"off-idle", got it.

I essentially agree with you, and honestly, I am splitting hairs when it come to off idle boost and the modified roots and twin-screw.

however, I still believe the modified roots will provide the quickest off idle boost, if only by a few hundred rpm compared to the twin-screw.

I will post more later when I have more time to post supporting info.

 

Wow!!! Who's website is that?

 

Yeah, I realized I was unclear and went back and edited the post to clarify.

From what I've both read and heard from people, there is really no discernable difference in off-idle power and behavior between the two types of blowers. The modified roots and twin screw have, for all intents and purposes, the same off-idle behavior. If the difference was over 100 rpm's I would be surprised. But if you can find otherwise, please let us know when/where you get your info.

 

WILCO (will comply), with reference to what I've heard; the twin screw takes a little longer to build boost.

 

thank god for half day friday's!
i'm home now and hunting and pecking a response.
First, I'd like to apologize to the topic starter for going off task a bit here. I am trying to stay mostly on task though as I've supplied some good info on turbocharging/supercharging and my bias toward a modified roots/Radix. Again mostly on task, with the exception of the twin screw/modified roots sidebar.
All important information/facts and opinions, nontheless.
Second, however, this has turned into a very informative thread for all involved.
Third, I will post more very shortly (currently researching, organizing thoughts).

 

Alright, read the responses. I'll be back in a couple of hours. Then it's game on

I also apologize for being off topic some (but I am the only one to help out the twin screw ) And especially for being a moderator and doing this...but as long as it stays a clean exchange of info., it's cool.

 

I beg to differ with you on that.

 

but I cannot leave you guys misinformed,
no one is misinforming here, just exchanging facts/ideas/and opinions, so others can formulate their own choices.
Let me find the rule where NHRA outlawed twin screw compressor usage in top fuel because they built boost too quickly. That is the only reason there is not a twin screw compressor on top of that top fuel engine. (that and beside the fact that more power=less chance of traction for them anyway)
I'd be interested in that
Here is a link to some real info about a twin screw compressor.
http://www.lysholm.se/PDF/diagram_lys2300ax.pdf
In this case, it references the Lysholm 2300ax. Maybe if Magnuson would come out with a detailed readout of their M-112 blower we could at least compare some real data
OK, lets anylize this a bit. We always hear about the Radix/Roots supercharger's main weak point right? "Heat". Specifically, heat at higher boost levels. Now, what about the Twin Screws main weak point? "Off Boost Conditions". Specifically, The Twin Screw is still compressing air internally even during non-boost conditions and this generates heat and has more parasitic drag on the engine during non-boost even with a bypass valve. Now, thinking about this logically, do you really think when sitting at idle (i.e. off boost) that the Twin Screw supercharger is able to overcome its biggest disadvantage fast enough to build boost quicker than a modified roots/radix supercharger in its most ideal condition? Meaning, off idle, a modified roots, only has productive boost to build, while a Twin Screw, has to overcome its biggest drawback before producing productive boost (even though once it starts to build boost, it is more efficient).
Hence, the reason Mercedes Benz does use some Twin Screw (Lysholm) superchargers on their super cars and they employ a clutch to disengage the supercharger from the crank shaft during non-boost conditions to counter the negatives of the Lysholm superchargers.
Lastly, on daily driven vehicles, how often are we at an "Off Boost Condition"? I say, half of the time, like when we come to a stop light, idle, or have to slow down for traffic. Advantage, Radix!
Food for thought...why have roots blowers become "modified" by twisting the impeller lobes?
This is why the rotors are twisted:
Eaton rotors are twisted 60 degrees. Helical rotors, along with specially designed inlet and outlet port geometry in the blower case, help to reduce pressure variations resulting in a smooth discharge of air and a lower level of noise during operation. This arrangement, also, improves efficiency. With helical rotors, the supercharger can run at up to 14,000 rpm, making for a small blower for a given boost and air flow output.
They leak air
A roots based supercharger does not "leak air". It is a different design, that's all. It pushes air, not much else to say about that really. However, In essence, what Eaton has achieved, is to bring the adiabatic efficiency up from a low 50% to over 60%.
Where a roots supercharger suffers from reverse flow, or pulse, the Eaton has a 60° twist designed into the rotors to effectively reduce this problem. By introducing the opening lobe cavity at an angle, as well as the introduction of an extra lobe to increase the frequency of this event, less reverse flow is experienced.
Really? This is what came off of one of your sources. "The primary reason that blowers aren't optimized by their manufacturers is that the tooling necessary would push the cost into an area which would make the units prohibitive to the OE marketplace, which is "the" big consumer". I know this was taken a bit out of context as they were mainly describing why the housings and such of blowers are not hand finished. But the tooling needed to make a twin screw lobe is several more times the cost of a roots lobe.
BTW: If that were the case, then why is Eaton getting liscense to sell twin screw compressors instead of roots blowers? Why does the new Pherd GT use a twin screw compressor?
As for production vehicles, my thoughts are: there is no more reliable way to supercharge a vehicle than with a roots type blower, this is fact.
So reliability is my #1 reason
Cost is #2. Note: however, if the Twin Screw was mass produced at the level the Roots type are they would be cost effective as well.
With its low production cost and its improved efficiencies, the Eaton dominates the world's production, accounting for over 90% of the superchargers produced.
Lastly, I am not saying a Radix is more efficient, I am saying it produces the quickest off idle boost..........which.......IMO........for street vehicles is the most rewarding

 

no worries about side topics.....it's all very informative for me.........give that topic starter a sticker

 

one gold star it is! with a Radix on top!