going to install a supercharge

[TrunkMonkey]

What I don't get is how can a root type blower produce full boost at low psi? I understand that a centrifugal s/c makes full boost at redline, that make sense to me. But have do a root s/c make full boost at 3000 or so rpm? Thanks.

in a nutshell, a roots style blower doesn't compress air, it only pumps it. regardless of the engine's rpm, a roots blower will always pump the same amount of air. so all that's needed to reach full boost is to be at WOT.

-demetrius

[Ubernoober]

A roots blower is an air pump. Every rotation of the rotors displaces a given amount of air, frequently listed in cubic inches though Eaton likes to use Liters. This is also why roots type blowers are also called "positive displacement". The key is to turn the rotors at a given ratio to engine speed. This ratio will be fixed by the pulley sizes. Once set, the roots will provide the exact same air-pump ratio across the entire RPM range. Since your engine is also an airpump this ratio determines your boost. If the blower is forcing 300 CFM into your engine that normally requires 150 CFM you build pressure. As the engine demands more with increasing RPM, the roots also spins up at the pulley ratio giving you a contant boost.

[Ubernoober]

Also, I should add that the by adding a helical twist to the rotors on a roots you increase its efficiency at the sacrifice of additional power required to turn it. Eaton blowers have 1/3 of a complete twist in the length of one rotor I believe. Kenne Bell blowers take this to an extreme by turning them from rotors to screws and by tapering the body of the blower. At that point, it becomes and actual air compressor and behaves as such. This doesn't mean much to the guy mashing the pedal but for us techno-geeks that are trying to design their own, it makes a huge difference in how you calculate your efficiency and boost potential.

http://automotive.eaton.com/pr...s.asp

[Red Lightning]

Ok, I think I understand it. Does a twinscrew s/c make full boost at low rpm? Since it's not just a pump, it compress the air? Thanks.

[TrunkMonkey]

Does a twinscrew s/c make full boost at low rpm?

yes.

Since it's not just a pump, it compress the air?

the only thing that really separates roots and twin-screw blowers are the clearances between the vanes (rotors). twin-screws have much much tighter clearances. therefore, it compress the air as it pumps.

-demetrius

[PMan_S13]

wow, did it really take over a year for bedbull to get his truck back from the body shop?

[Ubernoober]

Ok, we need to be very careful how we use the terms "compressor" and "air pump".Sometimes interchangeable, but in regards to superchargers and turbos, it has similar but different meanings. When we say compressor, we are talking about a device that presurizes an air charge and then introduces it to the engine. In a turbo the compression comes from the centrifugal action of the air as it passes through the vanes of the compressor and is compressed in the exit scroll. In the Kenne Bell and Whipple screw-type superchargers (both companies have sometimes called them compressors) they actually pull air from the rear and compress it by squeezing it tighter and tighter as it travels to the far end of the device. This is also why they are internally tapered. A Vortech type supercharger is just a vaned compressor being turned by the crank instead of by a turbine.

An air pump moves air by displacement instead of compression. The classic roots was just a giant egg-beater internally. Every rotation would displace a given volume of air. When you displace more air than the engine normally consumes, you develop pressure. Pump harder, develop more pressure. Note that the air pump does not internally compress the air charge, that is done upon introduction to the outlet side of the device. This is the geeky part that engineers use to differentiate compressors from pumps. If you add a helical twist to the screws, you add a compression aspect to a roots, allowing you to gain efficiency. Also a classic roots pumps from top to bottom, while the twisted screws pump from front to back.In both instances you are still developing pressure in the manifold, just the manner is different.

I don't know why I wrote all this. Bored I guess. I imagine someone will quickly come along and point out what is wrong.But the simple answer to your question is this: My Saleen is an Eaton M-90 supercharged 4.6 liter engine. Boost is there right off idle, but low. Full boost (7.5 PSI) by 1500 RPM. The computer pulls a lot of timing there though, so its better to roll into it up to about 2000 then mash like crazy. Not to mention the wheelspin.

[TrunkMonkey]

just to make sure everyone's on the same page...

roots and twin-screw blowers are both pumps. a centrifugal supercharger is a compressor.

a roots blower only pumps, but twin-screws and centrifugals both compress.

the difference between a compressor and a pump? a pump doesn't need to rely on the engine's rpms. it's going to move the desired amount of air to make boost regardless (roots and twin-screws). a compressor needs to spin at a certain speed (rpm dependant) to create the desired boost pressure (centrifugals).

this is why a twin-screw blower is so effective. since it compresses the air as it pumps, it's as efficient as a turbocharger, it has the ability to run high boost throughout the entire rpm range, and it does so with almost no lag whatsoever.

the twin-screw aka lysholm blower is king of forced induction .

-demetrius

[Red Lightning]

So a twisted root s/c is considered a twinscrew s/c, and it will still make full boost at a low rpm?

[NateDogg]

Red Lightning:

It's all about volumetric efficiency. Since the VE curve of a roots/twin-screw is similar to that of an engine, boost can be made throughout the RPM range. IE. Think linear...

I have been researching putting a twin screw on the cold side of the 240SX but the intake manifold (to get it to my standard) is taking more R&D than previously thought. Originally I was gonna hack off the runners as the peak and then add a plenum/air-water intercooler/blower adaptor but I'm having issues with substantially different intake upper and lower port floor lengths. It's do-able, all it takes is more time and money.

[Chezedik]

My friend and I have it drawn out, and will begin production shortly. From there, we will take care of asthetic issues, and do a few months of R&D, then we will work on marketing the kit.