supercharger without bypass valve

Sun Sep 04, 2005 9:38 am

is this a no no like running a turbo without a wastegate as i thought that superchargers run to a set rpm dependant on engine rpm so couldnt in theory overboost?

Sun Sep 04, 2005 10:04 am

The bypass valve's just there to unload the charger at idle and light throttle conditions.

Positive displacement superchargers are geared to provide a certain amount of air over what the engine consumes with something like 34% more air equating to about 5psi of boost.

If it overboosts in one place, it'll be overboosting everywhere!

Think the car wouldn't be quite so nice to drive around town without the bypass valve meself. Why you ask?

Sun Sep 04, 2005 12:06 pm

AndyBird wrote:is this a no no like running a turbo without a wastegate as i thought that superchargers run to a set rpm dependant on engine rpm so couldnt in theory overboost?

but when you take your foot off and the throttle slams shut the chargers still spining!
i had boost spikes in excess of 20psi in the pipes at light load trust me thats not good for the charger

Sun Sep 04, 2005 5:14 pm

lentec wrote:
AndyBird wrote:is this a no no like running a turbo without a wastegate as i thought that superchargers run to a set rpm dependant on engine rpm so couldnt in theory overboost?
but when you take your foot off and the throttle slams shut the chargers still spining!
i had boost spikes in excess of 20psi in the pipes at light load trust me thats not good for the charger

so a bypass valve is like a bov what about installing a bov downstream of the supercharger to stop that?

Sun Sep 04, 2005 9:16 pm

Thats what im running! It makes a hel of a noise at part throttle a recirculating dump valve might have been a better idea!
But yea that works

Sun Sep 04, 2005 9:22 pm

Not really.

A BOV takes it's vacuum signal from between the throttle and engine so when the throttle's shut, the vacuum causes the BOV to open and vent the unwanted boost pressure.

With a pos disp SC installation, the bypass valve also gets it's signal from between the engine and throttle. However, the SC is also between the throttle and engine and this bit of plumbing needs to be kept sealed, even if the SC is being bypassed.

If you had a BOV installed, the vacuum would cause it to open, this would let air in downstream of the throttle causing the vacuum to drop and the BOV to close, which would then increase the vacuum and cause it to open and so forth with this fluttering.

Basically, you need a bypass valve.

Sun Sep 04, 2005 10:14 pm

just done a google search and found this rather interesting documenthttp://www.jagweb.com/aj6eng/supercharging_article.html
look at the pic of "race or high boost bypass" thats allmost what im running execpt ive a dump valve to atmos instead!

Mon Sep 05, 2005 8:14 am

Have you still got your throttle downstream of the charger then Len?

Mon Sep 05, 2005 9:10 am

Sure have
For now

Mon Sep 05, 2005 1:30 pm

Aah that explains it!

Mon Sep 12, 2005 7:51 am

Right. after a bit of playing and modifing ive found that a recirc valve off of a volvo 740t

Works great with a supercharger! i can leave the charger turned on all the time with no funny noises!
And its adjustable!!

Mon Sep 12, 2005 6:15 pm

Where does it recirculate to exactly?

Mon Sep 12, 2005 11:54 pm

The other side of the charger
Asin air filter side