The turbo kit we are using...
The turbo kit we are using for this story is the Hellion Heat system. It is designed to fit '79 to '93 Fox-bodied Fords and make more than 500 hp at the wheels with the right tune-up.
Sometimes we have to wonder why anyone is trying to make N/A power anymore. We concede that there are a zillion racing rules to prevent power-adders from dominating, and turbos look kinda complicated. But you'll need to get over it. We realized this after getting hooked on watching those turbo small-block guys on YouTube beat the hell out of Vipers and any sportbike jockey willing to risk the road rash. Forget the big cam and loose converter; you won't need 'em. You don't even have to wonder how to stash a big-block under the hood or where to cut the blower hole. All you need is a turbo or two to make obscene power, and we're going to show you how to get one.
The Compressor
Big or small? On the pressure or cold side of the turbo system is the compressor. As spent air and fuel exits the exhaust port, it spins the exhaust turbine wheel which spins the turbo shaft that is connected to the compressor wheel. The size and pitch of the wheel and the shape of the housing determine where the combination of air flow and boost pressure is most efficient. The trick is to select the compressor size that delivers that efficiency in a usable rev range. A smaller compressor wheel will be more efficient lower in the rpm range but will create more heat at higher engine speeds. It will also restrict the flow at higher rpms. Too large a compressor will cause boost lag and possible compressor surge in the lower rpm range and be the most efficient at higher engine speeds. Since the compressor wheel predicts the horsepower needed from the turbine, it is very important to get the sizes correct. Too small a turbine spools fast but restricts at the top end. Too large a turbine can't deliver enough power to the compressor at the low end.
The key to the Hellion system...
The key to the Hellion system is the Turbonetics Custom 60 turbo with a 61mm F1-65 compressor wheel and a 0.81 A/R ratio. If you want, you can step up to the T66 or T76 compressor for about $595 extra if you want to make more than 600 rwhp.
The pressure ratio and corrected mass airflow are the two numbers you need to evaluate the compressor on a map. Select the turbo with a compressor map that puts the two plotted points between 65 and 70 percent efficiency for a street application. To get the pressure ratio, simply add the amount of boost in psi to standard atmospheric pressure (14.7) and divide it by 14.7. We will use 10 psi because it is nearing the threshold of safety for a nonintercooled pump gas engine. The pressure ratio for a 302-inch engine at 6,000 rpm is 1.68.
Looking at a compressor map, it is possible to make the mistake of simply multiplying the total engine CFM by the pressure ratio to get the corrected mass airflow and connecting the dots. The truth is that the corrected mass airflow number is a result of several complex calculations involving air density, pressure ratio, engine CFM, and even air density at boost. If you do manage to get through the math, you'll note that the final piece of the puzzle is the efficiency of the compressor itself determined by a table.
The shortcut to all this is what Turbonetics engineer Dave Austin calls tribal knowledge. Look at what other guys are doing and see if it works or simply call a reputable turbo company to get some suggestions. Turbonetics, for example, has a matrix of its popular turbo categorized by engine size and horsepower based on years of trial and error. The entire grid is too large to print here but you can access the knowledge with a simple email or call to the tech line. Just be sure to know all the details about your car and your plans for its use.
The key to turbo longevity...
The key to turbo longevity is cooling and lubricating the bearings. Oil that is not changed frequently can clog and turn to carbon, a term called coking the bearings. The Custom 60 has an in-line filter preinstalled to protect the bearings. Cooling the turbo with engine oil is traditional, but you can also buy water-cooled systems.
The Turbine
Picking a turbine involves choosing the wheel that is small enough to respond quickly and large enough to spin the compressor wheel fast enough to produce the desired boost pressure and minimize backpressure. The rule of thumb is to pick the smallest wheel diameter that still allows you to meet your horsepower goals without putting a kink in power. Modern turbos are ultimately tunable with replaceable and clockable turbine housings, so you can fine-tune the system if you miss the mark.
To help you choose a turbine housing to suit your needs, turbo manufacturers rely on a simplified tool called the A/R ratio. The A is for area and the R is for radius. The A/R ratio is the relationship between the center point of the cross-sectional area in the passageway and the radius from the center of the turbine wheel at the inlet to the volute. This is a simple division of A over R. As A gets smaller, air speed of the gas increases, as does its effect on the speed of the turbine wheel. If A gets too small, it will choke and not be able to deliver enough energy to the compressor, and the peak power will suffer. The backpressure on the engine will also get too high, causing back flow into the cylinder when the exhaust valve opens. As A gets larger, it will be able to deliver more energy to the turbine wheel at the expense of speed. The efficiency of the turbo and the design of the turbine wheel also have an effect, but usually it is the A/R and the turbine wheel size that determine the spooling, overall airflow, and pressure that are delivered. As a general rule, an A/R of 1.5 will deliver more power and an A/R of 0.5 will have better low-speed response. According to the matrix, engines between 5.0 and 6.0 liters will like between 0.68 and 0.81 A/R.