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Turbos Explained


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#1 Guest_SubaruJunkie_*

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Posted 28 September 2008 - 09:40 PM

VF13
Primary turbocharger used on the JDM Legacy MY93-95.

VF14
Secondary turbocharger used on the JDM Legacy MY93-95.

VF18
Primary turbocharger used on the JDM Legacy MY96.

VF19
Secondary turbocharger used on the JDM Legacy MY96.

VF20
Turbocharger used on the JDM Legacy MY97.


VF22
(460cfm at 18.0psi, 250-325whp, Bolt-On) Used on the V3 Subaru Impreza WRX
This turbo has the highest output potential of all of the IHI VF series turbos and is the best choice for those who are looking for loads of top end power. The top end power however, does not come without a cost. The VF22 spools significantly slower than the rest of the IHI models due to the larger P20 exhaust housing and is much less suited for daily driving than some of the other models. Although the largest VF series turbo, the VF22 is not quite optimal for stroked engines or those who wish to run more than 20PSI of boost. Expect to achieve full boost with the proper mods and a quality tune between 3200-3700rpms.


VF23
(460cfm at 18.0psi, 250-325whp, Bolt-On) Used on the JDM V3 Subaru Impreza WRX.
This turbo is considered a great all-around turbo. Like the VF22 it utilizes the largest P20 exhaust housing. This housing is mated with a smaller compressor housing of the of the VF24 for fast response and excellent low and mid-range performance. This turbo is considered optimal in applications with range from mild to slightly wild. It does not have the same top end power of the VF22, but spools up significantly quicker. Expect to achieve full boost with the proper mods and a quality tune between 2800-3300rpms.

VF24color=yellow]
(425cfm at 18psi, 250-325whp, Bolt-On) Used on the V4 Subaru Impreza WRX STi.[/color]
This turbo shares its compressor housing with the VF23 however, this housing is mated with a smaller (P18) exhaust side. The smaller characteristics of this turbo allow it to provide ample bottom end power and quick spool. This turbo is very popular for Imprezas with automatic transmissions and Group N rally cars. Expect to achieve full boost with the proper mods and a quality tune between 2800-3300rpms.

VF25
Primary turbocharger used on the JDM Legacy B4. Utilizes a thrust-bearing design and a P12 exhaust housing.

VF26
(400cfm at 18psi)
This is the standard equipment primary turbocharger used on the JDM Legacy B4. Utilizes a divided thrust-bearing design and a P14 exhaust housing.

VF27
(420 CFM at 18psi)
This is the standard equipment secondary turbocharger used on the JDM Legacy. Utilizes a ball-bearing design and a P18 exhaust housing.

VF28[color=yellow]
(425cfm at 18psi, 250-325whp, Bolt-On)
This turbo came standard on the STi Version 5. In terms of overall size, it is smaller than the VF22, VF30 and VF34, and about same size as the VF23. Expect to achieve full boost with the proper mods and a quality tune between 2800-3300rpms. 2002-2005 WRX owners will need fuel upgrades for this turbocharger and proper engine management is highly recommended for all vehicles.

VF29
(425cfm at 18psi, 250-325whp, Bolt-On)
This Turbo is nearly identical to the VF24, with the same compressor and exhaust housings. However the compressor wheel in the VF29 has been changed slightly. The changes made to the compressor wheel in this model are generally viewed as improvements, and as such this unit is typically chosen over the VF24. Has a different location for the pressure hose on the wastegate actuator.

VF30
(460cfm at 18psi, 250-325whp, Bolt-On)
This is the standard equipment turbocharger used on the JDM V7 Subaru Impreza WRX STi.
The VF30 is commonly considered the best bang for the buck turbo in the IHI VF series line. A relatively new model the VF30 features the same exhaust housing as the VF24 but a larger compressor side similar to the VF22. The combination of these two parts results in increased output potential without the lag associated with the VF22. Although it doesn't offer the top end supremacy of the VF22, the VF30 is a great compromise between these unit and the quicker spooling models. The VF30 is a thrust-bearing turbo that utilizes the P18 exhaust housing of a VF24 and the compressor housing sized between a VF23 and a VF22.

