blow off valve on supercharged motor
 

When i run my boat at 3500-4000 rpm's,i don't need to open throttle far enough to actually see any boost at manifold but when i mount a test gauge i actually have 7-10 psi backing up behind throttle body. I feel this extra load creates heat in the supercharger,puts stress on the belt and robs part throttle horse power turning the blower with no where for the air to go. My question is when procharger and all the other centrifugal supercharger companys make blower kits for cars/trucks they always incorporate a blow-off valve that bleeds off the boost until your manifold vacuum hits zero then it closes off but on their marine kits i don't see this being used. I figure that most boats are farther into the throttle and able to actually use all the boost at part throttle when using a smaller base engine. Is anyone using a blowoff valve with a centrifugal supercharger on here? Just curious,i spend alot of time cruising at part throttle and am looking for every possible variable to be correct. The other thing is but this doesn't hapen often is if I'm waay into the boost and you have to throttle back quickly i hear the blower huff and almost try to stall as the air its pumping has no where to go,Smitty

Re: blow off valve on supercharged motor
 

I believe the reason for no blow off valves is a Coast Guard regulated type of thing. If it backfired it could blow fire out the valve etc. Just a guess. I run Whipples and it does have a valve for lower rpm/boost condtions that recycles the air that is not getting used at cruise speeds. It vents the excess back to the output side of the blower and helps keep a constant smooth idle with no surging. If you search around you may find something that someone has made or whatever that will accomplish what you are asking for. Maybe you could make it work similarly to what is on the Whipples.

Re: blow off valve on supercharged motor
 

The coasties don't like backfire valves in closed compartment boats.

Re: blow off valve on supercharged motor
 

Maybe calling it a blowoff valve is the wrong word,what I'm talking about is a bleed-off or bypass valve at the centrifugal supercharger discharge. The air hasn't went thru the innercooler or flame arrestor at this point so it isn't for backfire protection or coast guard regs. Procharger says their systems don't need bypass/bleed off valves when you call BUT my boat makes considerable boost before the throttle blades at 3000-4000 rpm's. The motor doesn't need the airflow the blower pumps so it backs up causing the blower to build some heat and from the belt dust i see it must slip the belt a little too. A vortech engineering system for blowing thru a 502 efi plenum uses a bypas valve but they are the only ones. I'm trying to find someone with same problem who has adapted something,Smitty

Re: blow off valve on supercharged motor
 

my Vortech supercharger has a place for the back pressure valve but it has a plate bolted over it. If you look at the picture of mine it looks like you could use that same tube in between your blower and you intercooler and use the back pressure valve. You could call Vortech and see if they would sell you one?

Re: blow off valve on supercharged motor
 

You are describing two different situations. One where you have boost between the S/C and the throttle plate at small throttle opening and the other is when you are at WOT and close the throttle and boost builds up behind the closed throttle and the S/C and boost/air will bacu up against the S/C and actually push the impeller which could hit the housing and break and at less just cause a reverberation noise and try and stop the impeller and be why you have belt dust. You can use the Vortech blowoff/popoff valve and vent it back to the aircleaner. The valve has ports on it to make it open under high vacuum in the manifold and high boost before the throttle. It can be done.

Re: blow off valve on supercharged motor
 

Marty,the valve that vortech uses in apps like mine is set to open to prevent overboost conditions and when letting off the gas suddenly,its not vacuum referenced though like the auto valves. The valves they sell for autos are much smaller but work like you described,I'm hoping someone has installed something and will chime in,i hate to buy one for 300$ that is too small,Smitty

Re: blow off valve on supercharged motor
 

You could make something yourself pretty easily using a vacumn advance off a distributor, a swtich a relay and a solenoid.

Re: blow off valve on supercharged motor
 

My vortech unit has ports on both sides of the diaphram. You can put vac or boost on either side to get what you want.

Re: blow off valve on supercharged motor
 

Look at their website for the Mondo blowoff valve. http://www.vortechsuperchargers.com/...s/aircomp.html