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The difference between CFM and PSI


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#1 Lance_S

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Posted 17 June 2011 - 11:22 AM

Hey guys,

I am interested in your thought on CFM vs. PSI. Take for example a smaller turbo with lower CFM at higher PSI vs. a larger turbo at lower PSI. A turbo will only flow X (total possible CFM) but then we put it on an engine. The engine, depending on the bottleneck may limit CFM capability. Here is my question, does the turbo start to produce boost when it reaches resistance from the engine? In other words, if the engine is more efficient with air (i.e. intake porting, head porting, headers, etc.) will the motor produce boost at a later rpm because it hasn't run into resistance yet?

Question #2: Will a smaller turbo at a higher PSI produce the same air as a larger turbo at lower PSI? And if so, what is better? A smaller turbo at say 22 PSI vs.a larger turbo at maybe 15?
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#2 Chad

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Posted 17 June 2011 - 11:34 AM

It's very dynamic, the whole thing is driven by exhaust volume and pressure on the turbine wheel, which is dictated by the CFM's the motor is drawing in, which is forced in by the comperssor wheel.   You can't have one without the other, and they both effect each other.  

It's nearly impossible to predict any specific scenerio of x + y / z * w = a where you have pressure, CFM, RPM's, effeiency, and resulting power.

It's more of a relationship thing than an exact science.

yes, a small motor with a lot of exhaust pressure will cause a turbo to build boost due to bottle necking, but then a small motor won't likely be able to make enough exhaust pressure to accomplish this task in the first place.  

What you are describing there is a function ot turbine wheel to compressor wheel size ratios.  If they are perfectly suited to the task they can do just about anything , but it's all tradeoffs.

If you take 2 turbos where one is exaclty twice the flow as the other, the smaller one will make boost sooner and easeir, and the larger will be later but improve topend power.  If you compare HP at a the same pressure, the larger one will higher overal, but the smalelr one will accomplish it peak lower in the RPM band.  The large one represents less resistance to exhasut flow, and is thus going to make more power for a given PSI.  that is why bigger turbos spool later, but make more power.  Because they make more power and flow more, they also have more exhaust pressure/volume, which adds even more to the equasion and shows up as increased power output for a given (equal) pressure.  

It's like a 3 dimensional relationship.

#3 Frenchi934

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Posted 17 June 2011 - 11:39 AM

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#4 jszucs

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Posted 17 June 2011 - 11:44 AM

View PostChad, on 17 June 2011 - 11:34 AM, said:


It's like a 3 dimensional relationship.

Woudn't it be a 4 dimensional relationship since it's over time to?

#5 89PalermoSHP

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Posted 17 June 2011 - 12:16 PM

Would it be comparable to the amperage/voltage relationship?
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#6 Fuze

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Posted 17 June 2011 - 12:36 PM

View PostLance_S, on 17 June 2011 - 11:22 AM, said:

I am interested in your thought on CFM vs. PSI. Take for example a smaller turbo with lower CFM at higher PSI vs. a larger turbo at lower PSI.

In a nutshell, CFM (Cubic Feet/Minute)is a measure of volume and PSI (Pounds /Square Inch)is a pressure measurement.

One note on turbo size that I don't think has been looked at is heat and thermal efficiency. A smaller turbo at higher boost will heat the air up more than a larger turbo at the same boost level, and hotter air makes less power. Higher pressures also increase heat, and for example with the stock 12A, if you are making 20PSI you may not make more power than you would at 15PSI due to all the added heat from the higher pressure. Also, small turbos aren't really designed to operate in that boost range efficiently.

The larger turbo is more efficient in large part to the fact that it can move the same amount, or more air at a lower boost level due to the fact that the compressor wheel is able to grab more air and push it through with less revolutions. (more surface area on the blades, turns slower but moves the same amount of air.)

The drawback to the larger compressor is the greater inertia of the larger rotating assembly, and that's why it takes longer to spool up, you are spinning a heavier rotating assembly with the same amount of exhaust energy that is used with the smaller turbo. Now you can combat that greater lag effect by reducing friction within the turbo rotating assembly (ball bearing turbos do this), or increasing exhaust flow/intake flow (big exhaust, unrestricted intake, ported exhaust manifold/head, etc.)

