2000 Tran Zam

iTrader: (2)

Dont know much about superchargers... but I always hear people talking about this in reference to twin screws...

For example, you have a 6 liter motor and throw a 2.8 KB blower on there (lets say its a vette to be realistic) and then they say its like having an 8.8 liter...

Why is this? Is it because of the intake? Is it also true for centri superchargers also?

98turbls1

iTrader: (17)

Well I know that centri's aren't positive displacement because basically they're a belt driven turbo...

I've thought about this before and MY conclusion is that they are referred to positive displacement because they are positioned behind the TB... I don't know if you can infer that it makes a 6 liter into a 8.8L engine but that's a good assumption...

I would also like a better explanation of why they are named that too... Good question....

Speed

iTrader: (33)

Well it's a little of a misused term. Positive displacement in this scope refers to forced induction devices that displace X amount of air per revolution. A traditional blower opens up a volume, pulls in the air, moves it uncompressed into the manifold and squeeze the air out into the manifold by collapsing the volume. This is what they mean by positive displacement. To the same extent, this is why piston engines are called positive displacement engines.

Centrifigual devices like turbochargers and superchargers that use a belt driven turbocharger type compressor are mass airflow devices. They don't actually displace air by expanding and collapsing a volume. They are mass movers that push air and compress it dynamically. Airflow is induced and ambient air is drawn in to replace the air it just moved.

This is overly simplistic but it in a nutshell.

2000 Tran Zam

iTrader: (2)

so basically the difference is roots types setups compress the air going into the motor which means its a "positive displacement" setup, as opposed to turbos and centri blowers move uncompressed air?

jfman

iTrader: (6)

From what I gathered.... roots type compresses the air before it's released into the manifold and the centri type blows it like a leafblower.

2000 Tran Zam

iTrader: (2)

So is that why i makes so much tq down low and looses steam up top?

Speed

iTrader: (33)

Well not exactly. They both compress the air. A turbocharger compresses it within the compressor housing through to the back of the intake valve. A roots and roots derived supercharger does not compress it the air at all within the rotor housing. The air is not compressed until it's released into the manifold under the supercharger. If you notice how the two rotors intertwine between each other in the middle of the supercharger..

The as the rotor spins, it opens the volume between the lobes to the open air above the supercharger. It rotates and traps that volume of uncompressed air between the two lobes and the rotor housing. When it spins far enough, that volume of uncompressed air is opened the manifold below the supercharger. As the rotor continues to spin, the lobe of the other rotor fills the volume and forces the air into the intake manifold. It is at this time that the air is fully compressed. It is a positive displacement device because it will only move a fixed volume of air per revolution.

T A WS6 M6

iTrader: (1)

"Positive displacement" simply refers to any type of pump (our superchargers included) that displace (move) a certain amount of air per revolution regardless of ambient conditions (pressure, temperature, etc.) A centrifugal pump (or supercharger) is not "positive displacement" because it ramps up pressure in a non linear fashion.

ex

If a positive displacement moves 1000lb/min at 1000rpm, then it will move 4000lb/min at 4000rpm,

but

A centrifugal pump that moves 1000lb/min at 1000rpm (which will be much, much larger than a posie btw) will only move 2000lb/min at 4000rpm.

=>

a posie has characteristics that allow you to attach it to an engine in a manner that will force the engine to move, say, 1.5 times the air it would naturally throughout the rpm band,

and

a centri allows you to hook it up in such a way that it does not draw much power to turn down low becuase it is not really moving much air and it is more efficient when it starts to move air, but it must spin faster.

I hope that helps.

Dave

Speed

iTrader: (33)

Because you can gear a roots supercharger to provide X volume at Y engine RPM. When you go full throttle it can stuff the engine with as much air as you want. They tend to loose steam up top because they don't follow the VE of the engine like a turbocharger does. That's not to say you can't setup a supercharger to work within the parameters of your application.

MrDude_1

iTrader: (4)

they said it, but i'll try to rephrase.


a positive displacement supercharger displaces X amount of air per revolution.

if you turn a eaton M112 supercharger once (the one on the cobras, and lightnings) it moves 112 cubic inches of air.

lets pretend the blower spins twice as fast as the engine. lets also pretend the engine displaces 300ci (and therefore uses 150ci per revolution)

if you stuff 224 ci of air ( the M112 spinning twice) into a space for 150ci of air (the engine) then the air becomes compressed.


this is as simple as i can think to explain it...




centrifugal chargers and turbos work differently.
they fling the air around, shoving it in.. kinda like a leaf blower... but they dont move any specific amount of air per revolution... you spin it with your finger, and it moves almost nothing.. spin it the same number of turns at redline, and it moves ALOT of air.
the amount of air it moves, is more related to its speed, then the revolutions it makes.

