Im running twin tv75s on a bbf and IC it but every says go with 3.5" pipe and ICs but its going in a boat and the turbos only have 3"openings on the cold side plus Im running cold lake water thru the IC so you think 3" pipe and ICs will be ok cause the turbos can only push what it can push out of a 3' opening

 

well it's an aspect of the turbos compressor housings it's about cfm. Larger tubes will allow larger cfms. On my setup I go into the turbo with a 3" and then come out with a 3.75" pipe.

 

This is completely backwards. A turbo always has less airflow coming out than going in. If you are over about 15psi there is more than twice as much airflow going in as coming out of the turbo.

How fast will you spin the engine and what is its displacement?

 

The same volume of air that goes in the turbo is the same volume that comes out... it is just compressed. Sometimes it is okay to slow down the airflow... On my vehicle I went to a 3.75" pipe for many reasons... Mass air flow sensor, ability of more cfm, throttle body etc ... none of which I do not see are backwards or contradict anything that I said above... The cfm that can travel through a pipe is dependent on its size.

 

This is correct

This is not. The same mass that goes into the turbo is the same mass that comes out. Large amounts of airflow at atmospheric pressure goes in and smaller amounts of cfm at a higer pressure comes out. A turbo might have 600cfm in with 300cfm at 14.7psi out. A turbo does not increase the volume of air into an engine it increases the density.

 

Its a 466 bbf and rpm around 5800-6000 max

 

I never thought about it that way but it makes complete sense.
Thank's for the enlightenment on that.

Mark

 

466cid X 6000rpm X .9volumetric efficiency = 728cfm. I think dual 3" is plenty for this setup.

 

I don't recall ever saying that a turbo "increase (s) the volume of air into an engine," as if it created a larger amount than what first entered the turbo... That would go against law of conservation of energy in which no matter can be created or destroyed. I do remember saying that it compresses it therefore taking whatever went in- turning into a more dense composition.

Oh, I do think dual 3" are enough.