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BMW Power The debates between the relative merits of turbocharging and supercharging probably goes back to about the time hot rodders started putting them in cars and racing benches. Turbocharging is a form of supercharging in which the engine's exhaust gases are used to drive a turbine, which shares a shaft with a centrifugal compressor. As far as this writer is aware, the first successful use of turbocharging--then called turbo-supercharging--was on the P-38Lightening fighter plane, and was made possible by some of the same advances in metallurgy that were begining to make turbojet engines a reality. There were experiments in the 60's including Indy cars, a factory turbocharged Chevrolet Corvair, and the early work of Bob Keller, now president of Turbonetics, Inc. It quickly became clear that large-displacement turbo engines could easily make far more power than any production based driveline could withstand. What really brought turbocharging to the forefront of automotive technology, however, was the mighty Porsche 917, followed by BMW taking the lead applying the technology to F1 in the early engines. It wasn't long before seemingly every automaker had at least one turbocharged model for sale. Some became legendary, others were avoided in droves, but in any case, relatively few survived today. The technology for crankshaft-driven superchargers has been around longer than the automobile. Roots-type blowers with massive wooden rotors were used to ventilate mineshafts, and other types of compressors have similarly early "roots". It did not take automotive engineers pioneers long to begin applying the technology, either. Blower Bentleys are still an impressive spectacle for those fortunate enough to see them in operation. Hot rodders began using blowers stripped from GMC two-stroke diesels in what is now the infancy of the sport, and similar hardware can be bought new off the shelf today. In the 1980's the Eaton company developed a new configuration of the Roots-type blower, and has recently succeeded in having it incorporated in an expanding list of OE applications., such as Mercedes-Benz 2.3 liter four. So if the OE market is any indication, it might appear that superchargers are gaining the upper hand at present. Let us examine the relative merits of the two systems. Turbochargers typically have some efficiency advantages. A centrifugal compressor is more efficient than most positive-displacement compressors, so it heats the intake air less. A turbocharger is driven by energy in the exhaust gas, which would have just been wasted as heat to the atmosphere. A turbocharger itself is relatively compact and can be put almost anywhere, but that usually only begins to make up for the increased complexity of the plumbing for the required to have the intake and exhaust flows pass through the same area of the engine compartment. Turbos get hot, too. It is normal for the exhaust side housing to glow red under heavy use, and the associated high underhood temperatures can lead to innumerate other problems without proper engineering in place. Finally, the fact that a turbochargers speed is not tied directly to crankshaft speed has a significant effect on power characteristics, which can be either positive or negative. One way of seeing it is turbo lag, which can make a car feel as if it being pulled along by a giant rubber band if the driver is not used to it. The opposite way of looking at the same phenomenon is to say that a turbo cab spool ahead of the crankshaft, providing a notable increase in low-rpm torque. Witness the Audi 1.8T. Which of these phenomena is perceived would seem to depend primarily on the selection of the turbo. Superchargers are available in a variety of configurations, so the benefits of each may vary from those generally claimed. Having the compressor tied to the crankshaft speed eliminates lag, giving the engine a feel more like a normally-aspirated engine, for a given engine speed and throttle position, you know how much power you'll get. Positive displacement superchargers, such as the Roots type and its derivatives, move a constant volume of air per revolution, not unlike an idealized four-stroke cycle. In fact, this can compensate nicely for a real engine's variations in volumetric efficiency throughout the rpm range, giving a flat torque curve with a very responsive feeling from off-idle. Packaging is usually a little easier with superchargers, since only the intake flow has to pass through it, but we have seen some truly "creative" setups used in the past to get power from the crankshaft to the blower, and superchargers do tend to be physically bigger larger and heavier than the turbocharger that would place them. Most types of compressors used in superchargers are inherently less efficient than turbochargers, meaning they heat intake air more, and superchargers traditionally have been driven constantly, robbing crankshaft power even when they are not needed. To overcome this, the Toyota MR2 Supercharged used a vacuum-controlled solenoid to engage and disengage its Roots-type compressor, and the Eaton supercharger incorporates and internal bypass that recirculates air from the compressor exit back to the inlet under low-load conditions. The various pros and cons are given different importance by different people, leading to endless debates between those who have chosen sides. The points presented here are only a very quick summary of the lines that have been drawn in the sand over how to stuff more air into an engine. To see what could be learned by examining the hardware, we rounded up a pair of E36 M3s, one supercharged and one turbocharged, and a disinterested professional driver to wring them out at the dragstrip. We had no illusions of settling the debate, but were pretty sure we'd learned something we could pass along. Before going any further, though, let me point out that in spite of appearances, this wasn't really intended to be a "shootout". Both of the systems we tested are available in configurations advertised at well over four hundred horse power, so this was really just a chance to get an up close look at two excellent tuner systems and see what could be learned. |
