By: Haitham Alhumsi
The first time I ever heard of twin charging (using both a turbocharger and a supercharger on the same motor) was probably back in year 2000. At that time I was very interested in performance for the Toyota Celica and naturally I also read a lot about its sister cars (that shared some of the engines) such as the Camry and the MR2.
One of the most interesting aftermarket parts I ran across at the time was the HKS turbo kit for the 4AGZE powered 1st generation mr2. The 4agze (for those that are not familiar with Toyota engines) is a peppy 170 horsepower 1.6 liter engine powered by the Toyota SC-12 roots type supercharger. On this car Toyota used an electromagnetically clutched supercharger that could be disabled during low power requirements such as cruising, and engaged when the user demands it.
One of the most important parts of the HKS kit is the bypass valve. This valve was used to direct air from the supercharger to the engine at lower rpm/flow points. Once the rpm’s rise, and the engine starts to demand more air, and the turbocharger is fully spooled, the valve switches over gradually till the turbocharger alone is feeding the engine while the supercharger is completely bypassed.
The theory behind this kind of system is to use a small positive displacement (roots style) supercharger. Supercharger performance efficiency is typically its highest at lower engine and supercharger rpms (for example from idle to 4000 rpms). Above 4000 rpms the supercharger’s performance and efficiency starts to drop, the horsepower required to drive it starts to rise exponentially, and the air temperature coming out of the supercharger starts to rise dramatically limiting performance.
On the other hand, using a generously sized turbocharger will allow us to feed the engine efficiently with cooler air (than that from an overworked supercharger) and maintain high rpm performance. The problem with using a larger turbocharger is that a generously sized turbocharger typically doesn’t spool before 3000 to 4000 rpms giving us a limited power band and thus providing no performance boost at lower rpms.
The idea of twin charging is to use both a supercharger and a turbocharger to have each one do what it does best, have the supercharger boost the motor for low end torque, and as it runs out of steam, the turbocharger comes online to carry us through to redline.
There are three aspects to these types of systems that make them prohibitive to most tuners:
- Cost and complexity: Having a complete supercharger system as well as a complete turbocharger system on the same time is a lot of money to spend and a lot of parts to deal with and diagnose in case something goes wrong.
- The bypass valve used to bypass the supercharger (and yet hold in all the air pressure coming from the turbocharger) as well as being able to control this valve electrically or mechanically requires a custom made one off valve that isn’t quite available off the shelf.
- Since we are using two different types of chargers with two different efficiency maps, it can get very complicated to figure out how to tune the motor (especially with much simpler fuel injection systems that were used at the time) because the air density can vary dramatically at the same rpm point and pressure level depending on which charger is feeding air to the motor and at what proportion. This is also where the HKS turbo kit for the 4agze was at its weakest, namely at smoothing the transition point fueling between the supercharger to turbocharger switchover.
However, what is interesting to me, is that even with the advent of more efficient superchargers (such as the 4th generation Eaton chargers with coated rotors, higher tolerances, and lower friction drives capable of extending their working rpm range from 12000 rpms up to 16000 rpms), and with the availability of faster spooling turbochargers such as dual ball bearing turbochargers, or VTN (variable turbine nozzle) turbochargers that some people still attempt to turn twin-charger theory into practice.
Here are three examples:
Alta’s Turbo System for the 2002 to 2006 Mini Cooper S
“The Alta Mini Performance Twin-Charged Turbo kit for the Mini Cooper S features a Garrett GT3071R turbo capable of producing up to 400 horsepower! “
“Alta includes a 5% larger supercharger pulley in the kit to reduce the supercharger boost output. By pushing more boost through the turbo and less through the supercharger, the kit increases in efficiency and produces more power reliably.”
As I mentioned earlier, one the main things to understand about these systems is how the different chargers have different efficiency ranges, and based on that, require precise tuning especially at higher boost levels. But unlike 10 years ago, the technology is now available where you could very easily purchase install and tune from scratch a standalone engine management system within a week.
“A fuel computer such as the Turbo XS D-Tec or the Apexi SAFC are both simple options for fine-tuning your fueling. For those looking to take things further, Hydra EMS and Alta have developed a Standalone Engine Management system designed for use with this kit.”
The Twincharged 350z
“An ORC supercharger takes care of the low rpm boosting while a large Blitz/KKK K5 turbine comes into play at higher revolutions. The result? Power all the time with an impressively immediate throttle control. The engine remains completely standard, but even the low boost setting that is currently being used now is enough to develop an impressive 550 PS and a very full torque curve that peaks at 60 kgm.”
With these kinds of systems, the driving feel and engagement from the noise of the engine by is something that can’t be explained or debated over the internet like the ongoing debates of ‘fast spooling turbocharger vs twin-charger vs highflow ported superchagers’:
“The immediate acceleration and lack of lag are what first surprises; there is no trail off of low-down torque or high-end power, and the boosted V6 seems to pull all the time in any gear. It’s almost electric in character. There are all sort of noises being made: supercharger whine, large turbo spool up, an exquisite exhaust note and external wastegate chatter when you lift off the throttle.”
Ford Mustang Turbo Kit – The Hell Raiser !
Like I stated earlier, one of the complexities of twin-charging is sourcing a reliable bypass valve and controlling it for a smooth transition between supercharged and turbocharged power delivery.
One solution to this problem is sequential-charging where the supercharger outlet feeds the inlet of the turbocharger or vice versa. The advantage of this type of system is that low boost settings (for example a boost setting of 7psi for the supercharger which is a pressure ratio of 1.5) and a boost setting of 7psi for the turbocharger waste gate which again is a pressure ratio of 1.5) results in more than 14psi as the resultant pressure ratio is 1.5*1.5 = 2.25 which is around 18psi so that neither the supercharger nor the turbocharger has to carry the full load of boosting the engine up to a pressure ratio of 2.25. For the mustang for example, combining a factory set 6.5psi of superchager boost with turbochargers set for 13psi we get a combined 24.4psi of boost.
The disadvantage of a this kind of a system is that if the turbocharger has outlet temperatures 60 degrees above ambient, and the supercharger has outlet temperatures 80 degrees above ambient, then the combination of two chargers in series (rather than having them in parallel with a switchover bypass valve) results in that the air entering the engine can be as high as 140* above ambient temperatures which is horrible for performance unless you have extremely high octane gasoline or a really low compression ratio boost friendly motor.
In addition to using TWO intercoolers to cool the charge down, the hell raiser kit also uses water injection to further reduce the air temperatures and increase the effective octane rating of the motor’s fueling:
Find the complete articles here:
Alta Turbocharger Kit for the Mini Cooper (discontinued)
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