I am in the process of putting a 408 into a '03 Silverado RCSB and need help picking a turbo. The engine has a 8.7cr, p&p 6.0 heads and a 224/231 .595 LSA 114 cam. The turbo kit is a rear mount STS that came with a Garrett GT70.

 

How much boost, and RPM are you wanting to run? I would go on the smaller side being in the rear. I would think the 70 would work ok for low boost, and a 76 should support moderate boost.

 

15lbs max. As far as RPM I don't know but definitely not more than 7K. I just want to be in the low 11's.

 

I have a PT76 with a .96 a/r housing on my 408 and I'm pretty happy with it. It build full boost by about 2700 or so, and I can get 1 or 2 psi while cruising in 4th at about 2200 or so. 76 would probably be a good size to go with if you play with the a/r a little bit. 70 might be a little better though for building boost quickly. I know those rear mount kits take longer to spool up. Once it's spooled though, it won't make much of a difference.

 

Get a GT-76/PT-76 and go with a A/R housing that's bigger than .70 but smaller than .96. Not even sure what's between the two!
I have a rear mount 76 with a .68 A/R on a 6L and the turbo size is on the limit of being to big but the A/R chokes down at higher RPM, so I figure the 76 will work well for you but get a bigger A/R

 

I spun my GT70 .96 A/R to 15psi, but it was pretty inefficient there... I set my plug wires on fire

You definitely want at least a .96 A/R with the exhaust pulse from the 408 -- maybe bigger unless you want maggie like insta-boost and are willing to sacrifice going higher than 15psi.

Personally, I'd just cut to the chase and put an 88 on there

(There's a .81 housing, I believe, you will want more for the 408)

 

OK so a Garrett GTS-76 or a Turbonetics TC76 with a .96 A/R is the ticket for 12-15lbs on the 408? I should have ordered the STS kit with the 76 to start with. Now I have to sell the GT70 and buy a 76.

 

s85 with a 1.1 housing

 

I agree with two of ya'll. An S85 or bigger is what I'd go with. A 408 should have an 88mm turbo or bigger if you use a standard turbo. A T76 is more of a turbo for a 6.0L.

 

An STS GT70 will do 12psi all day long. You can get to 14psi if you take some steps (run headers and ! cats to help with the pressure) but may take a lag hit (I ran a set of bassani middies for a while with no big increase in lag and no big heat/pressure problems).

It's great for almost immediate boost... Not so great for huge top end power (even at 14 psi) I believe I gave up around 50-60 rwhp with the 70 at 14psi compared to Parish's 76 at 14psi (front mount) -- it was around 100, but I was awd on a mustang and he was 2wd on a jet, so...

Upgrading your kit to a bigger turbo is more than just buying the turbo. You'll need a new flange to step up, and a new #1 pipe. If you don't already have the big oil pump, you'll need that too.

If you keep the GT70 (or anything but the monster S85 with a 1.1 lol), you need a reverse split cam. Your back pressure will be too much for a forward split with that much overlap. Again, if you are not looking for all-out numbers...

If you haven't installed anything yet, contact STS directly to see if you can swap the turbo. There will be an upgrade fee, but it's cheaper than buying a new turbo and selling your 70.