very true

 

The "psi" that we are talking about is the manifold pressure caused by the charged air. (so the pressure of air that is pumped by compressor at the compressor end is arithmetically different than the pressure in the manifold. The difference is paralel to the loss of kinetic energy which changes into heat as air particles are rubbing to the internal perimeter of pipe and rubbing each other. That is why, the longer the charge pipe-the bigger the pressure difference, and the smaller the charger pipe-the bigger the pressue diffence)

The flow in our case can be evaluated as "compressible frictional-turbulent flow in closed pipes".

facts:

1)If you compress the air, it gets hot. The specific gravity (let say density) of air, =number of air particals in a given volume, changes with pressure and temperature.
So, the situation of air is a complex one with continuous changes in *density-temperature-pressure-velocity (see 2 below)

2)Air loses its pressure (holding the pipe d constant) directly proportinal to the pipe length and logarithmatically proportional to its velocity.

The power of our engine is basically the result of the air-fuel-spark reaction. The higher the air amount (assuming we have enough fuel and spark) the higher the power output.

amount of air=number of air molecules

Having stated all bad-organized facts above; the density of air inside the manifold, which charged by gt30, may be different than the one charged by gt35 (simply because velocity and charge temperature may be different even if the pressure is same).

Having (i)the same diameter of charge pipe-same volume and structure of intercooler as well as same bends in charge pipes (which is possible), (ii)same internal volume of intake manifold (which is practically not possible), (iii)same velocity of air at same engine revs (which is another impossible point-that is the difference that we see in compressor maps) it would be possible for us to observe same power output for gt30 and gt35 charged engines.

conclusion: psi is not equal to psi

 

Higher efficency = cooler = more molecules = More HP

Cold PSI =/= Hot PSI

 

Efficiency has a lot to do with the pressure built up in the exhaust manifold. We always talk about the pressure in the intake manifold, but not in the exhaust manifold.

As a really extreme example. If 10 psi of exhaust pressure was in the exhaust manifold due to the turbo efficiency, manifold design, etc... and you were running 20 psi of boost what would the effective boost you would see in the cylinders?

Same question if there were only 5psi in the exhaust manifold?

Obviously charge air temp, etc... also plays a role.

 

Yes but it'll be very hard, with a 30 series turbo. I was just saying that if you have a log manifold or want 500 hp that the gt35 is a better option.

If you have tubular or want less than 500, than the gt30 is best. If I were to put a GT35 on my car right now and boost the same amount I might make another 20hp but I wouldn't actually be faster than what I have until 8K rpms.

 

I thought you were talking about the 35 series

 

On the charge side you have to consider that 1psi≠1psi necessarily.

1 psi at 26.85C has 20% more oxygen molecules than 1psi at 101.85C. (will somebody please check the maths).

So the more efficiently the turbo creates the pressure (ie. the less heat it adds) the more powerful the psi is.

 

A post I wrote a while back explaining power and boost pressure:

Power vs Boost Pressure

 

This is not an easy concept to grasp, but it is "graspable". There are many factors that come into play when discussing the operations of a turbocarged engine but if you just focus on the compressor itself...

Two compressors of wildly different size operating at the same pressure ratio (pressure differential between the inlet and the outlet) will not move the same amount of air. Typically, larger compressors will move more air with a smaller pressure differential and at lower rotational speeds.