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 Post subject: Efficiency Vs Flow
PostPosted: Mon Oct 16, 2006 1:02 pm 
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Token Old Guy
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Ok time to start a new debate...

After much discussion about the benefits of the T3 turbine flange over the T25/28 flange on the GT3071R, I would like to raise another variable to this scenario.

The two turbos below both have different turbine housings, but the 3071 is a far more efficient compressor across a far broader band. At what point do we abandon turbine housing optimal size for a greater efficiency?

http://www.turbobygarrett.com/turbobyga ... 0382_3.htm

http://www.turbobygarrett.com/turbobyga ... 382_12.htm

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PostPosted: Mon Oct 16, 2006 2:07 pm 
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Not sure if you are talking about compressor or turbine efficiency here?

In terms of turbine efficiency, it basically means how well it can convert the kinetic energy from the exhaust gas into shaft momentum. As may or may not know, the 56.5mm turbine (on the T2 GT3071R) is actually a trimmed down GT30 60mm turbine, shoved into a bored T28 housing. The efficiency is down compared to the original 60mm and thus it means more lag. Also the flow is hardly any better than the NS111 found in the 2560/2871 too etc. So whats the advantage? Probably nothing.

As a rule of thumb from my research, for 1.0lb/min compressor flow you would want 0.5 lb/min of turbine flow (ie 2:1). When the ratio is greater 3:1, you got a 'mismatched' turbo. Ie a turbo which will generate a lot of lag and then not produce as much top end power as the compressor would dictate. GT2876R 0.64 or GT3040R anyone?

As omg stated, the GT3071R with a GT30 turbine is a sweet turbo. Its flow ratio is good (2:1) and the comp/turbine diameters are pretty close (71mm/60mm). Also the GT28RS (or the GT2530) is also another turbo with good matching between the turbine/compressor.

Alright thats enough essays from me from now on lol. Its bordering spam.


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PostPosted: Mon Oct 16, 2006 2:49 pm 
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I was refering to compressor efficiency... the 3071 has a far better sweet spot than the 3076. The question is at what point do we swap exhaust flow for compressor efficiency?

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PostPosted: Mon Oct 16, 2006 3:21 pm 
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depends on how much power you want to make, how much boost you want to run, how big your engine is, and how well it flows.

There needs to be a balance to all the variables, and sometimes the best way to find that is trial and error - with some help from the numbers.
Obviously trial and error is very expensive and time consuming, which is why people tend to recommend what's been tried and tested by people who have spent a lot of money in the past to come up with an answer :)

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PostPosted: Mon Oct 16, 2006 3:25 pm 
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lol OK got a bit confused. Well here's my take;

Well there is no "set point" that we would select efficiency over flow. Flow is flow at the end of the day - 42lb/min at 60% or 42lb/min at 70%. Just the former compressor is working harder and putting more heat into the charge. But I see what you are saying.. should we choose the more efficient compressor? Yes in theory... but theres another factor to consider. And thats compressor inertia - lag!! You'd want the smallest compressor that would comfortably meet the target. :D

So you'd start off by selecting your flow target and go from there eg 42lb/min. The 71mm 52 trim makes that, then we'd choose it that because there will be less lag compared to the 76mm 56 trim. Sure the efficiency will be down compared to the 76 (ie 76% vs 70%) but remember in the I/C calculator in the previous thread, a few % here or there in terms the efficiency is not such a big deal IMO with a good FMIC. Of course if the flow target is right on the choke line eg 45lb/min, then you're aiming too high or going too small of a comp. Changes in trim (eg 52 to 56 trim) can offer flexibility in the same compressor family.

Hope thats not too confusing. :(

EDIT - posted right after Dumhed but hes right, at the end of the day its real life results that matter. Numbers on paper are simply that - something we have to resort to when we don't have $$$ for our own dyno or turbos to choose from!


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PostPosted: Mon Oct 16, 2006 4:48 pm 
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No that makes sense... What would be ideal is if we could plot the whole boost curve through the map to see where it goes then you could compare them against other turbos to see which holds the best efficiency.

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PostPosted: Mon Oct 16, 2006 4:57 pm 
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one thing to keep in mind is that if the compressor is operating at lower efficiency, that extra heat it's generating on top of the air pressure is energy - which comes from the turbine.
So, if the compressor is less efficient, you need more power from the turbine, which means the wastegate has to be closed a bit more to provide more drive pressure to the turbine. That has the effect of more exhaust restriction, which reduces the engine's volumetric efficiency, and can also promote reversion, and hence higher chamber temps and detonation.
It's unlikely to be a big difference, but there are so many variables involved that it's not really possible to have an "ideal" setup - so you have to go with the closest thing available.
Due to the amount of trial and error people have done in the past, the closest thing available is pretty damn good these days!


