Courtshark

Rebuilt turbos are fine. I buy/send mine from/to turbochargers.com; get the Texas Monster Thrust Bearing upgrade.

http://www.turbochargers.com/store/i...3c08ff6e3c876f

atb

So here's some interesting math calculations on turbo sizing. http://www.turbobygarrett.com/turbob...o_tech103.html

I've made calculations for a 400RWHP (460 crank) and 300RWHP (345 crank) set up.

First, I wanted to determine how much air mass flow the turbo would need to flow to derive the compressor operating point:

Wa = HP * A/F * BSFC/60

HP=Target FLYWHEEL Horsepower
A/F = Air/Fuel Ratio
BSFC/60 = Brake Specific Fuel Consumption (lb/HP*hr) / 60 to convert to hours to minutes
(The constants for BSFC are between 50 and 60 depending on whether its street gas or race fuel, so I used 55)

400 RWHP
Wa= 460*12.5*0.55/60
Wa= 52.7
Compressor needs to flow 52.7 lb/min

300 RWHP
Wa = 345*12.5*.055/60
Wa= 39.5
Compressor needs to flow 39.5 lb/min


Next, I determined the absolute manifold pressure needed to reach the HP goals:

Absolute Manifold Pressure Requirement

MAPreq = Wa*R*(460+Tm)
VE*N/2*Vd

Wa= Air Flow
R = Gas Constant (639.6)
Tm = Intake manifold temp in Fahrenheit
VE = Volumetric Efficiency (.92 4v head / .85 2v head)
N = Engine RPM
Vd = Engine displacement in cubic inches


400 RWHP
MAPreq = 52.7 * 639.6 * (460+130)
.80*6000/2*271

MAPreq = 19887082.8
650400
MAPreq=30.58 (absolute pressure)

30.58 – 14.7 (sea level atmospheric) = 15.88 lbs if boost

300 RWHP

MAPreq = 39.5*639.6*(460+130)
.80*6000/2*271

MAPreq = 14905878
650400
MAPreq = 22.92

22.92 – 14.7 = 8.22 lbs of boost

And then, the Pressure Ratio (P/R) = compressor / compressor inlet pressure

400 RWHP
P/R = 15.88 + 14.7 / 14.7
P/R = 30.58 / 14.7
P/R =2.08

300 RWHP
P/R = 8.22lbs + 14.7 / 14.7
P/R = 22.92 / 14.7
P/R = 1.55

IcemanG17

Adam
Great data....so of course I figure out one for a 5L making 750hp (say 640whp).... I went with a higher BSFC of .6 but kept the same 12.5AFR.....you need a BIG turbo....93.75lbs/min at 16.95 psi with a PR of 2.15 will do it...so I checked it with this to determine the dynamic compression ratio
http://www.rbracing-rsr.com/compression.htm

which would be 21.65CR...or fairly high, but still okay on race gas, but not street gas....

Lizard928

actually most S4s have around 9.3:1 or so some say.

Which 17 PSI is doable with newer systems.

I ended up lowering my CR on my turbo build to 8:1.
Maybe you should see what type of power I "could" extract from that......

The two together should produce around 145 lbs/air at 1 bar of boost.....

atb

One variable that I wonder about is Garrett says that the volumetric efficiency of a 2v is 85%, and for a 4v 92%. I think they use the Mustang 5L and 4.6L for their calculations. When I think of my US 16v 4.5L motor with maybe 200 rwhp, I have a hard time believing that it would have the same volumetric efficiency as a 2v 5L mustang motor, so I pulled the VE down to 80%. This is a totally unsubstantiated number that I pulled out of the air, but it seems reasonable to think that my motor isn't as efficient as a current 2v engine. Am I off base on this?

atb

Here's the initial plotting of my 300rwhp and 400rwhp scenarios using the GT35R turbo pressure map. This is just the very beginning of choosing an appropriate turbo. My understanding is that you want the ulitmate efficiency to plot on the right hand side of the map, that way you won't be subjecting the turbo to surge at any point in the rpm range. The far left line is the surge line.
My 300hp set up plots within the 72% island. My 400hp plots right on the 72% efficiency island border line. That's a pretty good start.

