If any component can be truly described as the “heart” of a turbo system, then the APS Intercooled Twin Turbo LS1/LS2 system truly has the biggest heart of all!! Or, to be precise, TWO hearts!!
With the V configuration of the LS1/LS2 engine (two banks of 4 cylinders each), each bank drives its own turbocharger for truly balanced engine operation. The APS twin turbocharger approach completely solves the problems associated with single turbocharger system configurations that route exhaust gasses from one bank of cylinders, all the way across the engine bay to finally join up and drive the single turbocharger. The LS1/LS2 with a single turbocharger system configuration experiences massive discrepancies in exhaust back pressure, cylinder pressures and temperatures from one bank to the other and greatly compromises turbocharger response.
On the other hand, the APS intercooled twin turbocharger system is widely regarded by true LS1/LS2 performance enthusiasts and performance professionals alike as the ultimate in turbocharger systems for the LS series engine.
Twin, state of the art and liquid cooled 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 dynamics of the vehicle.
The highly specialized turbochargers are based upon custom Mitsubishi gasoline spec turbochargers with an Inconel 713C turbine wheel, liquid cooled bearing housing (50 lbs per minute mass air flow @ 18 PSI), APS single entry .84 A/R turbine housings with external waste gate port/provision and APS designed and manufactured .65 A/R compressor housing.
But engineering excellence doesn't just stop with the turbochargers alone.
High engine bay temperatures have always been the Achilles heel of forced induction engine performance. With each turbocharger tucked up under the chassis, heat is channelled away from the engine compartment for high engine durability and consistently high engine performance. With the APS stealth Twin Turbo configuration, under bonnet air temperatures are a fraction of traditional turbocharger designs. Let's face it, nobody likes to wait for extended cool down periods, when you can simply turn around and line up for another screaming pass down the quarter mile track.
The turbochargers are positioned behind the steering rack with the turbine side facing rearwards to ensure normal operation of the steering rack components and provide maximum protection of the many ancillary components both inside and under the engine bay.
A highly effective press formed protective tray continues on from the standard front tray and delivers highly effective protection. This tray smooths out the turbulent underbody air flow and delivers excellent splash protection.
Each liquid cooled turbocharger is directly coupled to the custom APS High Energy exhaust manifolds, cast in super tough high silicon/moly Iron for maximum durability and precision NC machined for leak free exhaust operation. In addition, this superior exhaust manifolding design transfers the greatest amount of total energy to drive each turbocharger and delivers instant engine response (even in part throttle openings) - for the most awesome ride of your life!
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 liquid 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.
Single or Twin Turbochargers on a V Configuration Engine
Whilst turbocharging can be a complex process, there is often a good deal of confusion in the market place regarding the benefits of single versus twin turbochargers - due mainly to the fact that some who dabble in turbocharging do not understand the issues involved.
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 "V8" configuration engine such as LS1 has 4 cylinders on one bank and another 4 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.
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.
There are 3 different scenarios to consider:
- To achieve similar low to mid RPM power and turbocharger response as the APS Twin Turbochargers one would need to specify a single turbo of around 3/4 the size of the total turbocharger capacity of the twin configuration.
- To achieve the same outright horsepower as the APS Twin Turbochargers, one would need to specify a single large turbocharger of equal (or slightly higher) capacity - which is a massive turbocharger for the LS1 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. This turbocharger would have an operational range starting relatively high in the RPM range (no useable power to speak of below that point). In this case, the engine would need to turn out to engine speeds well above the standard RPM limit to have a worthwhile power band. This single turbocharger may be viable in a competition engine which spends little time at low to mid 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 high up in the RPM range.
But getting back to production specification - a single turbocharger (at around 3/4 capacity of the twin turbochargers) - its power curve will always be below that of the twin turbochargers 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 LS1 engine will always have a power ceiling of around 3/4 flywheel horsepower of that of the APS Twin Turbo system.
Regardless of the single turbocharger size, the real issue pertaining to high horsepower on a single turbo conversion for the LS1 is the very limited space available to package an exhaust downpipe capable of producing high horsepower. This is where we see the real limitation of the single turbo design for the LS1 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 low horsepower due to a restrictive exhaust down pipe (that necks down to around 2.5"), 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 - who quickly discounted the single turbocharger approach as being fundamentally flawed in terms of packaging a system that produces high outright power and strong low to mid RPM response.
No doubt the twin turbocharger approach is the optimum configuration in terms of overall engine performance, and the slightly higher cost of a total solution twin turbocharger approach (complete with sophisticated fuel system) is a small price to pay for high horsepower, excellent engine response and street legal motoring.