Let's say you have a T-25 and a (bigger, right?) T-28 on a motor, both running, say, 8psi of boost. Techically, it should be the same power output, because it's the same amount of boost, right?

Nope. The T-28 will move more air (CFM, or cubic feet per minute) at the same pressure ratio. And as we all know, more air = more power.

 

Re: Re: Turbo theory?

Nope. The T-28 will move more air (CFM, or cubic feet per minute) at the same pressure ratio. And as we all know, more air = more power.

Okay, I see. So it's a balancing act of spool rate vs. ultimate power, assuming an equal boost level, right?

 

Here you go enjoy.
http://www.turboclub.com/aturbo.htm

 

Here you go enjoy.
http://www.turboclub.com/aturbo.htm

You rock. Thank you. BTW, I autocross with Dennis Grant all the time.

 

Re: Re: Turbo theory?

Nope. The T-28 will move more air (CFM, or cubic feet per minute) at the same pressure ratio. And as we all know, more air = more power.

nope, CFM is not a way to rate the power output of turbos. a T28 @ 10 psi does NOT flow any more air than the T25 because it is up to the engine to injest the pressurized air. the cfm will determine how much boost the turbo wil support, kind of (i dont want to go into the entire turbo theory, there are plently of websites and articles dedicated to this)

10 psi of air is 10 psi of air, regardless of turbo size. the reason a T28 @ 10psi will make more power has more to do with the efficiency of the turbine. The T28 will create less backpressure to make that 10 psi, increasing the VE of the engine, therefore making more power. also, the T28 will peak later in the powerband, and since HP is tq over time, you will have a higher peak HP number.

repeat after me: the turbine is more determinate of horsepower than the compressor. having a huge compressor on a little, restricted turbine will create a laggy, low HP turbo, because that poor little turbine will only flow so much. the more happy and freeflowing your turbine, the more power you will have, at the cost of lag (though modern turbines such as the DP T28 Rob Cadle has designed create almost no lag, yet support about as much as a T3 turbine)

read SCC's turbine efficiency and the Suck Squish Bang Blow series by Mike K. he also has a wonderful new article about turbo sizing in this month's issue of SCC.

i am sure Rob Cadle will chime in here, and fix any errors i may have made, as i am still an amateur compared to Mr Cadle's expansive turbo knoweledge.

 

I have read those articles, and they are excellent. Just wanted to see if I truly understood them. According to your post, I seem to be on the right track. Good info!

 

Re: Re: Re: Turbo theory?

10 psi of air is 10 psi of air, regardless of turbo size. the reason a T28 @ 10psi will make more power has more to do with the efficiency of the turbine. The T28 will create less backpressure to make that 10 psi, increasing the VE of the engine, therefore making more power. also, the T28 will peak later in the powerband, and since HP is tq over time, you will have a higher peak HP number.

For the most part, your post makes sense. However, if the T-28's compressor outlet (I believe it's referred to as A/R) is larger than a T-25's, then wouldn't the T-28 flow more air than the T-25 at 10psi? For example, if you were to apply constant 10psi of air pressure through a tube the diameter of a straw and a tube the diameter of a Cambell's soup can, wouldn't the tube the size of a Cambell's soup can flow more air?

Michael.

 

yes, but like i said, the engine is what flows the air, when the intake manifold is pressurized to 10 psi, the engine will ingest whatever air it needs. a larger compressor wheel will require less work to spin up 10 psi of air, making less demand on the turbine, creating less backpressure, increasing VE (volumetric efficiency) and resulting in more power.

the larger wheel that flows more will be able to support a higher level of boost. flow at 10psi from turbo to turbo is the same, because it is the engine that is ingesting that air.

there are other factors that affect flow, HP levels and boost levels such as aerodynamics, blade count, etc, but for the most part flow WILL be the same at similiar boost levels. i cannot iterate this enough.

 

if you were to apply constant 10psi of air pressure through a tube the diameter of a straw and a tube the diameter of a Cambell's soup can, wouldn't the tube the size of a Cambell's soup can flow more air

true, but lets say the straw could support a total of 14 psi of air (much like our T25s) and the campbell's soup can could support 25 psi of air (.83 A/R T3/T4 Stg III). they both can support the same 10 psi of boost, but the soup can will support way more due to its better flow. the soup can will also be less taxed than the straw, creating less backpressure and less work for whatever is supplying the air in this case.

 

Alright, that makes sense now...

Michael.

 

just go buy corky bells book on turboing, read it, learn it, school us

 

When I used to ask about CFM of turbo's as a way of rating them Rob Cadle warned me about using this as a way of measuring a turbo's performance. I think it was that equally important as CFM was lb's per minute. CFM is the volume of air, and LB's per minute was the weight of the air. You can have the same volume of air, but lower density of air, due to heat that a T25 would create. Where at the same time having the T28 producing a much denser "charge" at the same psi, meaning the motor could be able to injest technically more air because it is a denser material. So I would say (my opinion) that 10 psi from a T25 is technically less air than T28's 10 psi, while volumetrically being the same amount of air, it's weight is where the difference lies. This comes from my rudimentary understanding on this topic, so it could get proved wrong any sencond.
We all know 2 different objects with different density, yet same physical size can weigh 2 totally different weights.
CFM=size
Lb's per min=weight