Good screw S/C, Roots Blower info
06-10-2006, 06:26 PM
#1
Good screw S/C, Roots Blower info
An inside look at the PSI and High Helix Rootes
"Probably the most significant innovation for the classes of Top Alcohol Dragster and Top Alcohol Funny Car took place when Norm Drazy and Mert Littlefield decided to "build a better mousetrap." Specifically, we're referring to Drazy's PSI screw-compressor supercharger and Littlefield's improved Rootes blower, commonly known as the high helix Rootes.
Without question these two devices have vastly advanced TA/D and TA/FC performance levels. It could easily be argued that the first alcohol cars in the five-second zone - Bob Newberry in TA/FC, with a Littlefield-built high helix, and Steve Faria in TA/D (also the first to run 230 mph) with Drazy's PSI - were the direct result of the new supercharging units. Because within the last 12 months their use has led to such eye-opening performances, a look at the theory behind the PSI and high helix Rootes, as well as a glance at the hardware itself, is in order.
Clearly the most radical of the two new units is the PSI. The brainchild of Drazy, a former aerospace engineer (in 1986 he left Garrett AiResearch after 17 years to start Performance Systems Inc., from where the supercharger gets its name), the PSI is an entirely new supercharging concept, unlike the high helix, which is an improved version of the Rootes.
Drazy has been involved with drag racing since the Seventies when he was partnered with Chauvin Emmons on Emmons' TA/D effort. That racing experience, combined with a professional knowledge of screw compressor technology, has given Drazy the necessary tools to head down a different supercharging path.
The screw compressor, invented in 1943 by Swedish engineer A.J. Lysholm, is the standard in the compressed air industry. In fact, the top five air compressor companies in the U.S. feature screw compressors as state of the art in air compression.
Hardly noticed by a layman, screw compressors are widely used. They provide compressed air for pneumatic tools and machines (such as concrete-breaking jackhammers), air conditioning units for office buildings and factories, and are widespread in manufacturing where compressed air is needed.
Drazy came upon the idea of outfitting a screw compressor for drag racing service from two articles written by British engineer Mark Ransome. In 1965, Ransome proposed using a screw compressor on a Formula One engine.
Like the Rootes, the PSI supercharger fits atop the engine and is driven by the crankshaft. Both are positive-displacement pumps, meaning they move a given volume of air with every turn of their two counter-rotating rotors. However, when it comes to supercharging an engine, there is a significant difference between the two.
Drazy began his quest for a better supercharger based on the premise that the Rootes blower was never designed to handle the task of supercharging an engine with 35 to 40 pounds of boost pressure. Historically, he was correct in that assumption. The Rootes used in drag racing originally came from GM diesels.
They were designed by GM in the late '30s to blow fresh air -- at zero boost -- into the combustion chambers of their two-stroke diesels to aid in the exhausting. Early drag racers adapted stock GM, and later, aftermarket GM Rootes blowers, for the purpose of supercharging, or boosting cylinder volume above atmospheric pressure.
However, the Rootes was never designed to supercharge a drag racing engine to 30-plus psi levels. It can supercharge an engine, but not to the degree required in drag racing without the high cost paid in overall operating efficiency. A Rootes maintains fairly good efficiency up to 10 psi, but beyond that, its efficiency drastically drops off.
One of the problems with a Rootes is that it compresses air by back flow. Air is taken in at the inlet port at the top of the Rootes at 14.5 psi -- atmospheric pressure at sea level. It carries this 14.5-psi air in the spaces between its lobes and along the outer cases until it reaches the outlet port at the bottom, 180 degrees later. The air is deposited in the intake manifold where it piles up under pressure. That is where the problems begin.
First, the high-pressure plenum air continually wants to flow back into the low-air-pressure rotor spaces. This force tries to turn the rotors, and crankshaft, backwards. It takes a tremendous amount of power from the engine to turn the rotors against this pressure.
