It matters little whether you call them blowers, compressors, huffers, windmills, air transfer pumps, wheezers, Jimmys, or anything else. The supercharger in its various forms is almost as synonymous with high-performance enthusiasts as the beloved V-8 engine itself.
This Tech Tip is From the Full Book, HOW TO BUILD SUPERCHARGED & TURBOCHARGED SMALL-BLOCK FORDS. For a comprehensive guide on this entire subject you can visit this link:
SHARE THIS ARTICLE: Please feel free to share this article on Facebook, in Forums, or with any Clubs you participate in. You can copy and paste this link to share: https://www.diyford.com/roots-superchargers-guide-in-ford-small-blocks/
One of the more familiar superchargers is the Roots blower. It’s considered by most to be the one that started it all. The Roots blower was first designed as a water pump, but it soon became clear that it was more effective as an air pump. People used it to solve a coal mining problem, pumping fresh air in and methane gases out. One of the prerequisites for operating in such a dangerous environment was that the blower must not produce any friction or sparks. This turns out to be one of the reasons why we like the Roots blower so much.
In order to build something that will move plenty of air without any possibility of creating a spark, the design must be simple and hopefully not make any metal-to-metal contact as it does its job. The Roots blowers originally manufactured for ventilating mineshafts featured iron outer cases, but typically used wood rotors to actually move the air. When aluminum came into wide use, the non-ferrous metal was substituted for most of the components in the Roots, with the exception of the timing gears (manufactured from machined steel), which establish the relative positions of the two rotors within the case.
One of the major misconceptions about the Roots blower is that the air is moved straight down the center,between the rotors. That’s not true. In fact, the air is moved between the vanes of each rotor just inside the outer case of the supercharger. The rotors turn so that their vanes pass along the inside surface of the case in a downward motion.
The clearance between the rotor tips and the case must be small, but if it’s too small, the tips of the rotors will rub against the case, especially at higher blower RPM. This isn’t usually a problem on a properly setup Roots blower, but it will become extremely critical with certain other types of superchargers detailed later in this book.
There are other seals in a Roots blower that make a difference in its efficiency. In addition to the tip seals, there are annular or ringed seals located at either end of each rotor to seal them against the end plates of the blower housing. As you might expect, there are also oil seals on the rotor shafts to prevent the higher air pressure inside the supercharger case from moving oil away from the bearings.
There have been several rotor variations tried by the more innovative Roots supercharger builders through the years. The rotor configuration used in most of the early units, including all of the GMC types, involved a helical arrangement of the vanes and a tip shape (when viewed from the end of the rotor) that resembled that little paddle device your eye doctor has to hold in front of one eye as he tests the other. This vane shape allows one rotor to interact with the vanes of the other rotor, providing a fairly good seal against air leakage without the vanes actually touching. Without that unusual vane shape, the two helical rotors couldn’t turn against each other without grinding to a screeching halt.
Drag racer Larry Bowers (Bowers Blowers) spent many years developing and refining one of the first practical straight-vane Roots-type superchargers. Although it seemed much simpler than the cast-aluminum helical rotors, there were several problems that required serious engineering time to make the straight-vane rotors work. Bowers, motives in pursuing this course included the much lighter net weight of a CNCmachine-cut billet rotor, higher component strength, less rotor tip seal strength, and something we haven’t discussed up to this point: adiabatic efficiency.
On the inlet side of an engine, heat buildup is the all-conquering enemy. Depending on what type of supercharger setup you select, the air flowing through the inlet system may be heated significantly when it’s compressed.
Adiabatic efficiency is a number representing the amount of heat added by the supercharger system as a whole. In engineering terms, we’re talking about the thermodynamic efficiency of the system.
A given supercharger will often come with a manufacturer’s claim about its adiabatic efficiency, but the numbers for the whole system are a compound result of the individual components used between the air filter and the intake valve. In general, here is how the various power adders rank in terms of adiabatic efficiency, from lowest to highest (most efficient to least efficient):
- Roots helical-rotor blower
- Screw-type variants of the Roots design
- Centrifugal superchargers with external drive
The amount of boost employed, and whether or not the compression takes place within the supercharger, will have a great influence on how much heat is added to the inlet air charge. As a rule of thumb, the more compact the installation is, the less likely it will be to have a high adiabatic efficiency number
With a traditional Roots blower, the amount of heat added by the supercharger itself is very low compared with most of the other supercharger designs (Roots screw or centrifugal scroll types). The reason for this is that a Roots blower isn’t built to compress air as it operates – its effect is to feed the engine with (much) more air than would normally be contained within the intake manifold under normally aspirated conditions.
