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Old 04-10-2004, 02:16 AM   #1
M@St3R_D
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Question Choosing Turbo Size...

What are the calculations related to choosing the good Turbo size for a given situation??? I hate doing things halfway... I won't pick a turbo because it's a turbo, that ain't me... You probably know what it is when you're into something you LOVE to do... TUNING!!!
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Old 04-10-2004, 02:22 AM   #2
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Re: Choosing Turbo Size...

do u want a math equation or something or just wanna kno like the most popular turbo size? Well anyway u may want to specify what kind of engine u have. Then what kind of mods u have so far or plan to get. Then what u are looking to go into for example top speed or acceleration. Have fun w/ turbo buddy!
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Old 04-10-2004, 02:39 AM   #3
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I want math equations so that in the end, I understand what I need for what I wanna do... (ie: Top Speed, Acceleration, etc)
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Old 04-10-2004, 02:41 PM   #4
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To size a turbo you need to 2 numbers plotted on a compressor map.
Pressure Ratio and your Corrected Mass Airflow
Here is how you get those numbers.

Absolute pressure out of the compressor(Pco)
Pco=Boost+Atmospheric Pressure+Intercooler Pressure drop

Pressure Ratio(Pr)
Pco
Pr= ---------------------
Atmospheric pressure

Density of the air(Di)

Boost pressure + Atmoshperic pressure
Di= -------------------------------------------------------
R x 12 x (460+Post-intercooler temp)
R= 53.3(Constant)
12 is there to keep the unit in inch pounds
460 is there to convert degrees Fahrenheit into degrees Rankin(absolute temp)


Mass flow rate(Mf)

Di x Displacement(cubic inches) x RPM
Mf= --------------------------------------
2 x Volumetric Efficiency

Corrected mass flow(Cmf)

-----------------------------------------
/ Compressor inlet temp(in degrees Rankin)
Mf x √ ------------------------------------------
Ambient air temp(in degrees Rankin)
Cmf= ------------------------------------------------
Amtospheric pressure
---------------------------
Compressor inlet pressure



After all that you have your pressure ratio and your corrected mass airflow numbers, then you plot them on a compressor map to find out what turbo is best(most efficient) for you.

Sorry about the wierd format but it didn't look this way when I finished typing so if you have a question about a formula just ask.
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Old 04-10-2004, 04:28 PM   #5
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Absolute pressure out of the compressor(Pco)
Pco= Boost(Psi) + Atmospheric Pressure(14.7 sea lvl) + Intercooler Pressure Drop(Will I find this one in the intercooler specs???)

Pressure Ratio(Pr)
Pco
Pr=----------------------
Atmospheric Pressure
(I don't understand this equation)

Density of the air(Di)
Boost Pressure(Psi) + Atmoshperic Pressure(14.7 sea lvl)(This will give Di right???)
Di= -------------------------------------------------------(What does that mean???)
R(53.3) x 12 x (460 + Post(What's this???) - Intercooler Temp(Where do I get this???))(What's this equation for???)

Mass flow rate(Mf)
Di x Displacement(cc) x RPM(This will give Mf right???)
Mf= --------------------------------------(What does that mean???)
2 x Volumetric Efficiency(Will I find this one in turbo specs???)(What's this equation for???)

Corrected mass flow(Cmf)
-----------------------------------------(???)
/ Compressor inlet temp(In Degrees Ranking)(Where do I get this???)
Mf x √ ------------------------------------------(???)
Ambient air temp(in degrees Rankin)(U mean outside temp right???)
Cmf= ------------------------------------------------(???)
Amtospheric pressure(14.7 sea lvl)
---------------------------(???)
Compressor inlet pressure(Where do I get this???)
(I don't understand this equation)

It's maybe me, since I'm not used to these terms, that doesn't understand, but the equations with --------------------- I don't have no clue what this means, and some of the datas too. Maybe if you could give me these equations with exemples and with some descriptions on how we apply it and how it works could help me a lot... But thank you very much for this input spooleffect It seems that I will finally understand how to do my stuff, after I first got all of this of course
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Old 04-10-2004, 07:36 PM   #6
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Just imagine the -------- as a solid line. It means divide.

