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Thread: Blower Sizing

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    Blower Sizing

    -------------------------
    Blower Sizing and You.

    This article will serve as a means for one to better understand the sizing principles of centrifugal superchargers for your car. It will not serve as a course on what blowers door how they work. I am going to assume everyone knows the basics in that respect.

    To size a blower one must have discrete goals, and one must also understand the limitations of your blower assortment.

    Let's take for example the S65, since this is a very popular engine to recieve the centrifugal blower treatment.

    Basic engine specifications are:
    4 liters
    8300 RPM redline
    volumetric efficiency of 95% (this number is just a current assumption, we can change it later)

    Target boost, lets say were going for 7 psi so thats 21.7 psia.

    How big of a blower do I need?

    First you will compute the volumetric consumption of the engine at redline (or your target RPM).
    For the S65 at 8300 RPM that is an easy calculation.
    8300 RPM is 138.33 revolutions per second.

    What you will end up calculating is the following equation which I threw together for you.

    Click here to enlarge


    Where C is your engine displacement in cubic inches.
    RPM is RPM
    and VE is a percentage



    The S65 consumes 556.7 CFM of air at redline. I will not go into detail on the differences between mass flow and volumetric flow.

    Our target boost pressure is 21.7 psia (7 psig), which means we need to calculate charge density for us to know mass flow so we can use the handy dandy compressor map.

    Density is a function of the gas constant R = 53.34 lbf-ft/lbm-F, temperature in Rankine and pressure in lbf/ft2.
    We have R and P, we need to find T.

    We can infact compute charge temperature using isentropic relation for compressors which is:

    Click here to enlarge


    Where Tin is your inlet temperature in Rankine
    Tout is your outlet temperature in Ranking
    Pr is dimensionless
    and Gamma is your Cp/Cv approximately = 1.4 (constant)

    Assume our inlet temperature is 85F, it's a HOT california day.

    We need to know what the compressor efficiency for that equation to work, let's assume this blower is an S trim Vortech. Small, I know; but let's figure out where we are gonna be using this blower.

    Here is the S trim compressor map.

    Click here to enlarge


    Since our target pressure ratio is 1.47, let's say about 1.5. If we assume a 70% efficiency from the map (will get to this later), that gets us a charge temperature of 174.78 F.

    That's kinda high right?

    We know charge density from the following equation:

    Click here to enlarge


    Where:

    rho is in lbm/ft3
    P is in lbf/ft2
    R is constant (see above)
    and T is in Rankine


    And that comes out to be about 0.093 lbm/ft3 air density. This is blower outlet air with NO INTERCOOLER.

    Mass flow is then computed by multiplying 556.7 CFM by 0.093 density and you get around 51.43 lbm/min of air flow, if you want you can convert that value to SCFM by dividing it by ambient density (which is around 0.07 lbm/ft3 at 85F) and thIS S65 consumes about 705 SCFM at 7 psi boost.

    Back to the compressor map.

    The x axis of a compressor map SHOULD show you something called CORRECTED flow. Not mass flow, corrected flow; sometimes it is even called Reduced flow.
    It means, what would the compressor flow be if it was at ambient inlet pressure and temperature? Since we already are or are near ambient P and T, the reduced flow value for our application is close to the actual flow value. We will be using the actual flow value to read this compressor map.

    So we were at about 51.43 lbm/min.

    Observe where the lines cross below.


    Click here to enlarge

    Notice how we are close to the efficiency island between 70% and 72% which is close to our first guess. If we weren't, we would have to recalculate compressor exit temp and then recalculate density and mass flow again until we converge to a solution that satisfies both compressor exit temp and efficiency (implicit solution although you can make it explicit if you are slick).

    Anyway, this example worked well for 7 psi. Our impeller speed is about 33,000 RPM. Now we have a target impeller speed we know what size pulleys to get.
    The S trim uses an internal 3.45:1 step up. So that gives us a pulley speed requirement of around 9565 RPM. Since the engine spins to 8300, the ratio of crank pulley to blower pulley should be the ratio of those two speeds or about 1.15:1. So if you had a 4 inch blower pulley, in this case you would need about a 4.6 inch
    crank pulley.

    What happens if I have an S trim and I want to boost 12 psi?
    This is what happens. You max out the blower on this motor, and your IATs skyrocket. If you have any form of intercooling, you are inhibiting the onset of detonation by lower IATs.

    Let's prove this with physics.

    You have the same engine consuming 556.7 CFM. At 12 psi you are pushing 237.64 F air into the motor (compute using isentropic relation), assuming no intercooling and a 65% impeller efficiency.
    Mass flow only goes up to 57.57 lbm/min, our corrected flow must now be taken into account because we are straying far away from good efficiency guesses. We compute corrected flow to
    be 63.3 lbm/min. So the compressor feels like as if it is flowing 63.3 lbm/min but in reality it is only flowing 57.57 lbm/min.

