Turbocharging Basics 101: Session 4:Pumped Up
An article from Import Racer! By Dino G. Tadokawa


In this final session of Turbocharging Basics 101 we will uncover the brass ring or Golden Fleece of turbocharging—Racing Turbochargers. This subject is only for the hardest of the hardcore import racer, one that many of us dream of mastering, but only a few truly do. We will examine in detail, on a component by component basis, the DNA of a true racing turbocharger. From this, you will hopefully learn the differences between a high-performance sports turbocharger versus that of a true werks racing turbocharger. Most of us will never subject our production-based turbochargers to the extreme duty cycles and pressure ratio requirements demanded by a true racing turbocharger application. But in any event, it’s always good to know what is required or at least know what the major differences are [yeah, and its pretty frickin’ cool—Ed.]. This section applies especially to those serious import racers who are planning to take their racing program to the next level of ultimate performance.

Garrett, KKK, IHI, Holset, Mitsubishi…you recognize these turbocharger manufacturer names and see them plastered all over racing vehicles from Indy to Dakar. Well, we start by covering the features of these true racing turbochargers. Having gotten this far, you should have enough knowledge to properly size and match a turbocharger for a specific engine’s needs. This knowledge should help you meet all of your basic street performance needs. But racing applications have an entirely different set of demands, more stringent and extreme than anything you’d see on the street. Without getting overly technical, let’s start by doing a simple comparison of how a true werks racing turbocharging system varies from a production-based, off-the-shelf turbocharger, component by component.

Center Housing Rotational Assembly—As you know the CHRA is the heart and soul of your turbocharger. In breaking down the internal components we begin by taking a look at the turbocharger’s bearing system. Racing bearing systems by design are much more robust than any off-the-shelf setup. Thrust bearing and journal bearing systems are designed for higher capacity load bearing requirements. This is required in order to compensate for the higher speed and rotational loads typically seen in high pressure ratio applications. In racing turbochargers, which use standard journal bearing systems, higher capacity bearings are utilized to compensate for the increased axial (thrust loads) and radial dynamic shaft motion. Improved oil distribution is also designed into the tight tolerance, full floating bearing design. Thrust bearings designed for racing units also require materials with special properties as well as increased thrust pad area. Recent technological advances have made it possible to run higher boost pressure and faster rotational speed, which has led to greater performance demands on turbochargers. In order to meet these increasing demands, the latest bearing technology incorporates the use of ball bearings. Today, ball bearing technology remains one of the most closely guarded secrets in all of racing. Besides providing improved mechanical efficiency, ball bearing systems are able to withstand higher radial bearing and axial thrust loads. Ball bearing cartridge systems also help to greatly reduce the nemesis effect of “Turbo Lag.” As a result of advances in ball bearing technology, the transient response (time to boost) has been greatly improved. Ball bearing technology has proven itself many times, helping teams win many different types of motorsport competitions. True racers from World Rally to CART Indy Car racing rely on “BB” technology! Ball bearing technology is now even used on some production -based street vehicles as a result of this race-proven performance. Garrett and IHI are actively pursuing this low resistance technology in CART, Le Mans and World Rally Racing. HKS utilizes the same production-based ball bearing cartridge technology in their new lineup of GT Series turbochargers.

Center Housings—These are similar to production-based models, with the exception that some are made from rather exotic materials and cast in lightweight (very lightweight) configurations. For gasoline-based engines, a water-cooled housing can greatly enhance the life of the bearing system especially in high temperature and endurance race scenarios. The main benefit of water cooling is primarily after the engine is shut down [see “The Hard Life of a Racing Turbocharger”]. The thermal transfer properties of water cooling prevent “heat soak back” and oil coking damage that comes from shutting down a hot engine.

Recognize who takes ownership of this titanium based turbocharger?

Compressor Set—Due to different high rotational speeds and pressure ratio requirements, a variety of different compressor wheel and housing designs exist. Both production and performance compressor wheels are typically manufactured from high quality aluminum alloy castings. In addition, special high pressure, isothermal cure processes are used to further enhance the strength of production-based castings. For low boost race applications, a production-based cast compressor wheel will meet most high performance objectives. For high pressure ratio scenarios the story is quite different (and costly!). Full race compressor wheels in these situations are normally CNC machined on a five-axis mill from forged aluminum billets. Of course, the latest high efficiency blade curvature designs are also utilized in these situations. In certain instances, these billet wheels are even electrolysis nickel plated to help resist damage from ingested minor debris, as in CART racing. This plating tends to double the life of the compressor wheel in CART/Indy car racing. But an economical word to the wise: Never run your turbo without an air filter unless you have a lot of money burning a hole in your pocket!

Forged aluminum billets CNC machined on a five-axis mill into full race compressor wheels.