VF31
Utilizes a P11 exhaust housing.

VF32
The secondary turbocharger used in the B4 IHI VF32. On the exhaust side it uses a 46.5/35.4mm 9-blade turbine wheel, teamed with a 52.5/36.6mm 10-blade compressor wheel. It's rated at 180,000 rpm. Both the primary and secondary turbochargers use a floating metal centre bearing - not ball bearings.

VF33
The primary turbocharger used in the B4 is an IHI VF33 unit, which uses a 46.5/35.4mm 9-blade turbine wheel and a 47.0mm/35.4mm 6 + 6 blade compressor. At idle, the turbo spins at around 20,000 rpm and it can go on to a maximum speed of 190,000 rpm. It has a 17mm diameter wastegate opening to bypass excess exhaust gas. It utilizes a P11 exhaust housing and a divided thrust-bearing design.

VF34
(460cfm at 18psi, 250-325whp, Bolt-On) This is the standard equipment turbocharger used on the JDM V7 Subaru Impreza WRX STi Spec-C.
The VF34 is nearly identical to the VF30, with the same exhaust housing and compressor. However the VF34 goes back to the ball bearing design, and in doing so achieves full boost approximately 500RPM sooner than the comparable VF30. The VF34 is the most recent IHI design and as such costs slightly more than its counterpart. Top end performance and maximum output are identical to the 30.

VF35
(425cfm, 250-325whp, Bolt-On)
The VF35 has identical internals as the VF30 and it uses divided thrust bearings. However, the exhaust housing is a P15 which means this turbo will have fantastic spool characteristics. This turbo is standard on the new WRX Type RA. LIMITED SUPPLY. The VF35 is similar to the VF34. It utilizes the same compressor housing and the same compressor inducer size. The differences are in the divided thrust-bearing design and the P15 exhaust housing. This allows the VF35 to spool slightly quicker than the VF34 at the cost of less top-end performance.

VF36
(430 cfm, 250-325whp, Modification Required) This is the standard equipment turbocharger used on the JDM V8-V9 Subaru Impreza WRX STI Spec-C Type RA.
Roller bearing version of the twin scroll VF37, also has a titanium turbine and shaft for even quicker spool. Same compressor housing as VF30/34, however twin scroll P25 exhaust housing provides slightly better top end output due to reduced exhaust pulse interference. This turbo is good for 400HP and used on JDM STI Spec C from 2003 onwards. It is essentially a fast spooling VF34.

VF37
(430 cfm, 250-325whp, Modification Required) This is the standard equipment turbocharger used on the JDM V8-V9 Subaru Impreza WRX STI. (It is essentially a fast spooling VF30.)
Enter the age of twin scroll IHI turbos. Same compressor housing as VF30/34, however has a new twin scroll P25 exhaust housing that provides slightly better top end output due to reduced exhaust pulse interference. Twin scroll also provides better spool up for improved low down response over the VF30/34. This turbo is good for 400HP and used on JDM STI from 2003 onwards.

VF38
Twin scroll turbo with titanium turbine and shaft. Smaller compressor housing than VF36/VF37 provides tremendous spool up capabilities but less top end than VF36/37. The spool capabilities of this turbo are demonstrated on the JDM Legacy GT, which reaches peak torque at 2400RPM.

