RKSport Blower Kit

Chevrolet shocked everyone recently by introducing an inline five-cylinder engine to complement its four- and six-cylinder versions. The Vortec 3500 five-cylinder is positioned directly between the Vortec 2800 inline four-cylinder and the Vortec 4200 inline six-cylinder in both power and displacement. As a cost-cutting measure, all of the inline engines share the same bore and stroke (3.66×4.0), with the displacement and power output being a function of the number of cylinders. Of course, there is always the now familiar family of V-8s for those who want even more power. 

The current crop of V-8s is offered in 4.8-, 5.3-, 5.7- and 6.0-liter displacements, with the big-block family checking in at a massive 8.1 liters. While the power output climbs with the displacement, so does the fuel consumption. According to GM, the reasoning behind the five-cylinder Vortec 3500 was to combine the economy of a four-cylinder with the power of a six-cylinder. While we applaud GM’s reasoning for the 3500, we would like to suggest an even better combination, namely the economy of a five-cylinder with the power and performance of an eight-cylinder.

With gas prices skyrocketing every bit as fast as real estate prices, fuel economy has become almost as important as outright acceleration. This is especially true of daily-driven street machines, because no matter how much of an enthusiast you are, the reality is that your vehicle will spend much more of its life at part throttle than wide open. Cruising down the freeway, it is nice to have the economy offered by a small-displacement five-cylinder. You will be thankful for the purchase of the 3500 (over one of the V-8s) every time you pass one of those overpriced service stations. That said, there will be those times when the power offered by the inline 3500 just doesn’t get the job done. Whether that time comes while you’re accelerating onto a freeway, pulling a long grade or just ripping down your neighborhood back road, more power is almost always welcome. 

The ideal combination would be to have the power output of a larger V-8 while retaining the economy of the inline five. Apparently the folks over at RKSport agree, as they decided to offer a supercharger kit for the inline family of Chevy engines. That’s right: This blower kit is designed to fit not only the five-cylinder Vortec 3500, but the four- and six-cylinder versions as well. 

When comparing the Vortec 3500 inline five-cylinder engine to the larger V-8s, it is interesting to note that, although the larger V-8s obviously produce more absolute power, the smaller five-cylinder is actually more efficient. Compared to the 285hp 4.8-liter V-8, the 220hp inline five actually boasts a higher specific output. Producing 220 hp from 3.5 liters equates to 62.86 hp per liter, while the 285hp 4.8 manages just 59.38 hp per liter. The larger 295hp 5.3-liter V-8 is even less efficient than the smaller 4.8, producing just 55.66 hp per liter. Part of the improved efficiency offered by the inline five-cylinder can be attributed to the use of variable valve timing, which alters the cam timing to optimize power production over a much broader range. 

For most production motors, cam timing is obviously not optimized for any one specific rpm range; rather it is a compromise to provide a broad, usable power curve. The great thing about variable cam timing is that it all but eliminates the compromise associated with fixed cam timing. This system allows the cam timing to be optimized for not just one (limited) rpm range, but over a broad spectrum, from idle all the way to redline. The use of variable valve timing gave GM engineers the ability to produce an exceptionally broad torque curve, where 90 percent of the maximum torque output was available from 1,600 to 5,600 rpm. An elevated torque peak is impressive, but having near maximum torque throughout the rev range makes for impressive acceleration, regardless of the situation. 

While an elevated specific output is impressive, the fact is that when it comes to performance, horsepower is king. Despite the greater specific output of the 3500 inline five, the 4.8-liter V-8 has enough of a displacement advantage to offset the minor difference in specific output. Basically speaking, the bigger motor makes more power, and in any kind of acceleration contest more power will likely wind up victorious. Even if drag racing is not high on the list for your Chevy Colorado (or other inline-five-powered GM vehicle), having the extra grunt of a V-8 available is so much more fun than driving along wishing it were there. For most drivers, the time spent enjoying the fuel economy of the five-cylinder outweighs that spent wishing for more power, but why settle when you can have both? 

We got the opportunity recently to test a new supercharger system for the Vortec inline engine family (the kit is said to fit the four-, five- and six-cylinder Vortec motors) from RKSport. Our test vehicle was a fully decked-out 2005 Chevy Colorado. The trick truck featured a ton of mods, but our primary focus was the supercharger installation.

The highlight of the supercharger kit was, of course, the supercharger itself. The kit from RKSport utilized an efficient and silent Powerdyne centrifugal supercharger. Since centrifugal superchargers require much more impeller speed than engine speed, it is necessary to rev up the impeller relative to the engine. This is achieved by altering the drive ratio, first by running a smaller blower pulley than the crank pulley, producing somewhere between a 1.80-2.1:1 step ratio. While this may double the impeller speed, even more speed is required to produce usable boost pressure, so the centrifugal superchargers further increase the impeller with an internal step ratio. Using two different-size pulleys (or gears), the impeller speed is further increased. Unlike most centrifugal superchargers, the Powerdyne does not rely on a pair of gears to produce the internal step ratio, but on a belt to drive a pair of cog pulleys. This unique system ensures ultra-quiet operation, something that can’t be said of all gear-driven blowers. Of course, there is a limit to the impeller speed and ultimate power supplied by the belt drive, but the system has proven itself powerful and reliable for performance street applications. An additional side benefit of the internal belt drive is that it eliminates the need for an oil feed for the gear drive and, more important, an oil return, which usually requires removal and welding of the oil pan. 

