EJ8 GSR-T

Ok so i have heard that say 15psi on a small turbo is not the same at 15 psi on a big turbo. It makes some sense but i cant quite wrap my head around the idea. Say i have a supercharger on my car, i trade out a small one (850cmf) for a larger one (1250cmf). Everything else remains the same and the boost (read by the vacuumed lines) is kept the same. Is the there actually a greater volume of air, or is it simply related to pressure (psi)? Its a strange question and i probably didnt word it correctly but give it a shot if you have any constructive input.

base-op.1320

You know what is funny I have always heard people saying a bigger turbo with more cfm produces more power at same psi but couldn't ever understand why, to me a low cfm turbo at any psi is the same as a high cfm turbo at the same psi (excluding efficiency)- so I read your thread and decided to research this topic to see what I was missing and came up with this really good thread showing how I was always right and not missing anything https://honda-tech.com/zerothread?id=1469539

tony413

its an air density thing. atleast thats how i look at it. whichever turbo produces more dense air is going to make more power.
psi is just a measurement of pressure which is a force.
cfm is a measurement of flow over a time frame.

you can flow different volumes of air at the same psi. as psi is a measurement of force/stress.

adseguy

It's all about engine dynamics. With a smaller turbo you have a smaller exhaust wheel. This creates a lot more back pressure then a bigger turbo so the exhaust gases can't escape as easily. This is called Volumetric Efficiency (VE). Also as more air is demanded the smaller turbo, at some point, is going to try to spin too fast to blow air into the engine and this creates too much heat from the turbo also reducing the engine efficiency. For these two reasons that's why you get different power levels at the same PSI with different sized turbos. The volume of air between turbos stays the same at the same PSI since you are using the same intercooler piping. No more CFM comes from a smaller turbo then a larger one at the same PSI....except the larger one will flow more because the smaller one just can't do it at some point (past it's HP). The T28 for example can't do past 350whp so a bigger T3/T4 will flow more since more air is needed past that point. Hope that helps

adseguy

<TABLE WIDTH="90%" CELLSPACING=0 CELLPADDING=0 ALIGN=CENTER><TR><TD>Quote, originally posted by tony413 &raquo;</TD></TR><TR><TD CLASS="quote">

you can flow different volumes of air at the same psi. as psi is a measurement of force/stress. </TD></TR></TABLE>

No

gogunkergorilla

<TABLE WIDTH="90%" CELLSPACING=0 CELLPADDING=0 ALIGN=CENTER><TR><TD>Quote, originally posted by adseguy &raquo;</TD></TR><TR><TD CLASS="quote">The volume of air between turbos stays the same at the same PSI </TD></TR></TABLE>

Exactly, no matter if its a gt25 or gt45. When ANY turbo moves more air than the motor can consume, positive PSI is generated. The amount of CFM needed to generate a certain PSI is the same for all turbos.

<TABLE WIDTH="90%" CELLSPACING=0 CELLPADDING=0 ALIGN=CENTER><TR><TD>Quote, originally posted by adseguy &raquo;</TD></TR><TR><TD CLASS="quote">This creates a lot more back pressure then a bigger turbo so the exhaust gases can't escape as easily. </TD></TR></TABLE>

Thats why wastegate placement and efficiency are so important. A more streamlined path for the gasses from the manifold through the wastegate will reduce backpressure in the manifold and HP loss as RPM increases. But most manifolds are not designed that way and therefore backpressure accounts for a difference in HP.

<TABLE WIDTH="90%" CELLSPACING=0 CELLPADDING=0 ALIGN=CENTER><TR><TD>Quote, originally posted by adseguy &raquo;</TD></TR><TR><TD CLASS="quote">the smaller turbo, at some point, is going to try to spin too fast to blow air into the engine and this creates too much heat from the turbo also reducing the engine efficiency. </TD></TR></TABLE>

This is the only REAL reason. More heat, less oxygen density, less HP.


Modified by gogunkergorilla at 11:51 AM 11/11/2007

EJ8 GSR-T

So PSI is a has correlation to the amount of air moved in a given/constant space? And effectively the only different is going to be that generally the larger turbo will be able to move the same amount of air while spinning slower thus creating less heat? Will this hold true for a supercharger also?

tony413

<TABLE WIDTH="90%" CELLSPACING=0 CELLPADDING=0 ALIGN=CENTER><TR><TD>Quote, originally posted by adseguy &raquo;</TD></TR><TR><TD CLASS="quote">

No</TD></TR></TABLE>


explain to me why not.

psi is a measure of force and stress on an object
cfm is a measure of velocity

wouldnt psi be in respect to cfm ??? or is it the other way around

tony1

Not a different volume, a different mass.

tony413

<TABLE WIDTH="90%" CELLSPACING=0 CELLPADDING=0 ALIGN=CENTER><TR><TD>Quote, originally posted by tony1 &raquo;</TD></TR><TR><TD CLASS="quote">Not a different volume, a different mass.</TD></TR></TABLE>

ahh ok so i need to correct my wording.

RC000E

This is always a hard concept to wrap around, and I've seen it disputed on all kinds of forums.

The way I see it is, that both a large or small turbo take "x" amount of cfm to create "x" pressure on a given setup. If you look at the corresponding compressor map for that turbo you'll see that at that cfm and corresponding pressure the smaller turbo can handle and maintain it.

As rpm's increase and the motors ve increases, the smaller turbo will have to speed up produce the cfm in order to maintain pressure. This increased speed heats the air, reducing it's charge density, and on the corresponding compressor map you'll see the compressor moves toward the choke line.

What you feel is a small turbo that responds quickly, but just as quickly can't meet the demand and goes to the far right of the compressor map overspinning.

Also, a small turbo, with a smaller turbine housing, has a smaller capacity. The turbine housing controls how much energy the turbine can extract from the exhaust charge and transfer to the compressor wheel. This is why turbine housing size has such a large impact on how a turbo behaves.

A large turbo, with it's compressor taking a larger "bite" of air is able to maintain the cfm demands in order to maintain a set pressure. Large turbo's can put out so much so quickly that is why the surge line is so important, especially in small motors. As a large turbo spins it can produce a cfm at a given pressure ratio beyond what a motor can injest.

The point is, that a small turbo quickly moves through it's efficiency range in order to meet engine demand, and as it overspins it heats the air heavily. On the other hand a big turbo doesn't work nearly as hard, and has enough compressor and turbine housing capacity to continue to supply and maintain pressure.

That's tough to put into words, but I hope that clears it up.

RC000E

<TABLE WIDTH="90%" CELLSPACING=0 CELLPADDING=0 ALIGN=CENTER><TR><TD>Quote, originally posted by gogunkergorilla &raquo;</TD></TR><TR><TD CLASS="quote">
This is the only REAL reason. More heat, less oxygen density, less HP.</TD></TR></TABLE>

This is also the reason that an efficient intercooler can make up, to some extent, for the "lack of compressor".

gogunkergorilla

It won't make it up if your intercooler is already working as efficiently as it can. There will always be a difference relative to the difference in temperature coming from the 2 turbos.

That goes for the backpressure problem too. The wastegate for a smaller turbo will be less efficient than a larger turbo since there is more exhaust energy that needs to be diverted which causes more potential for turbulence.


Modified by gogunkergorilla at 5:35 PM 11/12/2007