VF39
(250-325whp, Bolt-On) This is the standard equipment turbocharger used on the USDM Subaru Impreza WRX STI.
Single scroll turbo used on USDM STI and latest 2.5L STIs released internationally. Smaller than VF30/VF34. It can be found on all model years from 2004-2006. The VF39 utilizes a thrust bearing design and the P15 exhaust housing.
Expect to achieve full boost with the proper mods and a quality tune between 3000-3500rpms. 2002-2005 WRX owners will need fuel upgrades for this turbocharger and proper engine management is highly recommended for all vehicles that utilize this turbo aftermarket. Though they are prone to cracking (wastegate hole), VF39’s can be had for very cheap if bought used. Not because they're an inferior turbo, but the exhaust housing can be near the wastegate hole.

VF40
Used on the USDM Subaru Legacy GT. It can be found on all the current model years from 2005-2007.

VF41
Used on the JDM Subaru Forester STI. It utilizes a P18 exhaust housing.

VF42
Exclusive turbo to the S203/S204 models, this features a twin scroll design with a slightly larger compressor than the VF36/37 turbos and different turbine design (more blades). The VF42 is a roller-bearing turbo and is likely of similar size to the VF22 turbo, but with twin scroll exhaust housing for faster spool and superior top end performance due to reduced exhaust pulse interference.

VF43
(250-325whp, Bolt-On)
Used on the MY07 USDM Subaru Impreza WRX STI. It can be found on both base STI's and STI Limited's. The VF43 utilizes a thrust bearing design and the P15 exhaust housing. The difference between the VF43 and the VF39 used previously on STI's is the size of the wastegate. The VF43 has a larger wastegate designed to reduce boost creep issues.

Taken off limited edition S401 Legacy STi BE5 Model.

Sequence Primary Secondary
Manufacturer IHI IHI
Turbo Type RHF428 RHF424
Model VF33 VF32
Turbine Blade no. 9 9
Turbine Rotor Size (inner) 35.4mm 35.4mm
Turbine Rotor Size (outer) 46.5mm 46.5mm
Compressor Blade no. 12 10
Compressor Rotor Size (inner) 35.4mm 36.6mm
Compressor Rotor Size (outer) 47mm 52.5mm
Max. Turbine Speed 190000 rpm180000RPM
Wastgate Port Diaphragm 17mm N/A
Wastgate Open Pressure 78 kPa N/A
Intercept Point 1900rpm
@ 760mmHg
A/R Ratio 11 18
Bearing Type Floating MetalFloating Metal

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#2 ams

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Posted 28 September 2008 - 09:49 PM

Thanks mate, awesome copy and paste skillz you got there ;) Very informative.

Ugh i could ramble on for forking days about this stuff.


#3 Guest_SubaruJunkie_*

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Posted 28 September 2008 - 09:55 PM

I Try I Try... ill try and get more of usefull info

#4 tangcla

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Posted 28 September 2008 - 09:56 PM

Thanks mate, awesome copy and paste skillz you got there ;)

+1 what he said, absolutely awesome! :lol:

Would love to know more info about the TD04HLA though, as seen on the auto gen4 Liberty, and how that compares to the VF38.
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#5 L1BER8ED

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Posted 28 September 2008 - 10:00 PM

good work jovan, stuffed if i knew there were so many different types.. guess you dont know much about something you dont have!! ha ha
he who laughs last .. thinks slowest....

TEAM BLUE OVAL

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#6 Soop

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Posted 28 September 2008 - 10:12 PM

Should we go into Aftermarket turbo's that are suited to an EJ turbo application? :P
TSM

#7 Guest_SubaruJunkie_*

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Posted 28 September 2008 - 10:19 PM

Should we go into Aftermarket turbo's that are suited to an EJ turbo application? :P



if you wish...

#8 Joe

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Posted 29 September 2008 - 03:23 PM

how about better aftermarket turbos to replace our factory twins jovan ? :)

#9 Soop

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Posted 29 September 2008 - 04:39 PM

Upgraded twins eh?
Have a look here matey.

