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Old 09-26-2012, 03:20 AM   #26
billyboy999
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I too am interested in the answer to OP's question. I think I get where he's coming from - if ALL else equal - that means manifold, piping, engine internals, valve timings, the tune... an increase in PSI should mean an increase in flow, because you're able to force more air into the cylinder. No change in PSI should result in no change in flow - what else is at play here? Alternatively, flowing more air will result in a higher pressure - when you blow through a straw, if you want to blow more air, you have to blow harder. Are you all assuming a larger turbo comes with upgraded supporting mods? Pressure absolutely should equal flow - all else constant.

Or - I'm really guessing here - imagine a hose attached to a pump, with a sprayer operated by a trigger at the other end. Let's say depressing the trigger represents the intake valve opening during operation. With a small pump, when you depress the trigger, pressure immediately drops to nothing, but once you close it back up, pressure rebuilds to 20psi. With a large pump, when you depress the trigger, the pump is able to flow more water and keep pressure constant at 20psi even while the trigger is depressed. Thus, two pumps maintain 20psi but flow different amounts of water. However, I would think a proper gauge would integrate the pressure properly and read the small pump's pressure as lower than 20psi.
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Old 09-26-2012, 05:59 AM   #27
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ya cant move as much air with a small fan as ya can with a big fan

RIGHT???

cubic
feet
per
minute

or whatever units ya wanna use....pounds or gramms or kilos or whatever

BIG fan....more air

and one of the reasons to go to a bigger turbo is so you can run LESS boost and make more power
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Old 09-26-2012, 09:20 AM   #28
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Quote:
Originally Posted by billyboy999 View Post
Pressure absolutely should equal flow - all else constant.
This is your issue. As the other posts have stated, the bigger turbo will pump out 20psi of much colder air. Colder equals denser, which equals more molecules of air going into the engine at the same pressure. PV=nRT.

People need to understand how compressor flow maps...
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Old 09-26-2012, 10:39 AM   #29
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PSI=Potential, if your electrically savvy VOLTAGE
Flow=Current or volume
Engine=Watts or Power

Physics 101, it's all basic physics.
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Old 09-26-2012, 10:54 AM   #30
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I'll tell you guys a story that pertains to this.

I work for a pharmaceutical company. We have large bio reactors that have tanks that can be up to 20,000 Liters. One day the facilities department came in to replace some gaskets. One of them noticed the pressure gauges were reading about 1-2PSI. Not thinking that was a lot of pressure (which it's not), they unbolted the manway cover. Wanna guess what happened? A stainless steel lid that weighed about 75lbs went flying about 1 story to crash into the ceiling of the building, 1-2PSI with 20,000L in volume is a LOT of gas. 1-2PSI in a 2mL cylinder won't even hiss when you release the pressure. PSI is nothing without volume.
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Old 09-26-2012, 11:01 AM   #31
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I was going to use a similar example with a scuba tank.
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Old 09-26-2012, 11:11 AM   #32
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Quote:
Originally Posted by erik11876 View Post
A stainless steel lid that weighed about 75lbs went flying about 1 story to crash into the ceiling of the building, 1-2PSI with 20,000L in volume is a LOT of gas. 1-2PSI in a 2mL cylinder won't even hiss when you release the pressure. PSI is nothing without volume.
Except you ignored all the relevant physics and added the ones that don't matter... The volume of gas has very little impact in how far the cover flew. The surface area of the cover is by far the largest variable. Lets say you had two PSI of pressure, and the cover was 2 feet across. Turns out there is 900 pounds of pressure pushing on that cover. The equation has no variable for the amount of volume behind the cover, it simply doesn't matter. That cover would have flown the exact same with 20,000l, 2,000,000, or 2,000l of gas. (yes, there is some propulsion effect that happens, but the difference would be negligble)
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Old 09-26-2012, 11:16 AM   #33
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Quote:
Originally Posted by billyboy999 View Post
I too am interested in the answer to OP's question. I think I get where he's coming from - if ALL else equal - that means manifold, piping, engine internals, valve timings, the tune... an increase in PSI should mean an increase in flow, because you're able to force more air into the cylinder. No change in PSI should result in no change in flow - what else is at play here? Alternatively, flowing more air will result in a higher pressure - when you blow through a straw, if you want to blow more air, you have to blow harder. Are you all assuming a larger turbo comes with upgraded supporting mods? Pressure absolutely should equal flow - all else constant.
Yes, but all else is not constant: The turbo is bigger. A TD04 at 20 PSI will flow less than a 20G at 20 PSI because the compressor on a 20G is bigger.

