I'm just a little confused about this, maybe because my theory is wrong. Lets say there are 2 turbo's: Turbo A (smaller turbo) and Turbo B (larger turbo). (This is a very basic example so no need to get too fancy). If each turbo is on a car at 20 PSI, the Larger Turbo would make more horse power than then smaller turbo... atleast from the reading I have been doing.

What im confused about is, shouldnt PSI be PSI, regardless of the turbo its coming from? At the end, it is the pressure inside the motor that produces the power for the motors, and whether you have a large fan or small fan pushing the air into the motor, shouldnt the SAME PSI create the SAME POWER?

help a noob understand the theory of the turbo.

 

It has to do with the efficiency of the turbo.

A turbo that struggles to make that boost (very small turbo) will be making a lot more heat than one that makes that boost easy (larger turbo).

A turbine side designed for fast spool will choke off the exhaust limiting HP. A turbine side that flows more will generally spool later but make more power at the same boost level.

Short and simple version.

 

While I cannot give you a detailed technical explanation, I will try to offer some insight. I could tell you that you can get, let's say 25 psi, out of a 16 G. You could also get 25 psi out of a 3076 or a 3582, etc... The 35 is working way less to make 25 psi than a 16 g is. That 16 g is spinning it's ***** off to make 25 psi. In other words, 16 g must spin more than a 35 to achieve that high level of boost. The 16 g heats the air more spinning this high, and I'm sure backpressure plays a role too... So, all in all, psi is psi. The main differences are the other factors.

 

Too late on my response, Scott beat me to it. At least I was right though!

 

That for the help you two!

 

You also forgot to mention that a turbo is not so much based on PSI, but rather in CFM or lbs/min. It is the volume of air that makes the horse power. Take a straw and fill it with 20psi of air, now take a 1" diameter pipe 3' long and fill it to 20psi. Same pressure, but larger volume.

Now add what I just wrote to what Mark wrote and you pretty much have a detailed explanation to your question.

 

Book... Maximum Boost by Corky Bell.

 

It's manifold pressure. Manifold or engine for that matter do not change in size. Straw/pipe size comparisons fail completely and people need to stop using them.

 

Unless you're switching to a 2.3 stroker when you swap the turbo!

 

True. And true.

I would not say they fail, - it is in fact true - it just doesn't really apply.

A bigger turbo will flow more air volume at the same pressure. More air. Plus more fuel. Gives more power.

 

It's oversimplified and only true when the smaller turbo is out of its efficiency range and the exhaust is choking on its hotside.
When both are in their efficiency range they will provide a very similar airflow at a certain boost (provided they can make that boost at those rpms) - it's the airflow that your engine can take at that point. Modify your engine and the airflow goes up at the same boost level.

 

T and VE, it's that simple.

It has nothing to do with the ability of a bigger turbo to flow more CFM or lb/min at a certain psi. Our engine size isn't changing.

 

Like others, I thought the same. I thought the pressure in psi shown on gauges was from inside the motor, which does not change in size?

If you use that example, i would think that a small fan putting 20 psi of air into a straw and a big fan putting 20 psi of air into a straw would generally mean the same thing. The object (straw, motor) isnt changing, the fan creating the pressure is.

Thanks for all the help guys