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Discussion Starter · #1 ·
I know that the Eclipse's T25 cfm rating is 265. I want to know what the SR20DET T25 cfm rating is. Or the GT-iR T28 CFM rating.
 

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Chris Colburn said:
I know that the Eclipse's T25 cfm rating is 265. I want to know what the SR20DET T25 cfm rating is. Or the GT-iR T28 CFM rating.
The BB DET T-25 compressor will flow about 27 lbs/min at a 2:1 pressure ratio (about 1 bar boost). That is about 390 CFM.

I don't know where you got the 265 CFM rating for the Eclipse's T-25. That sounds suspiciously low.

At 1 bar of boost, the GTi-R T-28 will flow about 32 lbs/min of air. That is about 465 CFM

You should be using lb/min, not CFM anyway from a compressor matching perspective. Volume is not conserved, mass is. It does not matter how many CFM the compressor can flow; it matters how much air the engine can breath.

Rob
 

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Discussion Starter · #3 ·
I always thought that cfm and lb/min where measuring the same thing but in different units. Like miles to kilometers, they both measure distance but with a different unit of length. The conversion I always used for cfm to lb/min was 14.5 cfm to 1 lb/min.

Now can you please explain to me the real diffence between the two units of measurement?

And for the Eclipse T25 cfm rating. I always am looking at turbo ratings. These is the numbers I have come up with.

T25-265 cfm=2nd Gen Eclipse (TDO4-9B-6CM2 / 265 CFM?)
CT26-26.4 lb/min (my conversion =385cfm)stock 2nd Gen 3S-GTE
14B-405 cfm=1st Gen Eclipse
16G-505 cfm
CT20B-37 lb/min (my conversion =535cfm)Stock 3rd Gen 3S-GTE
big 16G-550 cfm
20G-650 cfm
T67-850 cfm= Greddy turbo
T78-1000 cfm= Greddy turbo
 

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Chris Colburn said:
I always thought that cfm and lb/min where measuring the same thing but in different units. Like miles to kilometers, they both measure distance but with a different unit of length. The conversion I always used for cfm to lb/min was 14.5 cfm to 1 lb/min.
Nope. CFM measures Cubic Feet per Minute. Cubic feet is a measure of volume. CFM is a volumetric flow rate.

lb/min is pounds per minute. It is a mass flow rate. Mass and volume are not the same. It is like the difference between 'weight' and 'size'.

If you have a choice of carrying one cubic foot of lead to the top of a mountain, or one cubic foot of feathers to the top of a mountain, which would you choose? The feathers, right? But they are the same volume! Well, volume isn't a measure of much, mass is.

Your engine is what's known as a constant displacement pump. With each revolution, it breathes a volume of air equal to one half of it's displacement (ideally, anyway). So, spinning at 6000 RPM, your 2.0L motor is breathing 3000 liters of air per minute (at least ideally. In reality, you have less than 100% volumetric efficiency, so your engine breaths less air than that. But that is a side point that is irrelevant to this discussion).

Now, in order to make power, your engine must combine air with fuel in the proper proportion. The fuel then burns and creates power. The more fuel you burn, the more power you can make. The amount of fuel that you can burn is solely dependant on the MASS of air going into the engine. Not of the VOLUME flow rate going into the engine; the MASS flow.

Now, there are two basic types of fluids. Compressible fluids, and incompressible fluids. Water is an incompressible fluid. That means that the density of water remains approximately constant over a large range of temperatures and pressures. Think about it this way: one gallon of water weighs about 8 pounds. As the temperature of that water changes, the volume stays right at about one gallon. If you were to apply increased pressure to the water, say by pushing on the surface of the water with a plunger, you would not be able to compress it to a volume smaller than one gallon. This is what is meant by incompressible.

Air, on the other hand, is a very compressible fluid. If you have a fixed mass of air in a sealed balloon, you can vary the volume of the balloon by squeezing the balloon, or by changing its temperature. As a balloon is cooled off, it will tend to decrease in volume. As it is heated, it will increase in volume. In short, a fixed mass of air can occupy a whole range of volumes. This is because of air's compressibility.

