'We like the idea of a big centrifugal under the hood, especially if we can blow through a simple carburetor and avoid the tariff of EFI. And centrifugals deliver power on demand. Just think of a centrifugal as a beltdriven turbocharger without all that plumbing. It offers similar efficiency numbers compared to a turbocharger when it comes to compressing air without adding excessive heat. Packaging it is far easier than a turbo, too. The blower sits low off to the left or right of the engine, driven by a belt, and it can squeeze under almost any hood line. Why let the whole world know the deal? Leave that to the show-car guys.
Beltdriven centrifugals offer plenty of opportunity to make serious power. Our guide offers more than just cursory beltdriven recommendations. We took the plunge and stuffed an ATI PowerCharger on a 540ci Rat to make stupid power just to show you how's its done. Its the kind of engine that sets up the 6 o'clock news reporter to ask, "How much horsepower does this engine make?" Then he has to ask you again because he thought you said 900 hp. That's when you'll know you've arrived.
How Big?Just as with carburetors and cams, blower sizing is one of the more important questions you need to address when buying any kind of supercharger. Blowers move air, but how efficiently they move and compress it depends on proper sizing.As an example, ATI (ProCharger) offers 16 different centrifugal supercharger sizes, from the P600B that will move up to 1,200 cfm, to the monster F-4 that will push a massive 4,300 cfm-and according to ATI, up to 2,300 hp.
But not every engine needs a mongo blower. When searching for the right blower, the basic application starts with your engine's normally aspirated horsepower level. Smaller engines perform best with smaller blowers that can move air quickly, while larger-displacement engines need larger blowers to move more air. A big unit like an F-4 on a mild street 302ci small-block would still make power, but a smaller blower would actually perform better and make more torque.
Base engines that make 400 hp normally aspirated will demand a smaller size blower, something more like ATI's small P-1SC rated at a max airflow of 1,200 cfm and a max of 30 psi, or perhaps a D-1 that can help make as much as 925 hp. You can expect a blow-through centrifugal to add between 35 and 65 percent power to an engine. So if your engine is in good shape and makes 400hp, adding a D-1 for example could add another 200 to 250 hp to make 600 to 650 hp depending upon how well your cylinder head/cam/exhaust systems work. We've also seen centrifugals add 100 percent power, but that was with excellent heads, cam, and exhaust components where the engine was designed to be used with a blower.
Because a centrifugal is basically a beltdriven turbo, very high impeller speeds are a requirement. Centrifugals spin between 50,000 and 70,000 rpm, generally 8 to 10 times faster than engine speed. This is accomplished by combining the step-up ratio between the crank and blower pulleys and also from an internal ratio increase. Internal step-up ratios can vary even within the same blower size from 4:1 to 5:1. Higher impeller speeds do not necessarily equate to more boost. This is all tied in to the size and design of the supercharger. As you might have guessed, centrifugals do not make "instant" boost compared to a Roots but that may not be a detriment. Consider that boost at lower engine speeds can easily create more torque than your chassis can handle, so "instant" boost may not always be necessary.
One trick everyone wants to try is a smaller blower pulley that will spin the blower faster to make more boost. While this can be a benefit, the more common result of spinning the blower faster is more heat. This creates a situation where more boost results in less power because the added heat creates a reduced inlet-air density in the engine despite the greater pressure. A good rule of thumb is 1 percent more engine power for every 10-degree-F reduction in inlet-air temperature. As an example, if a centrifugal can reduce its discharge air temperature by 50 degrees, that's worth 5 percent more power. At 600 hp, that's 30 "free" hp.
Blow-Thru CarbsThe easiest way to make a carbureted package work is in a blow-through application. While it would seem you could use any old carburetor, experience has proven there are significant advantages to using a carburetor designed for blow-through applications.
We talked with Marv Benoit of Quick Fuel Technologies. Benoit has extensive experience with blow-through carbureted packages, and his comments were especially interesting. While you might think a big-volume carburetor would be ideal, Benoit suggested a 750-cfm mixer with annular-discharge boosters for our big 540 blower combination. He even has seen where a smaller 650-cfm carb worked best for a boosted blow-through Pontiac when nothing else would work. According to Benoit, the small venturi helps the carburetor maintain a safe, rich air/fuel ratio at higher engine speeds. The annular-discharge boosters are also more pressure-sensitive, which greatly improves fuel flow.
