Moving to the rear of the SpeedSource Mazda and it's clear that there is a lot going under the rear body work. At first glance you will probably notice a couple of heat exchangers, a turbo or two, some complex looking suspension bits, and much more. Let's zoom in a bit and take a closer look starting with my personal favorite part, the turbo system.
When looking at this shot of the two turbochargers it may not be immediately clear that this isn't a normal parallel-flow twin turbo system. The detail seen here that gives it away is that the turbine outlet of the upper turbo feeds directly into the exhaust inlet of the lower turbo. Generally, a compound turbo system like this utilizes two different sized turbos. The larger turbo is the low-pressure turbo that receives the fresh intake air which is then compressed and fed to the smaller “high-pressure” turbo before entering the engine. The reason for the differentially sized turbos is that they each operate at different pressure ratios even though they are flowing the same amount of air.
Now take a look at the induction portion of the turbo system where some imagination is probably required since many connections are buried. Basically, the fresh intake air flows from the airbox, into the low-pressure (bottom) turbo, through the right-hand intercooler, into the high-pressure (upper) turbo, over the engine, through the left-hand intercooler, and into the intake manifold. Like I said…pretty cool.
The low-pressure turbo sits extremely low just off the rear floor plate. This turbo is responsible for the initial compression of intake air and final expansion of exhaust gases. The other turbo, entire engine, and both intercoolers operate between the compressor and turbine housings of this turbocharger.
This shot of the high-pressure turbo better shows how the right-hand intercooler feeds directly into the inlet of the compressor housing. On the exhaust side you can see how gases from the first turbo combine with the wastegated exhaust flow from the wastegate located on the engine's exhaust manifold before heading back into the low-pressure turbo. Also check out the extensive use of thermal sleeving shown here. In endurance racing reliability is extremely important and one of the greatest threats to reliability is the engine's own heat generation.