Mass and volume are related to each other through density. In fact, density is defined as mass divided by volume:

rho = M/V

Density has units such as 'pounds per cubic foot' and 'kilograms per cubic meter'. From the definition of density, volume is mass divided by density.

Now, lets get back to how this relates to your engine: Your engine is a constant displacement device, which means that it breaths a certain volume of air with each revolution. How much fuel do you put in? Well, as I said, the mass of fuel you put in is dependant on the mass of air you have in the cylinder. The mass of the air in the cylinder is equal to the volume of air times the density of the air. So, mass flow rate is equal to the volumetric flow rate times the density of the gas.

Now it should be obvious why turbos are such wonderful things: compressing the air before you feed it to the engine increases its density. The engine can only breath a fixed volume of air; therefore, increasing density of the air it breaths will increase the mass flow rate of air into the engine. This allows you to burn more fuel for a fixed fuel-to-air ratio, and this in turn allows you to make more power.

Now, an SR20 running 7500 RPM with 90% volumetric efficiency will breathe 238 CFM of air. This is fixed, whether you turbocharge the car or not. The mass flow rate that the engine breathes in dependant on the density of air in the intake manifold; at 14.2 psi absolute pressure at 100 degrees F temperature, intake manifold denisty is about 0.0685 lbs/cubic foot. Therefore, the mass flow rate into the engine is about 16.3 pounds of air per minute.

If, on the other hand, you were running 20 pounds of boost on top of 14.7 psi 'ambient' pressure, with a 140 degree F intake manifold temperature, your intake manifold density would be .1562 lbs/cubic foot. In this case, the mass flow rate of air through the engine would be 37.2 lb/min. At 20 pounds of boost, the air flow rate is 2.3 times higher than it is at basically ambient pressure. This means you can burn 2.3 times more fuel, and you can make 2.3 times as much power (all else being equal).

Now, you need to match your compressor flow capability to your engine flow capability. How will you do that? Well, as I said before, the volumetric flow rate into the engine is fixed at 238 CFM, turbocharged or not. The mass flow rate into the engine is dependant on the intake manifold density (meaning boost pressure and temperature). Mass in conserved. Therefore, the mass flow rate into the engine is the same as the mass flow rate into the compressor.

Volume is not conserved. It is a function of mass and density (as I've said). The volume of air going into the compressor is *NOT* the same as the volume of air going into the engine, because the density of air going into the engine is much higher than the density of air going into the compressor (one would hope--otherwise, why turbocharge!).

So, why put your compressor map in terms of volumetric flow rate? It doesn't make sense, because the volumetric flow rate at the compressor inlet, the intercooler inlet, and at the throttle body are all different, because of the varying air density. it's best to plot the map in terms of mass flow rate, because mass flow is conserved, and it gives you a direct measure of the power the engine is making.

As for the conversion from lb/min to CFM, at 28.4 inches of mercury barometric pressure and 545 Rankin absolute temperature (Garrett's "Standard Day"), 14.46 CFM is equal to one pound per minute. On a different "Standard Day" (Mitsubishi may use a different standard), the conversion factor would be different.

Hope this makes sense.
 

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Rob--

Wow. That appears to be an entire year of Physics and Engineering classes, all balled up into your one post. Wonderful information, and written in a way that even I --who graduated from the Holley Double-Pumper School of Air Flow in the 1980s-- can understand.

Now then...let's relate all that to the proposed intercooler we're trying to do a group buy on. According to Forge Motorsports, it flows 253 cfm
cooling air flow 1200 ft / min
. Some have expressed concern over the cfm rating being low (compared to other units).

However...if I read your above dissertation correctly, the cfm rating isn't too terribly important, as our engines are 'fixed' at 238cfm, turbo or no turbo. As far as I can deduce, it's the mass of flow and not the volume of flow that is important in terms of intercooler performance, turbo performance, etc.

Or do I have this backwards?

