A couple of us got off topic in another post and i figured this was an advanced discussion. So, does 13 psi = 13 psi, no matter what turbo is producing that 13 psi? Thoughts?

yes, 13 psi is 13 ps1, like a ton of bricks or feathers, but is not equal to air DENSITY, which is really what you want

Can you elaborate on that a little bit?

PSI is pressure per square inch. So it's the force of the air. CFM is cubic feet per minute, which is the actual amount of air flow(power). Running 20 PSI on a t-25 isn't better then 15 PSI on a 20g. Since the 20g flows twice as much air, it is actually creating more power.

Think of it this way, PSI is like miles per hour. A geo metro going 60mph is the same as a semi going 60mph. CFM is like the force of the vehicles. The geo barely has any force (power) going 60mph, whereas the semi has tons of force.

No, Redsand187 you are wrong about that.

15 psi is 15 psi reguardless of the turbo that is creating it. The only difference will be the temperature of the air.

If one turbo can flow say 500 CFM, and another can flow say 200 CFM, and you hook them up one at a time to an engine, and set both of the turbos to 15 psi, as long as the cylinders can't move more than 200 CFM then both setups are going to be EXACTLY equal (assuming the temperature of the air is the same).

What you have been saying is that CFM makes the power. This is wrong. Just because a turbo has a bigger CFM rating doesn't make it more powerful. Where would the larger volumes of air go? The engine is still the same size and taking in the same amount of air at the same pressure.

An engine will use so many CFM of air at a certain RPM at a certain pressure, and as long as the turbo can supply the demanded CFM at the particular pressure, then it's all the same. If a larger turbo can supply more CFM, that doesn't mean it is going to, because you can't make more air go through the engine at the same pressure, understand?



I don't know if this is exactly "Extreme" Tuning, but it's apparently something that a lot of people are misinformed about, so it's probably important to have in a spot where people will take the information as the truth.

I'm also not trying to say that a bigger turbo isn't going to get you some benefits. Larger turbos can be more efficient, and have good porting work, less restriction, etc. And the air is also going to be a little cooler, which means more dense, and more powerful, but only slightly if you don't take advantage of what the larger turbo can do, keep up with a better tuned engine at a higher PSI. But if you just strap on a larger turbo, you probably won't feel any difference.

-Jesse

basically, you want to get an efficent turbo for the amount of air you need to push, say 600 cfm ok. a 14b is good for i believe 350 or so. but it will push 20 psi. now we have a ported and clipped b16g, may flow around 600cfm max, but 20 psi also. next we are going to have to look at efficency at that pressure ratio and air flow, and you will see that the 14b is way less efficent and creates alot more heat, meaning less dense air. to make it short, hot air gives you less oxygen per cubic foot (density) compared to cool, dense air, hence the intercoolers role.

Yes, that too. I'm just basically demonstrating if you just attach a bigger turbo without doing anything else

ok 300cfm at 15 psi and 600 cfm at 15 psi this is the difference between a big and small turbo. Wouldnt 600 cfm at 15psi be pushing 600 cfm of air through the system at 15psi. So there for the larger the turbo the more cfm it can push with less psi. So wouldnt that kinda make redsand right. If no think about it you bolt a 16g on a stock 1g and it adds 30 horsepower is this because of the lower heat? No its because a 16g flows more cfm at the same psi. Remember a engine is a big airpump the more air pushed through the more power. More air is equal to more cfm. It is just like carbs on old school #@%#@%#@%#@% 650 cfm carb makes more power then a 450 cfm carb because the 650 can let more air in the engine. Correct me if I am wrong

Drew

I don't know about comparing it to the carb theory as i'm not familiar with them, but that is more along the lines of what i was thinking. It is forced induction so i would imagine the more air flow, the more that gets crammed in, and thus the more power made.
Enigma, in your drawing you say where would the air go? Does the t25 max out the amount of space "available" at 15psi?
*shrug*

no, it doesnt run out of space. it makes alot more heat causing air density to drop and therefore the air going into the motor is less dense than would be efficent with a larger turbo. its like snow, when snow is snow (cold water) its fluffy and takes up alot of room (not dense,like hot air), make the snow melt and its heavier (more dense). as a reply to the 300cfm to the 600cfm post.......your on two different topics. one is pressure, the other is flow. pressure is different than flow all togather, its a measure of force. cfm is a measure of flow. you answered your own question, if a turbo cant supply the air flow required, how will it sustain the pressure also. for a while it will become inefficent making more heat which makes it take up more volume, which the engine will see as flow none the less. but there is less actual air in it for power due to the density changes. then after so much i will run out of breath all togather and give up.

