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Old 03-13-2004, 01:32 AM   #1
latinskllz
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Member#: 3461
Join Date: Jan 2001
Chapter/Region: South East
Location: orlando, fl, USA
Vehicle:
1998 BMW M3 Coupe
Estoril Bleau

Question Help with Choosing a Garrett GT series turbo...Need Clarifications!

Guys, I need your help figuring something out. I have been reading Maximum Boost by Corky Bell and I must admit excellent book. I’m reading this book in the process of turbocharging my car and trying to find a suitable turbo for my application. First off the current specs of the car:

- MY00 Impreza 2.5RS N/A
- TEC-II Engine Management System
- Vishnu Fuel Rails with 550cc Injectors and Aeromotive Dual Inlet FPR with Stainless Steel -6an lines
- Supra TT Fuel Pump
- ACT Clutch
- Gauges of Course (Boost, EGT, Oil Pressure, Water Temp)

Future upgrades on the car:

- STI Hardened Gearset
- STI Shortblock
- Titanium Springs and Retainers with a 3 angle valve job

Sticking with the current setup, based on Maximum Boost, here are the formulas I used and what I came up with.

Airflow Rate =
cid * rpm * .5 * VE
1728

cid for 2.5l = 150
rpm = stock 6250
VE = 85% or .85

So Airflow rate would be 231 cfm or 17.5 lbs/min for a N/A motor.

@ pressure ratio of 8psi (1.54 bar), my new airflow rate will be 356cfm or 27lbs/min.

@ pressure ratio of 10psi (1.68 bar), my new airflow rate will be 388cfm or 29lbs/min.

@pressure ratio of 15psi (2.02 bar), my new airflow rate will be 467cfm or 35lbs/min.

Keeping those numbers in mind, below are the specs of a compressor side I believe is ideal for my setup and would like further clarification.

If I understood the compressor map correctly (plotting Airflow rate and Pressure Ratio from above:

@8psi : The compressor will be working at around 79%
@10psi: The compressor will be working at around 79%
@ 15psi: The compressor will be working at around 79%



Am I looking at this right? All of the numbers are inside that 79% loop…Now for the Choke Line and Surge Line. Assuming that the desired pressure ratio is achieved at 50% of the powerband (3125rpms) at point cfm = 178 (8psi), 194(10psi), and 234 (15psi); then my surge line is:

5lbs/min or 71cfm(20% of 356)
6lbs/min or 78cfm(20% of 388)
7lbs/min or 93cfm(20% of 467)

all of these being plotted at PR = 1. So these “should” plot to the right of the surge limit line of the flow map right and I’m good right?

I don’t think I will be worried about a choke limit at this point, I’m just trying to understand the compatibility of what I want out of the turbo. I have a friend with a SR20DET running this same turbo, and I don’t see why it wouldn’t be feasible for a 2.5L application. Any thoughts or suggestions are greatly appreciated.

Please help me understand if I’m missing something.

Thanks,
Kevin
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Old 03-13-2004, 05:47 PM   #2
latinskllz
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Vehicle:
1998 BMW M3 Coupe
Estoril Bleau

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Damn I feel stupid
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Old 03-13-2004, 06:46 PM   #3
kastle
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Location: Phoenix, AZ
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1999 Outback Sport
Steel Blue Mica

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GT35R is too much... get a nice 30R with the 60 housing.. you will like it a lot... I have that on my car, incredible setup...
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Old 03-13-2004, 06:49 PM   #4
kaos200
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im looking at the gt30/40 with a .63 or .67
still gotta read up more on the compressor maps tho
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Old 03-13-2004, 07:01 PM   #5
latinskllz
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1998 BMW M3 Coupe
Estoril Bleau

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i need an internal wastegate setup with the flanges that of a gt25/28 since the pipes are already made....The turbo kit used to have a GT25R...

But can someone tell me if I am reading the compressor maps right??
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Old 03-13-2004, 09:10 PM   #6
no-coast-punk
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'05 R6 '95 BMW V8 hotness

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Your VE numbers are really low which will really up your airflow rate. That compressor looks a little big, if you plan on going with much more boost in the future (which it looks like you do) then stick with it, otherwise your going to see a bit of lag for a wheel that big. I would go with the .86 A/R turbine. And if you plan on going up to high altitude just remember you will be running closer to your surge line. Other than that everything looks good. If you want to cheat/play around with numbers easier you can use the utility I've been working on: *shameless plug*

http://forums.nasioc.com/forums/show...hreadid=512707

Also included in there is a graph of VE for a stock EJ25 NA motor, so using the EJ25 heads and a different bottom end it should be reasonably accurate (numbers are a little high for the compression drop)

I really screwed the pooch on the altitude correction so don't change them (it's accurate in it's current state, but gets really bad if you change altitude numbers).

But it looks like you made a good choice
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