If you want to use a pump analogy here, the traditional Roots design is a simple transfer pump that isn’t designed to raise pressure from its inlet to the outlet. Other supercharger types almost invariably are, indeed, built to elevate the inlet air pressure before the air intake charge leaves and goes on to the next part of the air inlet system. In engineering terms, a Roots is a pure blower, while almost all of the other supercharger types are actually compressors.
Screw-type variants of the Roots design actually compress air within their cases, and are usually followed in the inlet tract by an intercooler of some type. The intercooler counteracts the added heat from compression to keep the air dense.
Centrifugal superchargers also compress the air within their housings, but if the boost level is kept relatively low, they can still be used without an intercooler. For example, the Shelby GT-350 Mustangs that came equipped with the optional Paxton ball-drive, centrifugal supercharger had no intercoolers, and worked very well.
Turbochargers have gotten a lot more efficient recently because of highly sophisticated materials, but they still operate with the complication of bringing the inlet air within close proximity of the exhaust gases. The compressor side of a turbo raises the inlet air pressure and passes it along to the next component in the tract – usually an intercooler.
An intercooler is often required to maintain thermal-efficiency in a compressor, the payoff in measurable airflow at the intake valve almost always outweighs the added complexity in the system. As a total system, an intercooled compressor-type setup has a similar adiabatic efficiency to the Roots design, but with the advantage of higher net airflow.
Up to a certain point, Roots-type blowers like the GMC and all aftermarket units patterned after it move air in a linear relationship to their rotor RPM or tip speed. For this reason, a Roots blower will tend to maintain a more constant boost level at the intake manifold than certain other supercharger types because the rotor RPM and crankshaft speed are directly linked via the drive belt.
The drive ratio of a Roots blower is critical for establishing a given intake manifold boost level. When a Roots blower turns at the same RPM as the crankshaft, it is said to have a 1:1 drive ratio. If the blower turns faster than the crankshaft, it is over-driven at a rate governed by the size of the blower pulley versus the size of the crank pulley. Conversely, if it is under-driven (again governed by the size of the blower pulley), it turns slower than the crankshaft. An example of an over-driven ratio would be if the blower turns at 1.2:1 times the crank speed.
This linear relationship does not hold true for superchargers that compress the air as it passes through (i.e., an Eaton), neither does it always hold true for Roots types once they’re installed onto an intake manifold. The reasons for this are many, but the most obvious is that the engine will eventually form a restriction to the blower’s output, regardless of its rotor RPM. At this point, additional rotor RPM will make no difference and the boost pressure will not continue to rise, because its maximum point is limited by leakage back through the supercharger, around and along its twisted rotors.
Hopefully, it will never matter to you, but an engine equipped with a Roots blower will continue to operate even if the blower stops turning. Of course, the causes for this are many, the least hazardous (to the equipment and your wallet) being the loss of a blower drive belt. In such a scenario, the engine won’t run well, especially above a high idle, but it will continue to run, nonetheless. That’s because of the air/fuel leakage taking place at one end of the intake opening, along the rotor vanes, and down into the intake manifold at the opposite end of the rotor. This is particularly true of helical-type rotors.
The Roots mini-blower has been around since the early 1950s with one of the prime examples being the Latham Supercharger, which experienced limited success in full-size 1953-’54 Lincolns. But it wasn’t until the energy-conscious 1970s and 1980s that the small-displacement, high-RPM mini-blowers really came into their own.
The release of B&M Performance Products Street-Blower and Jerry Magnuson’s Magna-Charger were both strong indicators that the blower boys were starting to think outside the traditional street-supercharging envelope. The Magna-Charger should not be confused with the Eaton high-speed, positive-flow, compact street supercharger(s) or the Eaton/Magnuson-produced highspeed, positive-flow high-performance street blowers that are currently enjoying huge acceptance with OEMs and the high-performance aftermarket. We’ll discuss those later on. When we speak of a “Roots miniblower,” we’re basically talking about a compact version of the traditional Roots blow-through supercharger designed for use in tight engine compartments with a low hood line, as well as in vehicles equipped with a myriad of engine and emissions accessories.