Compressor Housings—A wide array of compressor housing configurations are available depending on engine requirements, fitment and so forth. Different aspect ratios (A/R) and diffuser designs can be matched to accommodate your specific engine’s performance needs. For extreme racing applications, ultra lightweight magnesium castings are used.

A custom five-axis CNC billet compressor wheel and Mar-M turbine wheel—very nice, but very EXPENSIVE.

Turbine Stage—As you learned before, this is the housing where the expended exhaust gases are converted into turbine driving power. Racing turbine wheels are designed and constructed out of exotic high temperature materials. High nickel, ceramic, even cobalt base materials have become the base line choice for racing applications because these wheels need to withstand the grueling effects of high exhaust temperatures. In rotary engines, for example, it’s not uncommon to see exhaust gas temperatures in excess of 2,100 degrees F! In addition, as part of turbine efficiency, low inertia and high flow are the key design parameters in racing turbine wheel design. On a related note, the balance of this rotational group (compressor and turbine wheel as a whole assembly) is critical for the longevity of the unit. Balance plays a major part in the life and durability of the turbocharger because typical racing units may see shaft speed in excess of 180,000 rpm. Excessive shaft motion, due to improper balance, at this speed is detrimental to the life expectancy of the unit.

High speed VSR balancing is required for extreme racing applications.

To improve turbine response, the use of ceramics and other advanced lightweight materials have proven to be very beneficial in assisting the turbine spool up. New technologies related to variable geometry have also contributed to the virtual elimination of “turbo lag.”

Turbine Housing—Designs of turbine housings are similar to that of compressor housings, however their duty requirements are quite the opposite. For cost control reasons, production turbine housing materials are usually made of variations of the standard gray cast iron. Higher corrosion-resistant and more durable high-ductility, ni-resis and inconel materials are utilized for the more extreme temperature applications. Racing units tend to have housings made of stainless steel in order to withstand severe duty and temperature requirements. Many full racing turbine housings are die-cast out of stainless steel for weight savings. However, these housings will not contain a wheel burst and therefore are for racing use only. Other advancements in ceramic and plasma coatings, diffuser designs and even internal extrude honed processes have increased turbine flow, efficiency and overall performance.

A top-secret turbine housing made of “stealth material”—don’t even ask because we can’t tell you.

That pretty much covers the turbocharger component DNA analysis. As a whole, racing turbochargers are subjected to extreme, and at times severe operating conditions. In comparison, a street turbocharger may be required to produce between 7-15 psi, while a modified performance super street application may see maximum spikes of 20 to 25 psi in short burst cycles (as in drag racing). In racing trim there are various rules and regulations that govern the turbocharger size as well as how much boost you can run. For example, in Indy Car racing boost levels are regulated to 7 psi. In endurance racing, 28-30 psi boost pressure is typical. In certain extreme racing applications, boost conditions from 56 psi and up are not uncommon. Can you imagine stuffing 56 psi into your Honda VTEC? Didn’t think so!

Another good comparison to point out is related to the turbocharger’s unit weight. Racers are always concerned with weight savings, however weight savings in turbo talk is very expensive. For example, an Indy Car unit weighs about 15 pounds—and that’s a complete unit! An equivalent sized commercial turbocharger weighs in at about 45 pounds. This is three times as much weight. To achieve this weight savings, magnesium compressor housings, lightweight centers and ultra thin-walled die-cast turbine housings are used in this ensemble, not to mention the titanium-based internals and ancillary sub-level components. Unfortunately, these types of turbochargers are not available to the public and are “for racing purposes only.” But console yourself, because even if they were available to you, you probably couldn’t afford them.

This is the highly sophisticated Garrett TR30R ball bearing racing turbo as used on this year’s Audi Le Mans winning race car. Note the huge 60mm TiAL wastegate which is required to control the boost level.

At this point we must touch upon tunability. For production-based street cars, the turbo system is typically designed for one car, one engine and, of course, stringent emission controls. The engine is usually built for one specific application and tuned accordingly to meet the variable requirements. Therefore, the engine is specifically sized for one turbocharger to meet this broad range of needs. For the enthusiast there are a variety of aftermarket upgrades and supporting accessories to fine tune it to the desired level of performance. In true racing applications, however, the engine designers and the race teams “tune” their racing turbochargers to match the various track and operating conditions. A typical Indy Car team may bring along six different turbine housings to a new track to “best tune to fit” their racing needs. In essence, what they are tuning for depends on the track configuration. Road or street courses require small turbines to help reduce lag coming off the corners, whereas at a super speedway, larger turbines are required to maximize top end power. Even at the highest levels of motorsports, there is always that tradeoff between “performance” and “time to boost response.” Everyone wants to have the feel of instant acceleration, but in reality it’s a given fact of physics; as turbo size increases (which increases rotational inertia and energy required) the more inherent turbo lag develops. That is a tradeoff you must consider when sizing a turbo for a specific application. But keep the faith—with new developments in ball bearing, ceramics, variable geometry, and air bearings, the elimination of turbo lag is just right around the corner.

