Those above who said it's the airflow volume are absolutely correct. Here are a couple of data points from the DSM world:

1g stock 14b turbo flows 450cfm @ 15psi - max ouput ~300hp
upgrade to a small 16g, 505cfm @ 15psi - max output ~ 365hp
upgrade to a big 16g, 550cfm @ 15psi - max output ~ 400hp
upgrade to a 20g, 650cfm @ 15psi - max output ~450hp

Each turbo makes more power than the previous at the same level of boost because it flows a higher volume of air (and is more efficient at the specified boost level) in CFM.

I understand the above, but doesn't that assume that the rest of the system can handle the increased flow? Doesn't the intake, engine, and exhaust system provide a limiting factor on how much flow can actually make it's way into the engine at a given psi regardless of the amount of flow that the turbos are capable of?

Maybe someone can show me how my water hose analogy just doesn't apply?

Before we go too far, take read here:

https://www.rx7club.com/showthread.p...ight=turbo+cfm

This was discussed before with a lot of information.

not exactly. Your thinking in incompressible fluids. air is a compressible fluid. Therefore, your ideal gas law isn't really correct for the situation. Mass flow CAN change.

Well sure, but think of the hose like this... Total airflow into the engine = Volume in CFM * Boost in PSI. This is important because to some degree they are interchangeable. If you have a small volume of air expelled from the turbo at a given PSI, as it exits the turbo outlet it can expand in volume (and thus become lower in pressure) to fill up the intake piping, intercooler etc. Because of this, if you have a giant I/C and wide piping, your small turbo might actually be pushing out 20 psi of boost at its outlet, but you're only measuring 15 at the intake manifold. A larger turbo (or in our case, twin turbos) will expel a larger volume of air at the same level of pressure out a wider outlet, and thus may require, and fully fill up a larger intake tract.

In terms of the hose analogy, then, I think you have to consider the faucet itself. If you have an small faucet opening, you'll end up with less pressure and less flow out the end of your hose of a fixed (larger) diameter than if you have a large faucet opening, both expelling water at the same pressure, but obviously not at the same volume.

Thanks. Reading now.

nah dude as others have said it's all about CFM. If you think about it's a huge variable.

In stock form: 10 psi with only the primary turbo on = less power than 10 psi with both the primary + secondary turbo. As we all know both turbos at the same time = more power.

If all other things are equal using a larger single that has a greater CFM = more power at the same boost level.

This doesn't work because rpms aren't constant.

Yeah, that is what so many have said. I still think the problem is in the "if all other things are equal" part.

read this thread and come to your own conclusion. (take some asprin before hand)
https://www.rx7club.com/showthread.p...ight=turbo+cfm

The simplest way to say it is a larger turbo, or in this case 2 turbos put more total air into the engine at the same pressure.

While the stock system could maintain 10 psi with one of the small turbos past 4500 it would generate more heat then power.

Anyone thats had a boost leak knows what this feels like. You might be getting 10 psi at your manifold but the turbo(s) are outputting maybe 20+ psi, way past their efficiency causing a huge loss in power making the car feel much slower.

Now the reason a larger single turbo generates SO much more power then the stock twins even at the stock 10 psi is because the larger compressor and larger exhaust housing greatly reduce the pressures both on the intake and exhaust side. This increases the volumetric efficiency (VE) of the engine. That means at the same pressure (10 psi) more total air is going into the engine. * imagine a 1" water hose flowing 10 psi of water, then compare it to a 4" water hose at the same pressure will generate alot more water.

The more efficient a turbo is at a given pressure the less heat is generated which allows for more total volume of air at that pressure.

This is the gist of my understanding after reading many threads about turbo's over the years including the one i posted above. I feel there is really no simple answer and that there are lots of factors that dictate the amount of HP a turbo will make.
I also think that while some say the manifold, throttle body, etc are the limiting factor on how much CFM of air the engine will take, that the turbo is an extension of this and plays a roll beyond just reducing back pressure. I also think the CFM a turbo is able to output plays some kind of roll beyond how much the intake charge is heated or not heated. (given the turbo is not way past its efficiency range)

The real reason a GT42R produces more power at 10psi is because the air comes off the compressor a whole lot cooler. So there are in fact more molecules of oxygen reaching the combustion chamber, and less backpressure on the hot side.

Dave

Correct but my post was just meant to explain in a real watered down terms on why we make more power with dual turbos at the same psi. Otherwise mazda would have given us only one turbo and not two.

My apologies if I made it too ambigious.

I'm not really understanding your point here. The "all things being equal" leaves out other variables that are attributed to power increase and thus leaves the turbo(s) as the only variable.

Boost pressure is just a reference to the CFM that a particular turbo produces. Nothing more. Gotta remember we are reading boost as the air is passing though the UIM, we aren't monitoring how quickly it's moving towards the combustion chamber. The faster you move that air (with the same boost level), the greater the number of oxygen molecules crammed into the combustion chamber.

^^ Exactly. The key point is that the DENSITY of the mixture is what matters. Since the combustion chamber's volume is fixed that means more MASS and more air molecules. You can increase density by raising boost pressure, but then that means more heat. More heat means higher temps and that means the air is less dense. There's a fine line between increasing boost pressure and increasing heat at the same time.

That's why compressor maps are so important, so you can choose a turbo (compressor) based on it's efficiency--which is really how well the turbo compresses air without heating up the charge.