You can think of it statically. If you have a sealed box that is 2' square, you will need a certain volume of air to pressurize it to 20 PSI. If you then have a 3' square box, you will need more air to pressurize it to 20 PSI because there is more box to fill. Same goes for a turbo, even thought the air is moving. A physically larger turbo will require more air to reach 20 PSI. Therefore it will flow more air into the cylinder. Remember 20 PSI is a measurement of the pressure at the turbo, not inside the cylinder.
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Old 09-26-2012, 11:19 AM   #34
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Originally Posted by dgm_wrx View Post
I don't know how to word it I guess but why does one person need 20psi to hit 300hp when another person can hit 500hp on the same pressure?
Because the person hitting 500 HP on 20 PSI has a larger turbo. When a larger turbo is used it takes more air to pressurize it to 20 PSI. Therefore more air will be flowing to the engine, and therefore it can make more power.
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Old 09-26-2012, 11:45 AM   #35
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Quote:
Originally Posted by erik11876
PSI is nothing without volume.
This.

I am a certified Instrumentation tech and industrial electrician.. I work at one of the largest chemical plants in FL and I deal with these items on a daily basis. Force is relative to the pressure and volume.

Perhaps later I can explain it in terms of water columns and psig (gauge pressure). What I quoted sums it up pretty simple though.
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Old 09-26-2012, 12:33 PM   #36
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Originally Posted by dgm_wrx View Post
I don't know how to word it I guess but why does one person need 20psi to hit 300hp when another person can hit 500hp on the same pressure?

Because the guy making 500HP is producing that 20psi with a lot less heat, and probably far fewer post-turbo restrictions.

The "pressure is not the same as flow" guys are missing the mark. Pressure *does* equal flow velocity, but not necessarily mass-flow.

Picture two containers pressurized to 20psi - one at 100*F and one at 300*F. The 300*F will hit 20psi without having as much actual air in it.


Remember that it's not pressure, nor volume, that makes power. It's mass. You need 1 gram of fuel for every 14.7 grams of air
to reach stoich.

That said, there are other factors too - the guy with the big turbo is probably going to have bigger piping, which means the pressure drop between the turbo and the intake valve will be lower. The 20psi pressure at the turbo outlet is going to be less than 20psi at the intake valve. You have a single mass-flow through the system, and a pressure of 20 psi at the inlet and 0psig at the outlet (tailpipe). Pressure is lost en-route through inefficiencies and pressure is used up performing work on the piston. There are also significant boundary layer effects, turbulence effects, and inertial effects that come into play to determine the actual pressure of the air being ingested.


Also remember that flow does not happen if there is no pressure gradient. If the pressure is the same throughout the system there will be no reason for air to flow from one place to another. You need a pressure gradient, where the pressure is higher in one place than another, to induce flow.

So, yeah...



Quote:
I am a certified Instrumentation tech and industrial electrician.. I work at one of the largest chemical plants in FL and I deal with these items on a daily basis. Force is relative to the pressure and volume.
Energy is relative to pressure, temperature, and volume (for an ideal gas). Force is relative to pressure and surface area. Neither is super useful in helping the OP understand his problem, since the problem is dynamic and turbulent and thus not bound by the ideal gas law or its derivatives.

