"wastegate" for centrifugal supercharger

[bear1a]

assume i want 9psi boost..i have a centrifugal supercharger...it puts out 9psi but i want it at a lower rpm..so i put on a smaller pulley...does the job..however at the higher rpms it produces too much boost...is there a way to blow off the extra boost?...similarly to what a wastegate functionally does on the turbine side of a turbo?...

 

[patprimmer]

Not really

The big disadvantage with centrifugal blowers is the exponential boost to rpm nature.

You could vent the manifold to atmosphere, but you would create quite a gale and waste a lot of power driving it. I expect it might also be quite noisy.



Regards

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[dcasto]

You need a blow off valve. It will vent the air to stay below 9 psig (or whatever). The problem it be getting the right location. If you AFM is up stream of the compressor and you vent the air after the compressor, the computer does not know the engine has less air and it will keeping adding fuel. If you vent the air back between the AFM and the compressor, the afm only sees the fresh air and not the recycle. The problem here is the air will be warmer than the ouside air and the computer will be off on actual air, unless you meve the temperature compensation measurement to the suction of the compressor.

One question, is your compressor rated for the new maximum rpm with the different pully?

 

[bear1a]

so you are saying that someone makes a blow off valve to regulate pressure for centrifugal superchargers?..do you know who?...it has a blow off valve now, but that is obviously for throttle off conditions...

 

[dcasto]

The BOV should do the same, whether its be cause of throttle off or over boost.

 

[evelrod]

I can't add anything definitive, but...If my memory is at all intact, back in the 60's I saw a McC setup with a variable speed pulley. If I remember correctly, it was setup with some type of Salsbury or CVT type drive. That might be an interesting idea to play with.

Rod

 

[MikeHalloran]

I suppose you could put a pressure- controlled butterfly valve in the inlet.



Mike Halloran
Pembroke Pines, FL, USA

 

[dcasto]

I believe you would put the compressor in surge and damage the unit if you put a control on the suction side.

 

[MikeHalloran]

Oops. I hadn't thought of that. Sorry.





Mike Halloran
Pembroke Pines, FL, USA

 

[GregLocock]

Given that a supercharger is creating 40 hp of pressurised air, your blow off valve will be glowing when it operates. This will then be followed by the sound of shredding V belts at a rough guess, at best.

Cheers

Greg Locock

Please see FAQ731-376 for tips on how to make the best use of Eng-Tips.

 

[Majik]

A pop off valve may work but a blow off valve... not a chance Besides centrifugal sc's need a bov anyway for rapid throttle closures unless you like to replace bearings.


Friend used to run a 14psi radiator cap on his intercooler piping as a failsafe if he over boosted. Goofy but functional. You'll need something bigger. Like a "wastegate". There's no reason you can't weld a exhaust waste gate to the intake pipe and then vent. Just recirculate the vent to the intake before the SC if it's a MAF car.

 

[DelSmith]

I would suggest a Diverter valve like found on Turbocharged engines. Your more complete Cent.Supercharging kits supply those recirc. valves. You can get them as cheap as 50.00 new from Autoparts stores. That will get you a Bosch type like on Turbo Porsches,Audi,VW and Saabs. You can get a Aluminum One for Less than 100.00 on EBay. That will Divert compressed air out of the discharge and recirculate it into the inlet housing. You will need to control that the operation of that valve. It is vacuum actuated and possibly could incorporate a boost controller to manage the vacuum. Are you wanting to regulate or simply protect overboost?
There are alot of contributing factors that would need to be taken into account and how much you have budgeted for this project.
MAF or MAP
Gas or Diesel
Common Throttle body?
Flow Curve of the Compressor?
Engine Displacement?

 

[r2800]

I would suggest controlling the air as it enters the compressor rather than trying to discharge high pressure hot air that has had considerable energy espended on it.
Assuming the throttle is between the compressor and the engine a secondary throttle body mounted up stream and ultimately controlled by manifold pressure via a servo will act as a restriction to the air ingested.This will result in wire drawing and hence limit flow at higher speeds and flows.