Compressor map question
Alright, i picked up Maximum boost few days ago, read it cover to cover and have been re-reading sections whenever i have time. i thought i understood how to read a compressor map however i guess not, or maby im just confused
In Maximum boost, they mention a 60-1 compressor wheel, and show a map for it and the cfm is listed going almost to 900, however looking here at this compressor map online http://64.225.76.178/catalog/compmaps/fig9.html it tops off at maby 65cfm? or is it a diffirent unit of measure? -Jacob |
Yes, its a different unit. The online map is using pounds of air per minute, which is a newer way of measuring. Maximum Boost is slightly outdated in this regard.
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Lbs/Min = 0.0756 x CFM ?
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You can just multiply your cfm by 0.069 to get a good ballpark figure for lbs/min. This isn't exactly correct, but I'm not sure if you want to deal with the "real" equations. :)
Two other things to note in the Maximum Boost book: 1) The Airflow Rate equation is for 4-stroke piston engines only. These engines are not very good (as we know, lol) and only fire half of their rated displacement per crankshaft revolution. Therefore, a "0.5" multiplier is in the equation so it calculates the fact that only half of the cylinders are actually firing per rpm. Rotary engines and 2-stroke piston engines fire ALL of their rated displacement per output shaft revolution because they fire ALL of their cylinders/rotors rather than half. Therefore, you need to either remove the 0.5 multiplier from the equation, double the rotary engine's displacement, or double your result in order to yield the correct cfm. 2) 1 Bar = about 14.5 psi, not 14.7 psi. This will not affect anything unless you are trying to convert your numbers to compare with engines outside the US. The other 14.7 numbers in the book are correct, since 14.7 is the international standard for sea level pressure, although this technically varies with altitude and ambient conditions (aka the high and low pressure areas shown on your local news TV station). Also, just for my curiosity, do you now understand boost better? For example, do you understand that a turbocharger multiplies pressure rather than adding it? I'm just curious how regular people view that book. |
Originally posted by CrackHeadMel Lbs/Min = 0.0756 x CFM ? That's air density at International Standard Atmosphere (ISA) conditions of standard sea level pressure 29.92inHg and 59degF, not corrected for humidity. http://www.aeromech.usyd.edu.au/aero/atmos/atmos.html |
Originally posted by Evil Aviator You can just multiply your cfm by 0.069 to get a good ballpark figure for lbs/min. This isn't exactly correct, but I'm not sure if you want to deal with the "real" equations. :) Two other things to note in the Maximum Boost book: 1) The Airflow Rate equation is for 4-stroke piston engines only. These engines are not very good (as we know, lol) and only fire half of their rated displacement per crankshaft revolution. Therefore, a "0.5" multiplier is in the equation so it calculates the fact that only half of the cylinders are actually firing per rpm. Rotary engines and 2-stroke piston engines fire ALL of their rated displacement per output shaft revolution because they fire ALL of their cylinders/rotors rather than half. Therefore, you need to either remove the 0.5 multiplier from the equation, double the rotary engine's displacement, or double your result in order to yield the correct cfm. 2) 1 Bar = about 14.5 psi, not 14.7 psi. This will not affect anything unless you are trying to convert your numbers to compare with engines outside the US. The other 14.7 numbers in the book are correct, since 14.7 is the international standard for sea level pressure, although this technically varies with altitude and ambient conditions (aka the high and low pressure areas shown on your local news TV station). Also, just for my curiosity, do you now understand boost better? For example, do you understand that a turbocharger multiplies pressure rather than adding it? I'm just curious how regular people view that book. Do you have a url or another book that would have this equation?, or maby its in the forum here already somewhere, ill search i spose Thanks for the info on the 0.5 multiplyer in the cfm forumla. I knew that the rotary had double volume but i wasnt 100% on if i could just remove the 0.5 or not. Yeah i really understand alot more about boost, i want to beable to understand exactly why im choosing what i choose for my car as i build it. There was alot of additional information that i didnt expect to see in this book. The only problem im still having is understanding a/r completly. Looking at Fig. 3.11 on pg 32 it shows how A6 over R6 is equal to A5 over R5 etc etc. However to me all i see the Area of a givin location in the scroll is equal to the radius of the turbin housing to the center of the scroll? I dont understand how a distance can be equal to an area i guess. I never had any highlevel math class's in Highschool so if this is something basic that im missing feel free to point it out :D Other than that i feel it was deffinetly worth the $30 something i paid for it -Jacob |
Originally posted by CrackHeadMel Hmmm, you make the 'real' equation sound a little difficult to understand but never the less if its the correct way to go about I should probably learn it Do you have a url or another book that would have this equation?, or maby its in the forum here already somewhere, ill search i spose You may also like this site. There are some math errors, but it gives you the basic idea. http://www.gnttype.org/techarea/turbo/turboflow.html Originally posted by CrackHeadMel The only problem im still having is understanding a/r completly. Looking at Fig. 3.11 on pg 32 it shows how A6 over R6 is equal to A5 over R5 etc etc. However to me all i see the Area of a givin location in the scroll is equal to the radius of the turbin housing to the center of the scroll? I dont understand how a distance can be equal to an area i guess. Example for .96 A/R ratio: A1 = 1.92 sq in R1 = 2 in A1 / R1 = .96 A/R A2 = 1.44 sq in R2 = 1.5 in A2 / R2 = .96 A/R A3 = .96 sq in R3 = 1 in A3 / R3 = .96 A/R Originally posted by CrackHeadMel Other than that i feel it was deffinetly worth the $30 something i paid for it |
