The obvious female anatomy aside, the old adage that if one is good then two must be better seems to be nearly universal in application. What could possibly top taking the hottest girl in class to the prom? Obviously taking the hottest girl and her equally smoking twin sister. What is better than having one dollar? Obviously having two dollars. Likewise for a day of vacation, a big screen TV, or Rolex. Can this double-trouble scenario be carried over to the automotive turbocharging world? After all, isn't more power always better? On the surface it seems reasonable to assume that if one turbo will make your motor more powerful, then two turbos make it twice as powerful. The logic is certainly there, but much like our double date, there is turmoil boiling just below the surface that needs to be reviewed, lest our prom night explode into a maelstrom of feline fisticuffs.
When it comes to turbos, two (twin) turbos will make more power than a single turbo, but there is more to the equation than one versus two. For most Fords (at least V-6 and V-8) applications, single turbos can be run just as successfully as twins. Twin turbos are not employed as a means to get additional power, but as a means for response rate and packaging. Twin turbo Cobra obviously sounds more impressive than just turbo Cobra, but the two situations may produce identical results. In most twin turbo applications, the two turbos are (by design) significantly smaller than the turbo employed on a single turbo kit. The idea is that the small turbos will respond quicker than a single large turbo, but it must be pointed out that while the smaller turbos will certainly be more responsive, they are being fed by only half the exhaust energy supplied by the motor. This means they would have to be more than twice as responsive to equal the larger single turbo. In truth, it is the turbo sizing (whether single or twin) that ultimately determines the response rate, and it is possible to have a single turbo system offer better response than a twin (and vice versa).
In some recent testing with the turbo guys from HP Performance, we combined twin turbos with a Roots-style blower to produce the trifecta of forced induction. If you want to improve the response rate of your turbo motor, try improving the power output of the normally-aspirated motor. All things being equal (including turbo sizing) a 400 hp motor will improve the response rate of a turbo or turbos compared to running the same turbo system on a 300 hp motor. That is why having the Roots-style blower on the Cobra motor greatly improved the response rate of the turbo compared to running them on the NA 4.6L. Before adding the turbos, the NA 4.6L produced roughly 300 hp, but adding the blower meant the turbos were fed the exhaust energy of an additional 100 hp. All that extra exhaust energy does wonders for the response rate of the turbos, but there is (as always) a downside to running this compound forced induction system. Having the Roots-style blower greatly improves boost response of the turbos, but so too does it ultimately restrict the efficiency, meaning the blower is in the way of the turbos making power. As we proved, the turbos alone will make more power at any given manifold boost level than having the blower and turbos. Taking triplets to the prom seems like the ultimate, but unless your name is Hugh Heffner, there are bound to be problems.
What we would like to see (meaning I will likely have to coordinate the test) is to improve the response rate of the turbo(s) by using a compound system (commonly used in diesel applications) that employs a pair of turbos. Rather than have a pair of turbos fed by each half of the motor, this compound turbo system feeds all the exhaust energy to a smaller (responsive) turbo. The exhaust from that small turbo (including from the waste gate) is channeled to feed a second (larger) turbo. On the compressor side, air enters the larger turbo and is fed under pressure through the smaller turbo. The smaller turbo is used to increase (multiply) the pressure ratio of the boost supplied by the larger turbo. The result (hopefully) is a twin turbo system that offers both impressive response (from the smaller turbo) and plenty of power potential (from the larger turbo). This compound turbo system should not be confused with sequential turbocharging where the exhaust energy is directed to a small turbo and then (using a diverter valve) redirected to a larger turbo once the small turbo is spooled up. It is the switching required for sequential turbocharging that creates difficulty. This is not to say that the compound system is without issues as well, but things look good on paper and besides, it gives me the opportunity to try some crazy stuff. Not as crazy as dating twins mind you, just like our prom scenario, we certainly expect some drama.