When Drazy began working on his screw supercharger several years ago, he built a supercharger dyno to measure, among other things, how much horespower a Rootes consumed at a certain rpm and boost pressure. He found that a 14-71 Rootes required 818 horsepower at 11,000 rpm and 35 pounds of boost -- that horsepower figure represents between 30 and 40 percent of an alcohol engine's horsepower output.
Second, back flow causes a tremendous amount of air heating. Moving pressure waves of air between the plenum and the rotor spaces at the rate of six cycles per revolution, according to Drazy, produces typhoon-like conditions inside the intake manifold. When the air is moved around to that degree, high temperatures are encountered due to friction.
As a result, air density is decreased, which makes the supercharger's job all the more difficult. (It was fitted to the engine to increase air density in the first place.) To make up for the rapdily thinning air, the Rootes has to be turned faster, which really takes a toll on efficiency.
Third, there is a significant pumping loss through the rotors as the high-pressure air pushes its way up past the rotors and out the inlet port.
Drazy was intrigued with a screw-type compressor design for a number of reasons. It could be readily adapted to drag racing use; it could be belt-driven like the Rootes for instant throttle response; it uses two counter-rotating intermeshing rotors like the Rootes; and its shape and location would be similar to the venerable Rootes. Therefore, a screw supercharger could be bolted on an engine with minimal change of hardware.
The efficiency of the PSI compared to the Rootes can be traced to the design of the rotors. The PSI employs a four-lobed male rotor and a six-lobed female rotor (the standard Rootes has a pair of three-lobed rotors). These two rotors are twisted at a high angle (the male has 300 degrees of twist and the female, 200 degrees) compared to the standard Rootes, which has a helical angle of 60 degrees.
Because of this high twist and rotor design, compression of the air begins almost immediately inside the PSI. Like the Rootes, the PSI fills from the top and discharges at the bottom; but unlike the Rootes, the PSI employs internal compression. This is why the PSI requires less horsepower to turn than the Rootes. Due to the extreme helix of the rotors, less and less of the rotor surfaces are involved as air is trapped in the rotor spaces at the top, compressed by the rotor's unique design, and deposited in the intake manifold.
With the Rootes, all of the rotor surfaces "see" all of the air pressure all of the time. Also, compression is a much smoother process and heat build-up is greatly reduced. According to Drazy, the overall efficiency -- mechanical, volumetric and adiabatic -- of the Rootes begins at 40 percent and drops off to 20 percent at the end of a run. Drazy says his PSI maintains an overall efficiency of 60 percent throughout the run.
Drazy's new supercharger was looked at with quite a bit of skepticism by racers when it was ready for on-track testing 18 months ago. The majority took a "show-me-first" attitude. Two who didn't were TA/D racers Mark Niver and Gary Southern, who drives for Dale Smart. The PSI made its competition debut at the California Nationals last year, when Niver qualified number one, with a 6.25, but lost in the second run on a holeshot. He also ran the top speed for the class at 220.53.
That performance did away with some doubts. But what really won everyone over was Southern's jaw-dropping win at the U.S. Nationals five weeks later. Southern ran the then-quickest TA/D e.t., a 6.12 in the semi's. It was the speeds the car ran that further stunned everyone; a best of 226.24 in qualifying, and in eliminations, a 223.38, 220.75, 223.32 and a final-round 227.96, the fastest speed ever for a TA/D. That seems to validate Drazy's claim that the more efficient PSI freed up approximately 400 horsepower on the top end.
Though the first two PSI's used billet aluminum rotors, Drazy's plan all along was to come out with cast-aluminum rotors. The reason was to save cost, weight, and development time. The paring of weight was critical, as the rotors represented rotating weight. (Racing 14-71 Rootes' use much lighter cast-magnesium rotors).
Fuel crewchiefs were especially concerned with the problem of how long it would take the heavy billet rotors to come up to speed off the starting line. The alcohol racers didn't have this worry as they brought the motor up to 4500 to 5000-rpm during staging. Not so with a fuel car; it sits on the line at a dead idle waiting for the light. Fuel racers can't afford to wait for the rotors to spool up before delivering the required boost pressure.
