I've always said twin turbos is a much more elegant solution than a single turbo, and with some of the members going with a single turbo setup at the moment, I though this article may create some discussion.
Single or Twin Turbochargers on a V Configuration Engine
It is no secret that APS is developing a single turbocharger system for the Nissan 350Z, however this new offering may create confusion in the market place regarding the benefits of single versus twin turbochargers. The following discussion sets out to explain the strengths of each approach so that APS customers can make an informed decision as to which application is appropriate for their requirements.
Turbochargers demand a good deal of high energy exhaust gas to drive the turbocharger turbine. Exhaust gas energy is a function of the mass flow rate of exhaust gas, gas temperature and velocity. The higher each of these exhaust gas parameters, the greater the energy available to spin the turbines. This means that the exhaust manifold design and the proximity of the turbocharger to the exhaust ports is critical in the overall performance of the turbocharger.
A "V6" configuration engine such as that found on the 350Z has 3 cylinders on one bank and another 3 on the other bank. This means that for the optimum turbocharger operation in terms of turbocharger response and resultant engine power over the entire RPM range, a turbocharger must be located in close proximity to each bank of cylinders - ie twin turbochargers.
In a twin turbocharger configuration, each turbocharger is located close to the respective cylinder bank for the optimum exhaust gas energy transfer to each turbocharger
A single turbocharger configuration on the other hand necessitates the exhaust gasses from each bank travel a longer distance than that of twin turbochargers located at each bank. The total distance travelled is determined by the placement of the single turbocharger but in short, exhaust gasses from one bank must travel across the width of the engine bay and merge with the gasses from the other bank before finally entering the turbocharger. This has a negative impact on the total exhaust gas energy available to drive the single turbine.
No doubt the twin turbocharger approach is the optimum configuration in terms of overall engine performance, however there are cost advantages to consider with the single turbocharger approach.
To offset the affect of lower exhaust gas energy available to drive the single turbine, the size of the single turbocharger must be reduced when compared to the total turbocharger capacity of twin turbochargers in order to achieve similar low to mid RPM engine performance to that of a twin turbocharger configuration.
Whilst the above discussion applies equally to any modern "V" configuration engine, when dealing specifically with the 350Z engine, we can now discuss turbocharger sizes that are meaningful and applicable to this engine.
The APS twin turbos have approximately 80 lbs per minute total air flow (resulting in 800 flywheel horsepower) where as a single turbo which would produce a reasonable low to mid range power curve would be limited to around 60 lb per minute mass air flow sized turbo (resulting in 600 flywheel horsepower).
There are 3 different scenarios to consider:
To achieve similar low to mid RPM power and turbocharger response as the APS Twin Turbochargers (80 lb per minute = 800 hp) one would need to specify a single turbo of around 60 lb per minute = 600 crankshaft maximum Horsepower.
To achieve the same outright horsepower as the APS Twin Turbochargers, one would need to specify a single 80 lb per minute turbocharger - which is a massive turbocharger for a 3.5 L engine. The down side is that the low to mid RPM response would be greatly compromised.
To achieve higher horsepower than the APS Twin Turbochargers, one would need to specify an even larger single turbocharger - say around 90 lb per minute mass air flow. This turbocharger would have an operational range starting at 5,000 RPM (no useable power to speak of below 5,000 RPM). In this case, the engine would need to turn out to over 9,000 RPM to have a worthwhile power band. This single turbocharger may be viable in a competition engine which spends little time below 5,000 RPM, but be unpleasant on the road in most driving conditions.
There will be a crossover point on the power curve if the single turbocharger is significantly larger than the twin turbochargers - and only in case 3 (where the single turbocharger is larger in total air mass flow rate than the twin turbochargers). This cross over point will be at some point around 6500 RPM (best estimate only).
But getting back to production specification - a single 60 lb per minute single ball bearing turbocharger (600 flywheel horsepower turbo) - its power curve will always be below that of the twin ball bearing turbochargers (800 flywheel horsepower total) at the same boost level. It's virtually impossible to achieve the same low to mid range power and turbocharger response from a large single turbocharger in a V configuration engine.
If the single turbocharger is matched to produce strong low to mid range performance (which would be the wise choice) then obviously the turbocharger specification will need to be precisely matched to the engine capacity. Bottom line, a large single turbocharger matched for strong low to mid range performance on the Z V6 engine will always have a power ceiling of around 600 flywheel horsepower (around 500 wheel horsepower).
Regardless of the single turbocharger size, the real issue pertaining to high horsepower on a single turbo conversion for the Z is the very limited space available to package an exhaust downpipe capable of producing over 500 wheel horsepower. This is where we see the real limitation of the single turbo design for the Z in comparison to the twin turbo approach, unless you're prepared to cut the body sheet metal and make some fairly radical mods.
It's all very well to have an large single turbo, but when it's limited to around 500 horsepower due to a restrictive exhaust down pipe, you'll never see the real potential of the large single turbocharger.
Hopefully this helps to put the single turbo in perspective - and to give one an idea of the challenges that are presented to APS as turbocharger system design specialist.
In our view, a well designed single intercooled turbo system with a well matched turbo would be a very streetable package on the V6 350Z engine up to around 600 flywheel horsepower. This is a great option for the 350Z enthusiast who desires engine performance that is superior to the lower cost supercharger options - but at a similar price point.
That said, the twin turbochargers utilized in the APS Intercooled Twin Turbo system deliver superior low to mid RPM engine power but with a higher power potential of up to 800 flywheel horsepower (rather than 600 flywheel horsepower of the single turbocharger configuration). This of course comes at a higher initial purchase cost.
APS is committed to delivering either option to the 350Z performance enthusiast.
http://www.airpowersystems.com.au/350z/turbo/turbo.htm
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". Thats my opinion anyway. 
Originally Posted by TTV6VR4
, I swear that car was haunted. 
