much air gets ingested by the cylinders when the intake valve opens. The cooler the air, the more dense it is, so more air gets into the cylinder. More air makes more power.

A small turbo will make 16 psi at say 6000rpm, but the if it is well outside it's efficiency range that intake air may be about 250F degrees. Compare that to a bigger turbo which may also make 16 psi at 6000rpm, but the temperature may be just 150F degrees. That's a 100F degree difference.

As an example, fill a balloon with air to almost bursting at room temperature, and then put it in the freezer and see how it shrinks as the temperature drops and the air inside gets denser.

Additionally, bigger turbos also usually have bigger turbines which flow more exhaust, and therefore produce less backpressure, which also allows the engine to breath easier therefore making more power.

Of course, sizing a turbo for your application involves a lot more.

 

I beleive it is because the larger the turbo, the more cubic feet per minute it can pull in. I know, sounds crazy, but bear with me.

A t25 turbo pulls in somewhere between 250-300 cubic feet per minute at 15 psi. A t3/t4 with a 50 trim compressor pulls in somewhere between 750 and 850 cubic feet per minute at 15 psi. At the same psi, the t3/t4 is pulling in nearly 3x as much air, and we all know that the more fuel you burn the more hp you make, and to make that fuel burn, you need more air.

So wouldnt it make sense that the larger turbo pulling more air, would be able to make more horsepower with at the same psi?

 

Phil - go play on Ray Hall's site for turbo selection and horsepower calculation: http://www.turbofast.com.au/tfcalc.html you find that the difference between say 65% turbo compressor effeciency and say 90% turbo effeciency is about THREE hp.

Andrew (93smgturbo) wins. Think of the turbo in a different sense. A CT26 is like a straw at 15 psi..blow through a straw at 15 psi, now blow 15 psi through a foot long water hose you'll get more air. Next try blowing 15 psi through a fire hose and you'll flow even more air. As the hoses (or turbos) get bigger the different boost/pressure levels move more air. This is a VERY VERY basic way of looking at it and I am 100% sure that this example could be proven wrong, but it gets the CORRECT idea across.

Aaron

 

so does the cfm increase if you raise the boost or is it a static number?

 

Air will be the same temperature at any given PSI on any turbo at a reasonable boost pressure. A CT26 will create the same amount of heat at 15 PSI as a T88. These things about backpressure and efficiency ranges arent very relevant either. Sure they are true, but you could just as easily have a big turbo with a small A/R that chokes the engine at 6000 as you could have a GT28 with a 1.06 A/R that lets the engine breath all the way through redline. That doesnt mean that at 10 PSI the GT28 will make more peak power than Most turbos that are sized correctly for the engine wont really ever get out of their efficiency ranges at a reasonable pressure. A GT28R and a SB50 are both in their islands on the 3SGTE, but the SB50 will move more air at a given PSI, and thus fill the cylinders more completely.

*edit*

And what Aaron said about the straws and hoses is a good basic representation, but what Ive been saying is trying to tell you why .

 

Your explanation is rather surprising Aaron. As I understand it, your example is not the same as the turbo equation for a car. The hose in this case (which is our intake system) always remains constant - it doesn't vary like a straw, a garden hose, fire hose etc.

As far as I know, 14.7PSI (1 atmosphere of pressure above sea level) is 14.7PSI, no matter what turbo produces that pressure. We measure our boost in the intake manifold, which tells us how much air pressure exists in the manifold above 1 atmosphere. The only variation is how much power it takes to generate that pressure, and the density of the air at that pressure.

 

The factor that most greatly impacts a turbo's ability to make more or less power at a given PSI is how it affects the engine VE curve. The turbine wheel and housing has the most to do with changing the VE. A "smaller turbo" will (typically) also have a smaller turbine housing and turbine wheel. This means that the VE of the engine will be reduced across the board when compared to a larger, more free-flowing turbine housing due to increased backpressure.

The CFM rating of the turbo's compressor has almost nothing to do with one turbo being able to make more power compared with another turbo at the same boost level. The engine dictates the CFM of air flowing through it, not the turbo. Granted, it is possible to choose a compressor that flows too much or too little air compared with what is required by the engine, but it is the ENGINE that dictates what the CFM value will be at any given RPM and load point.
This is why most CT26 turbo upgrades make little difference to the overall power output of the engine, even though many of them have compressor wheels capable of supplying a great deal more "CFM" when compared to the stock compressor. The reason is that 99% of all CT26 upgrades don't change the turbine wheel or housing at all. So, sure, the compressor wheel is a bit more efficient, so you pick up a few horsepower from a cooler charge, and the torque requirements placed on the turbine wheel are slightly less... but you don't see anything more than this.

