If any component can be truly described as the “heart” of a turbo system, then the APS 350Z system truly has the biggest heart of all!! Or, should that be TWO hearts?!!
Twin, state of the art and water cooled Garrett twin ball bearing turbochargers deliver ballistic power with bullet proof turbocharger reliability - and with a custom APS aerodynamic configuration, that ballistic power is delivered in an extreme efficiency and super compact package that is ideally suited to the 350Z vehicle dynamics. In fact, with a built engine, how about 800 hp air flow capacity at just 16 psi of turbocharger boost pressure!
Given that the engine with stock heads/cams will produce around 550hp flywheel at this boost, it can be seen that significant headroom remains in the available turbocharger capacity.
Or to put it another way, just add the APS Twin Turbo System for an instant 150 hp adrenalin shot at only 8 psi of boost pressure - obviously the turbochargers are extremely efficient and under-stressed at this power level!
Left hand side turbochrger shown above
Right hand side turbocharger shown above
But engineering excellence doesn't just stop with the turbochargers alone. Along with the APS cast Ni Resist exhaust manifolds that supply the maximum amount of exhaust gas energy to each turbocharger, APS cast Ni Resist High Flow Turbine Outlets scavenge exhaust gasses and ensure the highest possible energy transfer from each turbocharger. In addition, with the optimum internal outlet configuration, the APS High Flow Turbine Outlets deliver unsurpassed turbocharger wastegate performance for rock steady boost pressure.
Why Twin Ball Bearing?
Demands for improving acceleration response and for the reduction of so-called turbo lag are popular amongst performance enthusiasts who wish to take advantage of the enormous gains in power and torque delivered by turbochargers. In addition, bullet-proof reliability is required particularly at high turbocharger boost pressure levels as well as at extreme exhaust gas temperatures commonly found in high performance turbocharged engines.
In order to achieve crisp turbocharger response, a number of advances in turbocharger design have been utilized over the past decade. Primarily through the use of modern metals/ceramics in order to reduce the mass of the rotating assembly. However, significant gains have been made by reducing the friction of the rotating assembly - and this has meant a departure from traditional turbocharger designs.
Traditional turbocharger design employs a conventional plain bearing that runs on a film of oil. This is known as a floating metal bush.
The diagram above shows the turbocharger main shaft supported by floating metal bushes. Oil is fed through the bushes and forms a cushioning layer between the turbocharger shaft and the supporting bush. The shaft relies on a constant supply of fresh, clean oil over a very wide contact area in order to maintain sufficient clearance from the bush itself. A similar approach is used to support the turbocharger main shaft from thrust loads as well.
Whilst floating metal designs have served us well in the past, the frictional forces are relatively high. This results in sluggish turbocharger response and can be somewhat fragile in nature under extreme operating conditions.
Nissan attacked this very issue some 15 years ago on the GTR Skyline by developing a turbocharger bearing system that forms the basis of the true high performance modern turbocharger.
By utilizing robust ball bearings at either side of the turbocharger main shaft, this did away with the floating metal and thrust bushes.
|APS turbocharger rotating group above is a true twin ball bearing unit that not only delivers huge power and torque, but is also extremely robust and incredibly compact in size.
As seen in the diagram above, the turbocharger shaft is supported by two ball bearing assemblies. These again are fed with engine oil, but no longer rely on a thin film of oil over a wide area to support the turbocharger shaft.
The result is an outstanding reduction of frictional torque on the rotating turbocharger assembly in contrast to the old fashioned floating metal bushes. The improvement in turbocharger response, particularly in the lower to mid turbocharger speed range is phenomenal.
The graph above shows frictional torque versus turbocharger speed of both old fashioned designs and modern ball bearing turbochargers. Clearly evident are the improvements with ball bearing turbochargers - especially at the low speed range of under 60,000 RPM where friction losses are reduced by 40% to 50%. This translates directly into a quantum leap in turbocharger response.
To show this in another way, the above chart demonstrates the mechanical efficiency advantage of the dual ball bearing design compared to journal (floating metal) and hybrid ball bearing turbochargers. The improved mechanical efficiency of the dual ball bearing design results in exceptionally crisp and strong throttle response over that of lesser turbocharger designs. This produces an improved response that can be converted to quicker 0-60 mph times.
Reduced Oil Flow
The dual ball bearing design reduces the required amount of oil to provide excellent lubrication. This lower volume reduces the lubrication load upon the entire engine lubrication system and allows more oil flow to vital engine components - resulting in improved engine durability.
The oil flow through the turbocharger is contolled precisely through an innovative oil flow control module that also acts as a highly effective debris separator in order to keep potentially harmful foreign particles from entering the turbocharger bearing assembly.
Improved Rotordynamics and Durability
The ball bearing cartridge delivers exceptional damping and control over shaft motion, allowing enhanced reliability for both everyday and extreme driving conditions. In addition, the opposed angular contact bearing cartridge eliminates the need for the thrust bearing, commonly a weak link in the turbo bearing system.
And best of all for those who wish to push the limits, ball bearing design turbochargers provide significantly higher robustness by better supporting the rotating turbocharger assembly, as well as better spreading thrust loads over old fashioned methods.
Whilst turbochargers began to be applied to passenger cars in the late 1970's in response to the energy crisis, the first generation passenger car turbochargers were derived directly from commercial diesel engines. Engine oil was used to provide both lubrication and cooling and whilst this was an effective compromise between cost, durability and performance, in high engine performance applications durability suffered through fouling of the turbocharger bearings through high turbine and bearing temperatures.
By encasing the turbocharger bearings in intricate water passages, engine coolant is used to significantly reduce turbocharger bearing temperatures in order to eliminate the coking and lacquering issues that fouled old fashioned turbocharger bearings. Non water cooled turbochargers have no place in a high performance gasoline engine application and should be avoided at all costs.
The graph above shows the turbocharger bearing temperature leading up to engine shutdown and for 20 minutes following shutdown. The temperature is displayed relative to the coking threshold of high quality mineral based oil.
As is clearly evident, the old fashioned non water cooled turbocharger operates above the coking threshold when under high load and experiences a very high temperature increase through heat soak immediately after engine shutdown. The APS water cooled turbocharger on the other hand remains cooler than the coking threshold at all times and the bearing temperature increase through heat soak immediately after shutdown is reduced drastically.
|By specifying the latest in turbocharger designs that incorporate both water cooling and true twin ball bearing designs, the APS turbochargers deliver bullet-proof reliability and durability along with exceptional power levels and unprecedented no-lag turbocharger response.
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.
|Click Here for a discussion on how one turbocharger can deliver greater engine horsepower than another turbocharger at the same boost pressure.
|Click here for a discussion on turbocharger boost control and the affect a restrictive exhaust has on turbocharger boost pressure.