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A supercharger is one of the oldest methods of adding power to an engine, so it’s no surprise that racers around the world have always found ways to harness that power to make their cars faster. The supercharger has been a staple in drag racing since the sport began, from the first hot rods on the salt flats to the beginnings of Top Fuel racing. The two most common types of superchargers in drag racing are the roots and screw blowers that can be seen sticking out of the hoods of different racecars in venues around the world. Today, we’re going to take a look at these superchargers to discuss their internal workings and how they’re so similar yet so very different.
Because an engine acts as a giant air pump, the best way to make more power is to increase the amount of air that flows through it. The supercharger forces more oxygen into the motor, allowing more fuel to burn, and that in turn creates a higher power output. The roots and screw blowers you see at the track may look like they bring air into the motor the same, but there are actually a few differences in how they function.
Blowers have been a part of drag racing since its earliest days and old blowers can still be seen on nostalgia cars like these.
The Roots Blower
The roots supercharger acts like a large positive displacement air pump on the top of a motor and is what the nitro-guzzling NHRA Top Fuel and Funny Cars use to make their power. Inside the blower, there are two rotors that typically have three lobes each, and they’re what move the air inside the case. The air is held inside pockets that are around the lobes and is then moved from the intake to the discharge side of the lobe. As this is done, the rotors move more air than the motor can ingest, creating boost inside the manifold of the supercharger.
Darren Mayer, owner of DMPE Inc, is an expert when it comes to supercharging and its parts. He provides more background on what makes up the modern roots blower.
“The rotors inside the blower are a helical, three lobe design. The rotor has three male lobes and the area between them that accommodates the opposite rotor is called the root. All modern performance rotors have sealing Teflon strips machined into them that are replaceable. The case is a billet or cast housing that houses the rotors. The case provides a sealing bore for the rotors to run in and a location for rotor centers to direct air in and out of the unit, along with a way to mount the supercharger to the motor.”
Here are some examples of different roots blower case designs from DMPE.
Roots blowers are often referred to based on a number like 8-71 or 14-71. These are just random numbers assigned, yet they have a unique meaning. “The numbers are derived from the original GM diesel trucks that had a roots blower on it called a 6-71,” Mayer explains. “That blower had a 15-inch long rotor. From that day forward, a 15-inch rotor blower is called a 6-71, a 16-inch long rotor is called an 8-71, and a 19-inch long rotor is a 14-71 blower. As you can see, the bigger the number, the longer the rotor, and the more power it will make.”
The Screw Blower
The screw blower may look similar to a roots blower, however, it functions differently. The roots blower acts like a pump for the air as it goes into the motor, whereas the screw blower is more like an air compressor in how it forces the atmosphere into the engine.
A screw blower is able to deliver the air into the engine in a more efficient manner, as the rotors that capture the air reduce in size as they’re turned, which in turn causes the air to be compressed as it’s introduced into the manifold.
On this PSI screw blower you can see how much different the case and rotor design is compared to a roots blower.
The screw blower requires less energy to do the same amount of work compared to the roots blower. -Darren Mayer
Another difference; the screw blower doesn’t use the same numbering system for the size of the rotors that reside inside the case — it uses a letter classification like a C-Rotor or D-Rotor. The letter indicates just how big the rotor is, and the biggest one on the market is the C-Rotor and is typically used in forms of racing where there is no limit on the rotor size. The D-Rotor is the largest rotor size permitted in NHRA competition for those racers who use a screw blower.
The biggest advantage the screw blower design provides is how efficient it is over the roots blower when it comes to making power. “The screw blower requires less energy to do the same amount of work compared to the roots blower. Given the same 40 psi of manifold pressure, the screw blowers require on average 400 less horsepower to generate this amount of boost. This reduction of required power is delivered to the crankshaft for use in accelerating the vehicle, as opposed to operating the supercharger,” Mayer explains.
Roots Blower Vs The Screw Blower
When it comes down to it, the goal of a roots and screw blower is the same; move as much air into the motor as possible to increase power. How well that’s done between the two is where the big differences arise and show which blower can make more power.
