Dynamic compressors vs. Positive displacement compressors..

[Faultline]

huh???..this is just another way to say turbo vs SC...this forum is fairly Quiet, so I thou I would try to throw in a bone....turbos are dynamic compressors..they make compression by accelerating the air to high velocity and then slowing it down by diffusion. Diffusion converts velocity energy into pressure energy by slowing down the aiir w/o turbulence.An eaton roots style SC is a Positve dissplacement compressor-PDC.....the compressor pumpsthe same amount of air for each revolution of the engine regardless of speed. and beause it is a PDC, the the air must pass thru the engine.the advantages to this is that the manifold pressure remains the same at all engine speeds.In the old days, it was possible to remove an SC form the engine with a simple backfire if there were no safety valves between the SC and the enginne.....I thinkk of a roots blower this way.....It has two big lobes inter twined like an eggbeater puhing out chambers of air into the manifold...as the chamber of air goes into the manifold, It back pressures just a bit into the next chamber of air that is coming with the next turning lobe...but because it must win,mor air goes into the manifold because it must...but this little bit of turbulence creates heat,, and less efficiency...that is whyit so a misconception the Sc air is cooler than turbo air....It is allso partly why in the Vibe, 7.5psi manifold pressure from an SC creates 35%more power , when 7.5psi from my turbo creates over 90%,...I said partly because the other loss of power gains in the SC are becaus it uses crank power to make hp where as turbos use zero power to create power.But the sc give you imediate boost, where as my turbo has lag....I do not hit boost till 3000rpms...but I am rambling. on a bit...I just wnted to note, that pdc's air must go thru the engine , while the the turbos high speed air converts to pressure thru diffusion...so besides them creating manifold pressure...they do it differently....so ther you have it..

[PBFlash]

All I can say is Huh??!??

[Faultline]

yeah...I know...HUH??In my defense I will say that I normally do not talk this way, but before I decided to put a turbo on the vibe..I read way too much about forced induction,,..waaaaay,,,,,

[cohocarl]

quote:....turbos are dynamic compressors..they make compression by accelerating the air to high velocity and then slowing it down by diffusion. Diffusion converts velocity energy into pressure energy by slowing down the aiir w/o turbulence.I don't get it. I thought a turbocharger was a fan blade on your exhaust pipe that spins, then the shaft that the fan blade is on turns another fan blade that blows air into the front side of the motor. Then because the 2nd fan blade pushed more air/fuel into the motor, even mroe comes out the exhaust, which then spins the fan on the exhaust pipe faster which then causes the intake fan to spin faster pushing even more air/fuel in. This keeps building and building after a few seconds.

[satur9]

that is a semi simplified way of putting it.you could add that at idle low rpms there is less exhuast to push the turbo than at higher revs hence the lag.its actually a little more complicated but...hey faultline you forgot about centrifugal superchargers the "belt-driven turbo" such as vortechs they are rpm dependent and usually have intercoolers like turbos. youve probably seen the one for the s2000 that adds 117hp. and everybody... if you all need visuals check out "howstuffworks.com"

[Faultline]

Yeah its true, ...the centrifugal SC's are also dynamic compressors..like you said.. the Vortech...or the Paxton, or Procharger or , powerdyne..they are all in that group..what makes it tricky for these belt driven turbos is the engine rpms....turbos running from the exahaust spin over over 100,000 rpms, what kind of rpms does your enine spin?...up to 7,000 rpms?it not enough for the compressor... so I belive that not only are they geared with the corect ratios,,but they are able to shift....like an auto tranny...but I am not clear on this , maybe some one will set me straight...but any way,...coho carl..your right..axial fans are DC's....like a box fan at home.......and on the turbo....the fan that spins from the exhaust gas is called a turbine , which is connected to the same rod as the the impeller, which is the fan that moves the air...at high velocity...then the air gets smushed... ..in contrast ,.. the PDC's move big chunks of air, and because it is belt driven...is must go into the engine..because it corresponse to the crank speed.....I think of the turbo like a rushing mountain stream into a resevior than can only hold so much...dc's have to work at high speeds...pdc's are shoving it down the engins throat in gulps..like it or not...they do not need high speeds.....I dunno..thes analagies break down...but pdc's and dc's compress differently whether the source is crank, or exhaust...

[abe98gt]

haha i can think of a few ways to make your messages clearer. drop the analogiesuse paragraphsother than that, you got the main ideas right.