Single vs. Twin Turbo

[rlataille]

I am wondering what you prefer and Y? '

Heres the supra argument, big twin kits on 2jz ( hks 2530) start to spool a bit later in the power band, but they come on REAL hard. a big single has a much smoother spool up, goes over the entire band, thats why most road racing turbo cars in japan, gtrs and supras switch to big singles, in fact the most popular is probably the t78. But on the street the supras get real crazy with true twins (sequential from the factory) and the back end breaks loose real hard, so most go single, but its the opposite for the skyline gtr, they keep the twins more often because the awd keeps the car straight when the smaller twins come on and they launch like a $&$*#@(@) on the highway, ever seen mario's car?

....................................................................................

The way I see it is that with a V6 the easiest setup is to run twins, and acording to my research they have less lag. will our cars really max out 2
evo3's. How about a twin t3/t4. What are the advantages of going single (besides just having to deal with one turbo?

[Sully93VR4]

I disagree with your statement with "spool ups". My single turbo GT42 is like an on/off swith. The power comes outta nowhere, there isn't a smooth spool up with my car.

[rlataille]

I just copied that from the supra forum. My question is if you could get a single T4 setup or a twin T 3/4 for $4000 what would you get and why. would you but it for that price?

[Sully93VR4]

Do you have the other supporting mods? or does that fall into your 4K budget?

[rlataille]

SIG..... But Im doing some R&D on headers and DP, oil lines, coolantlines, ext... Im wondering what people would prefer; Im hoping that either setup should be availible for around $4k so i want to know what people like/ want and why.

[Sully93VR4]

haha, i didn't even look at your sig....oops

[DCIV]

yes we can max out evo 3's. Yes there at t3/t4 kits, look at DR. This has been talked about in great lenght. I really think that our v6 design works best with twins. supras do it because its easy and smart for them to do. People do sinle our cars and I dont enve the amout of work they have to do for it.


Coop

[rlataille]

well yeah when you can buy a single turbo headers for a supra for 250 on ebay I would go single too. How are the spool times on the twin T4/T3 like the DR kit?....WHAT DO you all want? TWIN / SINGLE?

[Kibwe Walker]

I got this a few years ago from sky vr4

Spool vs. Turbo Size:
The Infamous Single vs. Twin Debate

When you think about spool on a turbo, there are a few considerations to observe. Firstly, you want is as low as possible, for obvious reasons. This brings up the dilemma of using a single turbocharger or twin turbochargers. There are ideas out there that may cause you to lean towards one setup or the either, but most can be dismissed with a real look at the issue. Here are some examples:

-Some people argue “one turbo gets all the cylinders’ energy, twins gets half per turbo and significant exhaust pulses, which make the twins more inefficient”. This is mostly invalid because modern turbine housings are radially split and tangential, so the exhaust pulses are calmed upon entry to the turbine, and more effectively handle unsteady energy flow. Also, inertia for the most part prevents any major transience with (heavily) pulsed exhaust energy.

-You get more exhaust pressure with a single than twins, which spools it faster. This can be eliminated because exhaust pressure is not really dependant on the turbo, moreso the manifold. Additionally, exhaust pressure does not have a direct effect on exhaust energy. While it IS an indirect effect on exhaust pressure, nothing is going to add energy to the exhaust flow, especially making the engine pressurize the exhaust more. This not only makes the engine and turbo less efficient, it is useless. Looking at the thermodynamic equation for energy in a fluid flow:

Q + ∑mi(hi + 1/2 (Vi)2+ gZi) = ∑me(he + 1/2(Ve)2+gZe) +[m1(u1+1/2(V1)2+gZ1)-m2(u1+1/2(V2)2+gZ2)] + W

Where Q = heat transfer rate to the control volume
mi=mass exhaust flow in
me=mass of exhaust flow out
m1=initial mass of gas in the control volume (turbine housing)
m2=mass of gas in turbine at the end of the control volume process examination
h= enthalpy (1 = initial, 2=final, I=flow in, e= flow out)
u = internal energy (same conditions stated above)
V= velocity
Z= vertical height
g=force of gravity (9.807 m/s2)
W=work rate by the control volume


