recommended 4708r atmosphere turbine housing ratio?

C'roy78stroker · Jul 2, 2012

i'll be pairing a 38r and looking at a 4708r for the atmosphere charger due to the larger turbine wheel.. I'm wondering if someone could suggest a ratio for the turbine housing that would do well for decent response on the street and not still cause too much DP. Options are .96, 1.08, 1.23, and 1.39. I'm leaning towards the 1.23 ratio. external wastegate(s) will be used. 275/200's and either an srp 1.1 or a gen3 hpop are the plans for fuel and oil. Build will begin this winter. I'll be ordering parts before hand to have ready to go together as the build begins. Any input/information will be appreciated. Thanks for looking

Dieselboy. · Jul 2, 2012

Go on PSN, search Charles. He has oh a dozen threads on his twins. 38/47

C'roy78stroker · Jul 2, 2012

will do. Thanks Dieselboy

MadDiesel73 · Jul 2, 2012

Biggest housing u can get.

PowerstrokeJunkie · Jul 2, 2012

I would say the 1.23. Are you only wastegating around the 38r?

C'roy78stroker · Jul 2, 2012

May be a small external one on the 38. I'll have to try and find out how much the internal WG on the 38 will bypass. I'd be surprised if it would be enough. Kind of why I was asking opinions about the atmosphere housing. If I have to go smaller with it n gate it too I'd be willing to... More then likely will anyway.

TARM · Jul 4, 2012

The 1.39 from everything I have seen(Obviously no direct experience.) Some of this depends at least in some part on the total PR (boost) and how you plan to break up the total PR between the two turbos.

C'roy78stroker · Jul 6, 2012

thats been a question of mine as well, how much each turbo should see with the given size differences...i know this is where the wastegates come into play but i think ill probably call one of the turbo builders and try to get some of this figured out. thanks

TARM · Jul 6, 2012

Do you already have the 4708? If not looking solely at the maps the 4718 looks like a much better atmosphere turbo.

What you want to do is look at the maps and using those consider where you want them to be with also taking into account the ratio on the turbine side. First you need to decide on what your total boost goal. With that you get your total PR. To do PR for compounds the two are multiplied together so going from a total PR you divide one to get the other. Start with the atmosphere as you can easily get the boost and PR from it directly.

So say a 70psi boost gauge pressure. [70+14.7(atmosphere)]/14.7= 5.76

From here looking at maps you can play with some numbers. As an example take a look at almost evenly spreading the work between the two turbos to start. Of course the 4708 is going to be taking in a significant more air volume or air. Let say you want the 4708 to be pushing 20 psi boost or a PR of 2.36. total PR 5.78/2.36=2.44 PR for the 38r or 21 psi boost. See where that gets you with turbine drive pressure and ratio to boost. Then you can play with it from there based on your power goal, what the drive to boost ratios total and each turbo are etc..

Now to be accurate you would need to account for the actual pressure at your average altitude for where you will be driving as well as account for the filters resistance. But it seems easy if you just go with standard level for comparison.

Again this is just paper numbers for real world exp help you need to talk to the BTDT guys.