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About to pull the trigger on Twin G25-660HP Garretts

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Old 12-07-2017, 04:34 PM
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About to pull the trigger on Twin G25-660HP Garretts

Hey guys,


These are the latest to come out from Garrett, so new that I cant find any types of numbers from any platform. These look almost to good to be true in terms of their "size vs HP rating". In "Precision Turbo Terminology" these would be a "5449". The closest turbo PTE has to this would be a "5558" rated at 650HP, which is crazy considering we are talking about a much smaller exhaust wheel in the Garrett. In fact, these turbos are rated about the same as old school GT3582r turbos.


My goal is to have a 700+whp, 93 octane no meth setup with streetable spool up. Old school GT3076s have done this in the past, so if the new "G" series turbo ratings are legit, they should do this no problem.


My question to all of you is, do you have any doubt in Garretts HP rating on this new G25-660HP, because I honestly do based on simple physics.


Link to the turbo:
https://www.turbobygarrett.com/turbo...series-g25-660


*For the record I am not interested in a bigger VTG or SRM EFR stock manifold setup, looking to go outside the box. I get Garretts/Precision/BorgWarner at cost, so im not worried about the retail cost savings of a VTG stock manifold setup.
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Old 12-07-2017, 05:54 PM
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I have been eyeing these turbos myself. If I wanted bolt on I would do the Xona Alphas, if I want to fab I would definitely get these. I think they should be great for 1khp flywheel street cars.

The data supports the claims not sure what specific part you are wondering about. The compressor has a lot of backsweep and a large trim, much larger than normally released for a turbo supplier like Garrett. The turbine wheel is clearly designed for flow, not alot of work.
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Old 12-07-2017, 06:29 PM
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looking forward to hearing about your impressions and results.
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Old 12-08-2017, 02:45 AM
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Been looking at these too, interested in your results. 700w I think is very much achievable with these, interesting to see how they perform on this platform before choking out, garrett promising bigger sizes still. Finally they make something specific for the performance aftermarket ...
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Old 12-08-2017, 08:27 PM
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I've been looking at these quite a bit since they were released. I'm a bit sceptical on if a compressor that size can actually flow 60lb but being garrett I'm sure it does. The biggest thing I don't like is how small the turbine wheel is. Imo it's to small for the compressor flow. I'm sure it would be super responsive, but limit top end power. I think they are worth a shot though.
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Old 12-09-2017, 03:49 PM
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Quote:
Originally Posted by flewis763 View Post
I've been looking at these quite a bit since they were released. I'm a bit sceptical on if a compressor that size can actually flow 60lb but being garrett I'm sure it does. The biggest thing I don't like is how small the turbine wheel is. Imo it's to small for the compressor flow. I'm sure it would be super responsive, but limit top end power. I think they are worth a shot though.
Anyone know what size the vtg turbine is? Just curious. People always talked about it being the limiting factor.
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Old 12-09-2017, 04:34 PM
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Guys don't get so caught up on the turbine wheel size. We can make turbines of the same diameter flow vastly different tweaking the blade shape. Yes direction-ally larger wheels will flow more but for a given design.

The turbine stage on the 997 is a BV50 (50mm, however it actually measures 48mm). Everyone says its the limiting factor from a flow prospective however this statement is not 100% accurate. Look at your logs, you will see you are NOT at max open vane position. The turbine stage is a high flow turbine stage. You are making backpressure because of the efficiency it is running thus closing the vanes to make the turbine power required for your compressor operating point. Three things go into turbine power (temperature, efficiency, and expansion ratio). By commanding the vanes closed you are increasing expansion ratio because you are commanding a lower flow. If the turbine stage was more efficient, you would be running at a more open vane position (higher flow). We can go WAY deeper but I just keep seeing this statement over and over and its not exactly correct. The problem gets worse the larger compressor you hang on it or the more radial the compressor is. For a large compressor on this stage you would want something with a lot of backsweep.
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Old 12-09-2017, 05:34 PM
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Thanks for the info M3. Most of us are NOT turbo engineers so it's always appreciated when tech info is provided by someone that actually understands this stuff. You're right, people always talk about the hotside being too small. When I was looking at my logs I noted that even with high boost at high rpm the vane position didn't change (always in the 40% range) which made me wonder why so much increase in backpressure.
I've been reading turbo compressor maps and trying to digest the info and it hasn't completely clicked in my brain yet. Can you expand on the part where you said "by commanding the vanes closed...". What determines the efficiency of the turbine?