For blow-through carburetors, there are several ways to get there. Drop-leg-booster carburetors can be made to perform well in blow-through applications if extensive modifications are performed. If converting a typical 4150-style Holley carburetor, for example, Benoit recommends increasing the power-valve channel restrictions and reducing the size of the high-speed air bleed. Both of these mods will increase fuel flow at higher engine speeds. Other recommendations include applying epoxy to the main well plugs in the top of the metering blocks to prevent boost pressure from pushing those plugs out under high boost conditions. There are several carburetor companies that we found in our research that offer specially modified Holley carburetors for blow-through centrifugal-blower applications. Talking with these companies and doing research on your specific application will help you avoid the typical problems that can trip up the uninformed.
The "hat" or adapter into the carburetor is another crucial power component. For low-boost combinations, the hat and inlet plumbing is not as critical. But for you barnstormers running double-digit boost numbers, the hat design and inlet piping become crucial. Increasing the volume of the tube by adding pipe diameter from the blower to the carb can help slow the inlet-air velocity to give the carb a chance to work. Benoit's even seen applications where flow diffusers have been used inside the feed tube between the blower and the carb hat to stabilize inlet-air speed in an attempt to make more power. What can happen with excessive inlet-air speeds is the air "stacks up" on the backside of the carb hat and the net result is a lean air/fuel ratio. The large air boxes that encapsulate the entire carburetor, such as Paxton and Vortech kits, can be beneficial when dealing with high-boost-pressure applications. The difficulty with these boxes is that they make access to the carburetor cumbersome.
Boost-Referenced Fuel SystemsTo make power, you have to have fuel-it's that simple. But when you apply blower boost to the top of a carburetor, that pressure works against the fuel pump. With 10 psi of boost pressure pushing on the fuel in the float bowl, there's no way 7 psi of fuel-pump pressure will be able to push its way past the needle and seat. The solution is easy with the addition of a fuel pump capable of both high flow and high pressure matched with a boost-referenced fuel-pressure regulator. This style of regulator employs a small reference line routed from the carburetor hat to the regulator diaphragm that controls fuel pressure. As boost pressure increases in the intake manifold, this automatically "references" the fuel pressure. Most boost-referenced regulators are designed to maintain a 1:1 relationship, so if the regulator is tuned for 7 psi of fuel pressure normally aspirated, and the supercharger makes 10 psi of boost, then the fuel pump would have to generate 17 psi going into the carburetor to maintain a 7-psi fuel pressure difference. Often, the fuel delivery system will fall short of delivering at max demand, which is why keeping a careful eye on fuel pressure is ultra-important for blow-through supercharged systems.
A big-horsepower system needs a good fuel delivery system. This means a fuel pump with plenty of delivery capacity along with a large line size and no restrictions. There are many companies building high-capacity pumps and regulators that will do the job. We talked with Aeromotive president Steve Matusek to gain some insight for our project. Besides the Aeromotive pump, filters, and regulator he recommended, Matusek strongly encourages building a full-flow or return-style fuel delivery system. In this type of system, the regulator diverts excess fuel back to the fuel tank. The advantage to this type of system is that pressure does not "dead head" against the regulator and create large pressure fluctuations determined by whether the needle and seat is open or closed. A return system creates a more stable delivery of fuel to the carburetor, which is ideal for creating best power, and it appears that less fuel pressure minimizes foaming and aeration in the float bowl. Since we're trying to make 1,000 hp with a single four-barrel carb, this design should make a measurable difference. Our dyno-cell education on fuel delivery with our blow-through 540ci Rat has drilled home that even "mild" 500-600hp blow-through systems require an excellent fuel delivery system. Without it, you'll never come close to making the power.
Cam SelectionConventional wisdom contends that one key element to making blower engines work is a wide lobe-separation angle. This has to do with the overlap period that encompasses exhaust-valve closing and intake-valve opening. Most high-power, normally aspirated cams rely on overlap to improve cylinder filling. With a supercharger, we bank on blower pressure to push the air and fuel into the cylinder. With more overlap (where the intake tract is open to the exhaust side of the engine), intake pressure will push the boosted air and fuel right out the exhaust rather than filling the cylinder. So, the smart move is to employ a cam with wider lobe-separation angle to make more power.