Primary:
http://www.turbobyga...1_756068__1.htm

Secondary:
http://www.turbobyga...44_454082_2.htm

I have absolutely no idea how they'd perform. You would really have to investigate it. Who knows what the outcome will be though. Garret turbo's are a quality product.
TSM

#10 tangcla

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Posted 29 September 2008 - 05:26 PM

Any ideas with the TD04HLA?
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#11 Guest_SubaruJunkie_*

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Posted 29 September 2008 - 05:33 PM

Joe im looking at an idea of aftermarket ones for me, but soons as i source a set i will do some rebolting so will post the results. Nice find Paul. Tangcla TD04HLA i havent looked at it, i just havent got the time as yet. as my attention is focused on the TT setup of the Gen II / Gen III due to it being almost identical to the Legacy B4's

#12 Soop

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Posted 29 September 2008 - 05:59 PM

Tangy, Probably a good idea to post that in the "Single Turbo" forum :). Save having a mish-mash of information all over the place. Jovan - I was under the impression the Gen3 TT setup is actually a superior design to that of the Gen2. One thing that has caught my attention with you twin turbo guys, is the amount of boost being run. Whats Brendan running? 20-22psi? Whats his intake temperatures like? I would think being such small turbo's trying to maintain such high boost pressures, they would be well out of their efficiency range. Especially considering he's still using a top mount interheater. Consider that a VF23 is at its maximum efficiency at 18psi. It's obviously substantially larger than your secondary. Would it not be a sound idea to reduce the boost levels, in conjunction with a FMIC to give a colder intake charge. Which would actually result in a hotter more efficient burn of the air/fuel. By my reckoning it would also mean using less fuel, due to the fact that your not trying to combat the hotter air temperature by adding more fuel to it. Basically what I'm saying is. Would the theory: Less boost + FMIC = colder intake charge + less IDC + advance timing = more power, less detonation and more efficiency. Be a sound one? I apologise if this is unwanted in this section.
TSM

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Posted 29 September 2008 - 06:12 PM

Paul i cant speak For Brendans car, but i can safely say this- Intake temperatures on a TMIC are no highre when driving then 4-6 degrees over ambiant temp, ive only been monitoring mine on cold or cooler nights as i really wasent here to drive my car in winter. In summer time it does et a bit hotter but again not by much when the car is moving. ive also had temp as high as 15 degrees on ambiant as i dont have the plastic direction air thingy that sits from the bonnet to the TMIC as im running a bigger cooler it bends the fins so i have to chop it a bit. Where im getting this info from is my Greddy info meter which is pluged into the OBDII On your theory less boost- im not sure as most of us want more boost and a FMIC but dont want to spend big $$$ and the idea is to reduce the intake temps, ill have to dig up the info on detination and all that preignition that i have from tafe. Paul the Gen II/ Gen III setups are identical almost the major things that are different are Turbos, ECU, and few other things like the solenoid box, these are the things of top of my head. but mostly they are same

#14 Soop

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Posted 29 September 2008 - 06:19 PM

I understand your running standard boost levels though are you not? This would explain the cooler intake temperatures. How ever, once the boost levels are increased past the turbo's efficiency range the intake temperatures increase dramatically. So after a point all your doing is forcing hot air into the engine. This is when upgraded/more efficient/larger turbo's are the idea.
TSM

#15 Soop

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Posted 29 September 2008 - 06:25 PM

Sorry for the double post but this is too big to add to the other one.

I got this link form Pete.
http://www.gnttype.o.../turboflow.html

Turbocharger Compressor Calculations
John Estill
Introduction
The purpose of this little paper is to show the reader how to calculate the volume and mass of air moving through his engine, and how to size a turbochargers' compressor to move that quantity of air. It should also offer some enlightenment of the effects of temperature, pressure, and intercooling on the engine's performance.
Engine Volumetric Flow Equation

This equation is for finding the volume of air going into the engine. The displacement on our cars is 231 cu.in. We have a four stroke engine; the intake valve on a cylinder opens once every 2 revolutions of the engine. So, for every 2 revs the engine takes in 231 cu.in. of air. How many pounds of air is that? That depends on the pressure and temperature of the air in the intake manifold. But the volume is always 231 cu.in. every 2 rpm.