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| In the white corner...oops, this isn't a shootout. The white car was assembled by Race Marque Systems, typical of their Stage 1 kit using a Vortech centrifugal supercharger. Why didn't we dig up a positive displacement supercharger? Well, we didn't happen to know of anybody who had installed one on an M3, although we are sure someone out there will tell us. After seeing the clean underhood packaging Vortech blower, however, it's hard to imagine a Roots-type fitting as well. We asked RMS proprietor Osh Minelian about the choice. He said he prefers the softer low-rpm hit of the centrifugal compressor because it is easier on the engine internals, leading to the greater durability. foundation of the RMS kit is a version of the Vortech V-1 S-trim supercharger specially tuned for RMS by Vortech. The Vortech supercharger features internally step-up gearing, ands its lubrication is tied into the engines oiling system. To mount the supercharger, RMS uses CNC machining equipment to carve brackets and pulleys from 6061 T6 aluminum, which are then hard anodized for wear and appearance. All intake plumbing is constructed of mild steel tubing, which is finished in RMS black wrinkle powder-coat. All connections are made with high-temperature silicon hose. Larger Bosch injectors are installed. In this car, a MAF-air flow sensor from another high-end European car was used, because the stock sensor was running into its calibration limits prematurely. Some tuners might attempt to lie to the computer to make it do run the engine "mostly right", but Osh is able to reprogram fuel and timing maps to compensate perfectly for changes such as non-stock sensors, larger injectors, and massive increases in air flow. Now add-on or supplementary computer shenanigans are necessary.
One of the newest features of RMS's system is the water-to-air intercooler. It is essentially inline with the flow path from the supercharger to the throttle body, making for very efficient air flow. RMS experimented with an air-to-air intercooler, and found that with all the extra intake plumbing, plus the restriction of the core itself, 13 psi at the compressor outlet dropped to only 10 psi at the intake manifold. The water-to-air unit more efficient thermally, as well. It uses a 12 gallon-per-minute industrial-duty pump to move water from the intercooler core through a heat exchanger mounted in front of the A/C condensor. Electric fans were added, in can be controlled in a variety of ways depending on application. In this configuration, Osh tells us that during road testing the intercooler dropped intake temperatures 91 degree F at a constant 5 psi and full load for 30 seconds. With turbochargers, selection is 90 percent. The "other 90 percent" is everything else. Because a supercharger is driven by the crankshaft, however, a single unit becomes very versatile. In RMS' Stage 1 kit, the crankshaft pulley is 6 inches in diameter and the compressor makes 8.5 psi on a stock motor. Stepping up pulleys are available in 7- and 8-inch diameters, giving up to 20 psi, which requires a low-compression engine. Maximum possible boost depends on the flow characteristics of the engine, but the Vortech V-1 S-trim compressor is rated to 1000 cfm, which could feed as much as 680 hp in the right application, so theres plenty of room for RMS to work. It would not be practical to have such an "oversized" compressor with Roots-type blower, but the relatively compact centrifugal supercharger makes the overcapacity acceptable. Also, RMS says that the Vortech compressor is over 75 percent efficient, compared to roughly 50 percent for a Roots-type blower. |
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| The blue car features a Stage1 turbo kit from Active Autowerke. While with RMS' installation, the compressor is prominent, the Active Autowerke system is almost invisible at first glance period. The kit uses a special exhaust manifold cast from niresist alloy. It is a compact unit and tucks the internal-waste gate turbo charger down by the oil pan, out of sight below the slanted cylinder bank. From the turbo, air flows though a large, front-mounted intercooler and onto the intake manifold. Like the RMS system, all piping is powder coated, and silicon hoses are used for the connections. The Active Autowerkes turbo kit also includes larger-capacity fuel injectors and revised software. Special pains were taken to design the kit so the minimal drilling or cutting is required to install it. A hole is added to the oil pan for the turbo drainback, and the front bumper trim is cut to clear the intercooler. The car we tested is owned Tim Gergen. He is actually the second car this turbo kit has been installed in, and there are a few variation from standard Stage 1 trim. The previous owner had chosen a Stage 2 intercooler, and had installed a Greddy Type R blow off valve. As well, Tim had previously modified his car with a 3-inch high-flow catalytic converter and a Hamman 3-inch cat-back exhaust system.
