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[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.