volume of air (cu ft/min)= engine rpm x engine cid
(1728 x 2)
Ideal Gas Law/Mass Air Flow

The Ideal Gas Law is a handy equation to have. It relates the air pressure, temperature, volume, and mass (ie, pounds) of air. If you know any three of these, you can calculate the fourth. The equation is written:

PV=nRT

where P is the absolute pressure (not the gauge pressure), V is the volume, n is related to the number of air molecules, which is an indication of the mass (or pounds) of air, R is a constant number, and T is the absolute temperature.

What are absolute temperature and pressure? Do we care? Of course we do!

Absolute pressure is the gauge pressure (measured by a gauge that reads 0 when it is open to the outside air) plus atmospheric pressure. Atmospheric pressure is about 14.7 psi at sea level.

Example: a boost gauge reads 0 psi before it is hooked up. Hook it up, boost the car, and it reads 17 psi. 17 psi is the gauge pressure, the absolute pressure at sea level is 14.7 + 19 = 33.7.

A pressure reading is marked psia or psig. The "a" stands for absolute, the "g" for gauge. (The psi stands for Pounds per Square Inch). As we just showed, 17 psig = 33.7 psia. A perfect vacuum is 0 psia, or -14.7 psig.

The absolute temperature is the temperature in degrees F plus 460. This gives degrees Rankine, or deg R. If it is 80 deg F outside, the absolute temperature is 80 + 460 = 540 deg R.

The Ideal Gas Law can be rearranged to calculate any of the variables. For example, if you know the pressure, temperature, and volume of air you can calculate the pounds of air:

n=PV/(RT)

That is useful, since we know the pressure (boost pressure), the volume (which we calculate as shown in the first section "Engine Volumetric Flow"), and we can make a good guess on the temperature. So we can figure out how many pounds of air the engine is moving. And the more pounds of air you move, the more power you will make.

Here is the Ideal Gas Law rearranged to the two handiest forms, with the required constants:
To get pounds of air:

n(lbs/min)= P(psia) x V(cu.ft./min) x 29
(10.73 x T(deg R))
To get the volume of air:

V(cu.ft./min) = n(lbs/min) x 10.73 x T(deg R)
(29 x P(psia))
Volumetric Efficiency
If life was perfect, we could fill the cylinders completely with air. If we had 17 psi boost in the intake manifold, we would open the intake valve and get 17 psi in the cylinder before the intake valve closed. Unfortunately, this doesn't usually happen. With some exhaust remaining in the cylinder and the restriction offered by the intake ports and valves the actual amount of air that flows into the cylinder is somewhat less than ideal. The amount that does flow divided by the ideal amount is called the volumetric efficiency.

For your basic stock small block chevy, I think this number is around 0.85 (or 85%). Things like big valves, big cams, ported heads, tunnel rams, etc... get this number closer to 1.0 (or 100%). With tunnel rams some normally aspirated cars can get over 100% at certain rpms due to the ram effect.

To take this into account when we calculate flow into the engine, we multiply the ideal amount of air by the efficiency to get the actual amount of air:

actual air flow = ideal air flow x volumetric efficiency
Example

Time for an example. Lets calculate the pounds of air flowing into an engine for two different cars, an intercooled '87 and a nonintercooled '85. For both cars we will use a volumetric efficiency of 0.85. For both cars the engine is turning at 5000 rpm. What is the volume of air it is using?

volume, in cu.ft per minute = 5000 x 231 = 334.2 cfm
1728 x 2

This holds true for both cars, both intercooled and nonintercooled will be moving 334.2 cfm of air into the cylinders at 5000 rpm. As we will see however, the mass of air flowing is not the same.