cubic feet per minute that is the amount of air the turbo sucks in when it sucks this air in, there is air already there but it pushes it in there anyways compressing it PSI pounds per square inch. When the intake valve opens this pressure escapes into the cylinder for as long as the intake valve is open or to the same pressure as the air outside the cylinder. Another thing dense air isnt really a problem because compressed air is dense yes cooler air is denser wich is actually like adding more psi this is why we have fuel cut on our cars when its cold there is more pressure in the intake then air that was metered by the maf for that pressure or something like that. And what the hell is this about a turbo not running out of space for the air it trying to cram into the intake, this is where the boost controller comes into play when a certain psi is met it opens the wastegate causing the exaust to escape instead of pushing the turbine faster pulling more air into the sytem through the compressor. Redsand is right cfm is like the force behind the semi look at turbines the more cfm pushed by the wheel the more force is pushed out creating thrust, the air running through the intake system is measured in cfm. Having pressure in the intake is just helping push more cfm through the engine it pushes hence giving it boost while a normaly asperated car sucks air into the engine hence vacum. I am pretty sure I am right on this one.

Drew

When thinking about flow, try not to focus solely on the compressor side. As was stated, the power gained from a big turbo compressor is from improved efficiency and to feed the engine at higher rpm. But most of the time when a big turbo is installed it usually has a larger turbine wheel and housing. Exhaust flows out with less resistance. That's where you get more "flow" at the same psi. For example, lets say we put in a restrictor plate in the exhaust collector ( or anywhere in the exhaust) that has 5 cm area. (less than stock at 6 cm) No matter which compressor wheel you put on it won't "flow" any better, because of the bottleneck in the collector. 15 psi = 15 psi.
The flow is limited to how fast air can get through this restriction. Naturally when you open it up with something big like an 8 cm housing, it will flow much more. 15 psi @ 5cm< 15 psi@8 cm.
That's just an example. The question you need to answer is, where's the bottleneck in your setup? Exhaust? Turbine size? Cam duration? It could also be a cumulative restriction, such as the roughness of unported housings plus the extra bends in a pressbent exhaust plus ? Even poorly matched intake piping can hurt flow by causing too much pressure drop. HTH If I've misstated anything, please feel free to correct me, thanks.

Originally posted by smokehouse_83
ok 300cfm at 15 psi and 600 cfm at 15 psi this is the difference between a big and small turbo. Wouldnt 600 cfm at 15psi be pushing 600 cfm of air through the system at 15psi. So there for the larger the turbo the more cfm it can push with less psi. So wouldnt that kinda make redsand right. If no think about it you bolt a 16g on a stock 1g and it adds 30 horsepower is this because of the lower heat? No its because a 16g flows more cfm at the same psi. Remember a engine is a big airpump the more air pushed through the more power. More air is equal to more cfm. It is just like carbs on old school #@%#@%#@%#@% 650 cfm carb makes more power then a 450 cfm carb because the 650 can let more air in the engine. Correct me if I am wrong

Click to expand...

Well, you're right in a way, 600 CFM at 15 psi would be more powerful than 300 CFM at 15 psi BUT and a big BUT, just because you put on a bigger turbo doesn't mean you're going to get more CFM. The engine is what determines how much CFM you can move. If a turbo is labeled 600 CFM at 15 psi, if you put it up to a stock motor and set your boost controller to 15 PSI, you are not going to get 600 CFM through the motor. YES, the engine is a big airpump, but the amount of air it pumps doesn't matter what the CFM of the turbo is, it just matters what pressure the turbo is generating in it, but only up to what the turbo can supply. So, if your motor had large cams, unrestrictive intake/exhaust, and could take say 500 CFM @ 15 psi, but your turbo was only rated for 300 CFM @ 15 psi, then you'd be blowing a lot of hot air, and not be able to keep up the pressure.

Pneumo, we're all talking about on a very basic level here. People are thinking that because a big turbo says 600 CFM @ 15 psi that they are going to get more CFM just by using that turbo and doing nothing else, which they aren't.
All I'm saying is that putting a larger turbo on a car without changing anything else is basically going to do nothing.