Last edited by sniper1rfa; 09-29-2012 at 08:10 PM.
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Old 09-26-2012, 12:39 PM   #37
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Except you ignored all the relevant physics and added the ones that don't matter... The volume of gas has very little impact in how far the cover flew. The surface area of the cover is by far the largest variable. Lets say you had two PSI of pressure, and the cover was 2 feet across. Turns out there is 900 pounds of pressure pushing on that cover. The equation has no variable for the amount of volume behind the cover, it simply doesn't matter. That cover would have flown the exact same with 20,000l, 2,000,000, or 2,000l of gas. (yes, there is some propulsion effect that happens, but the difference would be negligble)
Who cares how far the cover flew? The point is, that was a hilarious situation .
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Old 09-26-2012, 12:41 PM   #38
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can we end this?


for incompressible flow to make things simple. First term is flow, second is location and the thrid is pressure. Add them all up to make constant flow in and out. The turbo just adds a pump and at a moment in time, you can just a height unit to one end.

{v^2 / 2}+gz+{p/rho}={constant}
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Old 09-26-2012, 01:24 PM   #39
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Definitely pick up "maximum boost". It's a good read and pretty cheap.

Uncle had it right, you can look at it in terms of fans. A larger fan will flow/push more air than a smaller fan at the same speed. It's more surface area, of course it's going to flow more.

If your asking why the engine is capable of accepting the increased flow, it's because it is forced into the engine. Hence forced induction. In a NA motor you can only get so much air flow through the engine (it is, after all, basically an air pump), a turbo/supercharger just forces more in than normal atmospheric conditions can.

Granted, you will have restrictions in the system (exhaust, intake, heads), but this can be overcome with higher boost pressure (or simply swapping parts). A good flowing system will produce the same power on less boost than a poor flowing system.

I experienced this particularly on my last build. Head work and proper hotside for the turbo produced crazy numbers on 91 octane out of a dom 3xtr at low psi (16psi, more than doubled my stock whp on 02 WRX). That's also the benefit of a freer flowing system, less heat, which correlates into more power.
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Old 09-26-2012, 03:39 PM   #40
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what about nitrometh?
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Old 09-26-2012, 07:31 PM   #41
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jesus children....pressure is irrelevant

FLOW is NOT irrelevant

idiots.......this thread is PROOF of idiots and idiot ideas and faulty, uneducated, logic

we are all d0000med....d0000med i say
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Old 09-26-2012, 07:40 PM   #42
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So say you were to go stage2, at say the stock boost level. If you were to change out your turbo for a larger one, run it at the same psi, it would make more power AND be safer for your engine correct?
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Old 09-26-2012, 07:42 PM   #43
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So say you were to go stage2, at say the stock boost level. If you were to change out your turbo for a larger one, run it at the same psi, it would make more power AND be safer for your engine correct?


lets just say dont mod the car until you understand what you are doing
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Old 09-26-2012, 07:54 PM   #44
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Let's just say I pray everyday for whoever you are an uncle to. Of all the post I have seen. Y you you insult and degrade everyone. Do everyone a favor and get off the forum. Some people here have curiosity about how things work and are eager to learn new things. I am a body man. Tell me to make a car show car worth in black and I cAn do that. But when it comes to engines, I know the basics so excuse me for asking as simple question about the on topic discussion. You say I shouldn't mod my car cuz I don't understand, then I guess you shouldn't talk to people cuz obviously you not know how to have a conversation with out insulting or being an azzhat.
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Old 09-26-2012, 08:11 PM   #45
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Ok people be nice. I'm going to go build a 1000hp subaru on 1.5psi boost because you only need flow flow flow
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Old 09-26-2012, 08:59 PM   #46
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Oh the fun.
I will give this one a few days to die down. Then start the next one. Lean, stoichiometric, rich and knock, detomation,and ping
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Old 09-26-2012, 09:25 PM   #47
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Meh. I'm bored and nowhere near an expert on the subject, but not exactly a newb either. I actually see the OP's point and I believe there is a misunderstanding... three pages of replies seems to confirm it.

