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My current combo is a 427 ci twin bw 76/82 using 2.5 inch up pipes to feed the turbos. Would there be any advantage to making 3 inch ones or even 3.5 inch ones, or would that actually have a negative effect on spooling?
So this is where you need to have some solid information to make this decision. By going with a larger pipe you reduce the velocity that feeds the turbine which will negatively effect spool. HOWEVER if you are suffering because it is being choked out then increasing the volume of air would be beneficial. The problem is that's not always easy to tell and if you don't have the necessary instruments to measure it then your best bet would be make some 3" piping and see what the difference is. From my knowledge of crossover pipe sizing though it would be a safe bet to say your 2.5" piping is probably fine as 2.5" piping from 2 banks feeding a single turbo of that size is safe.
He's saying that your hot side isn't that big so you don't need a 3.5" pipe. I'd honestly neck it down to create velocity then open it to whatever size flange you have right before the turbo. Truth be told it seems like you have wayyyy too much turbo. Because a pair of those is good to like 2000+hp.
Are you guys referring to the pipe that feeds the cold side of the turbo???
If so, I'd have to think that this is a very critical place to "get it right". I'm a big procharger guy and can tell you first hand that power can suffer in a big way with small tubes and restrictive air cleaners. I can't imagine that the inlet of a turbo is any less demanding than the inlet of a centri blower.
Also, I'm having a hard time seeing where an unrestricted inlet side would adversely affect spool times.
Most guys on the turbo inlet just run whatever size piping the inlet is. They run it to where they want to put the filter and use the biggest filter that will fit. That tends to be the best possible scenario
That's also only the inlet to the scroll volute. The exit of the scroll directing flow into the turbine wheel is much smaller.
What power level are you shooting for?
2.5 is more than enough. I'd bet you could go to 2.25"
Here is an example of what 2.5" pipe is capable of...
Quillens 482 cube engine makes 2800+hp on 2.5" uppipes and turns 8200RPM. Thats a 2.5" collector and 2.5" pipe feeding twin 91mm turbos. IT's also on methanol.
I feel like there is at least 2 different conversations going on here. The OP was asking about feeding the inlet side of the compressor on a turbo right??
I feel like some of you are talking about feeding the turbine side, then others are talking about the outlet side of the compressor.
What are we supposed to be discussing here again???
I feel like there is at least 2 different conversations going on here. The OP was asking about feeding the inlet side of the compressor on a turbo right??
I feel like some of you are talking about feeding the turbine side, then others are talking about the outlet side of the compressor.
What are we supposed to be discussing here again???
turbo up pipes would be from any collector to turbine inlet
Turbine would be the correct term for the hot side, right? Then when talking about the cold side, you refer to it as the compressor, right?
I'm no turbo guy, so I could be off here. You can see how this gets confusing. I believe the OP wants to talk about the cold side, or compressor side, of the turbo. So using the term turbine just creates confusion.
I have never heard or seen any reference to a turbo up pipe being anything other than the "hot side", collector to turbine inlet.
Yes that is the exhaust housing.
Downpipe would then be turbine housing to atmosphere, rear of car, wherever the final outlet may be.
Not sure why people are even thinking along the lines of intake air, especially given the sizes mentioned. ie 2.5", 3", 3.5", when most decent size turbos are 4" or above for the inlet, as per the SXE's he's using.
In theory you probably could step a 4" air inlet down smaller, especially if say the wheel was only a 64mm inducer wheel as that is around 2.5", but that would be a silly thing to do really when you want that side as free flowing as possible.
the hot side's I have been making have shown not to be an issue as far as limiting the engine's power. I have built a half dozen 2.5 hot sides that merge to a single 2.5 for about a foot and they seem to work very good. the fastest being my friends Camaro that has been 9.35 at 145 mph with a 5.3 triple 12 cam at 3550 lb race weight.
I did build a couple 2 1/4 setups with a merge to 2 1/4 single pipe before the turbo and it made 750 hp to the tires at 18 psi boost on a 6.0 so even that small it still doesn't seem to hinder top end power. I built them for truck use but the one went on a 6.0 in a Malibu and jfr 218 cam that made 750 hp.
from what ive seen the turbine housing still is the smallest point of the hot side or the bottleneck in the system.
here is a pic of the pipes I build they are all almost exactly the same in design
Ok I love you al but this conversation is kinda dumb. Simply spelled out
From exhaust manifold to each turbo 2.25 is fine
From turbo to exhaust match the size of the turbos exhaust
From fresh air into turbo match the piping to how large the inlet is
For how large the turbos compressed air through intercooler to throttle body
For a 102mm tb 2.75" is optimal
For a 90mm tb 2.5" is optimal
For a stock tb 2.24" is optimal
Those numbers are based on the piping for both inlets being the same size as the throttle body any larger the throttle body becomes a bottle neck. Any smaller and the piping is a bottle neck.
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Turbo up pipe sizing 