Insane147, I know exactly what I'm talking about, and I know I'm talking about two different things, flow and pressure. I'm trying to explain why that just because a bigger turbo can flow higher at a certain pressure that it's not really going to benefit someone by slapping a big turbo on their car.

GRRR... I don't think people are understanding what I'm trying to say, and I'm apparently not very good at saying it, so I'm just going to stop.

People who understand.. well that's great, those who don't, keep living on in darkness.

The short of everythign I'm trying to show is that putting a gigantic turbo on a car isn't going to make the car really any faster, unless the original turbo was very restrictive or bad in some specific way.

-Jesse

well I would assume we are talking about the 4g63 with the 200cfm of air and the 500cfm turbo. now I would say if the 4g63 can only injest 200cfm of air we have major problems on our hands, now I am not saying buy a T-66 because it flows 72lbs/min and run 15psi because it would make more power than the 20g at 15psi, for instance the 200cfm turbo(smaller than the T-25) would not work with the 4g63 it would work on a 5hp gokart, but that is not the point, I know the L3R a 65lb/min monster turbo made around 370whp on 15psi of boost and airfuel of like 10.8ish, now what I heard was that I can take a 14b motor and run 15psi and make 370whp??? and the only reason the L3R made more power is because the compressor is more efficient? no, the L3R would be somthing like 65%efficent at 15psi on a 4g63 now the L3R is 78% for most of the map but most turbos are at LEAST 75% efficent for there efficent island. so I am saying that the 4g63 stock will take in 50-55lbs/min stock add mods, porting, bigger valves and you will be able to run the T-66 at 35+psi and make monster horse power and the reason is that the turbo flows more CFM more LBs/Min, but there is a limit to what the engine will injest that is why you size your turbo to your motor you will have to figure out the 4G63 will take in 18lbs/min of air at 3000RPM at a pressure ratio of 1.2ish and for 7500RPM will be in the mid 55lbs/min at pressure ratio of 2, and running a pressure ratio of 1.3ish on the low side would almost be 20lbs/min and the high side 3.0-3.3 PR getting into the 60slbs/min. Now for Turbo sizing you will want to make sure 18lbs/min 1.2ish PR is on the graph and not above the surge line(where the compressor is unstable and makes knocking/banging noises and can appear to orbite in the housing as much as 1/4") then go to the top side of the map for the 55lbs/min at pressure ratio of 2 and the efficency should be about 60-65% for the ideal trubo for the motor, then look at the small turbos that come stock and the top side is way off to the right, but it still works though. There are hole books written on this stuff, but I belive that turbos that flow more CFM, Lbs/min, and run higher boost will make more power than smaller turbos with less CFM, less Lbs/min and depending turbo size will still make less power on more boost than the big turbo. other factors are Compressor A/R, standerd 360deg bushing VS Ball Bearing turbos, compressor housings, exaust wheel Stage, A/R of exaust housing, and compressor efficency at the boost/CFM on the map. I think thats it

not flaming just my thoughts


my 60trim seems small now

if it was easy to explain the physics behind the way air acts people wouldnt bother askin, and i know how you feel enigma_man.

15 psi is 15 psi...dont matter if its a mack truck or a geo metro.
300cfm is more than 600cfm....but compare when it makes that 600cfm at 15psi from a compressor map to see if its for you...
and air DENSITY is a huge portion of this, when you increase boost, you are trying to increase the density of air in the cylinders. when your pistons compress the air fuel mixture, it is making the mixture more DENSE...just as a turbo would...so a more efficent device that makes less heat doing its job , is making a more dense charge, which means it is flowing more cfm, or actual air into the engine...simple right...

15psi is 15psi, if you want to know more, do some research.
thats my $.02 (besides what galantvr41062 said).

Enigma- I understand what you're saying about just slapping a larger turbo on a car and expecting to make huge amounts of power. But i was more interested in the physics behind it. I understand what you've said and i now agree with you.

Let's see if I can straighten some 'stuff' up.

First a few comments:
-Just because a turbo CAN flow 600cfm, doesn't mean it WILL flow 600cfm.

In fact, many times more importantly is how efficiently it flows the cfm below its max rated. You will see why lower down.


-PSI is a point-in-time measurement. At that point at that time, there is X psi of force exerting on the system (in our case outwards on the intake pipes)

SO:

Saying 15psi = 15psi is essentially correct. This is a meaningless formula though has it has nothing to do with flow rate and volume.


-CFM is a volumetric flow, or volume per unit of time, measurement. Hence the 'M' in CFM.