Also just thought of another thing with the compressor efficiency.
When you're cooling air down in the intercooler, there will be a pressure drop.
The greater the temperature drop, the greater the pressure drop.
So, while we're talking about "boost" as measured at the intake manifold, really what's important from the turbo's point of view is the pressure ratio of the compressor, ie compressor intake versus compressor outlet.
The lower the efficiency at any particular boost pressure after the intercooler, the higher the pressure ratio you'll need at the compressor.
Intake restriction also plays a part.
If you have 1bar of boost with no intake restriction you have a 2:1 pressure ratio, but if you're getting signifigant restriction through the air filter, etc you could easily end up with a 2.5:1 pressure ratio.

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PostPosted: Mon Oct 16, 2006 5:50 pm 
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182Go wrote:
No that makes sense... What would be ideal is if we could plot the whole boost curve through the map to see where it goes then you could compare them against other turbos to see which holds the best efficiency.


Yep although i cbf doing it for an SR20, over at stealth316.com he plots the Mitsi V6 with a huge combination of MHI compressors.

Image

Only thing is it won't tell you how responsive a turbo will be. Thats a combination of a million factors.


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PostPosted: Mon Oct 16, 2006 6:24 pm 
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Busky2k wrote:
182Go wrote:
No that makes sense... What would be ideal is if we could plot the whole boost curve through the map to see where it goes then you could compare them against other turbos to see which holds the best efficiency.


Yep although i cbf doing it for an SR20, over at stealth316.com he plots the Mitsi V6 with a huge combination of MHI compressors.

Image

Only thing is it won't tell you how responsive a turbo will be. Thats a combination of a million factors.


I would have thought that there would be sufficient formulas available to put all this data in and spit out a nice little graph line.

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PostPosted: Mon Oct 16, 2006 6:50 pm 
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would be too much of a pain to input the compressor map data into a program that could do that sort of a graph me thinks

just do it manually, it doesnt take that long


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PostPosted: Mon Oct 16, 2006 6:56 pm 
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Oh yeah its not too hard to do. Just.. can't be bothered really. He's got the calculator on his site IIRC.


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PostPosted: Mon Oct 16, 2006 6:58 pm 
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what is IIRC ? it reminds me of mirc for some reason


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PostPosted: Mon Oct 16, 2006 7:01 pm 
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IIRC = If I recall correctly. Heheheh.... :D


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PostPosted: Tue Oct 17, 2006 9:53 am 
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mokompri wrote:
would be too much of a pain to input the compressor map data into a program that could do that sort of a graph me thinks

just do it manually, it doesnt take that long


I was thinking more along the lines of maping your needs and comparing them against the maps not the maps against your needs...

How do you do it manually that only gives you a peak HP figure doesn't it? Can you do incremental comparisions?

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PostPosted: Thu Mar 08, 2007 8:53 pm 
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Busky2k wrote:
Not sure if you are talking about compressor or turbine efficiency here?

In terms of turbine efficiency, it basically means how well it can convert the kinetic energy from the exhaust gas into shaft momentum. As may or may not know, the 56.5mm turbine (on the T2 GT3071R) is actually a trimmed down GT30 60mm turbine, shoved into a bored T28 housing. The efficiency is down compared to the original 60mm and thus it means more lag. Also the flow is hardly any better than the NS111 found in the 2560/2871 too etc. So whats the advantage? Probably nothing.

As a rule of thumb from my research, for 1.0lb/min compressor flow you would want 0.5 lb/min of turbine flow (ie 2:1). When the ratio is greater 3:1, you got a 'mismatched' turbo. Ie a turbo which will generate a lot of lag and then not produce as much top end power as the compressor would dictate. GT2876R 0.64 or GT3040R anyone?

As omg stated, the GT3071R with a GT30 turbine is a sweet turbo. Its flow ratio is good (2:1) and the comp/turbine diameters are pretty close (71mm/60mm). Also the GT28RS (or the GT2530) is also another turbo with good matching between the turbine/compressor.

Alright thats enough essays from me from now on lol. Its bordering spam.


I was wondering why there would be more lag on a smaller trim turbine?
A T25 .63 A/R (56.5 trim) should spool quicker than a T3 (60 trim) .63/AR GT3071R


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