Understanding Supercharging:

[BillyGman]

I found this article in a magazine called "DragRacing".....



Supercharged! The word sends the same message to all gearheads around the globe. Doorslammers around the world rely on some form of supercharging to increase the horsepower available to propel their hot rods faster than could be expected of any naturally aspirated engine. What are the popular means of supercharging today's doorslammers? Superchargers are simply devices that force air into an engine at a greater rate than mechanically possible under normal atmospheric conditions (thus the term "blower"). If one considers supercharging as a means of increasing the amount of oxygen per intake cycle, then it is safe to say supercharging can be done via mechanical or chemical means.

CHEMICAL;


This is a way to introduce a high pressure oxygen liberating chemical into the intake stream resulting in an increased oxygen content charge that is super-cooled. Nitrous oxide is the popular door car chemical that has been making power for many years dating back to warplanes in WWII. Nitrous oxide (N2O) is an inert gas that is neither flammable nor breathable, as the oxygen atoms are attached to a nitrogen atom. Until the oxygen atoms are liberated, the gas is not useful. After the oxygen atoms are broken from the nitrogen atom (via heat) the oxygen can be used as an ingredient in the combustion process. Because of the increased oxygen, an increased fuel quantity is needed to produce the increased power and also to stop engine damage from an oxygen-rich condition called running lean.
While nitrous oxide is impressive with the amount of power it can increase to the engine's power, it is not as effective as a mechanical supercharger.



SCREW TYPE
Mechanical superchargers are either positive displacement or not. A screw compressor is the most efficient type of positive displacement type supercharger available. The screw is just as the name insinuates the rotors appear to screw into each other. A screw compressor has a male and female rotor where only one rotor displaces air. Because of this, the rpm at which these units run is greater than a roots design. The internal tolerances of the screw compressor are very tight, and the inter-twined rotors act as a seal against any leakage up through the rotors. The screw has an inlet cavity that decreases in size as the rotors rotate. This compresses the air charge internally, because of this it can be called an air compressor. The screw is known to be a very powerful supercharger design, while the rotors are large and can be heavy it is commonly over 200 horsepower over a roots supercharger. The reason for this power difference is the combination of lower discharge temperatures and lower parasitic drag because of no rubbing internal parts.
ROOTS SUPERCHARGER
http://dragracingonline.com/technical/images/supercharger_schematic.jpg
The Roots supercharger is the old tried and true positive displacement design used since the GMC introduction on diesel trucks and even earlier in coal mines to move valuable air to the soon to be cancer victims at the bottom of the mine.
http://dragracingonline.com/technical/images/2blowers.jpgThe Hi-Helix (left) and Standard Helix superchargers.
The Roots has two rotors with three lobes per rotor. The Roots supercharger has two rotor types, the Hi-Helix and the Standard Helix. The high helix rotors are called 120-degree rotors and the standard rotors are called 60 degree. This is the amount of rotor twist over the length. Both rotors in the Roots design displace air, giving the roots a large displacement per rotation.
The size of the supercharger is based from the rotor length. A 6-71 supercharger has a 15-inch long rotor, and an 8-71 has a 16-inch one. This keeps going until the 16-71, which has a 20-inch long rotor.
http://dragracingonline.com/technical/images/Kobelco14_71.jpgThis is Kobelco's 14-71 supercharger.
Roots superchargers use Teflon sealing strips to help seal small clearances between the rotors and also to seal rotor to bore. These Teflon strips need replacing from time to time to keep the blower to its peak efficiency.
CENTRIFUGAL SUPERCHARGERS
Centrifugal superchargers are belt-driven turbine wheels that compress air centrifugally (outward from the center) with rpm. The impeller in the supercharger causes air to move from the center of the turbine and is accelerated outwards towards the turbine housing and is then directed to the outlet ducts. The rpm at which these superchargers operate is very high, upwards to 30,000 plus rpm. The centrifugal supercharger uses the same compressor theory as a turbocharger except that the means for driving the compressor is from the crankshaft via belt. The supercharger has a large gear ratio internally to bring the unit to its needed rpm. This type of supercharger notoriously makes good boost pressures, but is much slower than the positive displacement type superchargers at building this valuable intake pressure.



