Compound p/r understanding

N2BRK

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Dec 31, 2009
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This is just to satisfy my own head about his things work. I trust Mark and his expertise and he is the one tuning my truck to keep the magic inside the block.
Disclaimer finished, I am super close to getting back on the road and I am trying to educate myself on my first compound turbo setup. Specifically, I am thinking about how they share the load and where they can both operate efficiently while doing so. I've read rules of thumb kinda posts about wanting the setup that I have to supply the total boost 50/50, etc. I got hung up on the p/r math but figured out my mistakes and I think (hope!) that I have a decent grasp now. That said, when I look at the maps of the stock IHI and the S475, I don't see where the 50/50 recommendation is working for me. I've also read that you want to have both turbos at about the same pressure ratio, and that makes some sense. I suppose that you might want to play around with the contributions of each to find where the setup is most responsive, but the compressor maps and the p/r should perhaps dictate the playing field boundaries to avoid catastrophe or poor performance from surge etc.

My build: 2002 Lb7, SBE, 12mm pump, 65 over SACs, stock port/polished IHI and box stock S475 t6 1.32 a/r.

I am guesstimating max tune will have arguably about 45psi. I live at about sea-level.

(45psig+14.7psia)/13.7psia= 4.36 p/r
Primary making 15psi is a 2.17 p/r (halfway up the first island on the compressor map).
Secondary picks up the rest and delivers 30psi more. 2.19 p/r (halfway up the first island on its map).

Doesn't that seem better than 50/50 where the primary is at 2.72 (just leaving it's most efficient "island" on the map), and the secondary is at 1.64 (just below it's greatest area of efficiency on its compressor map)? Or is the 50/50 because the BTDT guys know that this is where everything responds best and it's within acceptable limits?

Just curious :)
 

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Chevy1925

don't know sh!t about IFS
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Oct 21, 2009
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60 overs and a 12mm, you can make more than 45psi lol. You can get in the 55-60range with that fuel. Now if you are capping yourself at 45psi, different story. You will have to do that with fuel. Probably put you in a strong 500rwhp area.

As for 50/50 boost. That was the general easy way to setup the stock turbo wastegate. Trent use to preach about setting up the wastegate to the coolest egt which works but can hurt spool up. I set mine up so the valley turbo makes about 20psi and then the big turbo starts kicking in. Gives me about a 5psi window above the 20psi depending on how I’m driving and keep the stock turbo in its map. Then it’s pretty linear with the big turbo adding boost to the system. At 60psi, the big turbo is making 35-40psi.

I still want to play with the gate some more.

Btw, when you start making the small turbo rely on the big turbo to stay in its map and you loose a boost tube, that stock turbo does not like it. Anthony found that out
 
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N2BRK

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Dec 31, 2009
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45 was a guess, since I made about 45 on my last truck with the 68mm and similar fuel @685rwhp.

For my “big” tune, I’m fine with as much Ricky Bobby as she’ll take without making banana rods 🤣.

Once I get to the point where I can play with this, maybe I’ll swap my PPE valve for something that I can adjust, and I’ll experiment some :) My replacement ihi stock turbo came with an adjustable arm for the wastegate, so between the two I should be able to confuse myself plenty, hahaha.
 

Chevy1925

don't know sh!t about IFS
Staff member
Oct 21, 2009
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Phoenix Az
You will need an adjustable regulator. It’s going to open far too late (I think, can’t remember)
 
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