Suppose the car an '85, so it isn't intercooled. The temperature in the intake manifold is about 250 deg F. The car is running 19 psi boost. What is the mass of air the engine is using?

Absolute temperature = 250 deg F + 460 = 710 deg R

Absolute pressure = 19 psig + 14.7 = 33.7 psia

n (lbs/min)= 33.7 psia x 334.2 cfm x 29 = 42.9 lbs of air per minute (ideal)
10.73 x 710 deg R

lbs air per minute actual = lbs/min ideal x vol. eff.
= 42.9 x 0.85
= 36.4 lbs air/minute

What if the car is an '87, it IS intercooled, so the temperature in the intake manifold is only 130 deg F. This car is running 17 psi boost.

Absolute temperature = 130 deg F + 460 = 590 deg R
Absolute pressure = 17 psig + 14.7 = 31.7 psia

n(lbs/min)= 31.7 psia x 334.2 cfm x 29 = 48.5 lbs of air per minute (ideal)
10.73 x 590 deg R

lbs air per minute actual = 48.5 x 0.85 = 41.3 lbs air/minute

Notice that the '87 car is getting MORE lbs/min of air (41.3 for the '87 to 36.4 for the '85) even though the boost pressure is lower. This is because the intake manifold temperature is so much lower. And more pounds of air means more power!
Compressor
The compressor is the part of the turbocharger that compresses air and pumps it into the intake manifold. Air molecules get sucked into the rapidly spinning compressor blades and get flung out to the outside edge. When this happens, the air molecules get stacked up and forced together. This increases their pressure.

It takes power to do this. This power comes from the exhaust side of the turbo, called the Turbine. Not all of the power that comes from the turbine goes into building pressure. Some of the power is used up in heating up the air. This is because we lowly humans cannot build a perfect machine. If we could, all of the power would go into building pressure. Instead, because of the design of the compressor, the air molecules get "beat up", and this results in heat. Just like rubbing your hands together will warm your hands due to the friction between your hands, the friction between the compressor and the air and between the air molecules themselves will heat up the air.

If you divide the amount of power that goes into building pressure by the total power put into the compressor, you get the efficiency of the compressor.

For example, if the compressor is 70% efficient, this means that 70% of the power put into the compressor is used in building air pressure. The other 30% of the power is used heating up the air. That is why we like high efficiency compressors; more of the power is being used on building pressure and less is used heating up the air. Turbos, Paxtons, and Vortechs are all centrifugal superchargers. The are called this because the centrifugal force of flinging the air molecules from the center of the housing to the outside edge is what builds air pressure. The maximum efficiency of these kinds of superchargers is usually between 70% and 80%. Roots blowers, like the 6-71, work differently and have much lower efficiency, like about 40%! With those, when you try to build lots of boost you have to put in a lot of power and more than half of it gets used heating up the air instead of raising pressure.

If the temperature goes up a lot when you increase the boost you can end up with fewer pounds of air going into the engine, so you lose power. That's why a Roots blower is bad if you want lots of boost. Screw compressors, like the WhippleCharger for the 5.0, have good compression efficiency. That's why the Top Fuel guys are starting to try them out, and getting good gains from them.
So? How Hot is the Air Coming out of the Compressor?
Well, I'm glad you asked. The equation used to calculate the discharge temperature is:

Tout = Tin + Tin x [-1+(Pout/Pin)0.263]
efficiency

Example: the inlet temperature is 70 deg F, the suction pressure is -0.5 psig (a slight vacuum), the discharge pressure is 19 psig, and the efficiency is 72%. What is the discharge temperature?

Tin= 70 deg F + 460 = 530 deg R
Pin= -0.5 psig + 14.7 = 14.2 psia
Pout= 19 psig + 14.7 = 33.7 psia
Pout/Pin = 33.7/14.2 = 2.373 (this is the compression ratio)

Tout = 530 + 530 x (-1+2.3730.263 ) = 717.8 deg R - 460 = 257.8 deg F
0.72

So the theoretical outlet temperature is 257.8 deg F. I sure would like to have an intercooler to cool that hot air down before it goes into my engine!

