so school me my brain doesnt get it. on any roots style blower how is the boost limited? i dont get it a turbo has a waste gate so that at Xpsi it opes and limits boost. now my gpt for example has a bypass so it will not boost but how does it know to only build 5 psi or whatever? cause it has 5 psi at lets just say 3000 rpm so in my head since it spins faster at 5000 rpm something has to keep it from makin more boost? if the bypass has some sort of pressure spring to open and allow only 5 psi then how the hell does a pulley swap work cause whatever kept it at 5pis with a 3.5 pulley should keep it at 5 with a 3.0 pulley....................

thanks for some real knewb lol questions lately

 

Positive displacement blowers work on RPMs.

The only thing limiting it to X psi with the first pulleys, was the RPM of the blower...

Change the pulleys and spin it faster.... then you get more boost.

The only exception to that is using a wastegate on the cold side, and a boost controller just like a turbo system does on the hot side....

Some people do that too

 

see that cannot be right because it reaches 5psi at 3000 rpms and still has that 5psi at 50000 rpms you just said spin it faster in makes more boost well its spinn almost twice as fast and is still at 5pis not 10 like a centrifigual (SP)............


check out this graph of a whipple 3700 rpm 8.9 psi 5700 rpm 9.1 psi
http://pdfcast.org/pdf/whipple-sc-dyno-results



im less concerned about pulley swap as i am how psi is limited

 

Could have easily had a wastegate...

 

my dead stock GTP makes same boost at 3k as it does at red line

 

so 126 views and nothing ?

 

The supercharger will produce more air as rpm increases, but the engine consumes more at about the same rate keeping psi pretty level. When you change the pulley you speed the blower up relative to the engine speed so the boost psi goes up all across the band.
As you spin the positive displacement blowers faster they lose efficiency because the fill time is getting fast and volume is reduced at the rotor, so eventually you will see boost fall off slowly as speed increases with blowers like your 3.8 GM engine. That type of blower is very good at low rpm.

Kurt

 

bbammmmm ther ya go man as soon as i read it i shook my head lol im retarded thanks for layin that out

 

Here's the deal. A supercharger is driven off the engines RPM with a relational spin rate of the blower to the crankshaft. When RPM changes a set of pulleys and a belt (most cases) tranfer the rotation of the crank to the rotation of the supercharger. This is a ratio value that is dependant on the size of the pulleys. Typically a bypass valve is used to limit compressor surge. This is a simple valve that allows the air being compressed to vent at a certain PSI level or to be fully open based on a "No Load" condition.

In modern superchargers they come built in with the bypass valves but are mainly used to eliminate backpressure of the supercharger during low load conditions. However they can also protect against compressor surge also. Let's say you want a maximum of 10PSI and if you see a surge to 11PSI you want that pressure vented. You could do that for the most part however don't even think that you could truly control beyond that.

But keep in mind there's not much flexibility to a supercharger when it comes to truly controlling boost levels. In a turbocharged application you can electronically set your boost levels on the fly. I can set mine from 10PSI-25PSI without making any other mechanical changes. It does this by adjusting the amount of exhaust gasses striking the turbine and thus spinning it at a rate that matches the requested boost level. A supercharger bypass valve can limit maximum boost but isn't designed to control it up or down on the fly as dictated by an electronic controller.

At the end of the day on a Turbo car you can be at 3000RPM and be pushing 25PSI then be at 8000RPM and can push 25PSI also. Hit a button on the dash and change it to 10PSI and it'll adjust on the fly. Can't do that with the Supercharger. It's strictly boost levels at a certain spin rate of the compressor lobes which is tied to a fixed ratio based on engine RPM.

So unless someone has invented a constantly variable pulley system you're outta luck in truly controlling boost.

 

But it is correct. You spin a motor twice as fast, it will gulp twice the air (assuming same VE).

 

oh no i fully understand the full how FI works lol i just had a massive brain fart on the use of air at higher engine speeds. iv had a tubro setup on my car and installed a couple prochargers i just was overcomplicating the whipple setup for some reason.

 

Yeah I never thought about low boost setups with fixed volume per RPM (superchargers) running out of steam at the higher RPM's.

 

dead on!