Newbie question on compressor maps...
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On The Tree
Joined: Jan 2002
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From: Raleigh, NC
Newbie question on compressor maps... Ok, I've got the book Maximum boost, but I've still got a question about compressor maps. In the book Bell describes how to determine your engines airflow rate in CFM. Most compressor maps I've seen use lbs/min, how do you convert or determine your engine's airflow rate in Lbs/min? And does the same procedure apply for pressure ratio?
Thanks!
BTW, anyone know the volumetric effeciency of an LS1?
Thanks!
BTW, anyone know the volumetric effeciency of an LS1?
Moderator
Joined: Nov 2001
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From: Raleigh, NC
Re: Newbie question on compressor maps... Volumetric Efficiency
Calculating your cars volumetric efficiency
http://eric.virginia.com/install_uni...n_9.012000.htm
Calculating your cars volumetric efficiency
http://eric.virginia.com/install_uni...n_9.012000.htm
Moderator
Joined: Nov 2001
Posts: 3,048
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From: Raleigh, NC
Re: Newbie question on compressor maps... Here's a quote from Stealth Peformance in a write-up about Fastoys:
"A typical stock fuel injected motor is rated at about 85 % V.E. This means that out of the 346 cubic inches of air your motor could theoretically take in, only 85 % of that volume enters the cylinders. This translates to about 294 cubic inches of air.
Some all out race motors can reach 100 % VE and supercharged or turbocharged motors actually exceed 100 % by forcing the air into the motor.
Not everyone can afford a supercharger but there is definately room for improvement for the stock motor if you make it easier for it to get air into the motor for combustion. This is what ram air does."
"A typical stock fuel injected motor is rated at about 85 % V.E. This means that out of the 346 cubic inches of air your motor could theoretically take in, only 85 % of that volume enters the cylinders. This translates to about 294 cubic inches of air.
Some all out race motors can reach 100 % VE and supercharged or turbocharged motors actually exceed 100 % by forcing the air into the motor.
Not everyone can afford a supercharger but there is definately room for improvement for the stock motor if you make it easier for it to get air into the motor for combustion. This is what ram air does."
TECH Fanatic
Joined: Nov 2001
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From: Las Cruces, NM
Re: Newbie question on compressor maps... Convert from cfm to lb/m multiply cfm by .0748, this is not 100% correct. Not sure what you mean by the same thing for pressure ratio. Pressure ratio is just that a ratio i.e. its unitless.
Gary
Gary
Re: Newbie question on compressor maps... I have a different turbo boost. There is a bunch of formulas that help choose and estimate efficiency. If you want to learn more, let me know.
Thread Starter
On The Tree
Joined: Jan 2002
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From: Raleigh, NC
Re: Newbie question on compressor maps... Redw ws6, where did you get the figure .0748? My other research has led me to the number .069. Aaarrgh, how in the HELL is someone supposed to read a compressor map with a bunch of mis-information! <img border="0" title="" alt="[Mad]" src="gr_images/icons/mad.gif" />
By pressure ratio I mean this. My bbok says that pressure ratio is 14.7 plus boost divided by 14.7. I.e.
14.7 + 10 = 24.7 / 14.7 = 1.68, a pressure ratio of 1.68
I am under the impression that CFM through an engine is derived like this:
cid x RPM x .5 x volumertric effciency (i.e. .85 for 85%) all divided by 1728. So my car =
(346 x 6200 x .5 x .85) / 1728 = 527.6 CFM
To apply that to a compressor map, you must take the total CFM and multiply it by the pressure ratio, at 10lbs it equals 886.4 CFM. i wanted to know if the pressure ratio was factored in the same way, or was altered by the conversion to lbs/min.
Aaargh, frustrating. <img border="0" title="" alt="[Mad]" src="gr_images/icons/mad.gif" /> <img border="0" title="" alt="[Mad]" src="gr_images/icons/mad.gif" /> <img border="0" title="" alt="[Mad]" src="gr_images/icons/mad.gif" />
By pressure ratio I mean this. My bbok says that pressure ratio is 14.7 plus boost divided by 14.7. I.e.
14.7 + 10 = 24.7 / 14.7 = 1.68, a pressure ratio of 1.68
I am under the impression that CFM through an engine is derived like this:
cid x RPM x .5 x volumertric effciency (i.e. .85 for 85%) all divided by 1728. So my car =
(346 x 6200 x .5 x .85) / 1728 = 527.6 CFM
To apply that to a compressor map, you must take the total CFM and multiply it by the pressure ratio, at 10lbs it equals 886.4 CFM. i wanted to know if the pressure ratio was factored in the same way, or was altered by the conversion to lbs/min.
Aaargh, frustrating. <img border="0" title="" alt="[Mad]" src="gr_images/icons/mad.gif" /> <img border="0" title="" alt="[Mad]" src="gr_images/icons/mad.gif" /> <img border="0" title="" alt="[Mad]" src="gr_images/icons/mad.gif" />
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From: Las Cruces, NM
Re: Newbie question on compressor maps... The .0748 was for converting standard cubic feet per minute (scfm) to lb/m. However the standard for the rest of engineering is slightly different than the automotive world. Use the .069 it is probably correct. You do not need to take into account pressure ratio when you are using lb/m (mass flowrate). Mass flow rate takes into account pressure, temperature, etc.
In your example you have 527 cfm which is ~ 36 lb/m (using .069). Converting this to flywheel horsepower = 360 hp (36 lb * 10 hp/lb). The last equation just gets you in the ball park.
Now I thought you were doing twins turbos. If so you get to multiply the flowrate (from compressor map) by 2 at whatever pressure ratio you think you can run.
Gary
In your example you have 527 cfm which is ~ 36 lb/m (using .069). Converting this to flywheel horsepower = 360 hp (36 lb * 10 hp/lb). The last equation just gets you in the ball park.
Now I thought you were doing twins turbos. If so you get to multiply the flowrate (from compressor map) by 2 at whatever pressure ratio you think you can run.
Gary
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From: san diego ca
Re: Newbie question on compressor maps... Keep in mind that a conversion factor is only accurate at one altitude and pressure, so the lower one may be for standard automotive conditions (80 degrees F?) vs. standard temperature and pressure (STP from chemistry/physics) 300 Kelvins and sea level <img border="0" title="" alt="[Confused]" src="images/icons/confused.gif" />
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Verdad Gallardo TECH Fanatic
Joined: Nov 2001
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From: Las Cruces, NM
Re: Newbie question on compressor maps... To use the conversion from scfm to lb/m (.074) everything has already been converted to standard conditions ie 70 deg F 1 atm. This is why compressor maps are in lb/m.
Gary
Gary