Drazy's first try with cast rotors was not very successful, to put it mildly. West coast racer Jay Payne tried the first cast-rotor version at an NHRA points meet in September of last year. When the car got to half-track on its first pass, the rotors disintegrated. The casing then exploded, spewing metal fragments all over the track.
It was discovered that stress cracks in the casting process caused the rotors to come apart. Drazy switched foundries, redesigned the rotors, and computer tested the new rotors. He then built a spin chamber, spun the rotors in excess of 25,000 rpm, and has had no problems with the hollow-rotored PSI since then. The move from billet to cast rotors has removed 27 pounds of weight from the PSI."
"Probably the most significant innovation for the classes of Top Alcohol Dragster and Top Alcohol Funny Car took place when Norm Drazy and Mert Littlefield decided to "build a better mousetrap." Specifically, we're referring to Drazy's PSI screw-compressor supercharger and Littlefield's improved Rootes blower, commonly known as the high helix Rootes.
Without question these two devices have vastly advanced TA/D and TA/FC performance levels. It could easily be argued that the first alcohol cars in the five-second zone - Bob Newberry in TA/FC, with a Littlefield-built high helix, and Steve Faria in TA/D (also the first to run 230 mph) with Drazy's PSI - were the direct result of the new supercharging units. Because within the last 12 months their use has led to such eye-opening performances, a look at the theory behind the PSI and high helix Rootes, as well as a glance at the hardware itself, is in order.
Clearly the most radical of the two new units is the PSI. The brainchild of Drazy, a former aerospace engineer (in 1986 he left Garrett AiResearch after 17 years to start Performance Systems Inc., from where the supercharger gets its name), the PSI is an entirely new supercharging concept, unlike the high helix, which is an improved version of the Rootes.
Drazy has been involved with drag racing since the Seventies when he was partnered with Chauvin Emmons on Emmons' TA/D effort. That racing experience, combined with a professional knowledge of screw compressor technology, has given Drazy the necessary tools to head down a different supercharging path.
The screw compressor, invented in 1943 by Swedish engineer A.J. Lysholm, is the standard in the compressed air industry. In fact, the top five air compressor companies in the U.S. feature screw compressors as state of the art in air compression.
Hardly noticed by a layman, screw compressors are widely used. They provide compressed air for pneumatic tools and machines (such as concrete-breaking jackhammers), air conditioning units for office buildings and factories, and are widespread in manufacturing where compressed air is needed.
Drazy came upon the idea of outfitting a screw compressor for drag racing service from two articles written by British engineer Mark Ransome. In 1965, Ransome proposed using a screw compressor on a Formula One engine.
Like the Rootes, the PSI supercharger fits atop the engine and is driven by the crankshaft. Both are positive-displacement pumps, meaning they move a given volume of air with every turn of their two counter-rotating rotors. However, when it comes to supercharging an engine, there is a significant difference between the two.
Drazy began his quest for a better supercharger based on the premise that the Rootes blower was never designed to handle the task of supercharging an engine with 35 to 40 pounds of boost pressure. Historically, he was correct in that assumption. The Rootes used in drag racing originally came from GM diesels.
They were designed by GM in the late '30s to blow fresh air -- at zero boost -- into the combustion chambers of their two-stroke diesels to aid in the exhausting. Early drag racers adapted stock GM, and later, aftermarket GM Rootes blowers, for the purpose of supercharging, or boosting cylinder volume above atmospheric pressure.
However, the Rootes was never designed to supercharge a drag racing engine to 30-plus psi levels. It can supercharge an engine, but not to the degree required in drag racing without the high cost paid in overall operating efficiency. A Rootes maintains fairly good efficiency up to 10 psi, but beyond that, its efficiency drastically drops off.
