Thank you guys for all the information. The CM/H to CF/M calculator will make the effeciency charts much easier to understand, thanks for the cool link!

GMC_DUDE, does leakdown between rotors increase proportional to blower size or is it a fixed amount?

Awesome thread guys, keep any comments comming!

 

With twin screw compressors the larger ones will work better. The slower you can turn it to get the boost where you need it the cooler it will be and less inertial forces will be involved with those spinning rotors (HEAVY). Also keep in mind that larger pullies get better grip on the belt.

The 3.3L Whipple (W200AX) is not too big for a truck at all. As an example, I just upgraded to a 2.3L (W140AX) compressor from a 1.6L compressor this year on the 383CID engine. I have a 2.625" pulley on it and I'm shifting at 5600RPM. The crank pulley is 7.5" meaning at my shift point I am hitting 16,000 RPM at the compressor. (The second rotor is over driven and hits over 26k RPM at this speed!) Even with all that I'm only getting 9psi of boost. If this was a Gen-III SBC and not a Gen-1e, I'd probably want to shift at 6400 which means I'd be restricted to a 3" blower pulley to not exceed its 16k RPM redline. The 3" pulley would give me only 5psi of boost on this engine. In other words, I just bought a brand new compressor a lot bigger than my last one and it's already maxed out. Go big on the compressor - room to grow. The 200CID compressor really is ideal.

 

If you look closr you can see a school of yellow tail....BOIL

 

This is funny, because I was actually in the process of talking to Whipple to see if they make some sort of adapter to swap out my 2.3L with the 3.3L and still keep it side mounted like mine is now. I am running a 2.5" pulley to make 7lbs of boost on this 496ci motor. Stock exhaust, stock cam, stock everything and i can only get 7lbs out of a 2.5" pulley (and intercooler). IMHO, a 2.3L blower is WAY undersized for my engine. I am hoping they can figure something out for me as I would much rather be spinning a 4" pulley to get my 7lbs than a 2.5" pulley. Like someone already mentioned, less heat as well. I need to find out how large the crank pulley is on this 8.1L and see if I am over spinning this blower. Remember, this sucker redlines at 4850. If anyone has time help me out with that calculation it would be much appreciated.

 

Damn this is a good thread with brief moments of laughter.
Q: Bigger blowers use bigger rotors and gives you more cubes right? So because of the added mass I'm assuming that it'll take more hp to get those moving? So wouldn't any added hp gained due to less heat build up in intake charge from the blower not working so hard be lost with the required hp to run the bigger unit?
This is based of my knowledge of bigger turbo's spooling up slower than smaller ones because of their additional mass.

 

Forgot to mention that I was thinking about the efficency of the blowers at lower RPM's where the blowers aren't using their full potential. Of course bigger blower will make more power given the correct application.

 

The bigger superchargers definetly take more engine power to turn, you get into 8 rib, 10 rib then cogged belts to drive them. The payoff from the larger compressors must be worth the increased power they consume, they make more HP.

I was a little curious what it would take to make 1K RWHP with a supercharger. There was a thread on the Corvetteforum that had a 408 with the KB 2.8 making 8xxRWHP. That was on ~21lbs of boost (unsure of pulley size), manual trans I think. It seemed that the set-up was pretty well maxed out, so I would assume a bigger compressor is needed to make more power. I can't wait to see some results from the trucks around this site!

James, do you have any belt slip or feel the need to upgrade the drive? If you were to run the 200ci compressor, what belt would you use?

 

Spoolin - Positive displacement and twin screw compressors spin with very little mechanical losses to friction. Under vacuum there's almost no parasitic draw on these types of compressors. Air moving against a surface creates friction. The faster that air moves the more friction and heat are created. Overcoming this friction transforms mechanical energy into heat. The heat is transferred to the air. This is why efficiency falls off at higher RPM - more energy becomes heat. In a roots or screw supercharger you have two counter rotating rotors. In a roots blower they spin the same speed, in a twin screw compressor the second rotor is geared up to spin faster. The mass of the rotors in both designs creates a lot of inertial load on the engine. In first gear, for example, when the engine is revving up fast, inertia of those rotors has to be overcome with input torque. It takes a lot of energy to accellerate a few pounds of aluminum from 1500 RPM to 16,000 RPM! Then, when the transmission shifts, you have all the inertial energy trying to overspin the crank pulley since the engine loses RPM much faster than the blower. That can cause the belt to jump off the pullies and/or even break the belt tensioner. The larger displacement compressors reduce inertial forces even though the rotors are more massive. There's a lot less accelleration of those rotors due to the slower rotation to achieve the same boost.

The belt around the 2.625" pulley on my W140AX is not slipping, amazingly. If I were running the larger W200AX compressor at the same boost levels I am getting from the 140, the puley would be much larger giving the belt more grip and more leverage on turning the rotors. The 6-rib serpentine system would work fine with that. My biggest problem is with engine decelleration adn the compressor overspinning the crank. It shifts all the belt slack to the driver side of the crank pulley and pulls the tension so tight it maxes out. This happens during shifts and after burnouts when the tires grab. (Any time the engine RPM rapidly decreases)

 

I think I just figured out why technically one is a compressor and one is not.

 

Between the rotors and the housing cavities of air are created. In a roots blower multiple cavities of air are moved around the outsides between the rotors and the housing from the top to the bottom. Putting a helix on roots rotors makes them more efficient in handling rear axial inlets, quieter, and smoother flowing.
In twin screw compressors the cavities of air move from the top rear to the bottom front. As they move through the housing they become smaller and the air is mechanically compressed. What's really interesting is that each cavity of trapped air actually surrounds almost the entire diameter of both rotors. It is only the mesh of the two rotors that separate each cavity.

Viewed most simply, cavities of air move from the rear to the front of a twin screw compressor and from the top to the bottom of a roots blower.

In a 4-lobe roots blower, 8 cavities of air (one on each side) are moved for each rotation - four pulses total per revolution.
In a Whipple or Lysholm compressor there are 5 lobes on the input rotor and 3 on the secondary rotor. 5 cavities of air are sequencially moved from the rear to the front with each rotation - five pulses per revolution.
In an Autorotor compressor there are 6 lobes on the input rotor and 4 lobes on the secondary rotor. 6 cavities of air are sequencially moved from the rear to the front for each rotation - six pulse per revolution.