There are certain facts and laws of nature that every diesel driver faces.  Among these are 1.  Ambient (atmosphere) air pressure is approx 14.7 psi at sea level.  2.  Turbochargers only function at their best in a limited band of RPM's.

When you combine these two facts together you come up with some interesting problems.  You have a few choices.  You can either choose a small turbo, which functions very well with your engine at low RPM's, but limits high end power.  You can get a large turbo which functions good at wide open throttle (WOT), but is horrible to just drive around town because it won't spool up (spool up refers to the amount of time it takes for the turbo to begin to produce boost), and has surging issues (trying to put more air into the engine then the inertia of the turbo allows, therefore causing the turbo to stop, reverse, and or slow down in pulses during operation).  Or you can choose a medium sized turbo which spool's up relatively well, and at WOT still has some exhaust restriction but allows a lot more power and cooler EGT's than the small turbo.  Yep, that's what your stuck with when you're choosing a single turbo.  Sure there are much better choices than others, for example our D-Tech Turbocharger, which offers a huge increase in airflow and performance than the stock turbo, as well as has amazing spool up.  But ultimately a single charger does have the restrictions listed above. 

Part of the big problem is the ambient (atmospheric) air pressure, we mentioned earlier.  If the boost pressure on your truck shows 35 psi, this is actually the gage pressure, or (psig), which means the zero on the gage is actually 14.7 psi  (at sea level) or atmospheric pressure.  The actual pressure is 14.7 psi + 35 psi = 49.7 psi, or actual pressure (psia).  What this means is that the pressure trying to get into the turbo is only 14.7 psi, and despite how fast you spin the turbo, there is only 14.7 psi pushing air in, and if you spin the turbo too fast it becomes inefficient at bringing new air in, while it becomes increasingly harder to get exhaust out.  To overcome this with a single turbo, people increase the sizes of turbines, housings, compressors, on and on, and may increase the amount of airflow, but ultimately hurt low end drivability and spool up, and all because they are up against those pesky laws of physics.  A certain size of hole (the turbo air inlet) will only flow a limited amount of air at a given pressure. Atmospheric pressure becomes a huge limiting factor.

So what's the solution...Twins (turbos that is) also known as compound turbos or sequential turbos, actually two turbos placed sequentially (one flowing into the other).  But not just any two turbos will work together, they must be sized correctly to complement each other or they can fight each other and not work properly.  These two turbos will consist of a smaller charger, and a larger charger.  The small turbo is the first to get exhaust from the engine, and the last turbo to touch the fresh air.  Fresh air enters a large (slower spooling) turbo first , then is pressurized, and then fed into the small (quick spooling) turbo, which then multiplies the already pressurized air, and then feeds the air into the engine.

The beauty of the whole staged, two turbo concept is this.  First of all you can have all of the benefits from a small quick spooling turbocharger, with more-than-all of the benefits of a very large turbocharger. 

Turbos multiply atmospheric pressure, not add it, but function by multiplying it.  Therefore if the small turbo as a single can take air at 14.7 psi, and produce 40 psi boost, it is multiplying the air by 3.72 times (14.7 psi x 3.72 = 54.7 psia, minus the 14.7 atmospheric gives 40 psig (gage pressure)).  The large turbo can do a similar job.  Therefore let's say that the large turbo multiplies by 2.2 times, it takes 14.7 psi (atmospheric pressure) and makes 17.6 psig (actual pressure 32.3 minus 14.7 atomspheric). (not taking into account adiabatic efficiencies), now the small turbo will see 32.3 psia at it's air inlet (instead of the 14.7 psia), but it think it's only seeing atmospheric pressure, or literally over double the amount that atmospheric would allow.  So we can literally cram over double the volume of air into the same inlet hole size in the small turbo.  So if the small turbo then multiplies the air by only 2.2 times you'll see 71.1psia - 14.7psia = 56.4 psig.   (Note this does not take into account any of the efficiency losses, due to heat, etc., which do come into play, but that requires a much more lengthy discussion.)

So what exactly does this all mean...Most compressor maps for turbos end between a 3.5:1 and 4:1 ratio because the efficiency of the compressor drops beyond that point so dramatically.  Most compressors have their highest efficiency at below 2.5:1 ratio.  Efficiency is the amount of energy which is converted into heat during the compression process.  The higher the efficiency the less heat is made from compressing the air.  Our D-Tech 62mm will run at 78% efficiency below 2.3:1 ratio, which is where it typically will run on twin turbos, running appx. 55 psi boost.  Whereas at 40 psi boost  when ran as a single turbo the ratio is 3.7:1 and the efficiency drops to 70%, or 8% less than at 2.3:1 ratio (which is still very efficient at that level compared to most other turbos but is significantly lower than the twins at a much higher boost level, the stock turbo at this same ratio 5% is less than that).  By 45 psi most single turbos are running off the compressor map at efficiency below 68%.  In other words our Twin Turbo Kit at 55 psi boost is 10% more efficient than any single turbo running at only 40 psi boost.  This translates into more usable, cool air entering the engine.  To show the increase in compressing efficiency, our Twin Turbo Kit at 58 psi of boost produces compressed air temps (before the intercooler) of appx. 375 degrees F, while at 35 psi the stock turbo produces air temps of appx. 465 degrees F.

But don't forget that because of the low ratio required by the small turbo, we can wastegate it much earlier on our Twin Kit vs. as a single.  By allowing the small turbo to wastegate early, the exhaust (drive) pressure goes way down.  Less drive, or back pressure also raises the horsepower while lower exhaust temps (EGT's).

So exhaust pressure drops by 10-15psi, while boost pressure is 55-60psi, this is 157% more air going into the engine, while allowing the exhaust to escape 20-30% easier, then with a single turbo.  On our properly engineered Twin Turbo Kit, all of these details translate into much more horsepower, much lower EGT's, better fuel mileage, a much broader RPM range (quick spool up, with huge high end WOT potential), and overall a much more drivable, high performance truck.

So next time you see a company trying to sell a single turbo, saying that it will compete with our Twin Turbos, ask yourself, if the laws of physics don't allow it, how are they claiming it.

Our customers all say that once they have installed our Twin Turbo Kits, they could never go back to a single turbo.

--Diesel Power Source