# Twin Garret G25-550's



## doyle4281 (Dec 10, 2008)

Hello all, has anyone put any thought into these new garret G series turbos? Their claim is that they are the highest flowing turbochargers for their size, which is I assume the GT28xx footprint. Give the GT28xx turbo is such a common "bolt on" upgrade, im surprised not to hear more chatter on these. There competition looks to be the EFR 6258, but there is no real data to compare them. Im not sure if they are bolt on, however I am sure they will fit low mounted in an OEM configuration. Any thoughts? What do you guys think they compare to as far as the other gtx28xx line? Thats also hard to compare as I dont see a lot of RB's running twin GTX turbos either. There is plenty of dyno info on the older GT28xx, -5, -7, -9, -10, esc, however not much in the way of newer generation garret twins


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## 120506 (Jun 23, 2015)

doyle4281 said:


> Hello all, has anyone put any thought into these new garret G series turbos? Their claim is that they are the highest flowing turbochargers for their size, which is I assume the GT28xx footprint. Give the GT28xx turbo is such a common "bolt on" upgrade, im surprised not to hear more chatter on these. There competition looks to be the EFR 6258, but there is no real data to compare them. Im not sure if they are bolt on, however I am sure they will fit low mounted in an OEM configuration. Any thoughts? What do you guys think they compare to as far as the other gtx28xx line? Thats also hard to compare as I dont see a lot of RB's running twin GTX turbos either. There is plenty of dyno info on the older GT28xx, -5, -7, -9, -10, esc, however not much in the way of newer generation garret twins


These are out..

https://www.tomeiusa.com/_2003web-catalogue/000_arms/product_arms_rb26-e.html


Ive heard people say they give disappointing results but I guess it depends on what you want from an upgrade so subjective opinion can mislead somewhat.


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## Sub Boy (Jan 28, 2008)

[redacted] said:


> These are out..
> 
> https://www.tomeiusa.com/_2003web-catalogue/000_arms/product_arms_rb26-e.html
> 
> ...


They do give disappointing results, laggy and not the powerful either. Would rather still have 2860-5's that the Tomei's


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## FRRACER (Jun 23, 2013)

Sub Boy said:


> They do give disappointing results, laggy and not the powerful either. Would rather still have 2860-5's that the Tomei's


Second that -5 or -5 reworked over the Tomei


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## doyle4281 (Dec 10, 2008)

Sub Boy said:


> They do give disappointing results, laggy and not the powerful either. Would rather still have 2860-5's that the Tomei's


Yeah, I havent heard anything good about those either. I was more referring to the later generation Garretts, like the G25 550's, the GTX28's, and the EFR 6258's. I actually have a set of EFR 6258's that I want to try to get in as much much of a low mount position as possible, but they are big and its not going to easy. I was expecting somewhere in the 700's with the EFR's, but I am really interested in what these later generation garrets can do. However, I have not seen any info at all on the G25's, and little to none on the GTX28 twins. Spool obviously is a big concern, as it is a 2.6. It is however built with great forged bottom end, resleeved, ported, high lift and duration cams, esc, esc. Why are the later generation garrets not very popular? Are twins that old news that nobody is even bothering with them?


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## Cris (Sep 17, 2007)

doyle4281 said:


> Yeah, I havent heard anything good about those either. I was more referring to the later generation Garretts, like the G25 550's, the GTX28's, and the EFR 6258's. I actually have a set of EFR 6258's that I want to try to get in as much much of a low mount position as possible, but they are big and its not going to easy. I was expecting somewhere in the 700's with the EFR's, but I am really interested in what these later generation garrets can do. However, I have not seen any info at all on the G25's, and little to none on the GTX28 twins. Spool obviously is a big concern, as it is a 2.6. It is however built with great forged bottom end, resleeved, ported, high lift and duration cams, esc, esc. Why are the later generation garrets not very popular? Are twins that old news that nobody is even bothering with them?


AFAIK there isn't a bolt on GTX option. It has been done and if memory serves the results we a bit better than -5s. Seemed a fair bit of cost/expense.

It could be that the low mount OEM pipework is limiting what these newer turbos can do but I don't have enough knowledge of such things to say more than this.