You'll have to ask one question at a time because there is way to much jumble for me to disect. This stuff is kinda hard to explain to others but I'll try to make it understandable.


To start, those equations are meant to figure out two numbers, Pressure Ratio and Corrected Air Flow(lbs/min). If you look at a turbo compressor map it is built on these two numbers throughout a graph plotted against boost pressure. So if you wanna run say 15psi and redline @ 7500rpm then you want a turbo that is effecient as possible throughout your engines rpm range. You'll need to look at a compressor map to get what I'm saying first. www.egarrett.com
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Old 04-10-2004, 08:10 PM   #7
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OK for what I think is your first question.

Pco- You should be able to inquire a comapny about the pressure drop number. It shouldn't be more than 1.5psi.

Pr= Pco/Atmoshpoeric Pressure(Pco divided by AP)

Di equation-
Di= Boost pressure + Atmoshperic pressure/R x 12 x (460+Post[- didn't mean subract] Intercooler temp) The formating got screwed by in my Orig post.
Post intercooler temp is the temp of the air charge after it has been through the IC. Adding 460 to the post temp convert the number to the degree Rankin which it needs to be.

Mf equation-
Mf= Di x Displacement(cubic inches) x RPM/2 x Volumetric Efficiency(of your engine)

Cmf equation-
First take the square root of the compressor inlet temp(in degrees Rankin) divided by the ambient air temp(also in degrees Rankin) then multiply that by your Mf number. I'll reffer to this answer as #1
Next Divide your Atmospheric pressure by the Compressor inlet pressure(Just use 13.95 as a good guestimate for the comp inlet temp) This answer is #2
LastDivide #1 by #2 and then you'll have your Cmf

If you still need some clearing up just let me know.
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Old 04-11-2004, 02:43 AM   #8
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Thanks a lot spooleffect!!! Now everything is clear... Sorry for rushing in all these questions I was in a eager to understand all of this and forgot my manners You helped me a lot with your inputs and I won't thank you enough for it Thanks again spooleffect!!!
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Old 04-11-2004, 03:06 AM   #9
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Re: Choosing Turbo Size...

i agree... very good post spool.. i believe that this thread should be stickified.. very good info
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Old 04-11-2004, 08:29 PM   #10
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No prob guys, glad I could help but what I wrote is too sloppy to be a sticky type thread. If someone would like to make a sticky out of this just PM me and I'll write a new thread that is a bit easier to understand.

Once again Im glad I could help.
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Old 04-15-2004, 11:29 PM   #11
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Nice!!! all we need it about 20,000 more people like you
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Old 04-19-2004, 02:30 AM   #12
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Quote:
Cmf equation-
First take the square root of the compressor inlet temp(in degrees Rankin) divided by the ambient air temp(also in degrees Rankin)
First Q: Do i take the square root of the Compressor Inlet Temperature alone or do I take square root of Compressor Inlet Temperature divided by Ambiant Air Temperature???

Quote:
Next Divide your Atmospheric pressure by the Compressor inlet pressure(Just use 13.95 as a good guestimate for the comp inlet temp)
Second Q: Atmospheric Pressure divided by Compressor Inlet Pressure or Atmospheric Pressure divided by Compressor Inlet Temperature??? What confused me is what you said in the bracket. In the equation you said Pressure and in the bracket you said Temperature...
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Old 04-19-2004, 02:51 AM   #13
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Re: Choosing Turbo Size...

Quote:
Originally Posted by M@St3R_D
First Q: Do i take the square root of the Compressor Inlet Temperature alone or do I take square root of Compressor Inlet Temperature divided by Ambiant Air Temperature???
Compressor Inlet Temp/Ambient Air Temp=X. Then take the square root of X.


Quote:
Originally Posted by M@St3R_D
Second Q: Atmospheric Pressure divided by Compressor Inlet Pressure or Atmospheric Pressure divided by Compressor Inlet Temperature??? What confused me is what you said in the bracket. In the equation you said Pressure and in the bracket you said Temperature...
Atmospheric Pressure/Compressor Inlet Pressure(Sorry I meant Pressure not Temp)
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