BOOST BASICS | TURBOS
by Dan Hankin

It's understandable to be intimidated by something you don't understand, but that's no reason to not dig in and learn something new! We talked to the pros at Millennium Autosports in Norfolk, Virginia to break down the mystery of turbochargers in simple terms. While not every aspect of turbos can be explained simply, understanding the hows and whys of their operation can go a long way towards figuring out the rest.

There are five main parts of a turbocharger (see photo above), the turbine housing and turbine wheel, the compressor housing and compressor wheel, and a connecting shaft. The turbo is mounted to the engine by bolting the turbine exhaust inlet to the exhaust manifold.

Exhaust from the manifold flows into the turbine and spins the turbine wheel which is connected via a shaft to the compressor wheel. The spinning compressor wheel then sucks ambient air into the compressor and forces it into the engine via a sealed tube, maximizing air intake. This is where we get the term "forced induction". We're forcing the engine to ingest more air than it could on its own. With more air in the cylinder, the engine is able to burn more fuel, making more power. An experienced tuner can alter fuel intake and timing to extract the most power and efficiency from a turbocharged engine.

Turbo Theory:
Many factors go into deciding the right turbo system for a vehicle. How much power you expect the engine to make, how much boost the engine can safely handle, where in the powerband you want the power to come on, and how big the engine is are all important questions to answer prior to selecting a turbo.A turbo that's too small for your application will spool fast, but will run out of boost early in the powerband. Too large a turbo will suffer from 'turbo lag', taking a long time to spool and possibly never reaching boost.

The main goal in putting together a turbo system is in selecting a turbine that will spool as quickly as possible and a compressor that will generate the highest pressure quickest. In order for this to happen, the engine must produce enough exhaust to keep the turbine full to generate enough power to keep the compressor turning at its optimum level. Turbo lag happens when there is not enough exhaust to spool the turbine and power the compressor. When the compressor is not doing its job, the vehicle is running naturally aspirated - as it would if the turbo were not there.

Another well known fact of making horsepower is that a cooler intake air temperature will yield more power. A hot intake charge, particularly in a turbocharged vehicle, can also cause detonation. Because of this, many people install intercoolers in their turbo systems. An intercooler is a heat exchanger placed in the flow of air into the front of the car, and plumbed between the turbo and the throttle body or carb. Compressed air from the turbo compressor flows down into the intercooler
and transfers heat from the intake air into the fins of the intercooler where it is then dissipated. The cooled intake charge is then sent to the throttle body or carb.A turbocharged vehicle should also run high octane gas to reduce the risk of detonation.

Turbo Vocabulary:
Blow Off Valve (BOV): A valve designed to reduce turbo pressure during deceleration or gear changes by releasing intake air into the atmosphere.

Bypass Valve: Serves the same purpose as a BOV, but releases air back into the intake system rather than the atmosphere.

Boost: The pressure a turbocharger creates in the intake manifold, measured in psi.

Boost Threshold: The lowest rpm where boost can be felt.

Cartridge: The center section of the turbo which houses the bearings.

Compressor: The half of a turbo that is powered by the turbine and forces compressed air into the engine.

Hybrid Turbo: A turbocharger mating a turbine and compressor of two different turbo units. The most common hybrid is the T3/T4, which utilizes the smaller turbine of a T3 with a larger T4 compressor to create a turbo that spools quickly and creates good boost.

Intercooler: A heat exchanger whose purpose is to cool the intake air charge before it reaches the engine.

Spool: The act of a turbo spinning up to make boost.

Turbine: The half of a turbo that is powered by exhaust and spins the compressor.

Turbo Lag: The time that passes between stepping on the accelerator and the compressor making boost.

Wastegate: A valve that controls the level of boost by allowing exhaust to bypass the turbine.

 

© Copyright 2006, Octane Publishing, Inc. All Rights Reserved.