Rear-Mount Turbochargers: Pros and Cons Of This Creative Boost Solution
Turbochargers can be an excellent way to add power to a wide variety of drivetrains, provided you've got the space for not just the turbine, but all of the associated plumbing and cooling that has to go with it.
Unfortunately, some vehicles are simply too cramped under the hood to make a turbo installation easy without major modifications. In other cases, they generate too much heat to be practical or safe in a performance application. For those unwilling to start sawing the firewall or adding outrageous scoops or power bulges to the hood, there's a unique solution that has been adopted by those desperate for boost and willing to take a chance on something a little out-of-the-ordinary: rear-mount turbos.
The Air Back There
Think about it for a second. While the front of your vehicle might be stuffed to the gills, chances are there's more than a little room at the rear - specifically, underneath the chassis, and tucked up behind the back bumper. This is the entire premise behind a rear-mount turbo design, which mounts the snails wherever is convenient but usually as close to the muffler tips outlets as possible to make exhaust routing simpler.
What are the advantages to this type of installation? First, it solves the 'no space in the engine bay' issue. Second, most rear mount turbochargers don't require a dedicated intercooler, because the charge is cooled by the ambient air passing alongside the piping that sends it back to the engine once it's been compressed. Finally, it prevents other parts of your engine—fuel lines, wiring, intake, cooling system - from being exposed to the significant amount of heat generated by a furiously-spinning turbo.
It's Not That Simple
There are some compromises associated with a remote mount turbocharger. Given that a turbo is driven by engine exhaust gases, which spin the turbine and compress the air that is then fed back into the engine intake, the farther the air has to travel in that cycle, the more lag there's going to be between stepping on the accelerator and making usable boost. There's also the cooling of exhaust gases and decrease pulse size once it reaches the turbine's fins, which makes a turbo's operation much less efficient than when installed closer to the header.
Because of this, it can be more difficult to use a rear-mount turbo on a small displacement engine, where the lack of torque can make lag a frustrating issue at lower rpm. There ways to tune around this, but it requires a strong knowledge of how to properly plumb the diameter of the tubing involved, which given the rarity of these installations usually means finding a specialist willing to work out all the kinks. In general, basic kits that offer low boost are simple enough to install and use with high-torque, large-displacement motors, but anything else is going require more than a little expertise.
A few other things complicate a remote-mount turbo installation. Finding room for the turbochargers is only half the battle - you'll also need to make sure there's space to run the tubing as well, and that it doesn't pass too close to suspension components or hang too low underneath the vehicle, where it risks being scraped or damaged. Then there's oiling the turbocharger, which will require an additional pump and line as well as typically a secondary oil reservoir that's nearby to the turbine.
Is It Worth It?
Given that rear mount turbochargers are more complex to install, less efficient than those located closer to the engine, and not usually suited for small-displacement engines, are there any compelling reasons to go this route?
Frankly, the strongest argument for a remote turbocharger is necessity. If you don't have space up front, and absolutely want to run boost, then it's really your only option. That being said, there are also some applications where turbo lag is not such a big deal—such as drag racing, where boost can be built prior to launch - which means you can piece together a decent homemade solution without having to worry too much about tuning and sizing the plumbing for standard street use and even response throughout the entire power band. The latter is particularly true if paired with a large V8.
Whether you choose to go with a pre-fabricated kit, or design your own system using off-the-shelf or junkyard parts, remember that caution is always the better part of turbo experimentation. Start out with conservative boost and maximize fuel delivery when initially tuning your setup, and work up gradually to the power level you have set as a goal, and you'll stand a better chance of avoiding any expensive engine failures at speed.
Looking for wilder turbo installs? Click here for a round-up of the craziest turbocharger setups we've ever seen.