Andy Severyn, John Sears, and Chelsea Clark are the core group of people who run PSI Superchargers, one of the biggest names in the roots and screw blower market. They have spent years perfecting both kinds of blowers and know what it takes to make big power with each.
Roots Blower Overdrive
A roots blower can have a certain amount of overdrive based on the class you are in. Janis explains how overdrive in a roots blower works. “While you would think that turning a supercharger at a higher overdrive would make more power, it’s not always the case. It actually will create a lot of heat in the supercharger, which then causes the intake temperature to rise, thus making your turn up much more difficult. In our particular case, the NHRA mandates that our supercharger has an overdrive limit of 16.5 percent overdrive. This means that with the pulley combination of the top pulley and the bottom pulley, the blower is turning 16.5 percent faster than the actual engine RPM. It goes hand in hand no matter what overdrive combination you are using.”
“The roots blower functions more as an air pump with parts that touch inside causing heat and friction, and can only spin to 13,500-14,000 rpm before it starts to create more heat than boost. That heat and friction causes the roots blower to be much less efficient and can’t move as much air as the screw blower.
“The screw blower is more efficient and you can spin them to 24,000 to 26,000 rpm and still have more room to create power, but there aren’t any cylinder heads out there that can use the level of air a screw blower can produce. Screw blowers are the way to go if you want to make maximum power because they’re efficient and it takes less horsepower to turn them.”
To further drive home the fact on the efficiency differences between the two blowers, you simply look at how much air a roots blower and screw blower can displace based on their sizes. The largest 14-71 roots-style blowers are able to displace over 540 cubic-inches of air, while a C-Rotor screw blower is usually displacing only 472 cubic-inches of air, but is able to do it more efficiently and produce a much cooler charge of air over the roots blower.
Mike Janis, Jr. of Mike Janis Superchargers has been helping people make huge power with both roots and screw type blowers for all kinds of racing for many years. He puts it into very simple terms exactly why the screw blower is able to make more power than the roots blower on a given engine.
“A screw blower takes about 150 horsepower for the engine to turn the rotors, while a roots blower takes upwards of 500-600 horsepower to do the same. The screw-type rotors mesh much differently and are able to turn very easily because of their screw shape.”
A set of screw rotors on the left, roots on the right.
A screw blower takes about 150 horsepower for the engine to turn the rotors, while a roots blower takes upwards of 500-600 horsepower to be able to turn the rotors. -Mike Janis Jr.
According to Severyn, you need to know what your goals are with the class you’re going to race before you choose a blower. “One of the big things you need to look at is the kind of racing you’re doing and where. We recently had a customer that loved his screw blower, but the problem was the tracks he was going to just couldn’t hold the power it was creating. Since the blower was too efficient at making power, it was better for them to go to a roots blower to get the most out of the combination.”
Another factor are the class rules. Due to how efficient a screw blower is, there is usually a hefty weight penalty attached to using them in a specific class, if they’re even permitted at all. “Many times a roots blower car can weigh a lot less than a screw-blown car — sometimes as much as 300 pounds less. Because of this, the weight penalty on the screw blower might play a part in choice due to what it would take to make the screw blower versus the roots blower work at that weight,” Severyn says.
Different organizations and eliminators have varying rules and limitations on superchargers; in some cases, both screws and roots are permitted with weight penalties depending on choice.
How the blowers are limited also makes a difference in how often they’re used, according to Janis. “It all depends on the class and application you’re running. For every 1,000 roots blowers out there, you are lucky if there’s one screw blower being used to that ratio. Roots blowers are much more common and accepted in the racing world overall.”
How the blower sits on the engine presents another difference in how they perform, and that’s where a setback blower comes in. “We set back the supercharger to allow the airflow characteristics of the supercharger to better feed all eight cylinders. You really want every cylinder to carry the same load. If the blower is too far forward and the front cylinders get proportionally more air, they will need more fuel to keep them happy. Therefore, they’re doing more work than the other six. By moving the blower, back we can square up the motor and make it do the work more evenly,” Mayer explains.