This shows that exhaust pressure does not directly affect the energy of the gas; it would increase the velocity of the inflow, but also increase the outflow, which would negate the effect since the mass flow rate should not change. Wastegates do not affect this since they exit the flow before the turbine. Arguing the pressure increases the heat may also be argued because of the same reason. The inflow enthalpy and outflow enthalpy are affected the same way: more heat in means more heat out. While is not equal out perfectly because of turbine performance, it is not enough to affect the thermodynamic flow of the turbine. Examine this closer and you might notice that increasing the pressure (by the ideal gas condition) with the (required) decrease in volume of the gas under inspection will have a very minimal effect. Raising the pressure by 1 bar (a lot) in a 40mm ID pipe (primary) requires a bit smaller then a 35mm pipe to keep the temperature constant. This is a 23.4 % decrease in pipe diameter, which is a LOT. This means that you would have to decrease pipe diameter by more than 23 percent to see any effect increased heat by increasing pressure by 1 bar. At 3 bar (44psi) of exhaust pressure, a 1 bar increase in exhaust pressure at an exhaust gas temperature of 600 degrees Celsius (873K) requires a radius decrease of ~10%. At 1000C (1273K) the radius must decrease of 11%. Thusly, you can see that a mere 100-degree increase in temperature requires a much larger change in radius that desirable. 100 degrees might net you some good energy on the inflow, but it will also increase the energy of the outflow. The difference might be 2%, which is a negligible gain.

Ideal gas formula:

PV=mRT
PV/T = constant
P1V1/T1=P2V2/T2 which yields the equations:

V2(T1) =(T1+100)*P1*V1 P1, P2, V1 known

and
P2T1
P2(T1)=(T1+100)*P1*V1 P1, V1, T1 known
T1*V2

Conclusion: more exhaust pressure is not as desirable as you’d like to think. As you will read, with forced induction you fit the exhaust size to the maximum torque output of the engine. Exhaust/intake pressure rations can be observed also. The effectiveness of aggressive valve timing can be absolutely eliminated if the exhaust pressure ratio is high. More on that later.

Now back to the turbo spool comparison:

There are two reasons to argue a single vs. twin turbos. If you compare the mass moment of inertia for a solid disc, you get 1/2*m*R2 on the axial axis based at the center of the circle (fig. 1) (call this z) and 1/4*m*R2 on the radial axis (x and y). For a circular cone (or circular cone section), the mass moment of inertia is 3/10*m*R2 along the axial axis centered at the center of the base circle (fig. 2), is 3/10*m*R2. The same is true for a cone sectioned by cutting it along the z axis with a plane parallel to the XY plane. Now observe the mass. In such a cone, with a constant density and a constant trim turbine wheel, the relationship of radius to mass is:

trapezoid: A = H*(R1+R2)
trapezoidal cylinder (frustum of right circular cone): V = pi*H*(Ra2 + Ra*Rb + Rb2)/3
Trim of turbine wheel: [inducer/exducer]2 =(I/E)2 = I2/E2
for the frustum, call the inducer Rb and the exducer Ra.
mass = volume * density
so, M1/M2 = (V1*p)/(V2*p)
with equal density, M1/M2=V1/V2




Now lets double the size of the turbine wheel (since trim is equal, the exducer doubles as well as the inducer.

V1= pi*h*(E12 + E1*I1 + I12)/3
V2= pi*h*(E22 + E2*I2 + I22)/3
set I2=2*I1 and E2=2*E1 ::
V2= Pi*h*(4E12 + 2*E1*2*I1 + 4*I12)/3
so M1/M2=V1/V2=>cancel terms =>(1+1+1)(4+2*2+4) = (3/12)=1/4

Thusly, doubling the radius results in4 times the mass.



This roughly simulates a turbine wheel, as close as simple calculations will go. Now if you take 1 large wheel and two small ones of half the diameter you get 1/4 + 1/4 = 1/2 versus 1. It is half the mass. This becomes even more important when you implement the mass moment of inertia. For a frustum of a right circular cone (solid of revolution of a trapezoid), the mass moment of inertia is 2/10*m*R2

If R2 is 2* R1, and hence mass 2 (large turbo) = 4*mass 1 (smaller turbo), then

I1= 3/10 * m1 * R12 and
I2= 3/10 * 4*m1* (2R1)2 = 3/10*4*m1*4*R1 = 3/10*16*m1*R1

So I1/I2 = 1/16. That means the large single turbo has 16 times the inertia. Inertia is the property of mass that is defined as “resistance to change in movement”. Like the old saying goes: “An object in motion tends to stay in motion, and object at rest tends to stay at rest”.

This makes single turbos look slow and laggy. But wait, there’s more.

Lets talk efficiency. Tolerances in a turbo are pretty much the same regardless of size. What this means is that the error can be twice as much area, which can be twice the percentage of error. This affects smaller turbochargers a lot more than larger one. This, as expected, results in 4 times the efficiency loss due to clearances. The big single makes “better boost” then the twins.

Hence, the twins will spool faster, but once spooled, the single is more efficient. If you’re driving a street only car and spool is your first concern, then twin turbochargers will suit your application better. If you are racing and know that you will not fall out of the powerband, and thus spool, the single will be a better choice.

Also keep in mind another adage, “spool only exists in 1st gear and there is [among other methods] a simple solution in a bottle!” The use of tuning to decrease spool time will be addressed in another article, as well as actual turbine performance and efficiency.

