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Again, it looks just a little bigger than a k27-7006 but with three hot side AR potions to allow one to tune it. I bet the 2.0 guys are using the smaller GT30 or if the larger one it would probably be with the smallest AR housing. But, that is just a guess.

Old 10-20-2010, 01:29 PM
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Laying on the parameters that I calculated for flow and pressure ratio, that Garrett won't be an upgrade over a K27-7200.
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Old 10-20-2010, 01:38 PM
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some are using the smallest .63 housing on the 2.0 motors
Old 10-20-2010, 02:06 PM
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Paul,

I appreciate your input. I really do not know. Just going on the wheel sizes and maps for the cold side.

Can you elaborate.

My thinking, for what it your conclusion makes sense for the cold side. Looking at the maps for the 7200 and Large GT30 they almost look the same.

What about the hot side? I suspect that the restriction on a 7200 is the hot side, not so much the cold side.

The 7200 and 7006 have almost the same size cold sides. The extra 20-30hp for the 7006 seems to me mostly from a less restrictive hot side.

The LARGE GT30 has a larger turbine than the 7200 and some choice of AR's.

I am thinking it is about the same size turbo as a 7006, maybe just a little bit bigger.

The hope is the faster spool ball bearing center section and maybe more modern and efficient wheels would give it the early hit of a K27-7200 and the HP range of the 7006 or just a little bit more.

The HF and GT35's would of course be a significant step up but maybe more than some of the CIS crowd needs.

The real test would probably have to be a back to back on a real car.

Just a thought.

Last edited by 911st; 10-20-2010 at 02:13 PM..
Old 10-20-2010, 02:11 PM
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Quote:
Originally Posted by 911st View Post
My thinking, for what it your conclusion makes sense for the cold side. Looking at the maps for the 7200 and Large GT30 they almost look the same.

Calculating mass flow of 52 lb and pressure ratio of 2.4 atmo (.9 bar of boost) generates a point that is right on the edge of the choke line of both compressor maps. Lay that same point onto the GT35r map and it is comfortably to the left of the choke line.
This is why the hot rod 964 Turbo S wasn't boosted above .9 bar by the factory. The air at 1 bar would be really hot, so that there was little hp gain and lots more thermal stress placed on the engine.
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Old 10-20-2010, 06:12 PM
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Paul,

Interesting info. What HP is that at the line you calculated? Not my area.

Per the Garret site:

Quote:
•What is my mass flow rate? As a very general rule, turbocharged gasoline engines will generate 9.5-10.5 horsepower (as measured at the flywheel) for each lb/min of airflow. So, an engine with a target peak horsepower of 400 Hp will require 36-44 lb/min of airflow to achieve that target. This is just a rough first approximation to help narrow the turbo selection options.
If this is true that should put the GT30-76R right in the sweet spot at TQ peak and still be at about 75% efficiency at 400hp.

I am not an expert but I have never see anything that Porsche every went over .8 bar (euro spec to .85 bar) on any of there cars. The exception being a hand full of special wish cars with adjustable boost and maybe the special 3.3 TurboS. Not the S2 or 3.6S.

There was a Road and Track article that I believe mislead some that a .9 bar spring was an option on the 3.6S.

I suspect the K27-7200 and 3.3 CIS fueling both had about the same design limits.

Per Bruce Anderson's book on modifying Porsche's a 3.3 CIS turbo with ports, intercooler, and SC cams should not exceed .9 bar if run on the track due to fueling and thermal limits. In this configuration it was about a 390-400chp car. 1 bar seemed acceptable for a street car duty.
Old 10-20-2010, 08:12 PM
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More info from the Garrett site:

GT30-small:
Horsepower 300 - 460
Displacement 1.8L - 3.0L



GT3071R:
Horsepower 310 - 525
Displacement 2.0L - 3.0L


GT35:
Horsepower 400 - 600
Displacement 2.0L - 4.5L

So is it better to have a 400hp motor with a turbo rated for 400 to 600 or a turbo rated for 310-525?

Last edited by 911st; 10-21-2010 at 09:52 AM..
Old 10-20-2010, 08:42 PM
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Too many people put turbos larger than needed for "later down the road". If you are shooting for 400hp get a turbo with a little head room not the one in surge at 400hp today but will make 600hp tomorrow.