We put the two cars through there paces at Los Angeles County Raceway in Palmdale. The track is roughly 2600 feet above sea level and notoriously traction-challenged. It was a hot day as well, so the times we go shouldn't be take as the best possible times we got for these cars. We enlisted James Hickerson to do the stab 'n' steer duties. He has raced on two wheels and four for many years, and is both faster and more consistant than any of us humble staffers. First up was the turbocharged car. James experimented with burnout and launch technique, eventually posting a best of13.635 seconds and 102.157 mph, with a worst ET of 14.452 seconds and 100.432 mph. Tim was disappointed with this performance, saying he has run 13.5 at Sacremento, and noted that with the high ambient temperatures, the car was making only about 5 psi boost. The white car, with blower performance, unaffected by temperature, was noticeably faster. Its first run was a 13.369-second pass with a 108.459 mph trap speed. James was unable to match this in 7 more runs, posting a worst ET of 13.917 seconds and 103.553 mph. Osh Minelian thought there was a software-controlled threshold being overrun, so he burned a new chip and put it in the car for one last dash down the strip. He was rewarded with a 13.339 second pass at 107.746 mph. The 60-foot times of the white car were consistent, neither as quick as the turbo car's best nor as slow as its worst. On the dyno, the white car was out in front again as the trap speeds believe you expect. At R&D Dyno Service in Gardena, the Race Maque Systems car 312.4 hp at 5900 rpm and 292 lbs-ft of torque at 5600 rpm to the Dynojet's roller. The Active Autowerkes car pulled a solid 286.4 hp and 274 lb-ft of torque at the rear wheels on Chassis Dyno's Dynojet. Once again, the blue car was only making about 5.5 psi boost, compared to 8.5 psi for the supercharged car. Active Autowerkes should produce 7 psi, which Tim has seen on cool days. He is obviously frustrated by the problems attaining proper boost levels, and plans to have a more sophisticated boost controller installed when time allows. Before declaring victory for the supercharger, we must point out that the supercharged car was under the full-time administrations of the tuner who designed the installation and made significantly more boost period. In contrast, the turbocharged car was in the hands of its owner. Tim Gergen is certainly no dummy but this car is his hobby, not his business, and it is being sorted out slowly as he has the time. He has only good things to say for the service provided by Acvtive Autowerke, and expects to be quite satisfied with his system when it is fully sorted out. To Tim's credit, The Dynotjet print out on Active Autowerke's website for the Stage 1 M3 kit shows only 291 rear-wheel horsepower at 7 psi boost, so he must be doing some things right. Tight Schedules prevented me from driving the turbo car, but i did get a chance to have a go in the RMS car, and i have to say its dang fast. Not the fastest car I've driven, but certainly the fastest car I've driven that I would want to drive everyday. With no turbo to chop up the exhaust pulses, more air and fuel burning, and a performance exhaust, the M3s usually low-rpm bark sounds downright mean. Blower whine kicks in between 2000 and 3500, after which point everything seems to blend together, sound building with power to a banshee-like shriek of fury at 7000 rpm. Though the engine is definitely about top-end power there is no lag, so its easy to drive. Throw in the luxury and handling of an M3, and its a very nice package. If I got to keep this car for a long time, I wouldn't complain. So which is better, turbocharging or supercharging? I told you not ask me me that. Would Tim's turbo M3 put another 30 hp to the wheels at 8.5 psi? Its seems likely. If we've done our jobs, you've learned something that will help you make a decision, or maybe you just have a whole bunch of new question to ask when trying to decide which way to go. |