One of the problems with a Rootes is that it compresses air by back flow. Air is taken in at the inlet port at the top of the Rootes at 14.5 psi -- atmospheric pressure at sea level. It carries this 14.5-psi air in the spaces between its lobes and along the outer cases until it reaches the outlet port at the bottom, 180 degrees later. The air is deposited in the intake manifold where it piles up under pressure. That is where the problems begin.
First, the high-pressure plenum air continually wants to flow back into the low-air-pressure rotor spaces. This force tries to turn the rotors, and crankshaft, backwards. It takes a tremendous amount of power from the engine to turn the rotors against this pressure.
When Drazy began working on his screw supercharger several years ago, he built a supercharger dyno to measure, among other things, how much horespower a Rootes consumed at a certain rpm and boost pressure. He found that a 14-71 Rootes required 818 horsepower at 11,000 rpm and 35 pounds of boost -- that horsepower figure represents between 30 and 40 percent of an alcohol engine's horsepower output.
Second, back flow causes a tremendous amount of air heating. Moving pressure waves of air between the plenum and the rotor spaces at the rate of six cycles per revolution, according to Drazy, produces typhoon-like conditions inside the intake manifold. When the air is moved around to that degree, high temperatures are encountered due to friction.
As a result, air density is decreased, which makes the supercharger's job all the more difficult. (It was fitted to the engine to increase air density in the first place.) To make up for the rapdily thinning air, the Rootes has to be turned faster, which really takes a toll on efficiency.
Third, there is a significant pumping loss through the rotors as the high-pressure air pushes its way up past the rotors and out the inlet port.
Drazy was intrigued with a screw-type compressor design for a number of reasons. It could be readily adapted to drag racing use; it could be belt-driven like the Rootes for instant throttle response; it uses two counter-rotating intermeshing rotors like the Rootes; and its shape and location would be similar to the venerable Rootes. Therefore, a screw supercharger could be bolted on an engine with minimal change of hardware.
The efficiency of the PSI compared to the Rootes can be traced to the design of the rotors. The PSI employs a four-lobed male rotor and a six-lobed female rotor (the standard Rootes has a pair of three-lobed rotors). These two rotors are twisted at a high angle (the male has 300 degrees of twist and the female, 200 degrees) compared to the standard Rootes, which has a helical angle of 60 degrees.
Because of this high twist and rotor design, compression of the air begins almost immediately inside the PSI. Like the Rootes, the PSI fills from the top and discharges at the bottom; but unlike the Rootes, the PSI employs internal compression. This is why the PSI requires less horsepower to turn than the Rootes. Due to the extreme helix of the rotors, less and less of the rotor surfaces are involved as air is trapped in the rotor spaces at the top, compressed by the rotor's unique design, and deposited in the intake manifold.
With the Rootes, all of the rotor surfaces "see" all of the air pressure all of the time. Also, compression is a much smoother process and heat build-up is greatly reduced. According to Drazy, the overall efficiency -- mechanical, volumetric and adiabatic -- of the Rootes begins at 40 percent and drops off to 20 percent at the end of a run. Drazy says his PSI maintains an overall efficiency of 60 percent throughout the run.
Drazy's new supercharger was looked at with quite a bit of skepticism by racers when it was ready for on-track testing 18 months ago. The majority took a "show-me-first" attitude. Two who didn't were TA/D racers Mark Niver and Gary Southern, who drives for Dale Smart. The PSI made its competition debut at the California Nationals last year, when Niver qualified number one, with a 6.25, but lost in the second run on a holeshot. He also ran the top speed for the class at 220.53.
That performance did away with some doubts. But what really won everyone over was Southern's jaw-dropping win at the U.S. Nationals five weeks later. Southern ran the then-quickest TA/D e.t., a 6.12 in the semi's. It was the speeds the car ran that further stunned everyone; a best of 226.24 in qualifying, and in eliminations, a 223.38, 220.75, 223.32 and a final-round 227.96, the fastest speed ever for a TA/D. That seems to validate Drazy's claim that the more efficient PSI freed up approximately 400 horsepower on the top end.