IF YOU IGNORE THE TURBO ALTOGETHER, and apply a constant 20psi to the engine via a magical, lossless means, the volumetric flow will be a constant. Your Pistons will suck in the same volume of air allowed by the valves in the time they are open. The problem is, volume doesn't mean crap. MASS does. Colder air is more dense at a given pressure than hot air. An intercooler or a compressor meant to flow more would provide a colder, denser charge on that same piston intake. Colder and denser means more mass in the same volume of air.

Now my question is, does that equate to an extra 200hp at 20psi? Or are there other factors such as turbine sizing (back pressure), cams, valve job, port job. i.e. has someone went from a 20g to a 35r and made 200hp more at 20psi without changing anything else. That doesn't answer the turbine sizing question unfortunately. I personally believe that is a big factor. More exhaust gas will be passing through the wastegate than through the turbine on a small turbo relative to a larger one. That bypassed gas is bypassed power.

My $0.02
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Old 09-26-2012, 09:44 PM   #48
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Originally Posted by dgm_wrx View Post
I know more air equals more fuel equals more power but you are still only pushing 20psi at 2.5liters. Not getting into all the tuning and requirements for a "safe motor" is it similar to NA motors and it all being about unrestricted flow in and out? Does compression ratio get changed? Do cams have longer duration or higher lift? I don't know how to word it I guess but why does one person need 20psi to hit 300hp when another person can hit 500hp on the same pressure?
The answer is mostly due to three things:
1) the RPM at which they reach 20psi... a stock turbo tune at 20 psi is not 20 psi at the RPM where peak power is reached. Peak power on a stock turbo 2.5L is around 5500 RPM and ~15 psi. 20 psi is a significant boost increase over a stock turbo "20 psi" tune. In addition you have 30% more power in RPMs if you can manage to make the "truck like" over-square short rod ratio 2.5L efficient at 7000 RPM.

2) Octane efficiency
Anyone making 500 WHP at 20psi from 2.5L is doing it with high octane fuel. This is a very significant benefit over pump gas. This can also unlock additional potential in terms of compression ratio.

3) breathing efficiency. This is largely what others have been saying in terms of flow vs. pressure. Pressure is indicative of restriction... removing restrictions means more flow at the same pressure. This can also be cams to make decent torque at higher RPM, or piston / valve / rode weight reduction to make more torque at high RPM. This is what is going to be necessary to make the subaru 2.5L reasonably efficient at high RPM so you can run the RPMs up.

There is a 4th factor... the dyno factor, that can inflate WHP to astronomical numbers.

combine those to get high power levels at "low" pressure.

You are correct that at some point 20 psi + 2.5L reaches a ceiling, but what I think you're missing is that most people's "20 psi" stock turbo tunes are really 15 psi tunes with a mid-range torque bump. When you talk about peak power, it's 15 psi with tapering boost, so there is even potential to have higher HP at 12 psi than the stock turbo on a "20 psi" tune. There is a lot more power potential for a turbo that can produce 20psi to high RPMs.

Last edited by Concillian; 09-26-2012 at 09:53 PM.
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Old 09-26-2012, 09:47 PM   #49
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Quote:
Originally Posted by 1BadP71 View Post
Force is relative to the pressure and volume.

Quote:
Originally Posted by sniper1rfa View Post
Energy is relative to pressure, temperature, and volume (for an ideal gas). Force is relative to pressure and surface area. Neither is super useful in helping the OP understand his problem, since the problem is dynamic and turbulent and thus not bound by the ideal gas law or its derivatives.
Force is relative to pressure and volume and is a measurement of energy. Pressure is relative to temperature in a fixed volume. As stated though this is a dynamic and turbulent (non laminar) flow.

It's way over his head by now.
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Old 09-26-2012, 09:54 PM   #50
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Different turbos have different flow rates. A VF39 flows around 425 CFM where a GT35R is flowing about 950 CFM. Because the compressor is larger, it can move a larger volume of air at the same pressure.
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