I believe you're talking seeing some dyno plots where a car makes 300RWHP at 10PSI of boost using a big turbo while a CT26 only manages slightly more than half that at 10PSI. So, now you know, bigger turbo = less backpressure at a given boost = increase in VE across the board = more power

 

You have to realize that Aeron isnt talking about our intake system after the turbo, but about the turbo itself. Pressure is pressure, actual airflow at that pressure can and will vary due to air temprature. Hot air take up more room that colder air, and have less oxygen in it as a result.
A bigger turbo, will move more air trough the compressor with less effort, creating less heat, and as such is more efficient. But the cost is more lag, since it takes longer to get it into its operating cycle.

T

 

Air will be the same temperature at any given PSI on any turbo at a reasonable boost pressure. A CT26 will create the same amount of heat at 15 PSI as a T88. These things about backpressure and efficiency ranges arent very relevant either. Sure they are true, but you could just as easily have a big turbo with a small A/R that chokes the engine at 6000 as you could have a GT28 with a 1.06 A/R that lets the engine breath all the way through redline. That doesnt mean that at 10 PSI the GT28 will make more peak power than Most turbos that are sized correctly for the engine wont really ever get out of their efficiency ranges at a reasonable pressure. A GT28R and a SB50 are both in their islands on the 3SGTE, but the SB50 will move more air at a given PSI, and thus fill the cylinders more completely.

*edit*

And what Aaron said about the straws and hoses is a good basic representation, but what Ive been saying is trying to tell you why .

Not exactly the way I understand it.

Let's look at thermal efficiency... The laws of physics and thermodynamics dictate that when air is compressed it gets hotter. This is referred to as adiabatic heat. Our turbochargers compress air, and therefore the compressed air discharged from the turbo is hotter than air entering it. In the ideal world the discharged air would be at the ideal adiabatic temperature, which we would equate with 100% efficiency. But, that is not the case, turbos aren't quite that efficient. Beyond adiabatic heat, our turbos transfer additional (polytropic) heat from turbulance, friction, etc. into the charge. This is why turbos are rated for efficiency, and turbo compressor maps show us how much polytropic heat is added to the air in addition to the adiabatic heat. As you look at turbo maps you realize the islands with the highest efficiency are in the middle of the map, and are typically around 75%. As you move away from the highest efficiency island the efficiency drops and near the edges it may be as low as 60%. From these maps we can determine how much polytropic heat is added by the turbo compressor as follows:

Let's assume inlet (uncompressed) air temperature is 70F degrees, and let's assume the adiabatic heat for our given boost pressure is 90F degrees. That means at 100% adiabatic efficiency the outlet (compressed) air would be (70F + 90F), which is 160F degrees. Now, let's see how much heat the turbo adds. We calculate this by taking the 90F adiabatic temperature and divide by the efficiency rating, let's say 75% (0.75), or in other words (90 / 0.75) which is 120F degrees. So in this example the turbo added another 30F degrees to the charge, and our actual outlet temperature is 70F (which is the ambient air temperature) plus 90F (adiabatic heat) plus 30F (turbo induced heat), which is (70 + 90 + 30) 190F degrees. This is the temperature of the compressed air that is fed into the intercooler inlet. Now, let's look at the lowest efficiency level of the turbo; 60%. Same inlet temperature of 70F, and same amount of adiabatic heat of 90F, yeilds (90 / 0.60) 150F. Therefore at the lowest efficiency level the compressor outlet temperature is (70 + 150) 220F degrees. So in this example, the difference between the highest (190F) and lowest (220F) efficiency is 30F degrees.

Now, that is just the compressor side. But I believe turbine design and turbine sizing has similar or greater affect on pumping losses. That is why for example the ATS TD06 kit with an 8cm turbine flows more than the ATS TD05 (with a bigger 10cm turbine), despite the compressors being identical.

 

The factor that most greatly impacts a turbo's ability to make more or less power at a given PSI is how it affects the engine VE curve. The turbine wheel and housing has the most to do with changing the VE. A "smaller turbo" will (typically) also have a smaller turbine housing and turbine wheel. This means that the VE of the engine will be reduced across the board when compared to a larger, more free-flowing turbine housing due to increased backpressure.

