Does the rated boost of a kit have any merit?

It does if, and only if, the conditions required to achieve that boost are defined and accurate. For example:

  • Was the gasoline used commercially available pump gas?
  • Were octane boosters used?
  • Was detonation present?
  • What was intake air temperature?
  • Is this the same boost-pressure setting the buyer will receive?

Considering the large power increases offered by the turbocharger, what keeps the entire structure of the engine from going south?

A proper answer to this question is a complete analysis of the inertial, power, and thermal loads before and after turbo installation. If this is performed, ' the conclusion will be two interesting bits of information:

  • The inertial loads in a modern internal combustion street engine are so large at maximum power that the power component of the total load is of little significance. For example, to induce as much power load into a conrod bearing as the bearing already sees from inertial loads, the actual power of the engine would need to increase approximately 50%.
  • The thermal load in an engine not originally designed for a turbocharger will cause an increase in component and cooling-system temperatures when operating under boost. The components and cooling system can handle the temperature increase for a limited period. This is true for Buicks, Porsches, Saabs, Volvos, Nissans, etc. It is also true for all aftermarket turbo kits. The time limit is subject to many conditions. Experience has led me to believe that the time limit at full boost is on the order of 20 to 25 seconds. This is an operational restriction but not one of any consequence. Consider, for example: How fast will you be traveling if you hold full throttle in a 325 bhp Toyota Supra for twenty seconds? The answer is obviously an impractically high rate of speed.

When should the turbo start producing boost?

In most cases, there are trade-offs between a low boost threshold and maximum power. To bias the turbo size toward low-speed boost capability generally means operating the turbo in a very inefficient flow range at the engine's top end. Conversely, if maximum power is to be achieved, the turbo will usually be so large that no boost will be available until the last half of the rev range. Compromise is obviously necessary. A reasonable balance between low speed response and top-end power is to size the turbo such that it begins producing boost at about 30% of the redline rpm, this varies with the cam profile.

How will the turbocharger affect driveability?

Driveability of fuel-injected engines will remain the same. Driveability of blow-through carbureted engines will remain virtually the same. The starting of carbureted engines will be degraded slightly. Please note that draw-through units will virtually always degrade drive ability and starting somewhat, with cold weather proving the Achilles' heel of a draw-through system.

Will the turbocharger hurt my mileage?

Yes. The turbo, when installed as an aftermarket item on a spark-ignition engine, is not an economizer and cannot be construed as such. There is no engineering basis for making such claims. If you are led into purchasing a turbo under the premise of improving your fuel mileage, be sure to get a written guarantee. When not operating under boost, a turbocharger is a small system restriction. This restriction causes a small loss in volumetric efficiency. Volumetric efficiency and fuel economy are definitely tied together. If your driving habits are about the same as most, your mileage will drop about 10% city and 5% highway. No miracles here.

Will the turbocharger affect engine wear and maintenance?

Certainly the turbo will affect engine wear. Do you really expect to add power and not increase wear? No miracles here either. If you drive vigorously but with some respect for the equipment, you can expect about 90% of normal engine life.

Will the transmission and drivetrain be adversely affected?

Very unlikely. Consider that the drivetrain endures more torque in first gear from the stock engine than almost any turbo can produce in second gear. Occasionally a clutch comes along that won't do the extra duty. Most clutch problems are going to crop up when shifting habits are less than acceptable. Not to worry.

What does it feel like to drive a properly set up turbo car?

A turbo can justifiably be called a torque multiplier: the more boost, the more torque. This situation is analogous to gear ratios. For example, a third gear with a tranny ratio of 1.4 will develop 40% more torque at the rear wheels than a fourth-gear ratio of 1.0. A boost pressure of6 psi will increase torque by about 40% (using an intercooler). Thus you can see that 6 psi boost will produce fourth-gear acceleration virtually equal to a stock automobile's thirdgear capability. Imagine what the proper turbo car will do in second gear! Another reasonable comparison is that a proper turbo car operating at 10 psi boost will do 0-60 in two-thirds the original time; i.e., 6 seconds versus 9 seconds.

From the Turbo FAQ on e30tech.com