jrussell

I moved to the Palm Beach County area a few months back.

blwnsmke

So pretty much all your saying is have a WG on the top charger set around 25-30psi and let the bottom charger take over? How does the s400 release pressure though?

cquestad

It does not need to in most applications...the AR is loose enough that it lopes around at up to 40 psi and moves ALL the air.

blwnsmke

So what should the top charger wastegate be set up? A htt 62 is whats gonna be on top. Also Say if I wanted to run more then 40psi or limit how much boost I want to run max then what should I do that make that work?

cquestad

It actually works better IMHO if you do it MPI's way...and the wastegate is actually controlled by a spring resisting drive pressure. Boost is what would be measured in the cold pipe between the the two turbos...
by 40 psi (total boost) you should see near 50/50 split. 40 total...20 in the cold pipe. That menas each turbo is making 20 psi boost. Mine match by 40 and climb almost identical till 80. THAT...is a perfect design.

Townsend600

what is done with the stocker and the wastegate. It is electronically controlled right? What keeps it from overspeeding with the s-400 pushing it? Does MPI provide some kind of an electronic unit to prevent the top turbo from working too hard?

jrussell

The stock electronic WG controller is replaced with a manual system that Nathan provides with his kits. It uses a simple spring and lever that is easily adjustable and very dependable.

You don't have to worry about overspeeding the top turbo because it's actually working much less than it would be if used as a single. You don't have to worry about overspeeding the S400 because it's such a big turbo with a huge exhaust housing.

Like Christian said, each turbo should be putting out 50% of the total boost. 50psi of boost on a properly tuned setup would mean each turbo is putting out around 25psi.

blwnsmke

Ok so what is the correct safe way to go about this? Using a HTT 62/71 on top. Depending on how much total boost I want to run depends on what I set the top turbo waste gate to? For the s400 do I need to put one or two blow off valves in the intercooler pipe? Do both of them work for just the s400? Also how does the blow off valves work, by spring tension?

jrussell

Yes, they work by spring tension.

Total boost is not controlled by the top turbo's WG. Total boost will be controlled by either your max fueling or whatever you have the pop-off valves set at. The top turbo's WG is only adjusted for tuning purposes (to hit that 50/50 ratio which is the sweet spot).

I would say you need 2 or more pop-off valves. They would be relieving boost pressure from both turbos since they're in the pipe leading to the IC.

blwnsmke

I saw your pic of the two blow off valves installed.. How do you know if both the blow off valves in IC pipe are set the same? Also how do you know when your at that 50/50 spot?

jrussell

They have numbers/marks on the side so they're easy to set.

To tune for that 50/50 ratio you connect a 2nd boost gauge to the port in the cold piping (between the two turbos). When that 2nd gauge reads 50% of what the total boost is then you're all set. This is adjusted using the WG on the top turbo.

blwnsmke

Ok gotcha. I see where he has a port welded into the pipe. How do you tell how much max boost to run? When the egts are under control?

jrussell

Generally speaking, with properly sized twins and head studs, you want all the boost you can get (while maintaining that 50/50 ratio).

The only time max boost would be a concern is if you don't have head studs or you are way over-fueled with a smaller set of twins.

blwnsmke

Ok gotcha.. So pretty much you just need to play with it alot and start off with the blow off valves..

jrussell

I only needed the pop-off valves because I still have stock head bolts.