Hey TurboForums, I'm looking for a little help at properly sizing my atmospheric charger.

14' Ram Ecodiesel 3.0L V6 turbodiesel, factory charger (GT2056VKR) is in a tight spot, and it's not really worth lifting the cab to replace the charger with a slightly larger charger, and no one makes a pedestal for a t3/t4 flange. 2 companies have already come out with compound setups, pretty similar but one company uses a BW s364.5/73mm turbine. The other company is using a BW s364.5/68mm turbine, but uses a custom machined part to use Tial GT35 housing because the BW wheel is likely longer.

The reason why I'm asking is because I purchased the Tial housing not knowing it used an custom part, so now I'm thinking of just getting a GTX3582R as its the biggest GT35 that fits the Tial housing or cutting the V-band flange off, buy a S364.5 and run the T4 flange. I would like to try something a little different, but I'm a little worried about back pressure and too small of a turbine. You can see I'm gating the small turbo, and have an 8# spring in the WG, would using a weaker spring create less back pressure if the turbo is slightly too small?

Wastegate setup.
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Charge pipe and Tial housing and waste gate.
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Down pipe.
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From what I have gathered, the atmospheric charger should be roughly double the air of the secondary. That doesnt mean double the size.
I would run as big of an exhaust wheel that you can. The increase in volume between the secondary and primary (exhaust) is crazy and will turn a little larger charger than you think.

As far as back pressure, things get complicated. How are you venting the wastegate? I cant really help until you answer that because it is a gamechanger.

 

The way I have sized compounds is to determine the hp level or desires. I agree we need to know how you are routing the gate. Is it to atmosphere or to the hot pipe to drive the primary harder?

 

I appreciate the response, I am routing wastegate back into exhaust to help spool big turbo. I settled on a GTX3582R w/ a .82 housing and 38mm wastegate and have since switched to a 1.01 AR housing and a 44mm wastegate, but I eventually run into back pressure issues if I stay in it too long. Currently running a #20 spring, but I think I'm going to add a boost controller.

I am roughly shooting for 350-400hp/600ft-lbs. Routing exhaust back to hot pipe to help spool big turbo.

 

How much pressure are you able to make on the current turbo?
How much pressure do you want to end up with?

You'll need to do a little math, its all laid out here: http://atptrucks.com/blog/compound-turbos-and-boost-numbers-the-math/

I'm currently helping with a 24 valve build, for that we determined the turbo size based on some other builds and the results, is there any info out there from others who have done what you're doing?

Blindly going with the biggest exhaust wheel you can find is not wise. In the case of the 24 valve we found that going with a 1.1 A/R 83mm s472 would be much more towing and street friendly than the 1.3 A/R 92mm s475. The 92mm would work better in a drag racing truck.

 

Stock turbo taps out at 29psi, but it hardly reaches it, with another turbo added, I was see 50+ psi of boost, but I'm usually around 60-80 psi of back psi, and most importantly, it seems like it run away psi, if I stay in it, it builds and builds.

Yes, there are a few kits, and I have largely copied them, however I did differ on turbo selection. I am running the smallest of the 3 kits, a 62.5mm/68mm, next is a BW 63mm/68mm and a BW 64.5mm/73mm, so I am close to them. I am looking for a little more street than strip, so it might be something I'll have to deal with until I get a bigger charger.

 

If its running away like that its because your feeding too much of the wastegate to the second turbo. The wastegate should bypass the second turbo straight to the exhaust. With the primary wastegate feeding the second turbo you have no way to control the big turbo and prevent the boost creep you are experiencing, you will have to run a second wastegate or plumb the small turbo gate differently.

The big trick in sizing compound setups is setting the big turbo up to spin the proper rpm using the leftover exhaust flow from the little one.
The simple solution is to keep your setup as-is and gate the big turbo, it sounds like you'll only need about 10-14psi off of the big turbo and you shouldn't have to bypass a lot of gasses, you should be able to run a pretty budgety wastegate.

Edit: I should have written that differently, talking about compounds gets confusing.

 

Yeah, I should have paid more attention to your build and what you would want from the turbo setup than I did.

I built a compound setup for our pulling tractor and there is only one way we run that, WOT! haha

From what I have watched on our datalogs, the back pressure will be high until the primary charger comes online. Then, depending on how the system is working, the intake pressure should overcome back pressure by a little bit. I see about 130 psi back pressure when running 155 psi in the intake. What you mention about the secondary turbo (small turbo) not making as much pressure now makes sense. First, you can only judge the secondary in a compound setup by what pressure ratio it is doing. Also, the secondary turbo now has exhaust pressure downstream of it to drive the primary turbo. The higher that pressure, the less differential pressure across it and the less it generates. When messing with the secondary wastegate, you also have to remember that it too is seeing pressure downtream of it.

To give you another example, at first, I had our secondary wastegate plumbed into the exhaust stack which bypasses the primaries. I would see 80 psi back pressure, 85 psi intake, 25 psi from the primaries, abd about 4.5:1 pressure ratio across the secondary. When I switched the secondary gate to go back into the exhaust upstream of the primaries, the very next pull, I had 120 psi back pressure, 135 psi intake, 60 psi primary, abd a 2.9:1 pr across the secondary.

After getting the wastegates (one on secondary and one on primaries) setup, I saw 135 psi back pressure, 155 intake pressure, 57 psi primaries, and 3.86:1 pr across secondary.

I dont know if those numbers give you much of an idea on how things work in our compound setup or not. As far as less than wot driving, we only have a few seconds of that at the beginning and I see higher exhaust pressure than intake pressure but our setup comes to life real quick.

 

Honestly it sounds like your secondary is too small and cannot move enough exhaust mass flow to keep boost (well drive pressure) in sane levels. Even with the waste gate plumbed to feed the primary your boost should level out and not continue to climb.