    Observe where the lines cross below. Impeller speed will be roughly 41,000 RPM.

    Click here to enlarge


    The blower is not operating in an ideal spot, sure it can go there and sure it can work. But you are taking a chance if you do this.


    So what now?

    You choose to get a YSi because you want to run more boost. How will that help?
    Look at this compressor map. Same motor and same situation at 12 psi. Let's focus on just blower capability.


    Click here to enlarge

    Compute the corrected flow to be around 64.82 lbm/min at the same 1.8 pressure ratio and this puts you in a 75% efficiency range. Your compressor outlet air temp will be reduced to 217.29F just by switching the blower, that is a 20 degree drop which is good for about a 20 hp increase. This is more profound the more boost you run and the farther out of range you get with a smaller blower.
    Impeller speed for the YSi is the same at about 41,000 RPM. The same pulley sizing criteria applies.
    Let's go in reverse.

    When will the YSi blower be maxed out on my 4 liter S65 that revs to 8800?

    At 8800 RPM the engine consumes 590.23 CFM.
    By looking at the compressor map, you would want to stay in this region shown below.


    Click here to enlarge

    If you do all the calculations in reverse order, and you were to solve for boost pressure knowing your mass flow (engine speed, mass flow requirement etc...) you will calculate about 18 psi with an impeller speed approaching 50,000 RPM.


    Here is where would be with that configuration.

    Click here to enlarge


    At this location on the compressor map, the outlet temperature is computed to be 267.51F, intercooling would be useful at this point.


    A lot of the assumptions I made in this article are 'quick hand calc' assumptions. Meaning, there will be a 10% error or more, this type of quick hand calc is used to get answers fast and help you understand where you are and where you need to go when sizing machinery.

    That being said, because I am human and I make mistakes, there may be mistakes in this article.

    If you see any, please point them out so I can correct the article. Thanks.

    ----------------------------------
    Some people live long, meaningful lives.

    Other people eat shit and die.

    I'm not racist, I hate everybody equally; especially fat people.


    Click here to enlarge

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    Moved to our newly created Advanced Tech... YAY!

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    You major in mathematics or physics dude? lol
    Burger Motorsports
    Home of the Worlds fastest N20s, N54s, N55s, S55s, N63s, and S63s!

    It is the sole responsibility of the purchaser and installer of any BMS part to employ the correct installation techniques required to ensure the proper operation of BMS parts, and BMS disclaims any and all liability for any part failure due to improper installation or use. It is the sole responsibility of the customer to verify that the use of their vehicle and items purchased comply with federal, state and local regulations. BMS claims no legal federal, state or local certification concerning pollution controlled motor vehicles or mandated emissions requirements. BMS products labeled for use only in competition racing vehicles may only be used on competition racing vehicles operated exclusively on a closed course in conjunction with a sanctioned racing event, in accordance with all federal and state laws, and may never be operated on public roads/highways. Please click here for more information on legal requirements related to use of BMS parts.

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    Click here to enlarge Originally Posted by fastgti69 Click here to enlarge
    You major in mathematics or physics dude? lol
    He's an engineer.

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    Click here to enlarge Originally Posted by Sticky Click here to enlarge
    He's an engineer.
    Oh yea... Totally forgot. He's the one with the TI 89 calculator.
    Burger Motorsports
    Home of the Worlds fastest N20s, N54s, N55s, S55s, N63s, and S63s!

    It is the sole responsibility of the purchaser and installer of any BMS part to employ the correct installation techniques required to ensure the proper operation of BMS parts, and BMS disclaims any and all liability for any part failure due to improper installation or use. It is the sole responsibility of the customer to verify that the use of their vehicle and items purchased comply with federal, state and local regulations. BMS claims no legal federal, state or local certification concerning pollution controlled motor vehicles or mandated emissions requirements. BMS products labeled for use only in competition racing vehicles may only be used on competition racing vehicles operated exclusively on a closed course in conjunction with a sanctioned racing event, in accordance with all federal and state laws, and may never be operated on public roads/highways. Please click here for more information on legal requirements related to use of BMS parts.

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    Click here to enlarge Originally Posted by fastgti69 Click here to enlarge
    Oh yea... Totally forgot. He's the one with the TI 89 calculator.
    The calculator probably did most of the work. Those things are insane.

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    Now this IS the $#@! that I live for on these forms.

    It was also the reason why I chose my Whipple charger back in 1992 for my 91 stang. As it was the ONLY blower that had an adiabatic effeminacy range that I was looking for.