Special thanks go out to the real turbo engineering guru’s at Garrett Turbo and Mazda (from right to left): Craig Gibbs, Jeff Lotterman and Tod Kaneko. Class dismissed!

And finally, we must address the cost of racing, though you might want to sit down on first. Ultra lightweight components, specially-designed full ball bearing cartridges, exotic high temperature materials, forged billet compressor wheels and micro-fine blueprint assembly specifications, titanium this and that can mean only one thing—DOLLAR$ and a whole lot of them. To approximate just how much this stuff costs the professional racing teams, we use what is known as “Racers’ Math”: Simply add another zero onto the price tag for a comparable street turbocharging system. For example, a top of the line production sports turbo will cost $1,000, while a real racing werks turbocharger will cost $10,000. During the high boost era of Formula One racing, a top team would exploit over a hundred turbochargers per season. That’s more than a million dollars in turbochargers alone! How? Well, if you use Racers’ Math and consider that a pair of turbochargers are often used up in a single ultra-high boost qualifying session, you see just where the money goes. The need for speed isn’t cheap!

As you can see a racing turbocharger costs much, much more than any turbocharger you are probably familiar with. But keep in mind that these demands on werks units are far more extreme than anything you could subject your own performance unit to. The combination of elevated temperatures, high rotational speeds and extreme pressure ratios make for an extremely stressful working environment, and necessitate all of the exotic materials, top secret technology and thousands of man hours. Class dismissed!

The Hard Life of a Racing Turbocharger

Racing turbochargers live a hard life. In general, a well designed and properly matched racing turbocharger is bullet proof, even though they are expected to operate at peak performance, without failure, under extreme heat and abuse. However, there are two situations that are known to damage, or as we call it, “murder,” the racing turbocharger (other than crashes...of course). They are foreign object damage (FOD) failure and heat soak back failure.

Foreign Object Damage Failure—Known in the industry as FOD, foreign object damage occurs in two situations. First consider that the compressor wheel is made from aluminum and has blade profiles which can be thinner than .02-inch at the tip. These compressor wheel blade tips rotate at hundreds of miles per hour, and the impact of even small particles can severely damage the blades. This is the first FOD failure scenario. Eventually the aero performance is hindered as well as an imminent wheel balance related failure.

But FOD failure as a result of turbine wheel ingestion tends to be less of a problem in the motorsports arena. Typically it is the result of a secondary catastrophic engine-related failure (such as piston or valve particles entering the turbine housing) that takes out the turbocharger. But even if your turbo somehow survives, your day is already over at this point. Ingestion of slight particles tend not to hurt the turbo since the turbine wheels typically have thicker and stronger nickel-steel alloy blade construction. Depending on the severity of the FOD in this scenario, the unit may just make it through the race. Inspection of both compressor and turbine wheels (as well as bearing stability) should be part of the normal racing engine maintenance program.

Heat Soak Back Failure—This is the result of rise in temperature a turbocharger bearing system sees after the engine is shut down after hard operating conditions. Both the turbine housing and wheel operate in very high exhaust gas temperature environments. When the engine is stopped, this “heat” flows or dissipates into the cooler bearing system areas. Residual oil cannot survive in these areas under these conditions. As a result the oil will carbonize (or “coke up”). Continuous build up of this carbon residue will eventually block critical oil flow and oil passages, as well as affect critical bearing clearances, thus the turbocharger will eventually fail.

The cure? Cool water. Turbocharger bearing systems that utilize water-cooled center housing sections have been found to be in perfect operating conditions at the end of an entire race season. The water-cooled center section is not for cooling the turbo while it’s running (normal oil cooling does a good job of that) but is intended for cooling the center bearing section after the hot engine is shut down. The coolant in the center housing carries the heat from the hot housing away from the bearing system, thus preventing the detrimental carbonizing effects of heat soak back. For less stringent applications, air-cooled centers are just fine. But for ball bearing applications, you must always run water cooling for long-term durability due to the low oil flow requirements (less bearing oil means less cooling effect). You may notice that many race cars are equipped with air-cooled ball bearing centers. But keep in mind that they replace (or rebuild) their turbochargers after every race, which is quite a costly proposition.

Take Home Message—Just remember, to prevent FOD failure, always run a good air filter. For you street guys, the cost of a slight inlet depression loss totally outweighs the cost of a new turbocharger! As far as preventing heat soak back failure, run a water-cooled center housing design where possible. Those with ball bearing applications must always run a water-cooled center housing system! In any event, always remember to let your turbocharged engine idle down for a few minutes after a hard session at the track. Following these simple rules will ensure that your turbocharger will live a long and happy life.

This turbine wheel shows both foreign object damage (FOD) and “heat soak back” damage. Note the FOD on the turbine wheel blades and discoloration of the wheel and shaft due to heat soak back.