Turbo Selection Discussion

[bmadd]

Hey, I have been lurking this forum for awhile, and usually find what I'm looking for, but now I have a question. A short introduction:

My name is Brendan, I am from Clarksville TN, USA. I am an Army helo pilot, serving next to some of your brothers in Afghanistan. I have three kids and a lovely wife, and cars are my passion. enough with that. I posted this in NICO, so I'm just copying and pasting.

I have researced this quite a bit, but I still haven't quite figured it out. I have read about how to read compressor maps, done the math using ballpark figures(ie who really knows the volumetric efficiency of their RB?), and have come to the conclusion that my goals lay somewhere between the GT3071R, and the GT3076R. But where I am stumped is all the other options that go along with the selection. I'll explain further.I try not to chase arbitrary horsepower numbers, but it still catches my eye. I know that the "supported HP number" that goes along with the turbo is just a flow number. that turbo can flow that much air to support the said HP number, given supporting mods. I also know that doesn't mean if I buy that turbo, I will instantly have said HP. The maps I have been able to find use only one size turbine housing. So, in what configuration does a turbo flow it's max HP number? Does turbine selection even correlate to the flow capacity? I have seen what numbers others are putting down with the combinations they are using, but me being the meticulous type A personality I am, I need more justification than what some one else did. I really want to understand why.

I have read people's comments that a .63 A/R turbine is too small for an RB25det. Then I have read that the .82 is too big. is a .82 good for a 3071 but not for a 3076? At what point does it become restrictive or create excessive lag? Does a 3071 with a .63 support as much HP as with a .82? I know it should spool faster, but do you trade top end for spool? I have been around these forums for awhile, and try to do as much research as I can before posting a question, so if this is a stupid post, point me in the right direction!

My car is an RB25DET, ported head, valve job, stock bottom end (checked bearing clearances, solid comp & leakdown), fresh gaskets and seals, FMIC, 3in turbo back, with plans for AEM ems, and injectors and cams to support new turbo. it will see track duty, and some drifting, so functional power is the goal.

Sorry for the long winded post, but a brotha's got nothin but time in Afghanistan. Thanks in advance!

[brent32]

all modded rbs are different and so are peoples goals best thing is to check the thread on your specific engine RB25 that have setups and dyno results only

i personally have .63 on a 3 litre and yeah its restrictive and doesn t have huge top end but excellent response and good torque suit drifting. Also lower revs = more durable.

[Dale FZ1]

Bigger turbine A/R without changing any other turbocharger specification will permit higher maximum mass flow rate for a given turbine inlet pressure. The cross sectional area is increased to give the bigger calculated ratio, so it’s a bit like running a larger diameter exhaust pipe. The exhaust breathing is more efficient. That should lead to higher maximum power capability, as the engine is scavenging its combustion chambers more efficiently. Turbine backpressure will effectively “back up” exhaust flow out of the chambers like a log-jam in a watercourse, so the larger A/R reduces the metaphorical log jam.

Supposing your power target is somewhere between 350-400rwhp based on turbo choice, the other consideration is power delivery. The smaller 0.63 A/R while tending to choke things as revs/power rises, will respond a lot “sharper” to throttle. So you could have something that tends to wheelspin easily. Maybe what you want for drifting? Also the 3071 will accelerate its rotating assembly more quickly/easily than a 3076, which could give an aggressive delivery too.

I’m now running a HKS 3037/3076 with 0.87 housing. It is proving to have a progressive delivery without wheelspin tendencies. A Garrett 3076 with 0.82 would be very similar in most respects.

There are other members around who are running different combinations eg 3076 + 0.63A/R, 3071 + 0.63A/R, 3071 + 0.82A/R. If able to be run on an “equivalent” tune, they would all produce power more aggressively than my particular spec. I know the chap running 3071 / 0.82 is about to change to a 3076 / 0.82 to cut down the amount of smoke he tends to produce…

Any of those specs will produce some pretty good results for a moderate $$ spent. The point is, they will make their power in different ways and will suit different applications or driver preferences.

Hope that’s a help

[discopotato03]

Garrett has turbine maps with lines representing the three GT30R turbine housing A/R sizes , you can see them at turbobygarrett.com .

Turbine housing size is a trade off between boost response and exhaust restriction , the lower the boost threshold the lower the maximum exhaust flow you'll have and vice versa .

With turbocharging often exhaust manifold pressure rise becomes the limiting factor because you start to suffer reversion/charge polution/charge pre heating proplems on top of pumping losses . The inducator is mostly detonation .

Possibly the best generalisation I know of is try to aim for lots of air and exhaust flow rather than lots of boost pressure .

We don't yet have conclusive results of a real GT3071R on an RB25DET as its only just being done as we speak .

The chief difference between a GT3076R and a real GT3071R is the extra speed you need to spin the 71R's compressor at to move enough air to make good torque . The one being attempted ATM is using a propper Garrett GT30 IW 0.63 A/R turbine housing and my gut feeling is that it will work quite well . It probably won't have the top end punch of a GT3076R but I think it will be more linear in its power delivery on an RB25DET .

Anyway real world results should be available soon , cheers A .

[bmadd]

Garrett has turbine maps with lines representing the three GT30R turbine housing A/R sizes , you can see them at turbobygarrett.com .

Turbine housing size is a trade off between boost response and exhaust restriction , the lower the boost threshold the lower the maximum exhaust flow you'll have and vice versa .

With turbocharging often exhaust manifold pressure rise becomes the limiting factor because you start to suffer reversion/charge polution/charge pre heating proplems on top of pumping losses . The inducator is mostly detonation .

Possibly the best generalisation I know of is try to aim for lots of air and exhaust flow rather than lots of boost pressure .

We don't yet have conclusive results of a real GT3071R on an RB25DET as its only just being done as we speak .

The chief difference between a GT3076R and a real GT3071R is the extra speed you need to spin the 71R's compressor at to move enough air to make good torque . The one being attempted ATM is using a propper Garrett GT30 IW 0.63 A/R turbine housing and my gut feeling is that it will work quite well . It probably won't have the top end punch of a GT3076R but I think it will be more linear in its power delivery on an RB25DET .

Anyway real world results should be available soon , cheers A .

Thanks for the responses, those were along the lines I was thinking. It seems as though everyone gets different results, sometims using very similar setups. I am just trying to understand, as opposed to just doing it because someone else did.