In a static system, you need pressure differential to have any flow. (every wonder where wind comes from?). In our engines its a wee bit different. You have a compressor source (turbo) as well as an evacuator source (the valves letting air into the cylinder). This gives you your flow. The volume of air being used in the evacuator source will give you your system flow. Ever tried to blow down a straw with your finger plugging the end? You can increase the PSI by increasing volume in the same space (blow harder) but unless you can evacuate the air and create a pressure differential between the ends, nothing will flow. All you do is pressurize the system.


-power is made by forcing as much volume as are as possible through the system which is a function of density vs temperature and results in a fluid mass. Mass gives you power.

Now you we have a pressure (15psi) and we have volume ( the capacity of the intercooler, IC pipes, intake plenum) so the only way to increase air flow is to increase the density which is a direct relation to temps and hence increase the mass of oxygen(air) that enters the cylinder.

Lets use a empty 2 liter bottle of Pepsi for example. Lets just cap it off so we have a defined volume. We have a volume (2 liters). We have a pressure of 1 atmosphere (14.7psi absolute). And let's say we have the air inside the bottle as 60 degrees F. Now in this configuration, there is only so much mass of air in the bottle.

Ideal gas law: PV=nRT

P = pressure
V = Volume
n = moles of gas (mass)
R = gas constant
T = temperature.

Now in our case, the volume (V ), the gas constant (R ), and the mass (n ) are all constant (remember the cap is on the bottle). So we can show relative changes like this:

P = T

As Temperature drops, so must Pressure to keep the formula accurate.

When we lower the temps, the bottle compresses. The sides suck in. Why? because since the volume and mass stay the same (the caps on remember) when temperature drops, so does the pressure.

Now lets take a look at our intake system.

V = constant since the pipes have a specific diameter as does the intercooler and plenum.
P = constant since we are assuming 15psi
R = gas constant which is....well.... constant.

That leaves us with:

a constant left side = n x T

So in order to keep the left side constant, as temperature decreases, n (the mass of air) must increase and if you recall, more mass of air (o2) means more power.

So, 15psi on a T25 IS NOT the same as 15psi on a Green because more mass flows from the lower compressor temps due to the more efficient turbo housing and wheel.

Discuss.



[Edit: One more thing as a Mod note. This better stay technical or I will lock it ]

Your makin perfect sense to me Enigma.

Originally posted by Enigma_Man
No, Redsand187 you are wrong about that.

15 psi is 15 psi reguardless of the turbo that is creating it. The only difference will be the temperature of the air.

Click to expand...

How am I wrong when that's what I said? I'm saying that 15PSI in a 20g isn't making less power then 20PSI from a t25, because power doesn't come from pressure. 15 PSI is 15 PSI, no matter what.

I was gonna do a whole long explanation about the gas law but it seems Sean beat me to it. Way to use scientific reasoning man

technically 15psi=15psi, simple numbers can't really argue with that. However, power is related to the flow rates, so is the psi. In two different turbos 15psi may not "equal" 15psi, because psi is an area derived unit. CFM is directly related to area as well because if you have a bigger opening the easier it is to move greater CFM through the opening. A larger area also means a lower psi. So a 300 CFM turbo at 15 psi, isn't putting nearly as much air through as a 600 CFM turbo at 15 psi. and that is regardless of area, but because the 600 CFM turbo will more than likely have a larger area it does depend on the area.


p.s. density and pressure (psi) are the same thing when talking about a gas, take my word for it (meteorology major), the barometric pressure is a measure of the weight of air molecules above the instrument. In order to increase the weight you have to put more molecules above the station or increase the density.

Originally posted by mdre21
pressure (psi) are the same thing when talking about a gas, take my word for it (meteorology major), the barometric pressure is a measure of the weight of air molecules above the instrument. In order to increase the weight you have to put more molecules above the station or increase the density.

Click to expand...

Wrong. Do not confuse the assumptions you make in meteology to apply as a constant law. Weight is a function of gravity x mass of the air. In any open system, pressure is not the same as density. Your barometric pressure instrumentation assumes a constants taht only apply to that realm.

<-- Mech Engineering major (Aerodynamics focus)

Thanks Tevenor, that was a great response.

weight= gravity x mass, more mass=more weight, greater density=more mass, in a system with gas molecules pressure and density are directly related. That's why the bottle has to suck it's sides in, to keep the volume proportional.