Since superchargers push air and fuel into the engine, valve lift is also an important consideration. Roller cams tend to offer increased lift compared to flat-tappet cams, but also cost more money. Ideally, a mechanical-roller cam will offer the most lift for the degrees of cam duration compared to a flat-tappet cam. You will also want to complement that high-lift cam with a set of quality roller rockers and good valvesprings. Good springs may not seem like a priority, but especially with big-block engines that use large-diameter valves (like our Rat), these components quickly become very heavy. Even at 6,000 rpm, this extra mass is tough to control. Consider, too, the boost pressure working on the inlet side of the intake valve. As an example, let's take a big-block 2.250-inch intake valve producing an area of 3.97 square inches. With 20 psi of boost pressure with the intake valve on its seat, we have 20 psi pushing against almost 4 square inches, which equals about 80 pounds of force trying to open the intake valve against a spring pressure of perhaps 130 pounds. That leaves only 50 pounds of spring pressure to keep the valve closed at high rpm when valve bounce is a common problem. On the exhaust side, typically the exhaust valve will be required to open against higher residual exhaust pressure in the cylinder, making the job tougher for the rocker and pushrod. As you can see, stuffing a blower on an engine makes it harder on the valvetrain components for reasons far beyond merely increasing cylinder pressure.
Boost Retard And Race GasThe simple act of specifying exactly when the spark plug lights is just as crucial on supercharged engines as it is with their normally aspirated cousins. Since we're talking street blower engines, we need something a bit more sophisticated than OE energy. The idea is to design a system to deliver an aggressive advance curve for excellent part-throttle performance while retaining the ability to retard the timing when the blower begins to make serious cylinder pressure. There are several ways to go about this, including a digitally programmable distributorless ignition from MSD or the equally trick Electromotive system. These options are pricey, but there are more affordable alternatives. MSD offers the combination of excellent spark energy with the affordable MSD-6A along with the ability to adjust timing based on boost. MSD calls this unit the 6-BTM, or Boost Timing Master. A dash-mounted control knob allows you to adjust the amount of ignition retard in degrees per psi of boost up to a maximum of 15 degrees of retard.
As an example, let's say we've determined that our blower motor prefers 42 degrees of total mechanical advance at WOT for best power normally aspirated on pump gas. With the blower, we know this much timing will create a major rattlefest with detonation on pump gas. So let's add the MSD 6-BTM and dial in 2 degrees of retard for each 1 psi of boost (e.g., 5 psi retards the timing back 10 degrees, from 42 to 32). The engine is happy, and we can now experiment with total timing with the simple turn of a dial. If you already have an MSD-6A box, you can purchase a standalone BTM module with the retard knob.
The advantage of a system like this is it allows you to run the engine with a stronger timing curve when the engine is at part-throttle on the street. It also allows you to quickly advance the timing if you decide to drop in a load of race gas that can handle more ignition timing without incurring the wrath of the detonation monster. Also remember that a powerful ignition is only strong if the plug wires and spark plugs are doing their job. Always use low-resistance plug wires to ensure all that spark energy is actually reaching the plugs.
Compression, Intercoolers, And Water InjectionEveryone has an opinion when it comes to combining compression ratio with supercharging. Some contend that low compression with lots of boost is the way to fly, while others project that a little more squeeze is better with a blower for more power. There are no wrong answers here as long as you avoid detonation. For a street engine, a little more static compression at around 9.0:1 to 9.5:1 with single-digit boost levels and no intercooler can be made to work well with pump gas as long as you don't go crazy with timing under boost with reasonably cool inlet-air temps. High inlet-air temperature into the carburetor increases the potential for engine-killing detonation. That's why intercoolers work so well.
We all know that intercoolers work exceptionally well in reducing inlet-air temperature and therefore make more power by allowing you to run more timing before the engine detonates. They are also expensive and require significant custom plumbing that scares many enthusiasts. As an alternative, you can always increase octane by mixing race gas with pump gas, but since fuel is a consumable, paying $5.00 to $7.00 a gallon for 10 gallons of fun can become a pain in the wallet.
Another solution that reduces detonation, improves power, and doesn't cost much is water/alcohol injection. This idea is almost as old as the internal combustion engine as a cheap source of power. For supercharged engines, the idea is to use the latent heat of vaporization characteristics of both water and alcohol (ethanol or methanol) to reduce the inlet-air temperature under boost by injecting this water/alcohol mix into the engine. It's even possible to introduce this mixture into the blower inlet. This allows the mixture time to homogenize into the inlet airstream and also by vaporizing in the high inlet-air temp to reduce the temperature somewhat. Have you ever applied rubbing alcohol to your skin on a hot day? That same cooling effect also occurs in the inlet tract of a blow-through system. The oxygen content in the alcohol will contribute a small amount to increased power. Combined with the lower inlet-air temperature, this idea is a proven winner.