TURBOCHARGERS
Turbochargers use the same type of compressors as the above and are centrifugal. The method for driving a turbo is with a turbine wheel in the exhaust tract. The heat energy and velocity of the engine's exhaust are used to accelerate the exhaust turbine that is connected to the compressor section.
In theory the turbo is the most efficient means of supercharging as the exhaust is already spent energy and therefore it is free energy being used for valuable intake compression. The downside to a turbocharger is the time required for the turbine to reach the optimum compressor rpm and also the heating of the intake charge from the turbine temperatures.
http://dragracingonline.com/technical/images/intercooler.jpgINTERCOOLERS
All of the methods of mechanical supercharging have the same goal: to increase the quantity of intake air charge resulting in increased horsepower output. All of the superchargers suffer from the same problem: heat. It is basic physics that anything which is compressed has an increase in temperature due to the act of compression. This is why the intercooler is a valuable tool in controlling the discharge temperature. The intercooler is simply a heat sink device that absorbs heat energy from the compressed intake charge and makes the compressed air charge cooler and more dense and workable



An intercooler can either be of the air-to-air or air-to-water design. Air-to-air simply means that the medium the heat energy is moved away from the intercooler is simply moving air. The popular drag racing intercooler is air-to-water. A super-cooled liquid can be pumped through the intercooler, thus dropping the discharge temperature dramatically. The centrifugal and turbocharged designs require the use of intercoolers to make the power required that is required to compete in doorslammer racing, partly due to the use of gasoline. The roots and the screw design will also benefit dramatically from the use of an intercooler, as you see in many marine applications using this type of supercharging.

[sweetair]

Great post Billy. Lots of info here. Just very well explained. Thanks..

[Leadfoot281]

Also worth noting; a 6-71 blower moves 426 cubic inches of air in one complete revolution (6x71=426). Each of the six rotors displace 71 cubic inches.

[BillyGman]

Also worth noting; a 6-71 blower moves 426 cubic inches of air in one complete revolution (6x71=426). Each of the six rotors displace 71 cubic inches. Now that's^ also very interesting. I didn't know that. Thanks.

[TooManyFords]

^^ Which is incorrect. The 6-71 is in reference to the engine it was bolted to. The 4-71 was on a GMC diesel with 4 cylinder, each 71 ci. The 6-71 (like mine!) was bolted to a 6 cylinder engine and the 8-71 to an 8 cylinder. This is NOT how much air is moved in one revolution of the blower. That is all determined by speed and and size of the lobes. Not all 6-71's are created equal either. The Wiend 6-71 uses a 2-lobe rotor where as most race blowers use the tried and true twisted 3-lobe design (again, like mine).

So, hopefully that clears up the GMC x-71 info. Anyone having more questions can just ask and I'll try to answer.

Cheers!

John

[Leadfoot281]

Ooops. Car Craft magazine must have had it wrong then, 'cause that's where I read that. Maybe you can help me out on this.

B&M sold "6-71's" that had two strait rotors also. Obviously the speed at which the blower is spun will help determine boost. Super heated air will create something I'll call "false boost", but that's another story all together...

A 6-71 on a common 350 Chevy is usually slightly underdriven (slower than the crankshaft) to create boost. Correct?

I always understood that a positive displacement blower, like a 6-71, displaces 426 cubic inches of air (or 7 Liters @ 61 cubic inches/liter). That into a 5.7 liter engine, even underdriven, would create boost. Am I wrong?

What is the actual displacement, per revolution, of a "positive displacement" 6-71, twisted rotor blower then? Thanks!