The examples used here are simplified for ease of calculation, but will work for any scenario. You probably aren’t going to have a turbine wheel twice the size of another, but interpolation and some calculations give a result that says a turbine wheel about 15% bigger has twice the inertia.

[Kibwe Walker]

Powertrip said this

I would disagree.

What I know about twin v. single, I learned on the Supra forums, of all places. Before I chose my setup, I was dead set on going single. Obviously, the majority of Supra owners prefer their singles, but when I discussed it with a couple of the "Supra Gurus", if I may call them that, the only people that, by the way, understood "basic" engineering and had experienced both big singles and big twins on the Supra, the opinion was surprisingly that twins are certainly better for the street, and, if selected properly, can even be better than a single for the strip.

The twins major advantage is their decreased moment of inertia which is very nicely described above. The twins major disadvantage is the gap between the turbine and compressor and their respective housings. As described above, this gap is kept to an absolute minimum to increase efficiency, but is usually constant between big and small turbos due to engineering limitations. Clearly, if you put a small turbine wheel in an inappropriately large housing the gap would be so large that the pressurized air would have a greater opportunity to go to the unpressurized side dramatically decreasing efficiency. Again, this gap is usually the same regardless of turbo size. The problem for twins comes simply down to pie times the diameter which equals the circumference of a circle. If for example, you had a set of twin turbo turbines or compressors with a diameter of 1 unit (the kind of unit, cm, inch, foot, whatever doesn’t matter) then the circumference would be 3.14 units. Since we are talking about a twin setup here, then the actual circumference of both wheels would be 2 x 3.14 = 6.28. Now look at a single turbo that will flow the same amount of air as the twins. To flow twice the air, the diameter of the wheel DOES NOT have to double…if that were the case that turbo would flow much more than the twins. In reality, the diameter does not have to increase much so that diameter is much less than 2 x the diameter of one of the twins. As a result, the actual circumference of the single turbo which flows the same amount of air as the twins is much less than twice the circumference of one of the twin wheels (but keep in mind the moment of inertia is much much greater). So, in a nutshell, the single has to deal with less gap area between the wheel and the housing than two twins and can therefore operate at a higher efficiency.

Here is the kicker though, turbos have become more and more advanced through the years, especially recently. Through various means, the efficiency of some of our more modern midsized turbos that are very suitable for a twin setup on a 3.0 liter Supra or 3S has come very very close to the efficiency of the larger turbos. Currently, you can get a medium sized turbo suitable for a twin setup with an efficiency range either the same or only 1 or 2% less than the comparably sized single. This small difference is further minimized by an adequate intercooler. With a properly selected turbo and IC, this efficiency issue can be of little or no significance, and often a smaller detriment than the increased moment of inertia of the larger turbo even when used on the strip.

One area where the single does have an advantage is that it often has a higher PR ratio and can achieve a higher level of boost than the common twin setup. Even this is becoming less of a problem, however, with more modern turbos. Take, for example, the Garrett’s GT3071 turbo which would nicely suit a twin 3S built for the track and even the street. It has a PR ratio of 4.2, and has produced over 40 pounds of boost just about as high as you can get with most Garrett turbos and more than enough for 99.99% of 3S members. Due to decreased lag, however, a twin setup can get to these higher pressures faster than a big single unless some other major discrepancy exists (it is really not very easy to achieve boost of 40 PSI on a 3.0 liter engine). So, again, if you choose your twin wisely (a comparable efficiency and PR as the single) the disadvantages of the single become greater than the disadvantages of the twins even for the track. If the efficiency and PR are the same or better for the twins, the twins would actually make a better choice for the track. The only possible exception is when devices are used to build boost off the line, such as a stutter box, decreasing the lag of the big twin. I, however, have yet to see a stutter box that will give you 30-40 PSI off the line, and, even if there were, I know of no drivetrain that would survive the shock, so lag from an increased moment of inertia of the big single is still an issue, but a much smaller one.

Still another consideration is the shorter runners and decreased exhaust gas heat loss on a twin header setup. Some of the single turbo setup headers can become quite complicated. Design headers for a 3S V6 and it becomes even more complicated…and expensive. All this results in even more lag for the single.

So, in the end, COST NOT CONSIDERED, there really are not too many situations where a big single can beat a PROPERLY SELECTED twin setup. A drag car with a stutter box that NEVER sees the street is the only one I can think of..

The real issue, however, is that the above issues is not what has made big singles so popular. Cost is what has driven the market and is 99% of the reason we see big singles used on cars like the Supra which are driven on the street only or on both the street and track. TWO TURBOS AND TWO HEADERS GENERALLY COST A HELL OF A LOT MORE THAN ONE TURBO AND A MASS PRODUCED HEADDER. All the engineering in the world will not likely make up for that.