The CFM rating of the turbo's compressor has almost nothing to do with one turbo being able to make more power compared with another turbo at the same boost level. The engine dictates the CFM of air flowing through it, not the turbo. Granted, it is possible to choose a compressor that flows too much or too little air compared with what is required by the engine, but it is the ENGINE that dictates what the CFM value will be at any given RPM and load point.
This is why most CT26 turbo upgrades make little difference to the overall power output of the engine, even though many of them have compressor wheels capable of supplying a great deal more "CFM" when compared to the stock compressor. The reason is that 99% of all CT26 upgrades don't change the turbine wheel or housing at all. So, sure, the compressor wheel is a bit more efficient, so you pick up a few horsepower from a cooler charge, and the torque requirements placed on the turbine wheel are slightly less... but you don't see anything more than this.

I believe you're talking seeing some dyno plots where a car makes 300RWHP at 10PSI of boost using a big turbo while a CT26 only manages slightly more than half that at 10PSI. So, now you know, bigger turbo = less backpressure at a given boost = increase in VE across the board = more power

Precisely! Well put.

 

Ok, so some turbos are more efficient at different pressures than others, but still, like Aaron said, it is hardly significant at all, and doesnt really answer the OP's question about why larger turbos make more power at a lower boost.

Again, yes, the turbine / housing size does affect power output, but that is if you go past what it was made for. At 10 PSI, the CT26 does its job very well. It doesnt have any more backpressure than boost pressure, which is just about perfect. A T67 will also see a similar condition at 10 PSI. That takes the turbine theory out. We have already established that compressor efficiencies dont make up for the vast differences we see between different turbos at the same PSI. So what else makes sense as to why different sized turbos, with adequate turbines / housings to efficiently flow this pressure all the way to redline, produce different numbers at the same pressures?

 

Okay, but now lets say youve got two turbos.l

One BIG one and one small one wich are both at 15psi at 4000 rpm (just an example)

Is it possible for the BIG turbo to make more power at the same rpm with the same engine at the same psi?

 

Now, that is just the compressor side. But I believe turbine design and turbine sizing has similar or greater affect on pumping losses. That is why for example the ATS TD06 kit with an 8cm turbine flows more than the ATS TD05 (with a bigger 10cm turbine), despite the compressors being identical.

The TD05 has a smaller turbine wheel than the TD06, the turbine housing is larger, but the smaller wheel is what (I think) causes the power to drop off at higher rpm.

In my straw story the straw represented the turbo, not the intake manifold. Hell, I can't explain it, but I can cite PROOF that a larger compressor wheel and better flowing turbine housing will usually make more power at the same boost level:

Same boost level, same ECU, same car, same everything except CT27 vs CT26 at 10 psi.

Ironically I can cite the opposite example too. On a car we recently tested with the TD05, TD06 and SB50 the biggest turbo made the least power.

Aaron

 

Okay, but now lets say youve got two turbos.l

One BIG one and one small one wich are both at 15psi at 4000 rpm (just an example)

Is it possible for the BIG turbo to make more power at the same rpm with the same engine at the same psi?

Well it depends. Is the small turbo making 15 psi because it is all it can make? Or is it making 15 psi because it is all the engine can ingest at that rpm. Same question for the big turbo...

Aaron

 

My tuner says both turbos would make the same power on the same car at a low psi wich the small turbo can hold to redline.

Bigger turbos are able to flow very big psi numbers, and hold it. Thats the only reason he says they produce more power.

I think i beleve him, he knows his stuff. They come to him from all over europe for turbo tuning.

Any comments?

 

Well it depends. Is the small turbo making 15 psi because it is all it can make? Or is it making 15 psi because it is all the engine can ingest at that rpm. Same question for the big turbo...

Aaron

They are making 15 psi because they are boost controlled.

 

10 PSI is a very bad example since, as has already been pointed out, even a small turbo will have very little negative effect on the VE of our engines at such low boost pressures. But, that doesn't negate the fact that VE is what makes one engine produce more at the same boost pressure when compared to another. When you see cars making 300RWHP at 10PSI it's usually because
A) They're revving the engine out to 8000+ RPM
B) They have done enough work to the head and intake manifold to allow the engine to maintain high flow rates even at very high RPM.
C) They have a turbo with a large enough turbine as to have little effect on the VE of the engine at high RPM.

