Turbo basics represents ground we have covered many times in this magazine. However, from the questions we get and some of the articles about turbos we read, the basics still need to be addressed. A turbo is an exhaust-driven compressor that compresses the intake charge, resulting in a denser mixture with more oxygen molecules compared with an un-compressed mixture. Exhaust gases spin the turbine wheel, which is connected by a common shaft to the compressor wheel. The inducer portion of the compressor wheel draws ambient air into the turbo from the intake tract. Once in the compressor housing, the exducer portion of the wheel compresses the air and moves it out the nozzle. A byproduct of compressing air is heat, which is why intercoolers are an integral part of the turbo system. For more on intercooling, check out the tech piece in the November '99 issue. Only when the compressed air is met with the proper ratio of fuel and effectively ignited will more power be generated. The more fuel an engine burns, the more power it will make.
Turbos cannot be rated by horsepower. There can't be a 230 hp turbo; instead, everything depends on which engine the turbo is paired with. A street Honda would do well to make 230 hp, but place this turbo on a V8 that already makes 345 hp and there is a problem. Manufacturers often say that a given turbo will "support" 500-550 hp. This figure is based on airflow, efficiency and in most cases, takes into account general rules of brake specific fuel consumption.
There is no real way to rate a turbo. The best way would be to measure airflow at peak efficiency. Airflow is measured in cfm and peak efficiency is the compressor wheel speed (rpm) that the turbo is compressing the air with the least amount of heat generation. Even with all this data, there is still a range of flow (called the peak efficiency island on a compressor map) where the changes in efficiency are so slight that no singular number can be affixed to a turbo. Since a compressor map that illustrates the efficiency of a given turbo is a bit over the head of most enthusiasts (due to establishing and understanding the proper pressure ratio), the "supports blank horsepower" statement is a good place to narrow the field.
Sizing is critical. Can you turn up the boost of a turbo and see no power gain? Yes, you can. A "T-2-small" turbo can max out its flow or compress the air so violently that any increase provided by the additional boost is negated by the coinciding increase in heat.
One can also see no real increase of power over stock on a turbo set at 25 psi. If it is too big and does not spool up until, say 8500 rpm, an engine that maxes out a 6500 rpm will see little useable boost. Turbo selection is a compromise-a smaller turbo will spool up quicker than a large turbo, but provide less flow at the top end. A larger turbo will flow more air, but take longer to provide useable boost. So the intended usage of the vehicle and the power curve of the engine play a key roll in sizing. Sizing comes down to two factors: housing size and wheel selection. A housing's A/R ratio is a value based on the area and radius of the housing. The higher the A/R value, the more top-end flow the housing will have. A lower A/R results in a more responsive housing with less top-end. When it comes down to it, wheel size (diameter), trim (contours of the blades) and even the number of blades impact a wheel's efficiency and backpressure characteristics.
The best way to boost your daily commute is to ask the experts. First you need to know the specs of the engine-stock or built-we're talking internals and head work. Know what you want in performance-street or strip-and have an idea of how fuel enrichment will be handled. A turbo specialist will be able to provide you with a few options and the understanding to make an educated selection.
If you own a car that has been figured out like a Supra, Z car, Eclipse, Honda or Turbo Buick, selecting a turbo is much easier. Those that have come before have had the trial by fire and you can now reap the benefits without the risk. For our Project Boneyard Buick, we contacted John Craig of Limit Engineering.
Craig has concocted a complete line-up of turbos designed for use on a variety of Buick applications. From stock to an 8-second, 4.1-liter Stage II engine, Limit can provide the best in boost. With a moderately modified engine with ported heads and a built transmission with a high-stall converter, we outlined our intention to run the car on the street. Responsiveness was key, crazy, top-end flow was not as important. The engine is set up to be smog legal, so we wanted a stock-appearing turbo. Craig said that we described his TA60V turbocharger. Designed for intercooled Turbo Regals, the TA60V incorporates a Garrett T04B compressor side with a 60-1 trim 60mm compressor wheel in a .60 A/R housing. The hot side runs a newer 71mm Garrett turbine wheel that has 10 blades, which is less than comparable to T04 turbine wheels, which have 11 blades. The reduction of blades means there is less backpressure.
Backpressure is bad for three reasons. First, backpressure can hinder turbo spool-up. The second reason is pumping losses-a loss of power due to the engine having to "work" to move or pump exhaust gases through the system. The third is extra heat at the exhaust valves. This heat can damage the valves or the valve seals. The TA60V turbo utilizes a dynamic type oil seal on the compressor side, which causes less drag than a conventional carbon seal and less drag means quicker spool-up. Peak efficiency for this turbo is between 16 and 23 psi; the turbo can support up to 575-600 hp. Craig has it down to where the type of downpipe on the engine is figured into the mix. Since we are running a Terry Houston pipe, Craig honed out the wastegate signal port to 1-inch in order to increase performance. If a stock elbow were to be used, the signal port would have been enlarged to .930 inches. Once ported, Craig contoured the inside of the port for increased flow.
The car's godfather, Lou Czarnota of Lou's Auto Service, played with the boost, but due to an ailing radiator and a hot-running engine, we are leaving boost at 15 psi. When a 130,000-mile engine is rebuilt and making more power, more fuel is being burned and consequently more heat is being generated than before the rebuild.
We wanted to leave boost at 15 psi anyway to illustrate another myth-more boost means more power. Because of sizing and efficiency issues, two turbos running identical boost can create different levels of power. It is a dilemma of pressure vs. volume. When a greater volume of air is introduced to the engine, no matter the psi, more power will be made (as long as there is proper fuel on hand). Since the TA60V will pump more volume, it should make more power than the stock T3 at the same boost level. How much more? What will the car turn in the quarter mile? Will we turn up the boost? See the answers to those exciting questions and more in the next episode of Project Boneyard Buick.
|Turbo ||TA60V |
|Compressor Wheel ||Garrett T04B 60-1 |
|Compressor Housing ||.60 A/R |
|Turbine Wheel ||Garrett 71mm (10 blade) |
|Turbine Housing ||.63 A/R |
|Flow (cu.ft/min) ||*655-946 |
|Peak Efficiency ||*68,000-90,000 wheel rpm |
|Optional Housings ||Compressor .50 A/R, Turbine .82, .85 A/R |
|Price ||$785 |
These numbers represent the peak efficiency island of a compressor map graph. The volume of exhaust gases present (read the size of the engine the turbo is used on) will ultimately determine the actual flow of the turbo.
Since we are running a Terry Houston downpipe, Craig honed the wastegate signal port to 1 inch to increase performance. If a stock elbow were to be used, the signal port would have been enlarged to .930 inches. Once ported, Craig contoured the inside of the port for increased flow.
Boneyard Buick's new TA60V turbo incorporates a Garrett T04B compressor side with a 60-1 trim 60mm compressor wheel in a .60 A/R housing. The hot side runs a newer 71mm Garrett turbine wheel that has 10 blades. This is less than comparable to T04 turbine wheels which have 11 blades.
The intake mounting flange is configured for 1986-'87 Turbo Regals. This design keeps the factory look of the turbo system.
The turbo's integral plunger-style wastegate runs an adjustable shaft. The threaded shaft can be adjusted to add or subtract boost pressure. However, we intend to manipulate boost with the ECU.