Due to their very nature, these miniblowers must spin at much higher RPM to achieve the same boost levels (up to 10 psi) as their larger-sized GMC-type counterparts. Of course, with higher RPM operating levels, these blowers require much tighter tolerances and higher-quality internal components.
Two examples of mini-blowers come to mind, although there have been a number of them over the years. One is the Holley/Weiand Pro-Street line and the other is the Blower Shop’s Low Profile (Street) Supercharger, both of which will be covered in the Buyer’s Guide segment of this chapter.
Unraveling Street Supercharger Mysteries
One of the most asked questions in regards to the Roots-type supercharger is, “what does it mean when you refer to a blower as being a 4.71, 6.71, 8.71, etc.?” To solve the mystery, we spoke with Craig Railsback, president of Blower Drive Service (BDS), one of the aftermarket’s oldest and most respected blower manufacturers.
“The ‘71’ connotation simply represents the cubic-inch displacement per cylinder for the motor designation originally used by Detroit Diesel. For example, if it were a GMC supercharged 2-cylinder inline diesel engine, the designation would be ‘2.71.’ If it was a GMC supercharged 4-cylinder inline diesel engine, the designation would be ‘4.71,’ and if it was a GMC supercharged 6- cylinder inline diesel engine, the designation would be ‘6.71’ to represent the number of cylinders the engine had!” Railsback continued.
“In later years, Detroit Diesel also came along with the V-Series blowers (V- 6.71 and V-8.71) to go along with their V- 6 and V-8 diesel-powered engines, as well as debuting their V-92 Series for use with their lager displacement diesel engines. Today, the nomenclature ‘71’ is used strictly to describe the type of blower, or size in question. It doesn’t have anything to do with the actual size, or type, of engine you’re actually installing it on.”
While we had Craig’s ear, we also asked him another question that many people often ask. “How big is too big?”
“It’s usually sized according to the application. For example, one of our customers runs a 14.71 BDS supercharger on a Jim Oddy-built, stroked 420-ci Ford 351W engine that runs on straight alcohol. Obviously, that is a special application, which requires a larger-size supercharger. Typically, for a street car, you’re looking at a small-bore 6.71 street supercharger because a stock-stroke 289/302-ci Ford engine really doesn’t need that much supercharger.”
What about compression ratios? How much (compression) is too much?
“Ideally, on street applications (due to poor fuel quality, etc.), it’s a pretty good idea to keep the boost and compression levels fairly low, and they (the engines) typically can’t hurt themselves. For example, on either a 289/302-ci Ford pushrod V-8 or Ford 351-ci pushrod V-8 engine, 8.5:1 compression and 8.5 lbs of boost is an ideal pump gas combination.”
And with this combination what would be an ideal (blower) drive ratio?
“If it’s a 302-cid engine, it would be advisable to run about 5 to 10 percent underdrive to make about 7 to 8 lbs boost. With a 351-ci engine, it would be advisable to run a 1:1 drive ratio to make about 5 to 7 lbs of boost. Remember, the compression ratio and the octane fuel you use generally limit the amount of boost allowable, because detonation is always a serious factor.”
How much carburetion is too much carburetion?
“The ability of an engine to consume air is in direct relationship with the cubic-inch displacement of the engine, the engine RPM, or operating level, and the amount of boost that you’re intending to run. We have a formula (which is published in the BDS catalog) by which you can actually calculate that out and determine what size, and the amount of carburetion, you’ll want to run on a particular engine.”
What about electronic fuel injection?
“The ability to ‘size’ a system is a by-product of the air control device being used and the ability to provide the correct size of fuel line and fuel injectors to actually feed the thing. Quite typically, we use the same formula that is used on a carbureted engine to size the system accordingly.”
The State of the Street Supercharging Industry
We’ve already established the fact that the traditional full-size Roots, or GMC supercharger, is the street and competition supercharger of choice with high-performance enthusiasts because of its looks, performance, and availability. With virtually millions of these blowers out there on the market (manufactured by GM’s Detroit Diesel Division until 1992), you could say a rather healthy street blower industry exists.