You could build an affordable water-injection system using a small nitrous solenoid, a high-pressure electric pump, and a jet to meter flow into the engine and trigger it with a Hobbs switch that senses when the engine sees boost. There are also several kits, including those from Snow Performance or coolingmist.com for supercharged applications for $200 and up. These companies also offer individual parts for building your own system. A slick aluminum tank partially hidden behind an inner fenderwell with a pump and a solenoid to feed a water-alcohol mix would be cool!
Plates And BracketsThe weird thing about centrifugals is that there are little companies out there making mounts for centrifugal superchargers not offered by the parent companies. As an example, while ProCharger doesn't offer mounts for small- or big-block Mopars, a company in Arizona called The Supercharger Store does. This company has built a business around making mounts for engines that take the road less traveled. They even offer a mount for ProCharger superchargers that include A/C for a 440 Mopar.
If you are building a dedicated blower motor and anticipate double-digit boost numbers, it would be beneficial to talk to your crankshaft company about a larger snout, especially for small-blocks. Beltdriven superchargers place a major load (up to 50 hp worth) on crankshaft ends, which effectively tries to rip the crank snout right off. Several companies offer big-block-snout cranks, timing gears, cam covers, and balancers for the small-block Chevy to accommodate these demands.
These extreme loads on the end of the crankshaft also place severe bending loads on mounting brackets. The beauty of a centrifugal is that if you are a talented fabricator, it's possible to build your own centrifugal supercharger mount, but be sure to reinforce the bracket. A single 31/44-inch-thick aluminum plate will probably not work. Several systems now employ dual 31/44-inch-thick plates in an effort to prevent the blower mount from flexing. This adds weight and complexity but does wonders for keeping the belt in place on the pulleys.
977HP Street 540 RatWe crammed everything we learned in this blow-through story into a 9.5:1, 540ci big-block Chevy using an ATI ProCharger F-2 centrifugal to see what kind of power we could make. The basic short-block has not changed since we used it to test Edelbrock heads back in February '05 ("Super-Sized Rat"), making 711 hp normally aspirated. Because we knew this would be an expensive package, we kept the price down by opting for a set of factory iron rectangle-port heads and changed the cam.
For the intake side, we went with a Dart single-plane intake and a 750-cfm Quick Fuel Technologies 4150-style carburetor. We also selected a Comp Cams Xtreme Marine hydraulic roller with 250/254 degrees at 0.050-inch tappet lift with 0.575-inch lift and 112-degree lobe separation angle using 1.7:1 Comp roller rockers. On top, we also added a pair of Billet Fabrication aluminum valve covers and TD Performance breathers. ProCharger supplied the blower-drive system, to which we added an Edelbrock aluminum water pump and March pulleys, plus a mount kit for the Bosch alternator. Rounding out the package is a set of 2.125-inch Hedman coated headers.
Dyno DayTest 1 was our normally aspirated baseline run on 91-octane pump gas with a Quick Fuel 850-cfm carburetor. This 9.5:1 Rat is no slouch, making 620 lb-ft at 4,800 and 623 hp at 6,000.
Test 2 began with a 50/50 load of 91-octane pump gas and VP C-16 race gas for roughly a 104-octane brew. At a maximum of only 11.75 psi boost pressure, the motor made 940 lb-ft of torque at 5,200 and 977 hp at 5,700 rpm. There's much more power left in the blower-a 540 with good heads can make 1,200 hp.
TEST 1 TEST 2 DIFF RPM TQ HP TQ HP TQ HP 2,{{{600}}} 529 262 599 296 70 34 2,800 546 292 612 326 66 34 3,000 567 325 645 369 78 43 3,{{{200}}} 588 358 685 417 97 59 3,400 602 389 726 470 123 {{{80}}} 3,600 610 417 766 525 156 108 3,800 613 442 805 583 192 140 4,000 612 466 841 641 229 175 4,200 612 491 873 698 261 207 4,400 614 516 899 753 285 237 4,600 618 542 919 804 301 262 4,800 620 566 932 851 312 285 5,000 618 586 939 894 321 308 5,200 608 600 {{{940}}} 931 331 330 5,400 592 609 933 {{{960}}} 341 350 5,600 574 615 915 976 341 361 5,800 560 620 882 973 322 353 6,000 549 623 832 950 283 327 6,200 - - 777 919 - - Avg. 579 451 815 694 Peak 620 623 940 977 Power/ci 1.15 1.15 1.74 1.81Note: Averages are taken from data recorded every 100 rpm using data not shown in the above chart.