Now, the effect the turbine housing has on the equation can REALLY be seen when you start pushing more and more boost. Even at 15PSI the exhaust manifold pressure with a stock CT26 is starting to climb rapidly. It may even be approaching twice the intake manifold pressure. A larger turbine housing will still be able to flow enough to keep the exhaust backpressure low, around the same as the intake manifold pressure, or less! The VE of the engine is highly sensitive to exhaust backpressure.

I'll say it again though, the CFM rating of the compressor has no direct impact on the power the engine will make at any given boost pressure, as long as the turbo is able to supply the engine with the airflow it requires without crossing the surge line, it won't make much of a difference.

Think about your fuel pump and injector setup. If you think of the pump as the turbo's compressor, and your injectors as the engine, the relationship is more obvious. If you use a supra pump, capable of flowing twice (or more) the fuel of a stock pump to feed stock injectors... do you suddenly make more power? No... The INJECTORS (size and duty cycle) dictate what the flow requirements are. Just like your engine dictates its flow requirements based on RPM, displacement, head design, camshaft profiles, and exhaust backpressure, among other things. If your engine can only consume 300 CFM of air at a given RPM, then guess what, bolting on a compressor that pushes 400CFM will not do anything, you can't magically transport that air through the engine. However, as I've said before, most often a compressor wheel that flows more air will be mated with a turbine wheel/housing that flows more air. It's the TURBINE that most affects the VE of the engine, not the compressor (Yes, the compressor has an effect, but not as drastic as the turbine section).

Bottom line: The turbo doesn't "make" flow, your engine (mostly the intake manifold, head and exhaust backpressure) does. The job of the turbo is to simply supply the required flow rate at a given rpm/load point.

This is why bolting a 50-trim compressor wheel into a CT26 turbo doesn't make you much more power than the stock CT26 did at the same boost level. But bolt on a 50trim T3/T4 with a .63 A/R turbine housing, and suddenly you're making more power at the same boost levels.

 

Its about airflow capacity.

At ~ 52-5400 RPM, the CT26 is no longer able to keep up with the amount of air the engine is consuming. And as RPM rises, and the amount of airflow needed increases, the boost falls, and torque drops off.

And since horespower is really just torque times RPM, a larger capacity turbo, that can continue to meet the engine's increasing demand farther up the RPM range, and is capable of making more horsepower.

If the engine needs 500 SCFM at 6000 to maintain 15 psi, then the turbo must be able to provide that airflow.

A turbo is simply a centrifugal air pump. When its flow rate no longer exceeds the amount of air the engine is using, then it can no longer maintain the boost level.

If you had a 40 GPM pump filling a swimming pool, and a 50 GPM pump draining it, the water level will drop.
If you have an air compressor, it must pump air INTO the tank faster than your air tools use it, or pressure will fall.

Same concept. The turbo has to pump air to the engne faster than the engine uses it.


And its not about efficiency, just capacity.

bill

 

10 PSI is a very bad example since, as has already been pointed out, even a small turbo will have very little negative effect on the VE of our engines at such low boost pressures.

We are using that example because we are trying to find other reasons than exhaust housing efficiencies. Even in the graph Aaron posted, in which the only thing that was changed was the turbo (bigger compressor, and yes, more efficient turbine), and in the lower RPMs before the CT26 starts to get choked by the turbine housing, the CT27 made more power. I believe its safe to assume that this was because of the larger turbine, and not having to do with the fact that the turbine is more efficient. So, besides the turbine / housing's ability to flow, why does a larger compressor make more power at the same boost level? What you say about the higher rev limits and such taking advantage of the larger turbine housings is true and will make more power, but it cant be used here because everything was the same, including cams, rev limit, and tune.

Look at that graph. Whenever the CT26 makes its peak power (~5600), the CT27 is making over 30 more HP. At 4000 RPMs, well before the choke point of the CT26 (torque is still climbing), the CT27 is making ~20 more RWHP, and torque is also much higher. So why, at right about the time both turbos hit full boost, does the CT27 make more torque over the entire RPM range, even before the CT26's choke point?

 

Aaron, the problem with your example is that the turbo shouldn't be the straw. If a ct-26 and a t67 both put 10 psi of pressure in the manifold at 7,000 rpms, the amount of air that rushes into the engine will be the same*. The air flow into the pistons is determined by the pressure in the manifold and the effectiveness of the cylinder head, not the capability of the compressor. The compressor pressurizes the manifold, the head/manifold let the air into the cylinders. The t-67 and ct-26 are both pushing pressure past the same sized straw.

*the increased backpressure in the exhaust from the ct-26's restrictive turbine are what reduce the airflow into the cylinder, because more exhaust gas stays in the cylinder

 

The compressor can do one of three things:
1) It is able to supply enough CFM of air at a particular pressure ratio to satisfy the engine throughout its operating range. (Most well designed turbo kits fall into this category)
2) It is unable to supply enough CFM at a particular pressure ratio to satisfy the engine's requirements. This happens to small turbos on free-flowing engines (big cams, aftermarket intake manifold, etc...). When this happens, the compressor will be unable to maintain pressure, since it MUST deliver the flow required, it has to lower the pressure to do so. This is what is happening when someone says the turbo "couldn't hold boost to redline".
3) The compressor supplies far too many CFM of air at a particular RPM point and pressure ratio. In this case, the compressor crosses the the surge line on it's compressor map any you get compressor surge. This happens on poorly matched compressor wheels such as the 57 trim TO4E with a stock VE 3SGTE.

So, the in a nutshell, the compressor can either supply enough air, too little air, or far too much air... That's it...

What bill is describing is pefectly accurate with regard to the CT26 on the 3SGTE. In our case, the turbo is operating under the second conditon I listed above. The observed pressure drop will obviously reduce power. However, this does not address the question at hand. The question is why a big turbo can make more power at the SAME boost. In the case of the CT26, the "same boost" part doesn't apply, since above a certain point, the CT26 cannot maintain the same boost pressure.

The reason the CT27 makes more horsepower across the board when compared to the CT26 is because it addresses the very problem I'm trying to point out.. Turbine restriction. ATS machines the turbine housing on the CT27 so that it is more free-flowing. As I said before, a more free-flowing turbine section (housing and/or wheel) will increase VE across the board. Not just beyond some "choke point".

I'll say this one more time. The main contributor to difference in power between two engines of the same displacement, same AFR, same ignition timing and the same boost level is the VE of the engine. (Yes, air density has a role to play, but as Aaron pointed out before, a drastic change in compressor efficiency will only have a minute effect on power output). If you increase the VE in ANY way, the power will increase, guaranteed. Big turbos have big, free-flowing turbine housings. This will cause an increase in VE across the board. With all other factors being constant, increased VE = more power.

 

Armstrom, I believe you are correct, but there are some small details which have a lot to do with appropriate turbo sizing and subsequent results; those details include the balance between the turbine and the compressor, and the engine's airflow capacity. We have to bear in mind that exhaust energy spins the turbine, the turbine spins the shaft that spins the compressor, the compressor flows more air into the intake, which creates more exhaust energy, which spins the turbine faster, and the cycle continues. If we swing any one aspect too far one way, it puts the whole thing out of whack. We really cannot look at just one subset or the other, we need to look at the entire package.

In your previous posts you seem to be suggesting that if you keep increasing the turbine size, and reducing the backpressure, you will continue to gain power, but that is not accurate. I'm sure this is not what you meant. The fact is, we cannot completely eliminate backpressure, we have to spin that turbine at sufficient speeds in order to flow more air. And how much air we flow needs to be gauged appropriately based on the engine's flow capacity.

So as we look at turbos we must try to create the ideal balance between compressor flow and turbine flow, all within the breathing range of the egnine. But say we mated a small compressor on a huge turbine, or a small turbine on a huge compressor, we would end up with a very poor performing system. Similarly, if we stick a mondo turbo (big turbine, big comp.) capable of flowing 1000CFM on a stock VE MR2 we will surely be disappointed with the result. So, optimize the magical balance, and you get the best performance.

A classic example is the recent ATS TD05 tests. As we saw from the dyno plot, the TD05 made less power, despite the fact the TD05 housing is actually bigger than the TD06 housing (10cm vs. 8cm). But as Aaron pointed out a couple of posts ago, the TD05 turbine wheel is smaller than the TD06. So in effect, we have less restriction with a TD05 than a TD06. But what I suspect is happening is a good deal of exhaust energy is flowing past and around the turbine instead of through the blades, so the turbine is spinning slower. subsequently the compressor is spinning slower, and therefore it flows less air. I bet if you hooked up an exhaust manifold pressure gauge you would find the pressure in the manifold is actually lower with the TD05 than with the TD06, which means the backpressure is lower, but too much of the exhaust energy is escaping without helping to spin the turbine.

As a general rule of thumb, compressor to turbine ratio should be between 1:1 to 1.25:1. Staying within this ratio while finding a compressor that flows enough air for our desired power seems to produce the best performance results between fast spool, low and high end torque.