[BillyGman]

One thing that I would have to disagree on in John's post is the comment about the speed of the supercharger dictating how much air is moved per each revolution. It wouldn't matter with a positive displacement S/Cer. yes, the faster the S/Cer is turning, then the more air it will move, but that still doesn't change how much air is being moved during each revolution. It only changes how much air is being moved per second, or per minute.

[TooManyFords]

I should not be in such a hurry to answer. I was thinking heat and typed about ci of air. Billy is correct and here's the 411 on air volume:

6:71 small diameter
Rotor dia=5.505", length=14.975", displacement per full turn
of rotor=339CI.

6:71 big diameter
Rotor dia=5.778", length=14.975", displacement per full turn
of rotor=411CI.

8:71
Rotor dia=5.778", length=15.905", displacement per full turn
of rotor=436CI.

10:71
Rotor dia=5.778", length=17.000", displacement per full turn
of rotor=466CI.

14:71
Rotor dia=5.778", length=19.000", displacement per full turn
of rotor=521CI


What you have to remember is that if you have a 281 ci motor, it takes two full revolutions to make up that volume. So, with a 6-71 running 1:1 it would be stuffing 411 cu.in. of air into 140.5 cu.in. of motor! Almost 300% positive and that is a TON!

That is why most people , myself included, will run it 10-15% underdriven. I'm planning on 10% under and that should put boost at about 15 lbs. The corrected compression ratio will run about 18:1 at this level. I'm also buying the 15% underdrive pulley just in case that is too radical or I can't tune around it.

Back to the heat issue. The reason I will be able to get by without the intercooler is that by underdriving the 6-71 a bunch, it spends more time pumping air and at a much slower rate and not creating that wild friction that a centrifugal or turbo does. If I wanted, I could have stepped up to an 8-71 and underdriven it even more to achieve the same amount of boost at a cooler temp because it physically would turn slower. And -that- was the gist of my previous msg. Oops.

Cheers!

john

[BillyGman]

Okay John. ;) Thanks for your reply.....

another thing that I can't help but notice are the massive displacement amounts of those big blowers, as opposed to our Eaton roots S/Cers being at merely 112 C.I.... but then again, the advantage is not having to cut up your hood, to fit the supercharger underneath it. Can't have it all I guess.

[John F. Russo]

[QUOTE=BillyGman]I found this article in a magazine called "DragRacing".....

Thank you very much!

Am I correct to say that a screw type supercharger will give the same pressure at any speed while the centrifugal will fall off at higher speeds?

Data: I have a Vortech and I notice that when I floor it the best I get at about 60 mph is 3 psig while I get about 7 psig when starting off the line. This means that I have less power at the higher speeds.

Am I correct?

______________________________ _____________________
2003 Dark Blue Pearl 300B (Canadian) w/Light Flint (reversed traction control, mini spare, trunked 6 disc CD changer, clock-in-the-radio, heated front seats/mirrors, hood light), Born 12/10/02; converted new then used 2/28/03
ENGINE: Kenny Brown: 6th “Signature Series” conversion (450 hp), Vortech supercharger (5 to 7 psig boost; pulley), 377 RWTQ, Dennis Reinhart Cobra engine cooling kit TRANSMISSION: Stock transmission upgraded with Performance Automatic "Super Streeter" transmission version, stub shaft (defective metal) failed at 30k miles; upgraded internals (except for torque converter) to Dennis Reinhart's design; Precision, triple disc, P/N469018-3, later upgraded to 3500 rpm stall speed); Ford Racing transmission aluminum pan SUSPENSION: Metco control arms (black powdered coated), coil from each front stock spring removed to produce the “same” effect as an Eibach spring DRIVETRAIN 4.10 gears, MMX Driveshaft, Ford Racing Stud and Girdle BRAKES:14 in. BaerClaw front, (two piston, slotted rotors), TIRES: Pirelli P-Zero Asimmetrico (front 255/45ZR18 99Y, rear 255/50ZR18 102Y), ACCESSORIES: Wheel locks (Ford), godshead valve stem caps, Dead pedal, Badgeless front grille by “Zack”; Zaino waxing; RainX; Autobhanded, FordChip, Ground clearance: 5 in.; 36,000 miles MISCELLANEOUS 18.5 mpg at a steady speed of 80 mph, one tank of gas
______________________________ ___________________________
1961 Ford Galaxie, 2 dr. Club Victoria, 390CID, 375hp, 4 barrel (gone)

[MikesMerc]

Am I correct to say that a screw type supercharger will give the same pressure at any speed while the centrifugal will fall off at higher speeds?