While many of the smaller blower shops still base their business on rebuilt GM cores, the industry itself is moving away from blower remanufacturing and concentrating on producing a better product by building their own cases, rotors, gears, end plates, and blower drives. In fact, these days it’s entirely possible to buy a complete “GMC” street supercharger using all new components.
“For a longest time, the standard has been to rebuild these old superchargers replacing whatever parts needed to be replaced along the way,” says Kevin Peters, president of Kuhl Superchargers. “Then, as useable supercharger cores started becoming harder and harder to find, the industry started manufacturing all new external pieces, (cases, end plates, etc.), but still used the core rotors and gears out of old Jimmy blowers. In recent years, however, the core quality of these parts has gone down dramatically. After Roger Penske purchased Detroit Diesel from GM approximately 12 years ago, he converted the product line over to turbochargers. So there have been no new Diesels produced with blowers on them for over 12 years. And that means no more service replacement parts. Now we manufacture our own rotors and gears, etc.; everything is brand new, front-to-rear!”
Not surprisingly, manufacturers like Kuhl, BDS, and others also supply these smaller blower shops with product(s), so what you’re actually talking about here is an industry within an industry. Now let’s take a closer look at the supercharger manufacturer’s (listed in alphabetical order,) who service, support, and motivate the Roots blower industry.
The Blower Shop
Beginning with the acquisition of Bowers Blowers in 1984, the Blower Shop primarily served the Top Fuel, Funny Car, and Top Alcohol dragster classes and drag boat market. The Santa Clarita, California-based company was instrumental in the development of leading edge Roots-blower technology. They manufacture examples of the famed supercharger in 6.71, 8.71, 10.71, 12.71, and 14.71 sizes.
The Blower Shop will build you a big-block (429-460) Ford street supercharger kit on request. More importantly, the company has new 192-cid and 250-cid low-profile street blowers. Blower Shop superchargers are CNCmanufactured from 6061, T-6-aluminum bar stock. The supercharger cases measures 85 ⁄8 inches from the top of the blower to the carburetor base plate (billet carburetor adaptor sold separately), which makes them ideal for any smallblock-powered early- or late-model Ford cars, trucks, and even street rods.
The input shaft and coupler are heat-treated 4340 steel. Its drive gears are also heat treated and specially ground for longevity and smooth operation. This unit uses a 16-rib drive belt, and a total of seven different top pulleys (available in either V-belt or serpentine) are available to provide up to 10-psi boost.
The Blower Shop only offers their 192-cid and 250-cid mini-blowers for small-block Fords.
The Blower Shop
26846 Oak Avenue, Units C&D
Santa Clarita, CA 91351
Phone: (661) 299-5483
Fax: (661) 299-5485
Blower Drive Service (BDS)
Established in 1969 in a one-car garage in Whittier, California, BDS has gone on to become the top manufacturer in the high-performance Roots-blower aftermarket. However, unlike many of your blower manufacturers who started out building race blowers, and then developed a line of street blowers, BDS started out as (and continues to be) a street blower specialty company, although they do build some impressive competition blower setups as well.
In regard to the street blower segment of the market, BDS is dual faceted. The company will service, rebuild, and update versions of the popular GMC supercharger. They also offer over-thecounter Jimmy’s using BDS-developed components. BDS was one of the first in the industry to manufacture replacement parts and upgrade components for these blowers, and today, they are clearly the largest.
BDS also manufactures their own blower cases (in everything from 4.71 to 16.71 sizes), along with their own intake manifolds, blower pulleys, blower drives, carburetor plates, fuelinjection plates, fuel-injection throttle bodies, air scoops, blower cams, engine management systems – you name it!
“Probably one of our real claims to fame is the fact that if it came equipped with a popular American V-8 engine, we probably offer an intake manifold and a kit to fit that particular engine,” laughs Craig Railsback, and he’s not kidding! All told, BDS offers street blower kits for a total of seven different automotive manufacturers, with a total of 106 applications! When it comes to Ford products, BDS manufactures 26 different setups ranging from the 289/302 up to the 429-460 Ford big-blocks – Boss 429 included.
