Horse Sense:
Heat expansion of the exhaust, not just exhaust flow, drives a turbocharger, thus hotter exhaust spins a turbo faster.
Of all the power adders used in 5.0 racing, the turbo-charger is by far the most mysterious and misunderstood of the bunch. Its name bespeaks power, as the shortened "turbo" reference has been used to emphasize the muscle of everything from lawnmowers to hair dryers and computer processors. Indeed, the word "turbo" is a household name--yet ironically, few 5.0 enthusiasts understand how turbo systems work, and even fewer have taken advantage of their almost-spooky power potential. In recent months, however, more and more attention has been drawn to turbo systems, as the racers who have started using them have, almost without exception, gone faster than ever before. A lot faster.
Not all compressor wheels are created equal. Although these two T-04 compressor wheels are the same diameter, the vane layouts are completely different.
Since turbo systems look like they're going to be the power adder of choice in the near future, we decided it was time we take a more in-depth look at them. For an education on turbos in general, we talked to Rick Head at Turbonetics in Moorpark, California. Turbonetics is one of the leading racing-turbo manufacturers in the country, and its turbos are bolted to some of the fastest door-slammers, including Hurley Blakeney's 6-second Silent Thunder T-bird. Head has vast experience helping Turbonetics' customers select the right turbo for their application, and has even designed many of the company's best-selling models. He does, however, emphatically point out that Turbonetics does not sell turbo kits, so we also contacted some of the leading turbo kit manufacturers to see what's available for your Mustang.
Choosing a Turbo
The four main components of a turbo are the turbine housing, exhaust turbine, compressor wheel (also known as an impeller), and compressor housing. The exhaust turbine and its housing are on the drive side of the turbo and receive the hot exhaust gas as it leaves the engine. The turbine shaft in turn spins the compressor wheel inside of the compressor housing, and the resulting compressed air is delivered to the engine's intake system. The design of these four components, their size, and the combination in which they are used determine how much power a turbo is capable of producing, and where in the engine's power band it will be produced.
The angle of the vanes contribute to the way the turbo performs; a more backswept impeller is more efficient at higher rpm. Compare the relatively vertical vanes of the wheel on the left to the vanes of the wheel on the right.
Historically, choosing the right combination of wheels and housings involved creating a compressor map, which takes into consideration criteria such as airflow, atmospheres of pressure, compressor efficiency, and shaft speed. "We used to sit down and do all the math," Head says. "But now that we are more familiar with the different components and their airflow capabilities, it basically comes down to three questions: engine size, maximum rpm, and the desired power level. That's the most important one. The power tells me which compressor wheel the customer needs. Engine size and rpm tell me which turbine wheel and housing will be necessary."
Selecting the compressor wheel is easy, Head says, because once you know how much power you need, you simply convert the power into airflow. The turbine, on the other hand, is more difficult because there are no turbine maps available to the automotive performance market. "That's where experience comes into play," he says.
Shown here are the stock T-O4 (left) and the larger T-series (i.e. T-70, T-76) compressor wheels.
The bigger, the badder. From left is a stock T-O4 compressor housing, T-series housing, and Big Thumper housing.
Housings are selected based on their area over radius (A/R), which is basically the amount of space inside the housing not occupied by the wheel's diameter. "A/R is a window of operation for the turbo," Head says. The bigger the number, the later in the rpm range the turbo will start to work and the higher it will top out. The lower the number, the sooner it starts to accelerate and the sooner it's going to top out or choke. The compressor housing can be especially important when you get into the higher-flow units, because it has to be able to flow the amount of air that the wheel is capable of moving. You really shouldn't use a wheel that is capable of flowing more air than the housing can handle."
Here is a side-by-side comparison of a standard T-O4 compressor wheel and the giant wheel used in the Turbonetics Big Thumper.
The area over radius (A/R) of an exhaust housing can affect where a turbocharger reaches peak efficiency. These two exhaust housings are of similar size, but note the vast difference in area. Similar to the intake port in a cylinder head, the A/R controls air velocity; so the bigger the area, the more efficient the turbo will be at high rpm. Conversely, a smaller A/R like the housing on the right will produce power at a lower rpm and top out sooner.
Intercoolers: Air-to-Air vs. Liquid-to-Air
Like us, you may have noticed that most centrifugal supercharger setups in Pro 5.0 competition use liquid-to-air intercoolers, while turbo systems typically use air-to-air. Why? "I don't know," Head says. "For an all-out drag-racing or land-speed-type application, liquid-to-air is still the best way to go because you can really bring discharge temperature down with ice water. For a street application, an air-to-air intercooler is simply more practical because you don't have the water tank, pump, heat exchanger, and all the plumbing to contend with."