IcemanG17

Hmm
Both your turbos will make 145lbs of air......DAMM....since 93lbs is good for 750hp...you must be sizing up for a serious HP output.....I'd do the math but I've been already so thats not gonna happen

Lizard928

That is both of them together.

And Adam. That turbo looks to be just fine for your app. It will help to have low lag as well.

Courtshark

To not spin tires? Just curious what you mean. In my book, lag is never good!

ptuomov

Try this:

http://www.squirrelpf.com/turbocalc/

By the way, has anyone explored lubricating the rear-mounted turbo using transmission oil?

atb

THAT is an awesome site, thanks for the link!
It automatically graphs your specs on the pressure maps on all of the turbos they have in their library. Too cool.

Don't know if anyone has thought about using ATF, is it conducive to lubricating turbo bearings?

entropy_engineering

I've got quite a bit of experience with stock and built turbo cars of different types. Smaller turbo's are going to come in almost violently quick leaving corners. A super T70 is not big I've run several and been thrilled. You have to consider the backpressure the exhaust will see. Generally, the bigger turbo will create less backpressure and spool slower, but the pound for pound horsepower will be higher (horsepressure?). I just used one on a built 3800 Grand Prix GTP that made ~500hp at the crank. It started making useable boost by 2000ish in the last three gears and made 17 psi by 4000. I'm assuming you mean with a .82 A/R turbine housing and a P trim wheel. I guess spool at the rear may be a little slower, but again you have a bigger engine with more low end torque anyway. The Brazilian made Master Power turbo's are nothing to fear either. They're cheap; if you wear it out replace it. I sell them and turbo xs I think it is to my customers with great results. They're not balanced that well from the factory (or consistently anyway). That's why some people say they have great luck and others hate them. I get mine rebalanced and ceramic coat the turbine housing they run for many miles of smiles at the boost level you'd be looking to run. BTW, T3/T4 hybrid turbo's are kinda' B.S. because it's supposed to offer the quicker spool of a T3 turbine section. In reality, the only reason they spool quicker is because the smaller (usually .63 A/R) housing. They cast the wheel opening bigger and still run an O or usually P trim turbine wheel, so it's essentially a downsized T4. The compressor on a 57 trim hybrid would likely get lazy at upper rpm. Oh, standard Garrett rebuild parts fit at least the off brand hybrid turbo's I remember, and I think the T70 as well. If you can afford a ball bearing unit do it, but I wouldn't sweat a second over it, especially as cool as it will run at the rear of a car.
If you run an externally gated turbo, which you probably will, spend your money where it counts...on a good (Tial) wastegate. If you have a cheap one and the diaphragm pops the boost will shoot up like mad. Same with the blow off valve. As long as the piping run is it needs to vent efficiently to protect the turbo.

entropy_engineering

BTW, for the sake of comparison, check this out. The GTP I mentioned above originally had a bunch of mods (headers, cam, ported T.B., exhaust, downpipe, air intake, ported blower, smaller blower pulley, reflashed computer, and probably a few things I'm forgetting). It ran consistently 13.7's and one 13.6. With a 100 shot it run 12.6. First pass with the turbocharger, not even intercooled, it ran a 12.7 without spray on the exact same boost the supercharger made. Admittedly, the blower was past its efficiency, but it makes you think a little about the efficiency...I'm not knocking superchargers, they have their advantages as well.

Lizard928

Sorry I meant that a smaller turbo will help to decrease lag.

And a good driver can live with some lag esp when tracking the car.

atb

Here's the T70 at 400rwhp. I like how efficient this turbo is for this projected use, but I think it's too close the the surge line to consider isn't it?