I think once you swap the oem pipework/manifolds etc most people go single. You'll be spending a few pennies on the new pipework so I guess having to only buy one turbo is pretty attractive. Likewise the plumbing/packaging looks a lot simpler than twin high mount.

Hope that this helps


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## joshuaho96 (Jul 14, 2016)

Isn't there a problem with turbo shuffle if you run something that builds boost too early on the stock twin piping?


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## doyle4281 (Dec 10, 2008)

joshuaho96 said:


> Isn't there a problem with turbo shuffle if you run something that builds boost too early on the stock twin piping?


I have heard that too, however piping can be redone to help alleviate that issue. I guess, it all boils down to the complexity of twins, and if the juice is worth the squeeze. I think that if your seeking power levels past a certain point, the single becomes so attractive that it really has reduced the popularity of twins. However, the older GT28xx are even today very popular, but I havent seen much data on the newer GTX28 versions. I think this again, boils down to cost and how much more attractive the singe becomes after a certain point. I myself are seeking reasonably high power levels 700-800 whp, however want to maintain a "stockish" appearance. I do have a pair of EFR 6258's, however, getting them low and tucked away is turning out to be very difficult, and now know why Full-RAce had to create such an odd looking configuration with their kit. My car is not a GTR, but I think I may have a bit more room for turbos in my chassis. But, back to the point, im surprised that nobody is talking about the G25 turbos, as they seem like a perfect "fit" to a high power twin GTR.


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## GT-R David (Mar 13, 2013)

doyle4281 said:


> ...but I havent seen much data on the newer GTX28 versions.


Lots of data at SAU forum.


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## David (Apr 25, 2003)

joshuaho96 said:


> Isn't there a problem with turbo shuffle if you run something that builds boost too early on the stock twin piping?


What's the logic behind that thinking?


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## joshuaho96 (Jul 14, 2016)

David said:


> What's the logic behind that thinking?


Eventually the problem ends up being that the stuff after the compressor can't handle how much air the compressor is trying to push. So you get compressor surge. Some function of the compression ratio, the profoundly terrible stock twin turbo pipe that merges air at near 90 degree angles, and static camshaft timing. Even if your turbo spools instantly you're limited by what the engine can actually breathe and the compressor map.

AFAIK Garrett hasn't bothered to modernize the -9s and -7s specifically because they need surge slotting on the compressor side if they build boost any faster and there's just no room for that in a bolt-on turbo. Source: https://www.sau.com.au/forums/topic...t-choice/?page=2&tab=comments#comment-7767211


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## FRRACER (Jun 23, 2013)

The -5 can use bigger turbine housings.


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## gtr mart (Mar 29, 2004)

[talking from an R35 VQ perspective] ~ I know Litchfield's are very interested to test the new Garrets to compare against the favoured EFRs. Their initial view was the Garrets looked small to flow the amount of air claimed but if they deliver then they should be excellent and another step forward in turbo performance, rather than just another turbo on the market.


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## Lith (Oct 5, 2006)

joshuaho96 said:


> Eventually the problem ends up being that the stuff after the compressor can't handle how much air the compressor is trying to push. So you get compressor surge. Some function of the compression ratio, the profoundly terrible stock twin turbo pipe that merges air at near 90 degree angles, and static camshaft timing. Even if your turbo spools instantly you're limited by what the engine can actually breathe and the compressor map.


I am of the opinion that a lot of the issues with stock location twins are also to do with the feed into the inlets of the turbos, both inconsistent and also fairly restrictive - ESPECIALLY the rear one. The trick is that to provide any decent amount of flow, the air needs to be moving fast at atmospheric pressure - the dodgy direction and diameter changes the flow will be subjected to is likely to cause all kinds of turbulence and there will end up being a choking effect... ie, a vacuum drawn at the compressor inducer because the inlet just can't support the full airflow being called on... so the compressor "sucks harder" to try and bring the speed up through the path where the air is actually able to effectively travel.