Factory turbos are usually very small but always have a few ponies avaliable when tweeked. Typically people loose that "factory response" when going larger and just live with it in the quest for more power. Very few size their turbos appropriately, I am in that group and considering the GT30-76 as my power goal on the stock motor is 480hp.
Old 10-21-2010, 09:17 AM
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I like the idea of the GT30r. I like it because it reqires less oil to support it, I like it because it's got dual BBs, I like it because it's sized right( the smaller one) I love the idea of great power off idle potential, or at least just above 2000rpm building instead of waiting for 3000. I Don't need to go over 400hp with good drivability off the track, but it can support more when I do want to pull that pin. It's a great fit foe the 930, especially the 3.0 930.
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Old 10-21-2010, 09:36 AM
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It isn't the desired hp that you want to run but the boost that you want to run and mass flow at max engine speed that will dictate the compressor size. Your better off running a GT35r compressor that's delivering cool, dense air at 1 bar than a GT30 that's running at the ragged edge of choke.
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Old 10-21-2010, 03:20 PM
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911nut - i agree to a point but larger wheels and housing a/r's need larger volumes of exhaust to turn them. If 400hp is the goal I see no reason a 600hp turbo would be better suited than a 500hp turbo. but i'm all ears
Old 10-21-2010, 03:29 PM
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Paul,

I guess the question is what is the ragged edge?

The GT35 is a great turbo! Especially for a 500hp EFI build. Just wonder if it is a fit for a 300-400chp CIS street racer build. Per there web site it is not for a fit for under 400hp.

I agree about fitting the compressor to run efficiently. Most street cars I believe we want the peak efficiency at the TQ peak. Some track builds try to move the efficiency island closer to the peak HP.

However, using a larger compressor wheel to run more efficiently has to be at the expense of response.
Old 10-21-2010, 04:41 PM
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Quote:
Originally Posted by 911st View Post
I guess the question is what is the ragged edge?
The ragged edge is when mass flow and pressure ratio converge at or very near the choke line.


Quote:
Originally Posted by 911st View Post
I agree about fitting the compressor to run efficiently. Most street cars I believe we want the peak efficiency at the TQ peak. Some track builds try to move the efficiency island closer to the peak HP.
However, using a larger compressor wheel to run more efficiently has to be at the expense of response.
The beauty of the ball bearing turbo is that you can have it all. The GT 35 will have head room for higher boost, good flow at high engine speeds and spool up quickly due to low friction.

Why spec a turbo that is good down low and runs out of breath up top? If that's the case, save your money and keep the K27.
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Old 10-22-2010, 03:44 PM
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Paul,

Thanks again. I did not phrase the question clearly,

What is the ragged edge for GT30-76R?

Am I reading things right. It looks like it is no where the ragged edge if under 400chp or so.
Old 10-22-2010, 04:42 PM
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Quote:
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Paul, what is the ragged edge for GT30-76R?
Keith, it's not even close on a stock engine. I would bet that it wouldn't be close on an engine with stock intake port diameters and a hotter cam. Plot those values in that I calculated and you'll see it's nowhere near the choke line of the compressor map.
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Old 10-23-2010, 07:26 AM
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Thought I would try to get this in one place, for what it might be worth.

Compartive compressor sizes:

53.1mm = Gt-30's except the 76R.

54mm = K27-7200 & 7006

56mm = new UMW K27 HySpool

57mm - GT30-76R

61 = HF

61.4= GT35




Above is subject to error, any correction appreciated.

Last edited by 911st; 10-23-2010 at 06:22 PM..
Old 10-23-2010, 04:11 PM
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Looks like there is a new Turbo coming from the HF guys that has the same size compressor as the GT30-76R.

K27 54mm, the new UMW turbo 56mm, GT30-76 is 57mm, HF is 61mm.
Old 10-23-2010, 06:11 PM
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Hey Keith, got your message, mine is a GT3071R, not a 76. I just looked it up, I don't know why but a 76 is actually cheaper? Yes it spools very quickly but at .5bar it barely gets whistling, it's pretty much silent even with the rarely late passenger exit muffler. You can still hear it spining a long time after you turn off the engine.

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Old 10-24-2010, 02:31 PM
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