I'm really enjoying reading your highly technical posts, Rob. For a non-Physics/Engineering person like me, they do help tremendously.

Now then...is that Forge intercooler worth buying or not? :D
 

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SERprise In WV said:

Now then...let's relate all that to the proposed intercooler we're trying to do a group buy on. According to Forge Motorsports, it flows 253 cfm
cooling air flow 1200 ft / min
. Some have expressed concern over the cfm rating being low (compared to other units).
Well, then maybe those who are concerned about the spec can explain to me what the hell it means. I saw the spec under the other thread, and it really doesn't mean s**t to me.

However...if I read your above dissertation correctly, the cfm rating isn't too terribly important, as our engines are 'fixed' at 238cfm, turbo or no turbo. As far as I can deduce, it's the mass of flow and not the volume of flow that is important in terms of intercooler performance, turbo performance, etc.
Well, for reasonably incompressible flow (you can assume air is incompressible as long as it as at a flow Mach number of less than 0.3 or so--this is useful for doing stuff like pipe flow analysis, but not for analyzing compressor wheels), pressure drop is proportional to density time volumetric flow rate squared. The volumetric flow rate is important because it is an idicator of velocity (for a fixed are and mass flow rate). Note the density term, though.

This just comes from Bernoulli's equation. Basic stuff.


So, dP = k*rho*Q^2, where Q is the volumetric flow rate, rho is the air density, and k is a constant.

If they are 'rating' an intercooler at some CFM number, what the hell does that mean? It means nothing to me. If if means something to someone else here, please educate me.

If you look at Spearco's catalog, they list a CFM rating at 1.5 psi pressure drop. That stat rates the F-Max core at 640 CFM. That's a lot more than the Forge core is 'rated' at. However, notice that the Forge CFM rating doesn't list the 'rated' pressure drop. Maybe Forge 'rates' their intercoolers at .25 psi pressure drop. Who knows?

Also, even the numbers that Spearco gives only tell a part of the story. The Spearco catelog also has charts of intercooler performance. There, they list pressure drop as a function of engine displacment. If you read the fine print, it says '@10 psi boost at 6000 RPM'. OK, that lets you get to a mass flow rate if you know engine size. So, they plot of curve of mass flow rate versus pressure drop. Helpful, but what about the density term? Well, if you check the fine print, it says 'for 20 psi, increase pressure drop 30%. For 5 psi, decrease 25%'. Well, for fixed engine displacement, changing boost changes mass flow rate. It also changes desity. So, the situation gets more confusing.

It is also worth noting that at the 640 CFM 'rating' that Spearco lists for the core, the pressure drop on the performance curve is less than 1.5 psi. This is probably because the '640 CFM' rating is not at 10 psi of boost.

The problem is that the Spearco catalog is trying to give you enough information to choose an appropriately sized cooler without giving so much information to confuse the hell out of everyone. Personally, I think they do a good job of it.

How does the Forge core compare to the Spearco core? Well, I don't know. Forge has not provided enough information to make that comparison. Their 'rating' of volumetric flow rate isn't specific enough to compare it to the Spearco rating.... one thing's for sure: the ratings are made in different ways. Understand the ratings, and then you can understand which one is better.
 

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SERprise In WV said:
Rob--

Wow. That appears to be an entire year of Physics and Engineering classes, all balled up into your one post. Wonderful information, and written in a way that even I --who graduated from the Holley Double-Pumper School of Air Flow in the 1980s-- can understand.

Agreed.Even if I have difficult to read english,your text is very clear to me!!:)
 

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This just comes from Bernoulli's equation. Basic stuff.
In your world, Rob, it's simple stuff. However, you've managed to explain things clearly, and for that I am thankful.

I would imagine that, just on cooling surface area alone, it is safe to say that the Forge unit has to be more efficient than, say, a stock Bluebird or even GTi-R intercooler. More efficient cooling, yes. But more efficient at moving air quickly? That appears to be the $450 question here.