Compressors do not always operate at the same discharge pressure. The discharge pressure that the compressor produces depends on the volumetric flow into it (not the pounds of air, but the CFM of air), and the rpm that it is turning. The performance of a compressor can be shown on a graph by a series of curves. Below is a compressor map from the Turbonetics catalog attached, it is the file called H-3.JPG. [The graph is included here, and is available for download via the hotlinks provided....Ed.]

This is for their Cheetah turbo; take a look at it. The bottom of the graph shows the lbs/min of air that the compressor is moving, corrected to a standard temperature and pressure. The standard industry practice is to put this part of the graph in actual volumetric flow (such as ACFM) since the compression is constant for a given volumetric flow and compressor speed, NOT for a given mass flow. Unfortunately they didn't do their curves that way, and to use the Turbonetics curves we have to figure out the pounds of air moving and correct it from the actual inlet temperature and pressure to their standard temperature and pressure.

The left side of the graph shows the outlet pressure to inlet pressure ratio.

There are two different sets of curves in the graph; efficiency curves and rpm curves. The area where there are lines drawn is the operating envelope. It is best to operate the compressor within its envelope. It will still run if you go to the right of the envelope, just not well. To the left of the envelope, where it is marked "surge limit", the flow through the compressor is unstable and will go up and down and backwards unpredictably. This is surging. Do not pick a turbo that will operate in this area! It can be very damaging.

The Turbonetics catalog says to pick a turbo that is close to the peak turbo efficiency at the engine's torque peak while still maintaining at least 60% efficiency at the maximum rpm of the engine.
Here's how to read the graph.

Figure out the pounds of air that you are moving through the engine. In our '87 example, we were passing 41.3 lbs/min of air, at inlet conditions of -0.5 psig and 70 deg F. Now correct that flow to the standard temperature and pressure.

Corrected flow = actual flow x (Tin/545)0.5
(Pin/13.949)

Note that I am using 13.949 because we are measuring everything in psia instead of in inches of mercury, which Turbonetics assumes.

13.949 psia = 28.4 inches mercury absolute.
29.92 inches mercury is atmospheric pressure at sea level, so 29.92 - 28.4 = 1.52 inches mercury vacuum.
That is their standard suction pressure.

Their standard temperature is 545 deg R, or 545 - 460 = 85 deg F.

So we are correcting the flow from 70 deg F and -0.5 psig to 85 deg F and -0.75 psig (or 13.949 psia, or 0.75 psi vacuum, or 1.5 inches mercury vacuum, or however you want to look at it.)
Again, temperature and pressure have to be absolute.

Tin = 70 + 460 = 530 deg R
Pin = -0.5 + 14.7 = 14.2 psia

Corrected flow = 41.3 x (530/545)0.5 = 40.0 lb/min
(14.2/13.949)

So we mark that point on the bottom of the graph, and draw a straight line upward from that point.

An alternate and better way of getting airflow at less than full throttle is the use of a scan tool. The scan tool (such as TurboLink™) reads the mass air sensor output. TurboLink™ gives this in grams per second. To convert that to pounds per minute just multiply by 0.1323. For example, if TurboLink™ says 18 gm/sec @ 45 mph, 18 x 0.1323 = 2.4 lb/min of air.
Correct that to standard conditions and plot that on the compressor map. Unfortunately the MAS will only read to 255 gm/sec. If you are moving more air than that, the MAS won't show it. That is why you need to go through the above calculation for full throttle air flow.

The next step is to figure out the compression ratio, using absolute pressures. Using our example, we had 17 psi boost in the intake manifold. Let's suppose the pressure drop from the turbo outlet to the manifold is 3 psi; so the actual compressor outlet pressure is 3+17=20 psig. The air pressure is 0 psig, but since the turbo is sucking air to itself the pressure at the inlet is lower than that.

