turbocharger selection help -- compressor maps

[zeeshan]

I have a 6.0L lq9 that I'm in the process of turbocharging. I've turboed cars before but I've just followed dyno sheets only and haven't properly sized a turbo myself. With the lq9 I'd like to be less redneck and more engineer:)

It will be a twin turbo setup which I am aiming for 700hp. I want a little bit room for more power in the future. I'd like to use the borgwarner s200sx turbochargers.

this is what i've come up with roughly:




why are the data points so low? what happens below the 69970 rpm line?

[Copper280z]

The points are so low because you're only planning on running 10psi(.7 bar) that turbo is MUCH happier in the 15-20psi(1-1.4bar) range.

[Jordan Innovations]

Yeah, easy solution - run those with more boost and the turbos will be happier

Conversely, run smaller turbos, but for what?  To have more torque at 2000rpm?  You can roast tires there without the boost.

[zeeshan]

I want to run low boost because i fear 14psi on those s200sx turbos will make too much power and that's not good for a relatively stock bottom end.

[zeeshan]

two t3/t4 50 trims look like a better fit (turbine specs: .82 a/r, stage 5 turbine wheel -- (bigger wheel)).

[forcefed86]

RPM is the killer in my experience, not boost.  Manifold pressure is resistance to flow through the engine.  Fearing "X" amount of manifold pressure is silly. To say an LQ4 "bottom end" is only good for 15i psi is nuts. Leave the motor totally stock, heads/cam/etc. You'd probably be close to your 700hp mark at 15-18psi with those turbos. I personally like to have a ton of power down low.  Then you can run a really tall gear, have great street manners, and load the hell out of the turbo.   

[twokrx7]

Bigger turbos are usually happier at lower pressure ratios (lower boost).  It's not that hard to select a compressor if you know the NA horsepower of the motor, don't forget to include effect of intake and IC when estimating PR.

[zeeshan]

i know bernoulli's equation which states that pressure will increase as the flow velocity decreases. i was using internet speak since that's what most people understand ;p

[zeeshan]

i've decided on two t3/t4 50 trims with .82 a/r turbine housing

[Copper280z]

i know bernoulli's equation which states that pressure will increase as the flow velocity decreases. i was using internet speak since that's what most people understand ;p

Bernoulli doesn't really apply to this situation.

[zeeshan]

you are right since we are dealing with compressible flows. navier stokes equation would be the correct one to have stated

[twokrx7]

None of that fancy stuff matters here, it's simple PR and mass flow and efficiency considerations on the compressor side and hot size sizing for spool and no choking on the big end.  Easy, motor makes 400rwhp NA, you can make about 600rwhp by adding another half an atmosphere at the valve.  Great thing about turbos, if you select turbo(s) right and fall short of the 600, crank the boost a psi and you are there.  Now other things matter but there is so much experience within the communities at these power levels that turbo selection should be a breeze. 

[Jordan Innovations]

^^^ Well said!