I've always wondered this...and I've never known for sure. If you have a tiny td04 turbo vs a t4/60-1 turbo, will 10 psi be the same amount of pressure from each turbo, or will it be different due to varying cfm rates? Any info's appreciated.

 

You are correct! 10 psi on a T25 is completely different than 10 psi on a T66. Think of this, try blowing as hard as you can through a straw and then try blowing out of a water hose. Even though you are blowing as hard as you can, you will get more air out of the water hose than the straw!


[Modified by TurbochargedSER, 4:30 PM 12/16/2002]

 

no, the bigger turbo will push more power per pound than a smaller one.

 

small compressor=small volumetric~mass flow rate
large compressor=large volumetric~mass flow rate=more power

 

thanks for the verification, just wanted to make sure

 

if you measure MAP at the intake manifold what difference does the turbo make. I can see how taking a measurment at the turbo would make a difference but most of us take readings from the MAP sensor at the TB.

at this point PSI is absolute, it irrelevant how it was generated. The smaller turbo will need to work much harder but at the end of the day how is 10psi not 10psi at the TB?

 

this only applies if all other factors are equal, piping, IC, etc...

 

So is there a diffrence in max power between 10 psi from a t66 or 10 psi from a T25 on the same set-up, i'm confused

 

I think the main difference is that a small turbo generates a lot more heat in building the same psi level. So the charge temp on the larger unit will be a lot lower.

But 10psi at the TB will flow the same CFM no matter what the source.

 

yes, the max air flow rate for those turbos is
T66=72lb/min
T25=31lb/min (for the ball bearing model with the 60 trim T3 wheel)
for the same psi a larger turbo will make more power. however, there a lot more factors that should be considered when sizing a turbo to an engine.

 

A big turbo makes more power cause it flows more air at a given turbine rpm, so yes it makes more power.

but if you have a 60mm TB, and at that TB you measure 10psi, the CFM from any source will be the same. the only other variable is charge temp.

if charge pipe and charge temp are equal, then power will be equal.

 

10 psi is the same pressure regardless on a tiny turbo or on a big turbo, pressure is pressure, what is different, is the flow.


[Modified by Mase, 10:35 PM 12/16/2002]

 

So a intercoolercooler is more important on a bigger turbo or a smaller one?


[Modified by therumblingteg, 6:38 AM 12/17/2002]

 

So you are telling me that the only difference is how much more HP a bigger turbo can produce compare to a smaller turbo where all psi level is the same HP, right?? For ex. both t3/t4e or t3/t4b at 10psi produce the same HP, but the t3/t4e can run (max out) at a higher psi (say max at 30psi) than a t3/t4b (say max at 25psi). Correct me if I'm wrong but I would really like to know if this is true. Also will both turbo produce the same lag or will the t3/t4e have a greater lag difference?

 

yes, charge temp must be considered. if picked correctly, a larger turbo is more efficient. this results in lower charge temp or a denser air charge. the density of the air charge(of larger turbo) multiplied by the volumetric flow rate results in a larger mass flow rate than a smaller more inefficient turbo operating near its max at the same relative pressure(gauge reading).

 

a t04e at 10 psi should make more power than a t04b at 10 psi, for the fact the t04e outflows the t04b.

also, since the t04e is a bigger turbo, it will create more of a lag

 

How is boost measure? I've always been curious as to how the pressure is not dependent on CFM.

As to why is TB reads 10psi from a T4 the same as a T25 when the T4 is pushing so much more air.

 

A friend of mine and I decided to debate this the other day, since neither of us were sure. we came to the conclusion that the only difference is thermal efficiency. We came to the conclusion that a big turbo pushing 10psi into an engine is almost identical to a smaller turbo pushing 10psi into the same engine, if they are both within their efficiency ranges.

When considering the air that is going into an engine, there are three things to consider; pressure, volume and temperature (which affects density). The volume of the engine is not changing, so the pressure that is induced by the turbo is proportional to the volume of air it is putting out, compared to the consumption of the engine. So, the only thing we consider is the pressure and the temperature of that air going into the engine.

To comment on turbochargedSER's analogy of blowing into a garden hose or a straw...the garden hose is like a larger engine, compared to the smaller straw engine. you will not be able to reach the same psi using a small turbo on a large engine, as if you used the turbo on a small engine. so, I don't think your analogy is correct, since I think you are really comparing engines, not turbos (your mouth is the turbo in that example, and it is obviously not changing size). If you are reaching the limits of a turbo's efficiency range a larger turbo will make more power, but I think this is merely because of the thermal efficiency, the small turbo is pushing eat at the same CFM that the larger is not.

What do ya'll think? I'd explain it better if I could, but it's late.

 

10 psi is 10 psi is 10 psi...

That's it.

But, look at the flow maps. you'll see that at a particular CFM and PRESSURE
you get different effiencies, at 5psi on a 1.8L motor the T25 is more efficient
(takes less power to spin) Than T4 at 5psi on the same 1.8L motor.

I realize that a T66 can push 72lb/min, but in order for a 1.8L motor
to flow that much you'd need to run 80 psi of boost @ 7000 rpm.
(the turbo cannot do this BTW)

Remeber you only use 900cc per revolution of air. (on a 1.8L)

CFM is controlled by the engine NOT the turbo.
(why a BIG turbo doesn't work well on a small engine, compressor surge)

There are other operating points to consider....
To consider just ONE operating point, (one pressure and one rpm point)
is short sided...

Jeffrey Atwood BSME

 

>>CFM is controlled by the engine NOT the turbo.<<

Can I get an AMEN!?

The universal solution to hp problems on this forum seems to be "bigger is better". It's nice to see someone post the truth for once.

Putting things in perspective, a T25 can push enough boost for about 300 hp. This is plenty of air for the vast majority of FI'd Hondas on this forum. Funny though, mention a T25 and it gets torn to pieces...

 

I'll be able to comment on this in about a week.
I'm switching from a small t3 to a t3t4oe.Nothing else is changing.

 

So on a 1.8 L engine, if you push 14 psi with a t-25 turbo vs pushing 14 psi on a t60-1 turbo, would the t60-1 turbo cause more wear on the engine?

 

It simply means cubic feet per minute which can be used to refer to a turbos max flow level or the heads max flow level. CFM isnt controled by anyhting, it just a measurement of flow volume.

 

>>CFM isnt controled by anyhting, it just a measurement of flow volume.<<

In his post, I interpreted his comment "cfm is controlled by..." to be "cfm is limited by...". In other words, the limiting factor with regards to CFM of air flow is the motor. At least in the *vast* majority of turbo cars around here...

Since we're talking semantics, it's reasonable to say that since the engine is the limiting factor in CFM of air flow, it "controls" CFM of air flow.

 

i see, i think there are a fair few motors on this board that are capable of hitting the CFM limits of their turbos (GE sleeves rated to 60psi) so i guess we should be specifc when refering to CFM as a limitation to making power.