When it starts drawing a "vacuum" at the compressor inducer, the pressure ratio starts going up disproportionately to the flow - so if you are reading 20psi of boost (~1.4bar) and you have 4psi (~.3bar) of vacuum being drawn then ... so your pressure ratio is (1atmo + 1.4bar boost = 2.4bar absolute manifold pressure) / (1atmo - .3bar vacuum = .7bar absolute inlet pressure) = pressure ratio 3.4. Check out what 3.4 looks like on the -5 compressor map:










Those are just numbers I've made up to illustrate the point, PROBABLY exaggerated but it at least shows part of what is likely going on which causes sticking with the stock low mount configuration to be something of a handicap - regardless of how good the turbos you put on there are.

In regards to the new G-series, I am certain that a certain amount of the improvement in their flow comes from the new housing designs - and also that the turbine design being more aggressive will result in the spool being significantly lazier than the GT28 turbine. I'm hoping that the balance of the "bits" that the G25 comes with mean it is a better compromise than the old GT28 and GT30 turbines *BUT* don't expect it to respond like a GT28.


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## joshuaho96 (Jul 14, 2016)

I wonder how much impact the Nismo plenum + air inlet + air duct to the airbox would help with resolving the issues you're talking about. The air inlet kit would at least fix the 90 degree bend that the stock piping uses and presumably resolve a lot of the flow issues you're talking about.


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## lightspeed (Jul 11, 2002)

Personally I like the weight reduction on the turbine wheels and the shift to Mar-m steel. Not as trick as ti-al but a healthy improvement all the same.


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## Lith (Oct 5, 2006)

joshuaho96 said:


> I wonder how much impact the Nismo plenum + air inlet + air duct to the airbox would help with resolving the issues you're talking about. The air inlet kit would at least fix the 90 degree bend that the stock piping uses and presumably resolve a lot of the flow issues you're talking about.


The Nismo plenum has nothing to do with it, but the air inlet kit and anything to free up the intake is totally relevant and would no doubt help things - though the space limitations down there make it hard for me to imagine that there wouldn't always be some kind of compromise with the piping down there when air flow requirements (through the intake piping) get up there. 



lightspeed said:


> Personally I like the weight reduction on the turbine wheels and the shift to Mar-m steel. Not as trick as ti-al but a healthy improvement all the same.


Yeah I like the concept a lot as well, I love the Ti-AL angle but the strength is a legit concern - however very keen to see how it actually plays out in real life. To be fair, the better design (if it proves that way) and heavier metal is more trick that just using the lightest material available which should handle what it's meant to do .... to a degree....


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## joshuaho96 (Jul 14, 2016)

Lith said:


> The Nismo plenum has nothing to do with it, but the air inlet kit and anything to free up the intake is totally relevant and would no doubt help things - though the space limitations down there make it hard for me to imagine that there wouldn't always be some kind of compromise with the piping down there when air flow requirements (through the intake piping) get up there.


Isn't the Nismo plenum supposed to help with the lean cylinder 6 issue? I would imagine that more even spool would reduce the chance that the two turbos end up fighting each other.

I'm inclined to agree that there's only so much to be done about the stock twin configuration, but it's a question of diminishing returns. If no bolt-on turbo can cause on-throttle compressor surge on an RB26 for pretty much any configuration then there's no reason to be concerned.


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## Warwolf (May 16, 2018)

joshuaho96 said:


> Isn't the Nismo plenum supposed to help with the lean cylinder 6 issue? I would imagine that more even spool would reduce the chance that the two turbos end up fighting each other.
> 
> I'm inclined to agree that there's only so much to be done about the stock twin configuration, but it's a question of diminishing returns. If no bolt-on turbo can cause on-throttle compressor surge on an RB26 for pretty much any configuration then there's no reason to be concerned.


The issue is on the other side of the engine, Josh. Plenum side will have no effect on the turbo side.


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## Warwolf (May 16, 2018)

And there are ways to mitigate shuffle issues.


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## anthonymcgrath (Feb 22, 2005)

FRRACER said:


> Second that -5 or -5 reworked over the Tomei


What's involved in reworking the -5 ? Didn't know that could be done?

Sent from my SM-A520F using Tapatalk


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## manzy47 (Apr 6, 2008)




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