Edited by Fuze, 17 June 2011 - 12:40 PM.

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#7 Chad

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Posted 17 June 2011 - 01:07 PM

View Postjszucs, on 17 June 2011 - 11:44 AM, said:

Woudn't it be a 4 dimensional relationship since it's over time to?


Yah, I'd actually argue 5 dimensions since temperature is also a large factor.  It's just hard to visualize 5 dimensions :)

The point is that it's not a linear relationship, and there are more than 2 variables in the origional question.  

It could be argued there are dozens of varriables, such as A/R of of the housings and wheel trims, geometry of the manifold, speed and temperature of the exhaust, B.S.F.C. of the engine, elevation, etc.

Shaft speed is also important, air can only move so fast, a bigger turbo can move a given volume of air with a lower shaft speed, so they are far more efficent at high levels compared to a smaller turbo asked to do the same task.  That is in part where a lot of the heat problems come from on small turbos turend all the way up, where a bigger turbo is probably a better choice.

#8 Chad

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Posted 17 June 2011 - 01:10 PM

View Post89PalermoSHP, on 17 June 2011 - 12:16 PM, said:

Would it be comparable to the amperage/voltage relationship?

Imagine such a relationship, but where the input and output affected each other dynamicaly, with additional  outside non-linear variables.  One where the load was not static, and neither was the source.

#9 Lance_S

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Posted 17 June 2011 - 01:25 PM

View PostFuze, on 17 June 2011 - 12:36 PM, said:


The drawback to the larger compressor is the greater inertia of the larger rotating assembly, and that's why it takes longer to spool up, you are spinning a heavier rotating assembly with the same amount of exhaust energy that is used with the smaller turbo. Now you can combat that greater lag effect by reducing friction within the turbo rotating assembly (ball bearing turbos do this), or increasing exhaust flow/intake flow (big exhaust, unrestricted intake, ported exhaust manifold/head, etc.)

Do any of the turbos that are bolt on for our system (14G, 16G, 18G, 20G, etc) have a ball bearing options or do you have to go to a T3 or T4 type setup to get that?

Thanks,

L
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My feedback:  http://www.starquest...howtopic=127993

#10 Chad

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Posted 17 June 2011 - 01:32 PM

Ball bearing is a Garrett design, and I don't know if any other manufactures are using it (like Mitsu).  

I do know the Bullseye Power  (borg warner) turbos have a DSM turbine housing that will bolt to a stock manifold, that gives you a lot of options there.   Most are a lot bigger than TBI will support very well though.

Ball bearing is a cool design feature, but they are also way more expnsive to rebuild, and cost a lot more upfront too.  There are so many other tricks that it's not really the best choice for many setups with cost and all the other options considered.    I won't suggest against it, but don't design the whole setup just to accocmidate just that one feature.

If you got a ball bearing turbo at the expense of some other important supporting mods, it's still going to be slow.

#11 Lance_S

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Posted 17 June 2011 - 02:24 PM

Good to know, thanks Chad.
Hardpipes, MAFT, GM MAS, 3" DP, 2.5" exhaust, 950/1050 Trilogy injectors, K&N filter, Machined and balance flywheel 23 lbs, short shift, 88/89 tranny, HD ACT clutch, DSM EVO3 S16g ported (21 psi), polished and clipped, NJV head (ported), larger SS Valves, Schneider springs, Ajusa HG, Adjustable FPR, MBC, BOV, MSD coil, Walbro 255, ABS, EGR and balance shaft elim, 88/89 ECU, UEGO 02, new or rebuilt everything.

My feedback:  http://www.starquest...howtopic=127993

#12 jszucs

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Posted 17 June 2011 - 02:49 PM

View PostChad, on 17 June 2011 - 01:07 PM, said:

Yah, I'd actually argue 5 dimensions since temperature is also a large factor.  It's just hard to visualize 5 dimensions :)


That is true to didn't think of that.  Man this is getting complex but great info.  Thanks.




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