The lack of oil supply and return lines makes the supercharger kit that much easier to install. To hammer home this important fact, the entire kit was installed in just a couple of hours during our dyno testing with MagnaFlow. Given that this Powerdyne blower is easily capable of supporting 500 horsepower (or more), it was more than sufficient to meet the needs of boosting the stock Chevy inline five-cylinder. While the highlight of the supercharger kit was the supercharger, it takes more than just a blower to safely provide boost pressure to a modern motor. The RSSport supercharger kit naturally included custom mounting brackets to locate the blower on the right side of the motor, adjacent to the power steering pump. We liked that the kit (shown in prototype form) included a secondary brace to triangulate the mounted blower. Rigid blower mounting is critical to ensure proper blower belt contact and life. The kit also included a free-flowing air filter system (using the factory MAF) to ensure adequate airflow to the blower, and a carbon fiber (wrapped) inlet tube from the blower to the throttle body. No intercooler was required due to the relatively low (five psi) boost pressure employed by this kit, but we wouldn’t be surprised to find an intercooled upgrade (with higher boost) at a later date. The combination of a polished blower and a carbon fiber inlet tube really stepped up the under-hood cool factor of an already impressive blower kit. 

While the blower was more than capable of flowing sufficient air, horsepower production also requires additional fuel. The RKSport supercharger kit supplied the necessary additional fuel via a tried and true (boost referenced) rising-rate fuel regulator. Once the regulator sensed boost pressure, the return fuel line was closed to increase the fuel pressure. The increase in fuel pressure was responsible for the additional fuel flow to augment the extra air supplied by the blower. Every bit as important as a safe air/fuel mixture is the ignition timing. Not surprisingly, the supercharged motor will tolerate a great deal less total ignition timing than the stock motor. Teaming with UniChip, RKSport was able to come up with a system to retard the ignition timing under boost to provide a safe but powerful timing curve for the supercharged motor. Making power with a supercharger is easy, but making safe streetable power is something altogether different. RKSport seems to have found the proper balance, as all of our testing was run on pump gas, to say nothing of the fact that this particular Colorado has seen plenty of street miles under boost and was driven to and from the dyno test.  

We mentioned that our goal for this supercharged five-cylinder was to equal the power output of a 4.8-liter V-8, thus providing the economy of a five-cylinder with the performance of a larger V-8. Whoever said that there is no replacement for displacement obviously didn’t have much experience with forced induction. Horsepower production is all about airflow. The more air the motor can process, the greater the power output. Increasing displacement obviously increases the airflow through the motor, but so does forced induction. In reality, a normally aspirated motor is subject to pressure, or to be more accurate, atmospheric pressure (roughly 14.7 psi). This is to say that the vacuum created by the downward-moving piston is filled by atmospheric pressure. Adding a supercharger to the equation increases the pressure in the intake manifold above atmospheric, and thus the vacuum created by the downward-moving piston is filled even more thoroughly, thanks to the additional pressure (nature abhors a vacuum). Thus the Powerdyne centrifugal supercharger employed on the kit from RKSport is used to force feed the Vortec 3500 five-cylinder more air than it could inject on its own. 

The relationship between the normally aspirated power output and the power produced once supercharged can be calculated based on boost pressure. Actually, the calculation based solely on boost pressure is an estimate, but testing has shown it to be a pretty accurate one nonetheless. In the case of our normally aspirated 3500, the five-cylinder (rated at 220 flywheel hp) produced 176 hp at the wheels, as measured on the MagnaFlow DynoJet. Since the normally aspirated motor was run at 14.7 psi (we are assuming a few things about the altitude, temperature and humidity), adding five psi of boost on top of that should produce a power gain of roughly 34 percent (5 psi/14.7 psi = .34). Using that formula, we see that our 176hp five-cylinder should produce 236 hp at the wheels at five psi. In reality, the blower kit produced 226 hp, but the motor refused to run to redline in boosted form. Given the shape of the power curve, we figure that the difference of 10 hp would be easily obtainable were the motor to rev another 300 rpm (as it had in normally aspirated trim). The formula also does not take into account the losses associated with driving the supercharger, which might account for 25-35 hp at this power and rpm level.  

Formulas aside, the RKSport supercharger did indeed deliver the goods, blessing the Vortec 3500 inline five-cylinder with power equivalent to the V-8’s. The blower kit added as much as 85 hp and 65 lb-ft of torque, and all from just five psi of boost. While the centrifugal supercharger can’t quite match the low-speed torque production of the V-8, this motor charges hard from 4,000 rpm to redline. The five-cylinder has a unique pitch, sounding nothing like a traditional V-8 or an inline four- or six-cylinder. Cruising down the road, you’d hardly know the blower was there—that is, until you stepped on it. Rather than a lackluster nudge, you are rewarded with an honest shove in the backside. Combine this impressive and altogether usable on-the-road performance with the ease of install and the visual appeal and you’ve got the makings of a real winner. Given the size and shape of the carbon fiber inlet tube, we couldn’t help but ask if an intercooler might be in the works, which would allow Vortec inline owners to further increase the boost pressure and performance. Since all we got was a smile, we suspect that an intercooled upgrade is already in the works. For now, we’re happy with the new math that shows 5 does indeed equal 8. TB