Thanks for the urls. Ive only quickly scanned them as of now but the 2nd one seemed easy enough to understand for me
As for the A/R thing, Now im completely understand what they were getting at. A pretty obviouse mistake made but atleast its answered When I was digging threw barns & nobles shelves looking for Maximum Boost i ran across his supercharger book and almost bought it, im sort of regreting not buying it now, but theres always next week. Do you know of more advanced books covering roughly the same topics? -Jacob |
Originally posted by CrackHeadMel When I was digging threw barns & nobles shelves looking for Maximum Boost i ran across his supercharger book and almost bought it, im sort of regreting not buying it now, but theres always next week. Originally posted by CrackHeadMel Do you know of more advanced books covering roughly the same topics? |
Originally posted by CrackHeadMel Thanks for the urls. Ive only quickly scanned them as of now but the 2nd one seemed easy enough to understand for me.... like evil said, some math errors (exponent is .286, not .268), and says 17 psi boost and then writes 19 in formula. But he has it al the steps right, but not smoothly presented. I have an excel spreadsheet to do all the math to generate the corrected mass flow and PR for garrett, mitsu, kkk, and holset 'test conditions'. Could e-mail it if u have access to excel. Moisture is less than a 1% densitity error, and not worth messing with imho. only used in dyno correction factor to get a 'zero humidity' common basis. |
Originally posted by KevinK2 Moisture is less than a 1% densitity error, and not worth messing with imho. only used in dyno correction factor to get a 'zero humidity' common basis. None of my aircraft performance charts has ever bothered with the humidity factor. It's more of an engineering issue rather than a user issue. However, it does affect the detonation threashold because it acts like natural water injection. |
Originally posted by Evil Aviator Hey, he asked for it. :) I just burst out laughing when i read that http://www.gnttype.org/techarea/turbo/turboflow.html like evil said, some math errors (exponent is .286, not .268), and says 17 psi boost and then writes 19 in formula. But he has it al the steps right, but not smoothly presented. I have an excel spreadsheet to do all the math to generate the corrected mass flow and PR for garrett, mitsu, kkk, and holset 'test conditions'. Could e-mail it if u have access to excel. Moisture is less than a 1% densitity error, and not worth messing with imho. only used in dyno correction factor to get a 'zero humidity' common basis. my email address is crack_head_mel@Hotmail.com This has to be one of the most helpful thread's i have ever participated in within this forum, hell any forum at all, thanks alot -Jacob |
Originally posted by CrackHeadMel .... Thanks for pointing out the errors, and yes i would love to have a copy of any of the excel sheets if you dont mind sending them my way ... |
I'd be interested in the spreadsheet as well please. I made up my own for use the a T-76 on my project as well. Just checking to see if I did it correctly.
The thing I still don't understand is how you calculate what turbine A/R to run vs. what RPM you will start seeing boost at. If you take a look at my graph, again, I think it's pretty close but would like other opinions. Go about 1/3 down the page. https://www.rx7club.com/showthread.p...hreadid=243960 |
Oops, forgot; my email is kerryb23@excite.com
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Originally posted by Turbo 3 ....The thing I still don't understand is how you calculate what turbine A/R to run vs. what RPM you will start seeing boost at. If you take a look at my graph, again, I think it's pretty close but would like other opinions. Go about 1/3 down the page. https://www.rx7club.com/showthread.p...hreadid=243960 Sent u my excel ... quick check has my flows much lower. |
Originally posted by Turbo 3 The thing I still don't understand is how you calculate what turbine A/R to run vs. what RPM you will start seeing boost at. Compressor maps are just that... compressor maps. This means that they only cover the compressor wheel, not the turbine wheel, shaft, scroll, etc. The compressor map will show you the theoretical (assuming a perfectly matched turbine) boost rise if you follow the surge line up to your boost setting line on the compressor map. |
Thanks Evil but that was the problem I was trying to deal with, avoiding the experience factor and finding something objective. Unfortunately, as you said, I haven't been able to find anything either that will be more definitive for my question. Add on top of that the total amount of BS I've dealt with on the 20B project and the wide range of answers, inexperience, etc, have made it just that much more fun! *sarcasm* At least the 2 rotor guys have a large amount of others' personal experiences to help them.
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Originally posted by Turbo 3 Thanks Evil but that was the problem I was trying to deal with, avoiding the experience factor and finding something objective. Unfortunately, as you said, I haven't been able to find anything either that will be more definitive for my question. Add on top of that the total amount of BS I've dealt with on the 20B project and the wide range of answers, inexperience, etc, have made it just that much more fun! *sarcasm* At least the 2 rotor guys have a large amount of others' personal experiences to help them. If you would like, I can post the Rick Engman method of choosing a turbo. |
please post it, myself and many other noOb'ish people would benifit.
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Originally posted by Mark'sMazda please post it, myself and many other noOb'ish people would benifit. No, I am not joking. I was very shocked to hear this at first, but then I began to realize that even the best rotary engine tuners will defer to experts in certain areas. This really changed the way I look at things. FYI the other impression made on me by Downing Atlanta is that Jim Downing helps his crew members push his car back in the paddock area. I would have thought he would let them do all the work, but obviously this is what it takes to be truly great. Hopefully the other noOb'ish people will benifit from this information as much as I did. |
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