Though the first two PSI's used billet aluminum rotors, Drazy's plan all along was to come out with cast-aluminum rotors. The reason was to save cost, weight, and development time. The paring of weight was critical, as the rotors represented rotating weight. (Racing 14-71 Rootes' use much lighter cast-magnesium rotors).
Fuel crewchiefs were especially concerned with the problem of how long it would take the heavy billet rotors to come up to speed off the starting line. The alcohol racers didn't have this worry as they brought the motor up to 4500 to 5000-rpm during staging. Not so with a fuel car; it sits on the line at a dead idle waiting for the light. Fuel racers can't afford to wait for the rotors to spool up before delivering the required boost pressure.
Drazy's first try with cast rotors was not very successful, to put it mildly. West coast racer Jay Payne tried the first cast-rotor version at an NHRA points meet in September of last year. When the car got to half-track on its first pass, the rotors disintegrated. The casing then exploded, spewing metal fragments all over the track.
It was discovered that stress cracks in the casting process caused the rotors to come apart. Drazy switched foundries, redesigned the rotors, and computer tested the new rotors. He then built a spin chamber, spun the rotors in excess of 25,000 rpm, and has had no problems with the hollow-rotored PSI since then. The move from billet to cast rotors has removed 27 pounds of weight from the PSI."
Last edited by moregrip; 06-10-2006 at 06:33 PM.
06-10-2006, 06:26 PM
#2
"The second supercharger common to alcohol classes was developed jointly by Littlefield and Andy Carl, an engineer for an Orange County, California, aerospace firm.
Carl had ideas of improving the Rootes but not the resources to go about building a better one. He approached Littlefield with two simple modifications that would improve the efficiency of the Rootes. Prompted by the loss of business to the PSI, Littlefield agreed to try Carl's suggestions.
First, he suggested that the helix angle be doubled from 60 to 120 degrees. Next, he wanted to restrict the outlet port to a small pie-shaped wedge similar in size and shape to the PSI.
Increasing the rotor helix angle changed one very important characteristic of the Rootes: It lowered the rotor Mach number. The rotor Mach number refers to the rotational speed at which the rotor tips go sonic, or are turning at the speed of sound (i.e., Mach 1). The resulting shock waves disrupt smooth air flow around the rotor tips. When the rotor tips rotate at Mach 1, around 12,000 rpm, volumetric and compressor efficiencies take a beating.
In the parlance of the racers, the blower "lays over." This usually happens at the top end of the run when the engine has really climbed in rpm. According to Carl's research, by increasing the helix angle, supersonic rotor tip speeds can be pushed up to a higher rpm (one that the engine and rotor don't reach during the run) resulting in a smoother flow of air inside the Rootes.
By restricting the outlet port on the Rootes, Carl and Littlefield were able to increase compressor efficiency. As was explained previously, the entire rotor is exposed to the high-pressure plenum air and must constantly work against the boost pressure. Sealing the bottom of the case from the plenum except for a pie-shaped port at the front (unlike the PSI which is at the rear of the case) reduces the high-pressure backwash.
Consequently, it takes less power to turn rotors. Less air leakage is also a beneficial result. The smaller outlet opening doesn't cut down on the amount of air deposited in the plenum either. Because of the twist and shape of Rootes rotors, most of the air is pushed toward the front of the case.
The contention from Carl and Littlefield is that a high helix is good for 100 horsepower right off the bat. Don't look for any more helix angle in future versions of the Rootes. In the interest of saving the racers the expense of buying another type of high helix, NHRA has banned any Rootes except the standard and 120-degree high helix versions.
TA/FC racers have taken favorably to high helix versions of the Rootes, which are now also made by Rootes builders Mike Kuhl and Gene Mooneyham. There are some concerns that to fill their larger cylinders, TA/FC racers will have to spin the PSI up to 100 percent over engine speed. Currently, the TA/FC Rootes are turned about 45 percent over engine speed.
