Some of the guys at ****************** have seemingly developed more boost (and therefore power) with an extremely simple and free mod.

http://www.******************/forums/...&page=12&pp=10

By removing and plugging off the vacuum hose from the solenoid that controls the superchargers bypass valve they have deactivated the boost limiting action of the bypass valve at high revs. According to some of them the valve opens at around 4500 RPM to limit boost to 12PSI, and by removing the hose the valve stays shut, allowing boost to keep building. Apparently one guy with a boost guage saw peak boost jump from 12PSI to 15PSI. If this is true (someone is dyno'ing on monday I think) that means more boost and more power with a simple, fast and free mod. WOOT!!!

Of course if it doesn't actually produce more power, it was definitely worth a try!

 

hope nothing goes wrong, theres a reason for a bypass valve, its to release pressure during shifts so your making 15psi then shift and your still at 15psi, kaboom.

 

Agreed. Without reprogramming the ECM many mods won't produce desires results. The computer will try to compensate in some way. I would hold off and let someone do some testing first. Hope it works, though.

 

I think you're mistaking a bypass valve for a blowoff valve, they are not the same. A Blow off (BOV) is to reduce compressor surge between shifts. A Bypass valve, bleeds off boost as a means of controlling max psi. The LSJ uses the bipass so that the engine will not see boost over 12psi.

 

Osmose or anyone that knows...can you then have a BOV on a supercharged car to release pressure? or would it serve absolutely no purpose?

 

Hey i want a BOV on my cobalt!

 

i thought it had a BOV , if not that would be my first mod thing arnt good when you cant releave pressue....excuise my pun hahaha

 

it will still open under vacuum, but the computer will never see max boost from the bypass valve since your unplugging the line to the PCM, the only issue i see there is the MAP sensor seeing too high of readings, but if it can handle what it's seeing at 15psi, then the computer should be able to compensate

 

Could one put a restriction in the line to give the solenoid a false psi reading, thus allowing the bypass to stay closed? Similar to a manual boost controller.

Wonder what bar the map sensor is?

Jay

 

since its a roots blower, its always at full boost.. if you power shifted, you would stay at 12 lbs the whole time..

 

well u can consider a bypass valve as both a boost controoler and bov, as it limits the maximum boost to 12 psi, and also reduces boost when it is not at wot. and also between shifts, its there for a reason not just to limit boost at 12 psi but so it releases pressure when it is needed.

 

Well, the answer is maby... It depends on the s/c setup. If it's a roots blower, then MOST LIKELY the S/C is after the throttle, therefore there is no need for a BOV or Bypass valve. If the S/C is before the throttle blades, then there would ABSOLUTELY need to be either a BOV or a Bypass valve. Do you think that Top fuel dragsters have them? No, because the throttle blades are before the S/C...

 

Good point, I think we are thinking there is a bypass valve to limit the boost from going over 12psi. I think what you are thinking is to keep boost from entering the engine when the throttle is closed. Chevy has supposedly put in a system to limit the boost pressure to 12 PSI even if the blower can make lets say 17psi.

I though the LSJ blower was a postive displacement so it produce full boost at lets say 2000 RPM, so I really don't see the need for a system to artificially lower the boost to 12PSI. They could just fit a blower that made 12 PSI boost MAX.

Any experts care to comment some more?

 

Yes it is positive displacement, and no it doesn't reach12psi at 2K RPM's.

The blower is not maxed out (in termes of rotaion speed) at 12psi, and yes GM did limit the boost pressure at 12psi. The blower will continue to produce more boost pressure until it reaches 12psi, then If the blower rotates any faster and produces a boost pressure over 12psi it will be bled off by the bipass valve so that the engine never sees a boost pressure beyond 12psi.

 

no roots blowers make instant boost, its not like a centrifgal..

 

superchargers run off the engine, so they make boost according to RPM... maybe it makes 2lbs at 2000, 4lbs at 3000, 6lbs at 4000, 9lbs at 5000, and 12lbs at 6000...

 

yes they do run off the engine, but a roots works different then a centrifgal..
roots is a big air pump giving full boost at all rpms, while centrfical is more like a fan building up boost with RPM..

 

Alright.... here's the scope... A positive displacement S/C puts out a given CFM at a given RPM, and the CFM rises linearly with RPM. Now, the engine wants a specific CFM at a given RPM, and that CFM rises linearly. So the ratio of the S/C output to the engines NORMAL demand is roughly the same. So, since the S/C is putting out more CFM than the engine wants, the pressure increases so that the CFM the S/C outputs is the same as the CFM the engine draws in... The presure rises roughly acording to the equation PV=nRT, Where P is pressure, V is volume, n is the amount of molecules, R is a constant, and T is temperarture. Now, that's for a roots style S/C.

A Centrifugal compressers (Be it a S/C, T/C, or fire engine pump) work differently. They use inertia to pressurize the fluid flow (be it water or air). What happens, is as it spins, fluid is "thrown" outward due to cintrifugal force and thereby has additional energy imparted to it. That energy does two things. First, it forces it out of the compressor, and second, it draws more fluid into the compressor at the intake. Now this works great, as long as two things happen, the fluid has somewhere to go, and the intake has plenty of fluid to supply. Now, because the compressor imparts energy on to the fluid, the rate of transfer will rise parabolicly from idle. Meaning it will put say, 100 cfm at 1000 rpm, 200 cfm at 2000, 400 at 3000, 800 at 4000, and 1600 at 5000 rpm. Now, since the engine demand rises lineraly, more pressure is produced at peak RPMs than at idle. This is overcome, by making limiting the peak boost, and using bypass valves to keep the pressure at something lower than peak.