Twin Turbo Conversion
- Chassis style not available in turbocharged trim in owner's region (2+2, Slicktop, and Convertible were all Naturally Aspirated in the US market)
- Investment in a car's interior, suspension, body that the owner wishes to retain.
- Sentimental attachment to a car.
A common misconception is that it's cheaper to convert your NA to a TT than it is to buy a TT outright. While this CAN be true (depending on the cost of cars, sources for parts, etc), the commonly accepted notion is that it's much cheaper to either buy a TT from the start, or sell an NA and pay the difference for the TT.
That said, it should also be made clear that you generally cannot convert an NA to a TT. Rather, owners must swap their NA motors for a TT motor and associated parts. The NA motor uses a higher compression ratio (10.5:1 vs 8.5:1), weaker internals, more restrictive heads, and more, which simply makes it unsuitable for forced induction. It can and has been done, but at the end of the day, the TT motor was built for boost and the NA wasn't.
Because of the sheer number and cost of parts that must be acquired for a complete TT conversion, it's common for owners to purchase a front clip from which parts can be used. However, a lot of parts end up being replaced or upgraded (turbos, intercoolers, piping, etc) so this is become less necessary as performance parts becoming cheaper and more readily available.
For a complete list of differences between NAs and TTs from the factory, see the article: NA-TT Differences.
For discussion purposes, the parts needed to convert are...
- Twin Turbo longblock and associated parts (oil pan, turbochargers, clutch & flywheel, etc).
- Oil Cooler
- TT Fuel Pump
- Intake piping, recirculation valves, and intercoolers
- Radiator, and (if desired) A/C condenser and associated piping.
In addition to the raw parts, there are a few slight modifications that must be performed as well:
- Lower Radiator Brackets relocated to accommodate TT radiator (unless installing an upgraded radiator, in which case a Radiator Adapter Bracket can be used).
- Various trim and caps removed (to make way for intercooler piping, oil cooler bracket, etc)
- Unless using a TT transmission, the bellhousing needs to be machined to accommodate the larger flywheel) and the starter needs to be shimmed to place it further outwards.
TT swaps are often performed when an owner acquires a TT parts car (one, for example, which has been wrecked), when an NA motor fails or becomes severely damaged, or when someone has a lot of time and money to blow on a project.
JDM Tyte Motors
Another common misconception in the import car community is that JDM motors create more power than USDM counterparts (this possibly stemming from the Honda scene, where this is often true), and that they are low-mileage, delicately used motors. Buying any used engine runs the risk of getting a lemon, and there is no reason to believe JDM motors are a better choice than a USDM engine. While it's true that JDM motors are often lower mileage than US counterparts, they're rarely as low as sellers claim (40K? get real, it's a 20 year old car).
Perhaps the most telling piece of information about JDM motors involves the government's regulation over maintaining a vehicle. Cars must be regularly inspected, and the inspection informs owners of any problems, cost to repair and maintain, etc. Because owners are taxed on a car's mileage (for emissions reasons) and are required to keep their vehicles safe and road-legal, it becomes costly to own and maintain a car as the miles rack up. The culture in Japan is that a car will likely be replaced within a few years, and therefore maintenance is often sporadic and neglectful. At the very least (and, again, this is true for all used motors), buyers should perform a dry and wet compression test and perform a 120k maintenance job.