Starting with the 289/302 pushrod V-8, BDS offers a total of three street blower kits based on the actual engine accessory packages (power steering, alternator, air conditioning, smog pump, etc.) you plan on using. They are:
p/n 526-3S1/2: BDS 289/302 3-inch 1v 6.71
p/n 526-3S2/2: BDS 289/302 3-inch 2v 6.71
p/n 526-3S3/2: BDS 289/302 3-inch 3v 6.71
p/n 566-3S1/2: BDS 351W 3-inch 1v 6.71
p/n 566-3S2/2: BDS 351W 3-inch 2v 6.71
p/n 566-3S3/3: BDS 351W 3-inch 3v 6.71
* In recent years, the popularity of the Ford 351-Cleveland (“C”) engine has been decreasing, due much to the fact that parts for these engines are becoming scarce, and with the implementation of EFI into Ford’s product line in 1984 (and the subsequent conversion of the 351 Windsor to EFI), the 351 Cleveland has sort of taken a back seat. Nonetheless, BDS still stocks a total of three kits for this engine, and another three for its second cousin, the slightly larger Ford 400M, which uses a larger supercharger. They are:
p/n 536-3S1/2: BDS Ford 351-C 3-inch 1v 6.71
p/n 566-3S2/2: BDS Ford 351-C 3-inch 2v 6.71
p/n 566-3S3/2: BDS Ford 351-C 3-inch 3v 6.71
p/n 578-3S1/2: BDS Ford 400M 3-inch 1v 8.71
p/n 578-3S2/2: BDS Ford 400M 3-inch 2v 8.71
p/n 578-3S3/2: BDS Ford 400M 3-inch 3v 8.71
Blower Drive Service (BDS)
12140 E. Washington Blvd.
Whittier, CA 90606
Phone: (562) 603-4302
Fax: (562) 696-7091
Holley Performance Parts recently acquired both the B&M Street Supercharger product line and the Weiand Equipment Company (say, “whyand”). This gave the carburetor king a staggering lineup of street-strip blower hardware, ranging from the ex-B&M Street (mini) Blower to the complete Weiand line of GMC Roots-type street blower kits.
Surprisingly though, the manufacturing giant only offers a total of three applications for the Ford small-block V-8 engines from the vintage Ford 21- stud Flathead V-8 (hey, that’s a smallblock) to 289/302 W-style engines. The proverbial centerpiece for these kits is Holley/Weiand’s low profile, 177 Pro Street long-nose supercharger. Holley/Weiand claims (horse) power gains of 25 to 40 percent are entirely realistic. In each application, this blower comes with its own dedicated intake manifold, along with a 10-rib blower drive, lower crank pulley, and all the necessary support systems accessory brackets.
p/n 6551-1 (satin), 6552-1, (polished): 177 Ford Flathead Pro-Street Kit
p/n 77-174FSBS-1 (satin), p/n 77-174FSBP-1 (polished): Holley/Weiand 177 Ford Small
Block Pro-Street Kit
The big news at Holley these days is the manufacturer’s 50-state legal Holley Thunder Power-Charger Kit for the 1997-2004 5.4L-equipped, 2-valve Ford Expedition, Lincoln Navigator, and Ford F-150s. This kit is reputed to boost the power of your 5.4L SOHC mod motor to an incredible 390 hp and 445 ftlbs of torque.
This kit comes with the Holley Thunder 144 Power-Charger supercharger, a dedicated intake manifold, 10- rib supercharger drive belt and tensioner, and fully integrated liquid-toair intercooler with Earl’s AN fittings. The air and fuel are handled by a Power Shot conical air filter kit, an 80-mm CNC-machined 6061 billet-aluminum throttle body, Holley boost compensator regulator, and a 225-lph electric in-tank fuel pump. A Holley Thunder Power Chip, all the brackets and fittings, complete instructions, and Holley Thunder fender logos are also included. Holley also hints of a 4.6L SOHC and DOHC Holley Thunder package for the Mustang GT and SVT Mustang Cobra models in the near future.
p/n 300-520: Holley-Thunder 144 Power-Charger
P.O. Box 10360
Phone: (800) HOLLEY
Tech Line: (270) 781-9471
Mike Kuhl, famed Top Fuel engine builder and car owner, established Kuhl Superchargers (just say “Kool”) (Kuhl & Olsen “Revell Fast Guys” AA/FD) back in 1970. Initially, the company only built blowers for use in drag boat racing and professional drag racing. Kuhl branched out into the street blower aftermarket in the early 1980s with his big-block Chevrolet street blower kit, which became a big success.
