The factory turbo diesels (1HD-T and 1HD-FT) respond well to a well engineered intercooler. On the 1HD-FT for example, an intercooler and better exhaust may increase power from 125KW to over 160KW and torque from 380Nm to over 470Nm (See also our dyno runs during a visit to Safari 4x4).
The idea behind an intercooler is to take the intake air which has been heated through the pressurization process of the turbocharger and cool it before it enters the inlet manifold.
Cooling is achieved via an additional radiator. Hot air from the turbocharger is routed through the radiator and then passed back to the inlet manifold resulting in a significant drop in inlet charge temperature. With high ambient temperatures, it is not uncommon to see inlet temperatures above 100 deg C. A well designed intercooler will drop this temperature to 50 - 60 deg C. The added benefit is less thermal stress on the motor.
As air is cooled however, the pressure drops proportionately (PV=nRT). As a consequence, the total boost upstream of the intercooler needs to be lifted by adjusting the turbo waste gate. The idea is to achieve similar to stock boost at the inlet manifold. If this isn't attended to, the boost compensator will sense a lower boost and not inject enough fuel to take advantage of the additional air available for combustion.
Exhaust is another area which will give a performance and driveability boost. A larger diameter exhaust (strictly speaking, free flowing exhaust with less back pressure) will create a greater pressure gradient across the exhaust side of the turbo, resulting in the capacity to spin up faster. This results in lower turbo lag and its benefit is clearly evident around town. There may however be exhaust noise issues with a free flowing exhaust.
There are a number of intercooler kits available for 80s with undoubtedly the best engineered kit from Safari 4x4. These guys have created an intercooler kit which is as close as possible to that which would come from the factory (if Toyota ever made one). Quality cast alloy components and kevlar tubing give maximum reliability and longevity. I have seen a number of other kits which do not make the grade and rely on off the shelf tubing and very little in the way of dedicated components. Safari also make a 3 inch exhaust to compliment the intercooler installation.
There is a good degree of interest in intercoolers amongst us in 80 Series Cruiser Owners OnLine and we're eagerly awaiting more of us to take the plunge. The Safari kit is not an inexpensive option and this fact alone has swung a few of our city dwelling members to opt for a less expensive option.