I would also imagine that having Forge build the same intercooler, but with 2.5" inlet/outlet would only make it more efficient. If for no other reason than doing away with a smaller restriction on both ends.

Thanks for your highly-informative info, Rob.
 

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SERprise In WV said:

I would imagine that, just on cooling surface area alone, it is safe to say that the Forge unit has to be more efficient than, say, a stock Bluebird or even GTi-R intercooler. More efficient cooling, yes. But more efficient at moving air quickly? That appears to be the $450 question here.

Well, intercooler effectiveness (not efficiency) increases with the heat transfer surface area. Meaning, if you want to transfer more heat, you need more surface area. That means higher frictional losses, which means more pressure loss. An intercooler with a very low pressure drop typically will also have a low effectiveness. The key is balance. Spearco publishes nice graphs showing the effectiveness of their cores. See if Forge can provide something similar.

I would also imagine that having Forge build the same intercooler, but with 2.5" inlet/outlet would only make it more efficient. If for no other reason than doing away with a smaller restriction on both ends.
Bigger pipes won't have any influence on the effectiveness of the core. They may have a small influence on pressure drop, but most of the pressure drop is in the core. Look at all those fins the flow has to pass through!

There are two things that define intercooler performance: effectiveness, and pressure drop. Don't get the two confused. The first is how effective the intercooler is at cooling off the gas. The second is how much pressure drop the flow incurs in going through the core.
 

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Discussion Starter · #11 ·
Well Rob I would just like to say thank you for your help. You really taught me something useful. I now understand the difference between lbs/min and CFM.

You did loss me on the "at 28.4 inches of mercury barometric pressure and 545 Rankin absolute temperature"

I understand that the weather an altitude affects air density but that is not impotant to me. Leave that to the weather men and people at Garrett.

The other statement you made that confused me and I think it is because you figured it out with an equation you did not list was

"The mass flow rate that the engine breathes in dependant on the density of air in the intake manifold; at 14.2 psi absolute pressure at 100 degrees F temperature, intake manifold denisty is about 0.0685 lbs/cubic foot. Therefore, the mass flow rate into the engine is about 16.3 pounds of air per minute."

I don't get where you get 14.2 (atmoshpere 14.7?) and how you figured out the intake density (lbs/cubic foot). I mean I understand how to take mass and divide it by volume but how do you know the volume of the intake manifold? Then I was seeing if you meant the motor itself and the whole 236cfm thing but I could not come up with an equation that made sense. Then I was going to recheck my book by Corky Bell "Maximum Boost" but I lent it to a friend plus I doubt it was in there.

So if you feel up to explaining more to me I would apperiate it but if not thanks for the help already.
 

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Chris Colburn said:

You did loss me on the "at 28.4 inches of mercury barometric pressure and 545 Rankin absolute temperature"
Inches of mercury is a measure of pressure. 28.4 inches of mercury is about 13.95 psi. This is Garrett's standard compressor inlet pressure. It is lower than ambient to account for inlet restriction from an air filter, etc. Rankin is a measure of absolute temperature. 0 Rankin is known as 'absolute zero', the coldest temperature you could theoretically have. Absolute zero is theoretically impossible to achieve. To go from Faranheight to Rankin, and 459.67 to the temperature. So, 545 R is about 85 F.

I don't get where you get 14.2 (atmoshpere 14.7?)
I assumed a 14.2 psi inlet manifold pressure for a naturally aspirated car (instead of 14.7) because I assumed there would be about .5 psi of pressure drop from the piping and the filter.

and how you figured out the intake density (lbs/cubic foot). I mean I understand how to take mass and divide it by volume but how do you know the volume of the intake manifold?
Intake manifold density is calculated from the ideal gas law:

rho = P/(R*T)

Where P is the intake manifold absolute pressure (boost pressure plus ambient pressure), R is a constant = 639.6, and T is intake manifold absolute temperature in Rankin (which is intake manifold temperature in deg F plus 459.67).

This will give you density in lb/in^3. Multiply by 1728 to get lb/ft^3.
 
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