In December 2001, Mike Kuhl sold his company to longtime employee/ engineer Kevin Peters. Peters expanded the Kuhl product line to 12 kits, with over 25 different fitments and/or applications.
For Kuhl’s small-block Ford street supercharger program, the Santa Ana, California-based supercharger manufacturer has taken a rather (dare we say it) “Kool” approach, with a decidedly high-tech twist.
“We offer two street supercharger kits for the 5.0L (302 ci) and 5.8L (351W) Ford small-block V-8 engines,” says Peters. “Rather than milling off the top of a conventional 4-barrel intake manifold and welding on a plate, what we’re doing here is using the 5.0L and 5.8L Ford truck rectangular EFI intake manifold base, and bolting an adaptor plate up to it! The welded two-piece carbureted (4v) intake manifold has been the standard in the industry ever since the first street blower kit was designed for this application. First of all, with this type of approach, rarely if ever do you get the intake runner length right due to the fact that you’re machining off an integral part of the manifold, so you’re sacrificing low-end torque and some degree of horsepower right off the bat. Secondly, with the use of a plate welded to the top of a modified 4-barrel intake, you often run the risk of warp-age problems due to it being welded.
“We’ve chosen the Ford truck base because it’s readily available. It features an excellent runner length and intake port volume, and due to the fact that you already have the fuel injector bungs plumbed into the base, you can either plug them up and run conventional carburetors (with carburetor plates available for Holley, Barry Grant Demon, or Edelbrock 4v carburetors) or set it up to run with electronic fuel injectors and twin 70- or 75-mm throttle bodies. Either application will also work with an Edelbrock, Weiand, or Trick Flow Specialties (TFS) EFI lower intake manifold, as they are all basically the same as the Ford. You can even use the stock Ford ignition and stock Ford fuel rails and fuel regulator with this setup. It’s a really neat system!”
Of course, the actual centerpiece with either setup is one of the company’s fully polished “Kuhl Kased” 6.71 superchargers.
“This blower is brand new from the ground up. It features one of our fully polished 6.71 Kuhl Kases, along with brand-new billet-aluminum end frames, a new front cover equipped with brand new gears, and rotors. It’s an all-new supercharger!
“Another interesting feature about our Ford kits is that we drive the water pump with a cog belt pulley, and we use the factor Ford damper rather than a crank hub so that the life of the crank will be increased (less vibration).”
Listed below are the part numbers for Kuhl Superchargers Kits:
p/n 02401-62: 5.0L/289/302 Street Supercharger Kit
p/n 02404-61: 5.8L/351W Street Supercharger Kit
p/n 02402-63: 351-C Cleveland Street Supercharger Kit
2222 West Second Street Santa Ana,
Phone: (714) 547-7071
Fax: (714) 547-4170
Mert Littlefield Blowers
Longtime Funny Car racer Mert Littlefield originally worked for Danekas Superchargers, but purchased the company in 1974. Annual sales of approximately 250 new Littlefield street and strip 6.71 to 14.71 superchargers a year, plus a thriving rebuild business, keep Mert and his staff of 10 very busy!
Customers include street machiners, nostalgia racers, tractor-pull racers, mud bog racers, and monster truck racers. Unlike the bigger guys, Littlefield likes to focus on one sale at a time. In regard to prepackaged small-block Ford street supercharger kits, Littlefield doesn’t offer one.
“We build our own superchargers and our own blower drives. However, we don’t offer a Ford small-block kit per se. What we usually do is purchase the intake manifold, and anything else the customer specifies, from a company like BDS, and then ship them their ‘kit.’”
1114 East Kimberly Anaheim,
Phone: (714) 992-9292
Fax: (714) 992-1717
Famed drag racer Gene Mooneyham has been building street and strip superchargers since 1951. The company offers a full line of both aluminum and magnesium blower cases in everything from 6.71 to 14.71 configurations, along with a complete line of support hardware.
Like Littlefield, Mooneyham manufactures his own superchargers but doesn’t offer a small-block Ford street blower kit. You’ll have to build one yourself!
1935 West 11th Street,
Unit N Upland,
Phone: (909) 985-4425
Fax: (909) 985-4435
Written by Bob McClurg and Posted with Permission of CarTechBooks