It seems that with the higher engine rpm and blower overdrive the Funny Cars run, a high helix is more suited to their needs. However, once some of the bigger TA/FC names (Pat Austin, Brad Anderson, Frank Manzo, and others) put laps on the unit, the PSI might find a home in that class as well."
Carl had ideas of improving the Rootes but not the resources to go about building a better one. He approached Littlefield with two simple modifications that would improve the efficiency of the Rootes. Prompted by the loss of business to the PSI, Littlefield agreed to try Carl's suggestions.
First, he suggested that the helix angle be doubled from 60 to 120 degrees. Next, he wanted to restrict the outlet port to a small pie-shaped wedge similar in size and shape to the PSI.
Increasing the rotor helix angle changed one very important characteristic of the Rootes: It lowered the rotor Mach number. The rotor Mach number refers to the rotational speed at which the rotor tips go sonic, or are turning at the speed of sound (i.e., Mach 1). The resulting shock waves disrupt smooth air flow around the rotor tips. When the rotor tips rotate at Mach 1, around 12,000 rpm, volumetric and compressor efficiencies take a beating.
In the parlance of the racers, the blower "lays over." This usually happens at the top end of the run when the engine has really climbed in rpm. According to Carl's research, by increasing the helix angle, supersonic rotor tip speeds can be pushed up to a higher rpm (one that the engine and rotor don't reach during the run) resulting in a smoother flow of air inside the Rootes.
By restricting the outlet port on the Rootes, Carl and Littlefield were able to increase compressor efficiency. As was explained previously, the entire rotor is exposed to the high-pressure plenum air and must constantly work against the boost pressure. Sealing the bottom of the case from the plenum except for a pie-shaped port at the front (unlike the PSI which is at the rear of the case) reduces the high-pressure backwash.
Consequently, it takes less power to turn rotors. Less air leakage is also a beneficial result. The smaller outlet opening doesn't cut down on the amount of air deposited in the plenum either. Because of the twist and shape of Rootes rotors, most of the air is pushed toward the front of the case.
The contention from Carl and Littlefield is that a high helix is good for 100 horsepower right off the bat. Don't look for any more helix angle in future versions of the Rootes. In the interest of saving the racers the expense of buying another type of high helix, NHRA has banned any Rootes except the standard and 120-degree high helix versions.
TA/FC racers have taken favorably to high helix versions of the Rootes, which are now also made by Rootes builders Mike Kuhl and Gene Mooneyham. There are some concerns that to fill their larger cylinders, TA/FC racers will have to spin the PSI up to 100 percent over engine speed. Currently, the TA/FC Rootes are turned about 45 percent over engine speed.
It seems that with the higher engine rpm and blower overdrive the Funny Cars run, a high helix is more suited to their needs. However, once some of the bigger TA/FC names (Pat Austin, Brad Anderson, Frank Manzo, and others) put laps on the unit, the PSI might find a home in that class as well."
06-10-2006, 09:11 PM
#6
Originally Posted by CHEVY6000VHO
Damn those were long Moregrip!! 
Now......what does all that mean in one sentence!
Jim
Now......what does all that mean in one sentence!
Jim
What I found fascinating was the High Helix(120 deg rotors) for a roots blower. "A better mousetrap" as was stated.
So for a given displacement, the high helix rotor group produced a more efficient blower which ultimately equaled more power
06-10-2006, 09:43 PM
#7
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Originally Posted by moregrip
regarding what? so much info in there I don't think I could do it justice
What I found fascinating was the High Helix(120 deg rotors) for a roots blower. "A better mousetrap" as was stated.
So for a given displacement, the high helix rotor group produced a more efficient blower which ultimately equaled more power
What I found fascinating was the High Helix(120 deg rotors) for a roots blower. "A better mousetrap" as was stated.
So for a given displacement, the high helix rotor group produced a more efficient blower which ultimately equaled more power
Jim
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