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Old 09-09-2005, 08:52 PM   #1
Scooby Specialist
Member#: 498
Join Date: Nov 1999
Chapter/Region: South East
Location: Charleston, SC
1981 jdm swapped gc

Default so is 5psi well...5psi?

i have a rhb5 turbo, running 5psi, if i swapped to a bigger turbo and ran the same boost will it be the same hp wise? or will the bigger volume of air matter?

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Old 09-09-2005, 08:57 PM   #2
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Join Date: Sep 2000
Chapter/Region: Tri-State
Location: Irvington, New York, USA
1999 gc8 ver7 6mt
GT35R 504.7whp;)


Yes it will still be 5psi technically. You are right in assuming more air will be flowed though...but that depends. If the smaller turbo is inefficient at 5psi and the bigger turbo is more efficient that holds true. BUT if it's a gt42 then at 5psi it would probably be less air. At least that's my understanding of it, hope that's somewhat clear.
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Old 09-09-2005, 09:39 PM   #3
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Chapter/Region: TXIC
Location: Fort Huachuca, AZ
2003 Corvette Z06


5psi on a GT28 turbo is less power than 5psi on a T3 for example. its still 5psi but you cant switch turbo sizes and expect to run the same psi. you might BE able to, but say on a T3/T4 hybrid youll be making more hp at 5psi than on a GT25.

so the moral of the story is, if you have a smaller turbo and go to a bigger, you should probably decrease the psi unless youre tuned for it. either way though, if youre buying a new turbo that is either bigger or smaller and youre not tunning the car to go with it, if not for motor safety than for efficiency and power, there is something wrong.
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Old 09-09-2005, 10:30 PM   #4
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Location: Holly, MI
2004 Forester XT


Yes.. 5 psi is 5 psi. The only way you would have a difference in power was if the bigger turbo was more efficient at that flow and that pressure ratio since you would then have a higher density ratio. The density ratio is what's actually determing how much more air you are using over an NA engine.

Bigger turbo are an advantage because they usually flow more air.. not necessarily support that much more boost but they will be able to hold a certain pressure ratio into higher rpm's (more air) and at a decent efficiency.
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Old 09-10-2005, 01:44 AM   #5
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Chapter/Region: RMIC
Location: Inside your carnot cycle
1998 RST V8 STi swap
'05 R6 '95 BMW V8 hotness


If you are running a MAF style ECU things may change a bit. If you are running a speed density system all you have to do is make sure your IAT sensor is between the IC and TB.... of course you should have done this from the beginning right? If the IAT sensor is right before the TB your fuel/timing maps should change not a wink.
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Old 09-10-2005, 11:41 AM   #6
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Location: Delta, BC, Canada
2000 RS-T -> RSTi-8
Blue Ridge Pearl


5psi on a bigger turbo will generally push more air than 5psi on a smaller turbo, so that means more power and requires more fuel. Depends on how your EM does fueling - if your EM uses MAF, or MAP and compensates for IAT, extra fueling should be accounted for already. But if you're like me who is running MAP without IAT, you will need to add more fuel across the board.
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