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-7 or -5 turbos?

  • -5 for power - gimme gimme gimme!

    Votes: 150 57.5%
  • -7 for spool-up - it's a street car after all!

    Votes: 111 42.5%

  • Total voters
    261
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Discussion Starter #61
hehe, I read your other thread about the tubs Kismet! I found it quite amusing that running them @ 2bar was so low on your list of suspects!!!

when I get my -5's later this year, I don't believe I'll run them anywhere near that kind of boost!!! 1.5max - promise!:nervous:
 

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Some guy on T/M claimed to be making over 500hp ATW's with GTss's tuned by Soichi / R34 GTR
Insisted it would make 700hp (crank) ...
:
 

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Hi All,

I have read a lot of posts regarding turbo comparisons, and I was wondering about the affects of what boost pressure you are running, on an engine making the same power?
I am wondering if there is, say, more stress on an engine if you are, for example, making a 320awkw with 2860-5/2530s as opposed to a GT-SS setup making the same power.
What I mean is, the 2530s would be making that same power with less boost pressure required, yes?
So therefore, would the GT-SS setup that is running higher boost to achieve that number be putting more stress on the engine, or is there no difference, because to make that power the engine is at a certain stress level regardless of boost?
Make sense? :nervous:

Not sure if this has been answered before in one of the myriad similar "Which Turbo" topics, so apologies if it has, but I have read a lot of them and didn't see that particular question asked before...

Cheers!
 

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Discussion Starter #68
good question! as a rough guess, I would say that all other factors being the same, regardless of what turbos, to make your theoretical 320kw atw, you would need to be pushing x amount of boost into the motor, so whether you have small turbos going gangbusters to make y-psi of boost, or big-ass tubs idling along at y-psi boost, the stresses on the rest of the engine are much the same...

the only differences would be exhaust backpressure due to different tubs and possibly oil temp/pressure due to the same.

now, for an expert to come on here and correct me, cos I figure I must be missing something fundamental!
 

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extra boost can stress the head causing the head to lift (usually fixed by uprated head bolts), blowing out headgaskets, cracking heads at real high boost etc.

best way to do an engine and keep it reliable is to get the most power for the least boost pressure.
 

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Discussion Starter #71
yes, but you're missing the point - Moelders question was any different stresses on the engine with big vs little turbos making the same power - so my view is that to make the same power, you are making the same boost - ergo the pressures and stresses are the same, only the effort of the turbos changes!

discuss.
 

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^^^ Exactly.

To maybe further clarify exactly what I am asking here's an example (and bear with me, I will just pull some general numbers, that might be wildly off the mark, but just as an example!).

GT-SS - 1.4 Bar of Boost to make said 320awkw
2530s - 1.1 Bar of Boost to make the same power...

Does that mean the GT-SS setup is more likely to cause gasket issues etc. (as mentioned above as examples of problems caused by high boost) over that of what the 2530s would cause running at the lower boost threshold?

OR (as Frosty? said) would it be the same amount of air actually being pushed into the engine from both setups and therefore putting the same stresses on the engine?

Any further opinions on this from those "in the know"?? :thumbsup:

Cheers :)
 

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Discussion Starter #73
Frosty will be fine!! :)

however, I do not agree with your 1st statement regarding 1.1 vs 1.4 bar. As I said in my 1st post, ALL other factors being the same, regardless of what model turbos you have bolted on the side of the engine, you will require exactly the same boost pressure to make a given HP at a given RPM - regardless of whether that said boost pressure is at the bottom, top or middle of said turbo's compressor map - ie it doesn't matter if the turbos are flat out making, say, 1.4 bar or if it's in the middle of its range making 1.4bar.

Remember tho, this only matters for a "snap shot" - i.e. 1.4 bar of boost at 6000rpm makes 320atwkw on this motor - regardless of what turbos are making that 1.4 bar.

What WILL cause variance, is the how the turbos spool, their efficiency etc etc.

make sense?
 

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So therefore, would the GT-SS setup that is running higher boost to achieve that number be putting more stress on the engine, or is there no difference, because to make that power the engine is at a certain stress level regardless of boost?
This is my understanding
No difference.
x hp is x hp = x amount of stress on "internal" components.
hp = volume of air and fuel shoved in the cylinder and ignited.
The valves might wear different tho.?

If this volume is achieved from 2bar using small turbo's or 1.2 bar using a big one / no different.


The "other" question , higher boost = more stress on the turbo.

Its all about the volume of air moved and the time it takes to move it.
isnt it ...
A bigger turbo pushes more volume per impellor segment which is why it makes more power with less psi.