98turbls1

iTrader: (17)

SOOOO what I'm gathering from this is that the "positive displacement" superchargers are going to follow the torque curve of the engine closer because it is only able to turn X amount of rpms vs the engine rpm...

But the turbos and centris have a step up rate that spins faster as the engine goes which is linear to the rpm of the engine itself... I know that this is also dependent on the size of the engine and supercharger/turbo... I.E. how some "1500hp" Supras torque curve look like a half U because of lag but a properly sized TT LSX's torque curve looks like more of a table once it's actually spooled or spooling... I was just using these as references..

I know it's very GENERAL but I've looked at enough graphs to know how turbos, centris, and eatons/twin screws work... I just never knew EXACTLY how the positives worked in theory...

Speed

iTrader: (33)

Actually, just the opposite. Positive displacement devices don't care anything about what the engine is doing, it will flow X air vs rotor RPM regardless. That is also somewhat true about centrifugal superchargers in that they will spin the compressor wheel to flow ~x air vs wheel rpm regardless of what the engine is doing.

That is where turbos have such an advantage is they are slaves to their wastegates and will attempt to pressurize the manifold to X pressure. This disassociation of compressor and engine means that as the VE changes, the gate with automatically adjust exhaust flow to the turbo to keep the signal source at a fixed pressure. The disadvantage obvious is that the disassociated nature requires the engine to produce enough exhaust energy to overcome the inertia in the turbocharger. This is why turbos build boost exponentially or on a bell curve while superchargers are linear and tied directly to engine rpm.

Speed

iTrader: (33)

That's a good point actually. The reason supra's have that .5U shaped curve is, as I said above, the engine has to do it's own work to create the exhaust energy to get the turbocharger working. There is a break point where the additional energy used to spin the turbocharger is less than the extra energy added by the combustion of extra air/fuel and this is where the turbo 'takes off'.

Since supra's only have ~3 liters to work with, that engine has to work twice as hard as an equivalent 6 liter engine to get enough exhaust energy to spool the turbo. Also, the 6lt engine has the same .5u shaped curve but it occurs at 1/2 the RPM and is (iirc) 4x steeper than the 3lt engine. Anything after the wastegate takes over is nothing more the same engine living in a different atmosphere.

2000 Tran Zam

iTrader: (2)

So is there any way to get the rediculous amounts of air shoved into the motor from a twin screw setup, but have it flow freely and exponentially like a turbo setup?

Is that what compound induction does?

Speed

iTrader: (33)

No not really because of the nature of the supercharger drive and the dynamics of a positive displacement blower. That's why they get that instant boost response compared to a turbocharger.

And a turbo shoves the same rediculous amounts of air but requires a precursor to get it all started (N/A exhaust flow).

That said, they can overdrive a blower and use a pressure release to tune X amount of boost. It's inefficient but it works.

MrDude_1

iTrader: (4)

depending on how its done, its not as inefficent as you think.

most of the modern OEM superchargers do this... for example ona lightning, the bypass valve is open unless you're near WOT...
the supercharger is just freewheeling, and its taking less then 5% of the power it normally draws. (according to eaton)

Speed

iTrader: (33)

See I've heard that too and have wondered how exactly that works. They still have to turn those big rotors and push the air out of a discharge valve. Is there a clutch on the drive system?

MrDude_1

iTrader: (4)

no.. its very simple really.

if the "big rotors" are just free spinning and not doing any work, they take almost no power to spin.. you could spin them by hand that fast.


what happens is a small throttlebody like butterfly valve opens up.
this opens the bottom plenum (under the rotors) to the top (above the rotors)
air is free to move around the impellers.. if it goes in, instead of pushing boost, it pushes air around back out the port... no backpressure means it can almost freewheel doing it.

its really quite simple... kind of a "why didnt i think of that" kind of thing.

98turbls1

iTrader: (17)

I believe this is how the "valet" mode on the Kenne Bell blowers kind of work... It's so some punk kid that's changing the oil ( I don't why he'd be changing your oil if you have a KB on there anyways) can't get all crazy and wreck your car... But that's actually a mode that you have to put it into... any other time it's building boost..

MrDude_1

iTrader: (4)

most "valet mode" setups are electronic.
its possible if the valve on that car is electronically controlled. on most fords (they're the only OEM roots cars i personally know) the valve is vacuum controlled.

but that does sound like a good idea.

i know if i leave my car at a dealership for warranty work, the PCM is going to get flashed with a 15mph topspeed and a 2500 redline. theres no reason to exceed either of those in a parking lot.