When I see turbos in pictures it looks like ALL the exhaust gases would go through the turbine causing it to spin all the time. Maybe I'm missing something from the pictures I've seen...
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Old 12-09-2017, 05:52 PM
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A quick google search led to a nice article in simple terms:

http://theboostlife.com/2014/02/16/t...stegate-works/

Is there a wastegate on the vtgs or is that what the brv is for?
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Old 12-09-2017, 06:39 PM
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With the VTG turbine stage we pass all the exhaust gas through it, there is no wastegate. The BOV / re-circulation valves you see on your compressor are purely for the charge air side, not the exhaust side. With the VTG imagine we are achieving the flexibility of multiple turbines in one stage. The concept would be like you having a .40A/R to .90A/R on your Xonas just to help you visualize, its more complex than that but gives you the idea. By moving the vanes, open or close, you are choosing a different flow curve. More things are happening but lets keep it high level and simple for this discussion. Below is a picture of another Borg Warner VTG that is published. You will see the flow range from 100% open to likely 10-5% open.

The number being reported back in your log is actually actuator position but it will translate into % closed. So if you see your duty cycle at 40%, it means 40% closed vane or 60% open. The more closed the more expansion ratio you will make , more backpressure and turbine power. The more open the more flow dropping backpressure but also dropping turbine power. This is where if we had more efficiency in this turbine stage, we would be able to run more open, less backpressure, but have the turbine power required to hit the compressor operating point. Recall turbine power consists of exhaust gas temperature, expansion ratio, and efficiency.

The map below is mass flow as the Y / expansion ratio as X, each line is a flow curve at x vane position. The 140 - 430m/s number is turbine wheel tip speed in meters per sec. There is more information missing from this map to complete it which is efficiency. With that information we would make a countour plot of all this information to evaluate the match.

There is a ton more to this but hopefully this wets your appetite lol.
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Old 12-09-2017, 10:57 PM
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cool, thanks. What I don't get is this: if ALL the exhaust goes through the turbine of a vtg then why isn't it always making boost? is it because the velocity of exhaust at steady state or mild acceleration is low and the turbine isn't spinning all that fast?
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Old 12-10-2017, 06:44 AM
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At a part throttle light load condition you will have low exhaust energy so not much work you can extract. If you tried to always have the turbo spooled you would have poor fuel economy because we would be closing the vanes bringing the velocity up but have extra unneeded pumping work.

In a typical wastegate layout you would have all the flow going thru the turbine at this condition as well. Same thing here you are at low flow low expansion ratio not much power available. Your wastegate open scenario is at higher engine loads where flow is up and we have extra exhaust energy we don't need for te desired compressor operating point.

There are some setups where we purposely undersize the turbine stage but run the wastegates inverse. They are vacuum-actuated not pressure actuated. So we keep them open until Boost is required and as we hit boost targets we reopen them again to regulate pressure. So by keeping them open at part load we are dropping pumping work getting us better fuel economy. The undersized turbine will get us better transient response. There are issues with this setup as well, the industry learned a lot of hard lessons but it's getting off topic.

Hope this helps
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Old 12-10-2017, 08:51 AM
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M3, it's always fun to read your posts. RE: VTG vane positioning during cruise, I think that they have two "default vane positions" 1. when in normal mode...this has a less aggressive vane position which keeps shaft rpm down a bit with the benefit being slightly better fuel econ during cruise since there's less pumping loss 2. Once the sport button is pressed, the vanes assume a more aggressive angle during cruise so that shaft rpm is increased and transients are decreased as a result. I think this is part explains the different exhaust note when in sport mode as well.

My M3 actually does a very similar type deal, though with a standard wastegate and electronic actuator. When in sport mode, they "pre-tension" the turbos, which is just marketing BS for a more aggressive default wastegate position to get the shaft rpm up for increased transient response at the cost of a bit of fuel econ.

Last edited by A418t81; 12-10-2017 at 10:00 AM.
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Old 12-10-2017, 03:07 PM
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Pretensión as in closed wastegates like most wastegate turbos lol. I don't speak German but I wondered if the translation had something to do with it. But I think most likely the strategy is close the wastegates instead of having them open I agree.

Probably another tweak is in the ramp rates for the actuators.

Lots of options for the 997 market.
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Old 12-10-2017, 10:50 PM
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If a wastegate "fails" and gets stuck closed it could be a big problem. I suppose the EBC should be able to tell if an overboost situation is occurring. It's amazing to think that the turbine is rotating at variable speeds for its entire life. I know Pascal (K24f) had a turbo failure on his a3071s. It sounds like the Xona design is fairly robust, fingers crossed!
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