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2 Stage Variable Twin Turbo Math 
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LQFP112 - Up with the play
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Joined: Tue Jan 17, 2012 11:21 am
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Location: Ponte de Lima, Portugal
Hello,

As someone of your know I am trying to build a near lagless rb30 + rb25det on the long run. A great setup would a 2 stage turbo system, being them variable geometry, most diesels around here have these setups and I must say they do work wonders.
Image
http://www.autozine.org/technical_school/engine/Forced_Induction_3.html

But man, i am totally clueless about the math to calculate the turbo sizes.

Lets start dumping some information here, and gather as much as relevant information as possible.


Tue Apr 15, 2014 11:07 pm
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LQFP112 - Up with the play
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http://www.gtisoft.com/upload/BorgWarne ... ystems.pdf

Here is a presentation about the subject on diesels, plenty of data, but not much information to back it up.


Tue Apr 15, 2014 11:20 pm
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QFP80 - Contributor

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First off, how much power are you after? If it's less than 600bhp I can assure you that it can be done with a single turbo with zero lag whatsoever.


Tue Apr 15, 2014 11:41 pm
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LQFP112 - Up with the play
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Hmm, i am thinking on about 450-500 hp at the wheels, so that would 600ish on the crank, now, elaborate on that one too please :)


Wed Apr 16, 2014 12:12 am
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LQFP144 - On Top Of The Game
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what exactly do you mean by zero lag, define turbo lag for us first.


Wed Apr 16, 2014 12:52 am
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LQFP112 - Up with the play
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I mean real lag, timespawn between throotle close and reopen, obvious i want a decent and low boost threshould too, I mean i now have the skill driving offroad for long enough to keep in the right rpm range on a certain obstacle, but if i get lag, Its just useless I has i need a decent throotle response to keep it going.


Wed Apr 16, 2014 1:01 am
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LQFP112 - Up with the play
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http://forums.tdiclub.com/showpost.php? ... stcount=22


Wed Apr 16, 2014 2:25 am
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LQFP112 - Up with the play
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I'd prefer spending more cash on a really good single turbo that spools faster than adding the complexity and associated higher chance of failures by going hi-tech twin turbo.
I have no personal experience with these newfangled turbo's, but the Billet turbine turbo's are said to spool very fast.

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Wed Apr 16, 2014 8:39 am
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El Porto, 600 is single turbo territory for a 3 litre engine. What you're talking about isn't lag, it's boost threshold. You want 15psi at 2k and 7k and no turbo can do that. A R2S system, could, though. It's kind of compound, but kinda not. I love it, and have loved it since it was released maybe 6+ years ago? Keen to do that on my truck too. Keep the existing holset for the big one, and introduce valving and a tiny one for the other end.

In my experience with my large truck holset on a small petrol 4 cylinder spool time is negligible indeed. If you jump on the throttle at 5k, it's near instant. If you lean on the throttle at 3k it will *never* build more than a psi or so. This is NOT lag. It's boost threshold. You can plot possible boost vs RPM on my engine and it's not linear in the slightest. I know my holset will do 50+ psi, where it ends, who knows, but this should illustrate the point:

Image

A curve like this is a fact of life for any traditional turbo setup, hence all OEM setups have tiny turbos, 'cept the mitsi evo and subby STi, and they're still pretty small. Putting on a tiny turbo pushes that curve down lower and makes it steeper. Instead of 12psi at 4k you have your 12 at 2500 and will make a few psi at idle RPM and more in between, etc. The down side is it chokes the top end. You can't really win if you want linear torque, except by going to an R2S like system.

Fred.

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Wed Apr 16, 2014 9:43 am
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QFP80 - Contributor
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masterkorp have you consider twincharging? Turbo blowing through the supercharger? That's my ideal setup for a hillclimb car, and am a fan of this Evo (800hp from memory): https://www.youtube.com/user/evomadmac/videos

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Sat Jan 17, 2015 12:34 am
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