    Thanks DBFIUClick here to enlarge

    Now you know why PV=NRT is my favorite physic lawClick here to enlarge

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    Keep in mind guys, that the above equations for VE, volume metric efficiency are for the intake and cylinder heads ability to breath. Where as the blowers adiabatic effeminacy is its ability to compressor the air, without adding too much heat.

    Roots style are usually down around 0.5, where as whipple, aka Lysholm Screw are up in the high .75-.78 range.

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    Click here to enlarge Originally Posted by mramg1 Click here to enlarge
    Keep in mind guys, that the above equations for VE, volume metric efficiency are for the intake and cylinder heads ability to breath. Where as the blowers adiabatic effeminacy is its ability to compressor the air, without adding too much heat.

    Roots style are usually down around 0.5, where as whipple, aka Lysholm Screw are up in the high .75-.78 range.
    where does the KB fall?

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    First off, my previous post which I can not edit?, was off slightly.

    VE can be BOTH for the intake and heads, and of the supercharger. The VE for the supercharger is its ability to compress air without leakage. AKA 10 liters in, 7.2 liters out. THAT is what the above graphs are showing, me bad. Namely the the rough circles on the graph are showing the blowers different VE's under different operating parameters.

    Now the adiabatic effeminacy is NOT just heat related, but also the blowers requirements to generate boost. AKA the blowers overall effeminacy. Namely how much power does it take to turn the blower at a given speed to generate a given boost. THIS is what is VERY useful when selecting a style of supercharger. And that is what I was stating about roots vs Lysholm screws, aka whipples.

    FYI: Whipples ARE Lysholm screw style superchargers, just a different name to call it the same thing. AKA WHipple chargers=Kennebell superchargers. SAME design, SAME effeincy, different name.

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    http://www.jagweb.com/aj6eng/supercharging_article.php


    Good article about supercharger types

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    Click here to enlarge Originally Posted by mramg1 Click here to enlarge
    First off, my previous post which I can not edit?
    You can not but you will be able to soon with an optional package if you wish to have it. Edit time will be extended to a month in the package.

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    Click here to enlarge Originally Posted by LostMarine Click here to enlarge
    where does the KB fall?
    That is a twin screw.

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    Click here to enlarge Originally Posted by Sticky Click here to enlarge
    You can not but you will be able to soon with an optional package if you wish to have it. Edit time will be extended to a month in the package.
    I'm in Sticky, thank you!!!

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    Click here to enlarge Originally Posted by mramg1 Click here to enlarge
    I'm in Sticky, thank you!!!
    My pleasure.

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    Yes roots blowers are positive displacement and in theory should flow linearly with RPM. Because there is leakage around the lobes they lose efficiency in that regard. Also, as roots blowers heat up during operation the gaps close up and they run more efficiently, in some cases the lobes will touch.

    The centrifugal blowers are NOT positive displacement they DO NOT flow the same amount per rotation. Think of it like this.

    If you turn a whipple blower by hand, one revolution will displace say, 2 liters of air.

    If you turn the equivalent sized centri blower by hand one time, you will probably not have displaced any air at all, because centrifugal blowers create a pressure ratio that relies on dynamic pressure head (velocity) but the positive displacement blower pressure falls out as a function of flow.

    If anyone here is an electrical engineer think of this, a centri blower is a voltage source and a roots blower is a current source.
    Some people live long, meaningful lives.

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    Click here to enlarge

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    Click here to enlarge Originally Posted by DBFIU Click here to enlarge
    If anyone here is an ELECTRICAL ENGINEER think of this, a centri blower is a voltage source and a roots blower is a current source.
    I really do hate you my friend.

    Just remember, its not the voltage that kills you but the current, LOL

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    Click here to enlarge Originally Posted by mramg1 Click here to enlarge
    I really do hate you my friend.

    Just remember, its not the voltage that kills you but the current, LOL
    Hahaha I will tell you what I rep you for this.

    When I was an intern back in the day, I used to work in facility that tested power distribution equipment. We're talking 1000 amps and up. I was rooming with 5 other interns and we worked for 3 months in the lab. One time I was having a conversation with my roommate about how current is very dangerous, but voltage is really nothing more than just the electrical analog of pressure.

    He would argue and argue all day how dangerous voltage was. I tried to convince him that voltage means nothing really, you dont die from it, the result of high current going through your body is what kills you. He still insisted that voltage is what kills you, in fact voltage is what allows current to cross over and grab you, but the death comes from the current passing through your lungs and heart which ultimately stops your heart. He did not have a good physical understanding of pressure vs. flow.