Let's say it is -0.5 psig at the inlet. Then the compression ratio, Pout/Pin is :

Pout/Pin = (20 + 14.7) = 2.44
(-0.5 + 14.7)

So then we find about where 2.44 is on the left side of the graph and draw a line horizontally from that point. Where the two lines meet is where the turbo will operate.
Look at the efficiency curves, which look like circles. Our point is just a little inside the 72% curve, so when we are running at 5000 rpm and 17 psi boost with 70 deg air outside and 130 deg air in the manifold then the compressor efficiency is a fraction over 72%.

The other curves are rpm curves. Our point is above the 105,500 rpm curve, so the turbo has to spin about 108,000 rpm to get the pressure up to 20 psig from -0.5 psig. The Turbine has to provide enough power to spin it that fast.

Change any of these numbers, and the point at which the compressor runs at changes. More engine rpm means more air flow, so the operating point moves to the right. Colder intake temperatures means more pounds of air which moves our point to the right. Raising the boost probably means more air into the cylinders, but also the compression ratio goes up so our point definitely moves up and should move right. And so on.
Summary
So, how do tie all this together? Well, suppose you are in the market for a new turbo. Which one to buy?

* First, I would pick about 4 different operating scenarios. Highway cruise, part throttle acceleration (say 2/3 @ 2700 rpm), full throttle acceleration at 3500 or 4000 rpm, and full throttle acceleration at 5500 or 6000 rpm sound like 4 good points to me.
* Second, calculate the volumetric flow for each one of those cases. Then, making estimates of the intercooler outlet temperature (or turbo outlet temperature if nonintercooled), turbo discharge pressure, volumetric efficiency, manifold pressure, etc.. calculate the mass air flow for each case. You may also want to check the difference between summer and winter, ie air temps at maybe 90 deg F and 40 deg F. This will affect the manifold temperature and so the air flow. Note that when cruising and at idle, even though the manifold pressure is at a vacuum the turbo discharge pressure is not. It has to pump up the air some, even if it is only to 0.5 psig or so. You can check it out by moving your boost gauge to some point upstream of the throttle body. Besides the mass air flow, calculate the Pout/Pin for each case.
* Third, and this is the hard part, find the compressor maps for the turbos you are interested in. Turbonetics has maps for their Cheetah, 60-1, and 62-1 in their catalog. The other vendors may not want to let you have the maps for theirs. Plot the points from the 4 cases on the compressor map.
* Fourth, evaluate the proposed compressors performance. Are the idle/cruise operating points to the left of the surge line? Then this turbo will surge and isn't a good choice. Is the 5500 rpm point so far out to the right that it is off the map? Then this turbo doesn't flow enough for your application. You want all the operating points within the map, and preferably at as high an efficiency as you can get.


TSM

#16 twinturbosubaru

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Posted 05 October 2008 - 10:37 PM

Remember though, you are running 2 turbos, so your real limitation on a twin turbo is exhaust flow. I played with my gen2 setup for years and ended up sticking with 1.1bar as everything worked well there. More boost than that and the drop in pressure as the secondary came into play was more noticable, and became even more so the more boost you ran. The GEN3's also have no wastegate on the secondary, the gen2's do, however it never gets used hence the removal of such. After 270,000 kms my primary and secondary were still fine, proving that you can safely run more boost sensibly on the primary turbo. I ran 1.1bar for 7 years and 210,000km's with many track days and super sprints. Possible correction Jovi, VF13/14 combo was live until MY97 legacy, my 96 model had them in my first and second engine, second engine was out of MY97, I think from memory only the GT-B received the VF18/19 combo and then the MY98 had VF18/19 all round. Paul
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#17 Kosti

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Posted 08 October 2008 - 09:11 PM

This looks familiar :D
I prefer TWINS!

Now anyone up for a friendly game ?

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