Top Fuel Dragsters and Funny Cars both use setback-style blowers to help distribute the air better into the volatile engines.
The transmission behind the motor, no matter the blower type, must be able to handle the level of power the motor is putting out. In Mayer’s experience, the type of transmission doesn’t matter to the blower one bit, but it comes down to how the power is put down. “The supercharged motor won’t care what transmission it has been attached to, but the vehicle will definitely care. How you multiply the power and use it is a large part of drag racing, so making sure the transmission can utilize the blower power is key,” Mayer explains.
What RPM level you spin the motor and blower to is what makes the difference on transmission selection and how the power is applied. “A clutch or converter setup will react differently in how you apply the power and where. Another difference with the transmission is how you stage the car; a screw blower likes to stage at higher RPM because of how the blower makes power,” Severyn says.
Blower Power Basics
The roots and screw blowers are just a small part of the high-performance equation that’s under the hood of a forced induction racecar. The engine is built around the blower and, given that both roots and screws are allowed in whatever class the racer is going to participate in, their options can be fairly open on what route they go. The best choice in heads, manifolds, and engine size is more directly affected by the class and what blower they will allow, because the combination must match what will work best with that particular blower type.
“In the old days, you would need to get a small-block Chevy to use because they could deal with the higher RPM you would throw at it with a blower. Now with these modern engines, like from Brad Anderson, Alan Johnson, or Noonan, you can spin a bigger motor to a higher RPM. It’s all about keeping that valvetrain together. Compression ratios also play into it based on the combination; you might have one guy running a roots blower motor on 8:1 while another is running 10:1 with a screw blower on alcohol. Talk to different guys, and you’ll get a lot of different answers on what works best, but there’s no real definitive answer,” Severyn explains.
The style and design of injector hats varies across the board for both types of blowers.
The injector hat also plays a role in how the air flows into the blower, and that can influence just how much power the blower is able to create, regardless of supercharger type. According to Mayer, where the injector hat is placed has just as much to do with how much power it produces as the size. “If you put a Top Fuel hat on a door car it really won’t make more boost. The injector hat’s job is to get the best air charge to the supercharger as possible. A dragster requires the hat to be above the driver, so that it’s positioned in clean air. A door car and Funny Car can take advantage of air accelerated by the shape of the body as it drives through the air.”
Janis agrees with Mayer when it comes to injector hat technology. “Today’s injector hats, in general, provide much more power than any of the older hats. There are so many different shapes and sizes, which can be made out of carbon fiber or aluminum and help direct airflow differently and create the smoothest transition possible into the supercharger.”
The big cubic inch motors, like 900 cubic-inch nitrous motor, can’t spin hard enough to really use the blower since they’re RPM limited. -Andy Severyn
“There is really no reason to put a blower on an engine that has a huge displacement. The blowers make so much torque and horsepower that when you start getting that big, you would actually start going the other direction when it comes to making power,” Janis says.
It doesn't matter if the racer uses a roots blower or screw blower, they don't need a ton of cubic-inches to make power with either!
According to Severyn, this is illustrated even more when you look at how a screw blower behaves with an engine size, and why you don’t see mountain motors in the blower world. “The big cubic-inch motors, like a 900 cubic-inch nitrous motor, can’t spin hard enough to really use the blower since they’re RPM-limited. If you try to spin one of those big motors hard enough to use a blower, the valvetrain would fly apart. The screw blower is so efficient it doesn’t need all the extra cubic-inches to create the horsepower like a big cubic-inch nitrous or turbo motor does.“
Both roots and screw blowers are great ways to make big power in any class of racing, and they’re perhaps the most tried-and-tue form of power adding in the game. The choice on which blower to use really comes down to how comfortable you are with a combination, the style of racing, and what the rules are for the class. Each blower has its own advantages and can help push your racecar to new levels of speed and performance on any race weekend.