Not really. Most of the time, centrifugal blowers tend NOT to fall off at higher speeds as they make boost exponentially with blower speed and produce higher output as engine rpm (and therefore blower rpm) increase.

When you are analyzing "boost," you need to consider that superchargers do NOT produce boost. They simply pump air and are rated in specific cfm at given blower rpms. How much air is pumped depends on blower design and the rpm of the rotors, screws, or impeller.

Roots blowers pump large quantities of air at lower speeds, produce cfm in a more linear fashion, but fall of at higher speeds do to the limits of design efficiency.

Screw type blowers pump air through the whole range, low to high, and again produce cfm in a more linear fashion. The don't pump as much way down low at slower speeds like roots, but come close. However, they do not fall off at higher speeds.

Centrifugal blowers need lots of rpm to "throw" the air out into the volute to produce cfm. They are meant to spin at very very high impeller rpms. The harder they are spun, the more the centrifugal process works, the more cfm is pushed (to a point). Eventually they do fall off if over spun.

You engine consumes air. It consumes less at lower rpms and more at higher rpms.

BOOST is the difference between the cfm of air your supercharger pumps less what your motor consumes.

So, when analyzing boost production, its best to look at it like two graphed curves laid over one another. One line being air consumed over the engine rpm range, and the other being the cfm produced by the SC over the same engine rpm range. Engine air consumption is pretty linear with engine rpm. Accordingly, the boost characteristics of a blower set up is dictated primarily by blower design. But understanding how much cfm is consumed at various points in the engine rpm range in relation to how much cfm the SC is producing makes analysis of boost curves easier.

Although SC design dictates most of the equation, you can change the relationship between engine consumption and SC cfm production through pulley changes and by-passes.

If you are seeing boost fall off at the high end, what we are really saying is that the engine at higher rpms begins consuming more cfm then the blower is producing. Since centrifugal blowers make boost exponentially as rpms rise this can only mean one of three things.

First, that your blower is under spun. This means its not turning fast enough and needs more rpms to continue growing cfm production up the engine rpm curve. I doubt this answer applies here. Seeing that boost is "falling off" says the blower is producing enough cfm at mid range, then dropping. Since centrifugal blowers produce cfm exponentially with rpm increases, its doubtful the blower is underspun as it would not have produced enough cfm in the mid range if it was. Eliminate this answer.

Second answer might be that the blower unit is simply too small for the application and cannot feed the motor. This explanation is possible. You would see good mid to high range, then a fall off. However, if you are running one of the blower kits made for the marauder, this option becomes doubtful as well. All the SC kits produce more than enough cfm for our 4.6 motors.

Last, your belt is slipping. This is the likely answer. As boost pressure builds, their is a "back pressure" force in the intake tube between the blower and motor. This pressure makes the blower harder and harder to spin as boost builds. Accordingly, the SC belt can slip on the SC pulley at the highend. Because centrifugal blowers produce boost exponentially, the boost and back pressure build quickly and forcefully which causes the belt slip.

The cure is a belt tightening or a new belt. Also consider that how much boost you can make without the belt slipping depends on how many ribs the blower belt has. You can only get so much twisting force with various rib designs before they begin slipping. 6 ribs belts are popular but they aren't going to get you into the higher boost ranges easily without some slip. 8 ribs can handle higher boost ranges in the 16-18 psi range.

