Why do larger turbos output more power than small turbos at the same PSI?

[jeb1517]

How can big turbos supply more air but be at the same pressure if the volume of the intake manifold hasn't changed? 10 psi going through the manifold should be the same amount of air.

Obviously I'm missing something since bigger turbos do offer more power so can someone please explain it to me?

EDIT: Here's exactly what's confusing me:

Ideal gas law: PV=mRT

In order to make more power, I would have to increase the amount of gas (air) going into the engine (increase m). R is constant and we are holding P constant (10 PSI let's say). The intake manifold is unchanged so V is also constant???? But If I increase m, either V has to increase, or T has to decrease. I would think T remains relatively constant (so maybe this is what I'm missing?)
Modified by jeb1517 at 7:55 PM 10/2/2009

[ravera]

Larger turbos are more efficient. This is why a single big turbo will produce more power than two turbos half the size of a single.

Smaller turbos tend to blow lots of hot air (increase in T) at the maximums of their flow rates.

Larger turbo's offer less back pressure at the exhaust than a small turbo does, allowing easier flowing exhaust.

Larger turbos also have larger outlets, and as one would assume, larger piping to accompany, this means it's moving more air at that psi.

Finally, to stay it their efficiency range, larger turbo's have to move more air for the same PSI or they will go into compressor stall or "surge"

[jeb1517]

Finally, to stay it their efficiency range, larger turbo's have to move more air for the same PSI

Hey, thanks for the reply. I agree with most of what you're saying, but remember, the intake manifold is the same. All that air still has to go through the IM. So, if more air moves through the same volume, the pressure has to increase. That's what I don't understand. Why does the pressure remain the same but flow rate increases? And I would hope intercoolers minimize the difference between air temperature seen by the IM from large vs small turbos.

Maybe I'm just over-complicating it. It seems to me like flow rate and pressure are independent here and that seems just weird lol.

[ravera]

facepalm.

Like I said, larger turbos usually come with larger ducting. Until the air gets to the mani, it's restricted by the piping. A smaller turbo has a smaller outlet, and in many cases, the pressure feed to the WG is located at the end of the turbine outlet. Since it's a smaller hole, less air will be moved at that psi.

Larger turbo's have a larger outlet, and require larger ducting, unless you choke it down at the exit and then you've completely removed the use of the larger turbo.

And you're only looking at one side of the turbo. The hot side is larger on big turbos and allows more air to flow through, with substantially less backpressure.

You can toss 1000psi into a motor and it still wont make any power if it's got nowhere to go.

[jeb1517]

and in many cases, the pressure feed to the WG is located at the end of the turbine outlet.

OH! lol facepalm...I think this is where I was confused. For some reason, I kept thinking the WG would be referencing the IM. So I'm guessing the pressure in the IM actually does increase? Thanks alot for breaking it down man.

[RBbugBITme]

I don't really understand what your saying with the larger tubing thing...

There are 2 main points in my mind that cause this effect.1) Temperature drops a little so mass flow increases due to density change2) Turbine restriction affects volumetric efficiency

If you compare a gt30 and gt40 turbine map you'll see flow capabilities are vastly different.

[ravera]

I don't really understand what your saying with the larger tubing thing...

There are 2 main points in my mind that cause this effect.1) Temperature drops a little so mass flow increases due to density change2) Turbine restriction affects volumetric efficiency

If you compare a gt30 and gt40 turbine map you'll see flow capabilities are vastly different.

They flow that assuming they're allowed to flow that. If the tubing and piping will not allow the turbo to push the air it wants to at that pressure ratio, than the turbo will up the pressure ratio until it gets to that flow rate. There's a reason you don't see big singles running the same tubing as a GT28 powered car. If you want the turbo to spool in it's peak efficiency you need the correct plumbing to support it

[seang]

The original question here reminds me of how air compressors are rated. One air compressor will support 150psi @ 90cfm, and the other one (which is larger) will also support 150psi, but @ 140cfm. IDK if these numbers reflect actual air compressors, but you get the idea.

[jeb1517]

The original question here reminds me of how air compressors are rated. One air compressor will support 150psi @ 90cfm, and the other one (which is larger) will also support 150psi, but @ 140cfm. IDK if these numbers reflect actual air compressors, but you get the idea.

Yea, but the larger ones usually have larger tanks to go along with the larger compressors. You could think of the larger tank as the larger piping for the larger turbo I guess.

The reason I got confused is because people say larger turbos give you more power at the same psi. To me, that's a false statement because it really depends where you're measuring the pressure. If you measure it at the intake manifold, I think the pressure will increase by installing a larger turbo. At least that's what I've gotten from this thread lol.

[crzycav86]

How can big turbos supply more air but be at the same pressure if the volume of the intake manifold hasn't changed? 10 psi going through the manifold should be the same amount of air.

Obviously I'm missing something since bigger turbos do offer more power so can someone please explain it to me?

EDIT: Here's exactly what's confusing me:

Ideal gas law: PV=mRT

In order to make more power, I would have to increase the amount of gas (air) going into the engine (increase m). R is constant and we are holding P constant (10 PSI let's say). The intake manifold is unchanged so V is also constant???? But If I increase m, either V has to increase, or T has to decrease. I would think T remains relatively constant (so maybe this is what I'm missing?)

Modified by jeb1517 at 7:55 PM 10/2/2009

It has nothing to do with where you're referencing pressure from. It has everything to do with how much the air heats up as it's compressed. Larger turbos are more efficient, meaning they heat the air up less than a smaller turbo does. (you can look at a turbo compressor map for this info) It has also been mentioned that a bigger turbine will reduce backpressure and increase volumetric efficiency.

Take a look at this thread: zerothread?id=381746

You've got the right idea with the ideal gas law. just remember that we're doing with air FLOW not a constant mass that gets compressed and uncompressed.(that's essentially all you can do with PV=mRT) What you want is to use P*volumetric flow= mass flow * R * Temp. .volumetric flow is equal to engine displacement*rpm*volumetric efficiency/2 (the 2 is because there is a power stroke every 2 revs). So you end up with this:

P*engine displacement*rpm*volumetric efficiency/2= mass flow * R * Temp

You want to maximize mass flow, so you maximize pressure(boost), minimize air temperature(intake temps) and you want to maximize volumetric efficiency. However, these three variables are interrelated - you just need to strike the right balance to make the most power.

hope that helps.

[flinterman2000]

Larger turbos are more efficient, meaning they heat the air up less than a smaller turbo does.

In layman's terms. Bigger turbos push cooler air and are freer flowing. If you want to get into the nitty gritty of it the you would have to take into consideration atmospheric pressure, air density, humidity and all what crzycav86 said then some more.