wondering what are some ppl's thoughts or experience on what size tubin to go from the IC to the TB. i have a 90mm tb. my outlets on the IC are 3" i already have a 4" elbow. i was thinking of stepping up to 4" right off the IC and run 4" to the TB.

or run 3" from the IC to a Reducer and then into the 4" elbow?

 

Hmm..this is a good one. How far will it be from the IC to the TB? How many bends? What kind of cfm/hp are you looking for? This is for your 408 right?

 

yes my 88mm on my 408

i figured it to be 90 from IC then 90 up behind the radsupport, then 90 over and straight into the elbow or might do 90 from IC then 90 up behind the radsupport then 45 then 45 again then straight into the elbow

i want 9xx rwhp anything higher is a plus

 

4 all the way -- in fact, get your IC changed over at a local aluminum shop.

 

I don't know anything about turbos or forced induction but I know a little bit about flow dynamics, from several classes in college. Any time you increase the size of a pipe, 3in->4in, you will lose fluid velocity which I think will translate in to a drop in PSI.... For example the same air mass in a 3in pipe might be 10 psi, the same air mass in a 4in pipe would be 8 psi, those are just off the wall numbers. Another thing that will drop fluid velocity is sharp turns, so if you can get away with 2 45* with a small straight piece inbetween them your fluid velocity will not drop as much as a sharp 90* turn... IMO I would try and keep the pipe size the same and just enlarge the pipe at the very end to 4in via a reducing elbow 3in-4in at the TB.. A simple way to compare it is driving a car. It is easier to make a 90* turn in a car via a rounded turning lane, you can still keep some speed. Rather than a T intersection where you basically have to come to a stop to make the same turn....

Once again I am no expert but the physics of fluid flow rates and fluid dynamics is something you might want to look at. The hot side should be be the same, gradual bends to keep the air velocity up, which should translate into faster spool times.

I used to have a piece of software where you could model different types of tubing and angles and it would calculate the drop of velocity and PSI for each individual turn and give and input number and output number. I will try and find it again...

 

I say 4" then. 900rwhp is enough to necesitate it. You'll need to move 101lb/min or roughly 1450cfm. If you can get away with it, do what Erik said too. Sure your flow velocity will drop, but you'll still be moving the same amount of air, just at a lower pressure. The air velocity will be around 44% lower with 4" tubing vs. 3", but the cfm will be the same. It's kind of like how guys with SC see a boost drop when they put on headers and exhuast. They're not making less power, just less restriction. If anything they're making more power with less boost. Frictional losses aren't that big of a concern in a FI aplication when the tubing is pressurized, but if it has a million 90's it is. I think you'll be alright. It doesn't make sense to have charge piping smaller than your TB IMO when you're shooting for 4 digit numbers.

 

yea thats what i thought. PSI doesnt mean squat, HP figures is what matters. 12psi with restriction and etc might only be 8 psi with less restriction but more hp...this is an off the wall statement. and the comparison with the blower guys. i always thought as you just written. they worry about boost when they are making more power

 

i agree with erik. change the outlet on your intercooler to a 4" and run 4" the whole way.

if you were running a 78mm tb then i would vote for a 3" but with a 90mm you might as well match things up.