Correct me if Im wrong
lol
 

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Frosty will be fine!! :)

however, I do not agree with your 1st statement regarding 1.1 vs 1.4 bar. As I said in my 1st post, ALL other factors being the same, regardless of what model turbos you have bolted on the side of the engine, you will require exactly the same boost pressure to make a given HP at a given RPM - regardless of whether that said boost pressure is at the bottom, top or middle of said turbo's compressor map - ie it doesn't matter if the turbos are flat out making, say, 1.4 bar or if it's in the middle of its range making 1.4bar.

Remember tho, this only matters for a "snap shot" - i.e. 1.4 bar of boost at 6000rpm makes 320atwkw on this motor - regardless of what turbos are making that 1.4 bar.

What WILL cause variance, is the how the turbos spool, their efficiency etc etc.

make sense?
Sorry, this is not true at all. Did you learn how turbos work before making that post? Turbochargers compress air. What happens when you compress a gas? That's right, it heats up.

Two turbos running the same amount of boost, one larger than the other, will NOT result in the same mass of airflow going into the engine because the more efficient compressor will heat the air less.

You can model the amount of airflow into an engine using the ideal gas law; PV=NrT where P is pressure, V is volume, N is the number of molecules, r is a constant and T is temperature. From this you can see that increasing the pressure will increase the molecules or Mass of air, increasing the volume will increase the mass of air, and decreasing the temperature will also increase the mass of air! Pretty basic high school science concept.

Now, apply it to the sizing of turbochargers:

Take a GT2860-5 at 30lb/min airflow and a pressure ratio of 2.5 (1.5 bar boost). That compressor wheel will be operating at 76% efficiency (look it up on the compressor map at turbobygarrett.com

Take those same specs and look at a GT2860-7. You are going to be outside of 60% efficiency - it's pretty close to the edge of that 'land' so we'll call it 58% efficiency.

76% vs 58%.

Now if you look up compressor efficiency vs ΔTemperature, you can see how the inlet air temperature will change. The chart I am referencing (page 29 of Maximum Boost) is in Fahrenheit, so bear with me.

According to the chart at 75% efficiency you would be heating the air about 305 degrees. And at 58% efficiency you'd heat it about 365 degrees. So that's 60°F hotter air at the same pressure! Your intercooler is going to work to bring that charge temp down, but it makes sense that if you start with colder charge air in the first place, the intercooler will be able to bring the charge air temp that much more.

What does all this mean? Everyone knows that colder air is denser air and denser air makes more power. There it is, your incontrovertible proof that larger (more efficient) turbochargers will flow a greater mass of air, and therefore make more horsepower, than a smaller turbo at the same boost pressure.
 

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Discussion Starter #76
your sig is appropriate!!

what you have said is true, I don't argue that. And yes, I know how turbochargers work...

The point I was making was general rule of thumb, not feckin NASA calculated numbers.... Glenn puts it well, saying regardless of how you achieve it x HP = x stress on the engine.

Obviously, all rule of thumb - as you say, different turbos = diff temps etc etc

*yawn* - too much science!!
 

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Thanks for the replies :thumbsup:

Sam: Although a lot of the theory that you talked about is far to technical for a non-mechanically minded person sch as myself, your overall explanation seems to agree with what I was suspecting, that being, for engine life expectancy, it would be better to get say, 330kw from a 2530 or equivalent Garret running at ~1.2bar than it would be getting 330kw from GT-SS turbos running at say, ~1.5Bar.
As surely the hotter the air going in, the harder the engine is working and more detonation prone it would be (I know it depends on the tune, but surely that would mean it would be an easier "Safe Tune" on the larger turbo equipped car?)??

Anyway, thanks for the reply, because for me, that would definitely sway my answer to this poll towards the 2860-5 (or 2530s). I would happily take the 500rpm or so loss of response for the ability to make the same power at less boost, causing less stresses on engine and turbos, and the further benefit of then being able to push more power later once the engine internals are strengthened etc. :thumbsup:
 

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Moelders: Your interpretation of my overly technical post is absolutely correct on every point and includes a few of the reasons why I recommend the -5's/2530's as the smallest turbo that should be put on an RB26, and will suggest it EVERY TIME over a smaller turbocharger.
 

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Good stuff :thumbsup:

P.S. These forums are evil... Only just shelved out the $$ for an R34 V-Spec and already thinking of getting 2530s etc.! Will try to resist the temptation, and enjoy the car as it is... for a while anyhow!
So damned tempting though!!! ;)
 

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According to the chart at 75% efficiency you would be heating the air about 305 degrees. And at 58% efficiency you'd heat it about 365 degrees. So that's 60°F hotter air at the same pressure! Your intercooler is going to work to bring that charge temp down, but it makes sense that if you start with colder charge air in the first place, the intercooler will be able to bring the charge air temp that much more.
Can I be difficult and query why GT2560R-9s aren't more popular?
 
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