In any case, Head maintains that a certain amount of pressure drop across an intercooler core is a good thing. "You actually need a minimum of a 1/2-pound pressure drop across the core in order for the intercooler to 'scrub' heat out of the air. Two to three pounds of pressure drop should be the maximum--beyond that, the intercooler should be considered a restriction."
There are two basic styles of turbine housings. On the left is a tangential housing, on the right is an on-center housing. On-center housings are not as efficient as their tangential counterparts, but are often easier to package in some vehicles.
Turbine wheels are selected based on their tip height and exducer diameter, which will determine the turbine wheel's flow capability. The diameter of the wheel, meanwhile, can have an effect on the operating speed of the turbo. Unlike with compressor wheels, there are no maps available on turbine wheels for the automotive aftermarket, so experience is key in selecting the right turbine wheel for a specific application.
Here, a T-04 exhaust housing (left) is compared to the housing on the Turbonetics Big Thumper.
Turbos vs. Superchargers vs. Nitrous Oxide
Naturally, Head is a proponent of turbocharged power, so he is naturally biased toward the beloved hair dryer. However, the facts are the facts, and his comments are well founded.
"A belt-driven supercharger is speed-limited because of its gear box," he says. "Because of this, a supercharger typically uses a much-larger-diameter compressor wheel that is more radial (straight-bladed) and not as efficient (as a turbo compressor wheel). Since the turbo is driven by otherwise-wasted exhaust gas energy, it can run a smaller-diameter, more-back-swept wheel that spins higher rpm and provides better efficiency."
This cutaway of a Turbonetics T-04 reveals the exclusive, ceramic ball bearing used in T-04 and T-3 turbochargers. Originally, these turbos utilized a bronze thrust bearing which, according to Head, has always been a weak link in high- performance, gasoline-burning engines. The ceramic ball bearing is an angular-contact bearing that contains thrust and eliminates thrust failure common with the conventional bronze thrust bearing. A side benefit is that the ceramic ball bearing reduces frictional losses, so a turbo equipped with this bearing will spool up quicker than the same unit fitted with the conventional bearing arrangement.
The standard T-04 (right) is dwarfed by the Turbonetics Big Thumper. The Big Thumper can be had with an 88mm, 91mm, or 100mm compressor wheel--depending on whether you want 1,200, 1,400, or 1,600 hp. Yikes!
By way of comparison, most centrifugal superchargers are limited to 60,000 rpm. Head maintains a turbo is capable of speeds up to 120,000 rpm (depending on size and model), and that the backpressure losses of a turbo are insignificant when compared to the parasitic losses from a supercharger in the same application. How much difference is there? Depending on the size and boost level of the supercharger in question, Head maintains it can take 200-plus horsepower to drive a big centrifugal at 20-plus pounds of boost, while the biggest Turbonetics turbo, the Big Thumper, takes roughly 30-50 hp to drive at its maximum boost potential. This could explain why Junior Ibanez recently went from a centrifugally urged best e.t. of 8.10 to 7.80s with a Thumper on board.
Once the airflow and pressure requirements are known, a compressor map such as this one may be used as a tool to find out if that compressor wheel is going to work, or if another wheel would be a better choice. Head says it's a common misconception that being right in the middle of the map is the ideal spot, but in actuality, this fantasy island can sometimes make the response time lazier; sacrificing a little efficiency may often produce a better all-around power curve. A caveat is that the intercooler will have to work harder to scrub off the temperature differential between efficiency islands. 5.0
Another misconception, Head says , is that turbochargers have a higher discharge temperature than superchargers, a notion that stems from the turbo's exhaust-driven nature. "Typically, a turbo will not have as high a discharge temp because the compressor wheels are more efficient," he says. "When you compress anything it gets hot, but the more efficiently you compress it the less temperature rise you get, and the less the intercooler has to work, bringing more efficiency to the entire system."
So what about the champion of the low-buck racer--nitrous oxide? It's the one power adder that has no parasitic losses--so shouldn't it produce the most power? "The only parasitic losses from nitrous oxide come from your wallet," Head says (only half-kidding). "Nitrous oxide and even superchargers create a violent 'hit' on the engine when compared to a turbocharger. The power curve of a turbo is linear, and because of this, it is much more useable."