    You know, it only takes about half of one amp to kill an adult right?

    But you can get shocked with 100,000 volts and if its low current your hair will just stand up lol...
    Some people live long, meaningful lives.

    Other people eat shit and die.

    I'm not racist, I hate everybody equally; especially fat people.


    Click here to enlarge

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    Since you used an S65 for example, I guess it would be ok to ask this here.

    Why do companies choose to use Centri blowers over roots on the S65? I would think everyone would want the added boost to make up for the lack of low end power..

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    Click here to enlarge Originally Posted by LostMarine Click here to enlarge
    Since you used an S65 for example, I guess it would be ok to ask this here.

    Why do companies choose to use Centri blowers over roots on the S65? I would think everyone would want the added boost to make up for the lack of low end power..
    Sticky and I had this conversation way back when. Maybe if he can dig that up and merge it here it would be nice.

    Basically here is my take on it, and I could be wrong because I dont know the ins and outs of the S65 specifically.

    A roots blower makes instant boost, tuning for part throttle is difficult with those type so of setups because they are not as predictable as a centri (linear boost vs. RPM).

    Also, packaging. I dont know how much room there was under the power dome for the roots blowers.

    Lastly, the individual throttle bodies. Those guys are evenly spaced almost directly over every cylinder, a roots blower pulls air in from the front (or the back for fords) and dumps out the air on the other side. That would mean, you would need a sufficiently large plenum in between the blower and the ITBs to allow the flow to redistribute evenly or else the front 4 cylinders will get more air then the rear 4.

    Here is a picture of a roots blower discharge: See how it is offset to one side?

    Click here to enlarge

    Not all blowers look like this, but this is a potential problem IMO.
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    Click here to enlarge

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    Click here to enlarge Originally Posted by LostMarine Click here to enlarge
    Since you used an S65 for example, I guess it would be ok to ask this here.

    Why do companies choose to use Centri blowers over roots on the S65? I would think everyone would want the added boost to make up for the lack of low end power..
    With a positive displacement like a roots you want to pull through a throttle body whereas with a centrifugal you can blow into them and relieve pressure with a blow off valve. Not to mention a roots won't like to spin super high as it would need to with an S65 and the efficiency drops off. A twin screw might be able to do it but a centrifugal matches the powerband of the M3 really well and has a hell of a top end continually building boost which a positive displacement does not do.

    Also keep in mind adding torque up top is where you want it so a centrifugal won't have as big of a traction issue down low as a positive displacement. Make your torque work for you and the higher you rev with the more torque you can make at that rev range the more horsepower is multiplied. This was the old M way.

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    Click here to enlarge Originally Posted by DBFIU Click here to enlarge
    Hahaha I will tell you what I rep you for this.

    When I was an intern back in the day, I used to work in facility that tested power distribution equipment. We're talking 1000 amps and up. I was rooming with 5 other interns and we worked for 3 months in the lab. One time I was having a conversation with my roommate about how current is very dangerous, but voltage is really nothing more than just the electrical analog of pressure.

    He would argue and argue all day how dangerous voltage was. I tried to convince him that voltage means nothing really, you dont die from it, the result of high current going through your body is what kills you. He still insisted that voltage is what kills you, in fact voltage is what allows current to cross over and grab you, but the death comes from the current passing through your lungs and heart which ultimately stops your heart. He did not have a good physical understanding of pressure vs. flow.

    You know, it only takes about half of one amp to kill an adult right?

    But you can get shocked with 100,000 volts and if its low current your hair will just stand up lol...
    You would LOVE my job then. I am currently rebuilding numerous sub stations around Pittsburgh. Our typical feed lines are 138KV or 345KV.

    In one yard, the bus is soo low that when you walk through the yard, all of your hair stands on end, kind of like being near a vandergraf generator,LOL. This yard is what we call separating the men from the boys, as the rookies ALL duck low when they walk through, and the veterans stand tall and proud looking like Carrot top, ha, ha.

    Most people still don't get the voltage/current idea, as more people die from common 120 home voltage than ALL others combine.

    PS: THANKS for the rep!!!

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    I'm trying to soak this all in. I suck at math.

    Anyway, I would love to see what the S65 would do with a F1-R procharger.

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    Click here to enlarge Originally Posted by Sticky Click here to enlarge
    I'm trying to soak this all in. I suck at math.

    Anyway, I would love to see what the S65 would do with a F1-R procharger.
    It would be impressive no doubt, but where would you find the room to fit it along with the cog setup?

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    Click here to enlarge Originally Posted by Blaizon Click here to enlarge
    It would be impressive no doubt, but where would you find the room to fit it along with the cog setup?
    That is the problem. Maybe someone could figure it out...

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