One note, some slip is a good thing. The slip allows for some "cushion" between the blower and motor rpms. On sudden engine rpm drops (shifts or lifting off the gas pedal at high rpms ) the slip allows for the blower rpm to "catch up" or adjust to current engine rpm. Without some some slip, the sudden rpm changes can destroy a blower. This is why cog set ups are dangerous and only meant for blowers designed for it.

Sorry for the long post. Edited for terrible spelling :)

[Glenn]

Great reading, thanks for putting it together.

Glenn

[BillyGman]

Am I correct to say that a screw type supercharger will give the same pressure at any speed while the centrifugal will fall off at higher speeds?

Data: I have a Vortech and I notice that when I floor it the best I get at about 60 mph is 3 psig while I get about 7 psig when starting off the line. This means that I have less power at the higher speeds.

Am I correct?

I know that dave addressed your question already, but in short, the answer is no. You're wrong. In fact, above 5,000 RPM, centrifugal blowers are the best out of all three types of S/Cers (which isn't to say that they don't make boost under 5,000 RPM. But just that, their strong point is 5,000+ RPM). They start off at the low RPM and even in the lower midrange RPM scale with less boost pressure, but they continue to make more and more boost as the RPM increases (as Dave stated).

That also isn't to say that a roots supercharger won't give your engine considerably more power above 5,000 RPM than it had when it was naturally aspirated either. because it will. Just not as much as a centrifugal S/Cer will, given the same amount of boost pressure. Just remember that PEAK HP and PEAK Torque levels aren't the only things that propel your car faster down the road, nor down the quartermile drag strip. It's the power levels throughout the entire RPM range that does that, and roots blowers make the best power BELOW 4,000 RPM with all other things being equal. So it's a trade-off.

Twinscrew superchargers are said to offer the closest thing to the best of both worlds. however, it's also said by some engine tuners (Like Lidio of Alternative Auto) that roots blowers still offer a little more boost and therefore power below 3,000 RPM than even the twinscrew S/Cers do. However, if you ever plan on going all out, race-only with your car, then keep in mind that the roots design becomes so ineeficient when more than 13 PSI is used, that it isn't even worth going above that boost level. 14+ PSI of boost is where centrifugals shine, and even more so, twinscrews. ;)

I've read many a story on the SVT board of 700+ HP Mustangs running 18+ PSI of boost with twinscrew superchargers from Kenne Belle, and from Whipple. FWIW, it's also my understanding that the Whipple twinscrews make considerably less whining noise than the Kenne Belle twinscrew S/Cers do, which are said to be very loud.

[John F. Russo]

Not really. Most of the time, centrifugal blowers tend NOT to fall off at higher speeds as they make boost exponentially with blower speed and produce higher output as engine rpm (and therefore blower rpm) increase....Sorry for the long post. Edited for terrible spelling :)


Thank you very much for the education and probably the explanation for the inadequate performance of my Vortech supercharger.

I will have the belt checked out.

[John F. Russo]

[QUOTE=BillyGman][B][COLOR=#0000ff]I know that dave addressed your question already, but in short, the answer is no. You're wrong. In fact, above 5,000 RPM, centrifugal blowers are the best out of all three types of S/Cers (which isn't to say that they don't make boost under 5,000 RPM. But just that, their strong point is 5,000+ RPM). They start off at the low RPM and even in the lower midrange RPM scale with less boost pressure, but they continue to make more and more boost as the RPM increases (as Dave stated)...


Thank you Billy for the added information.

Do you agree with Dave that my boost issue is almost certainly caused by a slipping belt?

[BillyGman]

Thank you Billy for the added information.

Do you agree with Dave that my boost issue is almost certainly caused by a slipping belt? If you have a 6 rib blower belt, then that is very likely the case. The Trilogy blower set-up as well as some of the better centrifugal set-ups use the 8 rib blower belts, because they're better for gtip.

I wonder if it can be possible that you have a leak in the S/Cer itself. :confused: