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Discussion Starter #1
for some reason it just randomly occurred to me that i have a 70mm TB on my car which feeds into a CL 75mm plenum. car is an '01 single turbo hellion car on 14#'s. once i upgrade the wg spring i will likely bump it to 16# (still have plastic lower intake). Should i upgrade the TB to a 75mm or maybe swap in the bbk 78mm tb/plenum all-in-one, or is this worth fooling with at all?
 

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for some reason it just randomly occurred to me that i have a 70mm TB on my car which feeds into a CL 75mm plenum. car is an '01 single turbo hellion car on 14#'s. once i upgrade the wg spring i will likely bump it to 16# (still have plastic lower intake). Should i upgrade the TB to a 75mm or maybe swap in the bbk 78mm tb/plenum all-in-one, or is this worth fooling with at all?
You can measure pressure drop across, to determine if needed
 

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Discussion Starter #5
I came across a return stock 78mm bbk for almost half price. Went ahead and bought it since i can sell my old plenum and tb for close to that anyway. Its on the car now but had to order a different size tclamp as the original 3.5" was too small. I kind of figured it wouldnt make much difference since it is forced induction but wasnt sure. Thanks!
 

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fyi, the throttle body airflow will change with an increase in size

this can affect closed throttle operation, like idle and decel
 

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Unless flow is grossly turbulent, flow through a restriction with a given characteristic media is proportional to pressure delta across that restriction. This works with head ports, exhaust, oil pumps, and everything else including your throttle body.

If the throttle body is flowing enough for NA on that engine and you increase boost, the flow will go right up with boost. If you go from a N/A pressure drop of 14.7 psi between atmosphere and perfect vacuum to 20 psi boost with no port or throttle body or temperature changes the air flow increases (14.7+20)/14.7 = 2.36 times. You get 2.36 times the base power minus the pumping power required to run the compressor.

The bottom line to this is if a throttle body is good enough at NA for your base power without being a choke point, it is also good enough for the boost. The throttle body only needs to be larger if it is a choke point or restriction without boost in the same combo.

This is the same reason a high volume pump can't "pump the pan dry" unless the high volume pump adds a tremendous amount of additional oil pressure. Flow is always proportional to pressure differential across the restricted path.
 

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YouTube search turbo Windsor throttle body. They switch out a 75mm for a 90 on the dyno with only minimal power increase
 

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YouTube search turbo Windsor throttle body. They switch out a 75mm for a 90 on the dyno with only minimal power increase
and all they had to do, was measure for a pressure drop

to justify even trying a 90
 

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Friend ran 8.67 at 163mph with a 65mm TB in a 3200lb street car with a GTS 76mm. And the cold side piping was only 2 1/2" fwiw.
 
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