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

Re: blow off valve on supercharged motor
 

Smitty,
I was looking into this same thing when I had my PC setup. I found some nice blow offs that were available in different pressures at one of the turbo sites. I never got around to it but I'll see if I can't find the site.
Dave

Re: blow off valve on supercharged motor
 

Blow off valves are meant to keep the compressor out of surge when you snap the throttle shut at high RPMs. If you are doing that in a boat (throttling on big waves) then the valve is a must. Vortech recommends them once you get over 6-7 psi. The other situation you describe is better handled by a second throttle valve after the blower that works in reverse to the main throttle linkage. This will reduce the discharge pressure seen by the blower and therefore the heat before the intercooler, but since the compressor will actually be handling more lb/min of air you may not see any reduction in parasitic loss. At low RPM and air flow the centrifugal compressor has very low parasitic loss to begin with. Since the intercooler is sized for WOT, it is really oversized for part throttle conditions. Hope everything else is running well.

Tom

P.S. The throttle is the biggest variable in air flow restriction and when you move the throttle you are really creating different system resistance curves on the same compressor map. For sudden throttle movements you can consider that engine/blower RPM does not change right away, so this momentarily creates very different operating points on the same blower RPM line. Have a look below.

Re: blow off valve on supercharged motor
 

Tom,the boat is running good,I took it to tyler last week and we drove from port huron to harsens island and back with a wide band 02 sensor in one of my manifolds. He made small adjustments to all the fuel tables at 200 rpm increments while underway,my a/f ratio was at 10 -1 from 5000 rpm's up,he took substantial fuel out on the top. It was pretty hairy holding the boat at 5000 rpm's up steady in 200 rpm increments for a few minutes at each step while encountering cross chop and waves where I normally would let off. Ol' Tyler kept plugging away on his lap top though,I was worried I was going to glance back and he would be gone(we were catching some serious air! I put boat on rev limiter(6000 rpm's) with 30 pitch prop and full tk of gas and a couple of BIG dudes in my boat the next day so the top end is strongest yet,I might see 91mph :D when I try a 32 pitch labbed at hardy pond,Smitty

Re: blow off valve on supercharged motor
 

This is where a data logger works wonders, you can make your changes in much calmer conditions :D ... Your a brave man Smitty :eek:
Sounds like it's running very well :drink:
Dave

I spoke with ProCharger when I was building by 515 ci BB in my 272 Baja with a M3. I was running the 7 psi pully and they said I was on the verge of needing a blow off valve but could most likely get away with it. Cars need them because of the higher boost presures. The engine ran 968 hp on the dyno and has also run 90 mph on GPS. I also did not use the carb enclosure, but the ProCharger Comp blow throught bonnet. I had a custom blow through carb built from the Carb Shop for the engine.

I was thinking about one myself....going to be running a procharger on a HP500 and do the bonnet vs the box as well, seems to me talking to procharger not many there have much "real" info..maybe just who I spoke to or me ??

Not sure how relevant this might be for you guys, but...

We run MoTeC engine management on our race cars and were instructed to run a supercharged (Whipple) V8 by the OEM partner.
I overcame the part throttle and decel issue by installing a turbocharger wastegate on the back of the intake plenum, and controlled manifold pressure with the MoTeC ECU (and relevant inputs) by continually adjusting the duty cycle of the wastegate during real time running.

The turbocharger wastegate allowed us to open the plenum to reduce manifold pressure when we wanted to (depending on throttle input), create different torque curves within the engine by creating different map tables for wastegate control dependent on rpm / throttle position / manifold pressure, and also allowed real time switching of those maps from a dash mounted switch.

I thought is would be awesome for a boat after we did it. Mild boost for cruising with the family, turn the switch on the dash and add 10lbs of manifold pressure when its time to go fast :)

To be fair though, the capability isn't cheap. MoTeC stuff is badass, but it gets real spendy quickly.
Not to mention the time on the dyno creating the map tables etc.

But if anyone was to get serious about this, it has been done before and worked amazingly well.

Seems to me bleeding off the boost at low RPM and cruise would just hurt throttle response and create more tuning issues. Procharger makes a blow off valve for fast deceleration, I have one sitting in a box. I wonder what happens when the blower is spinning 50000 rpm and all of the back pressure is released.

I don't believe carb bonnets are USCG "legal" for marine usage. I think you have to run the carb "box" to be compliant. Performance and tuneability should be similar with either method, although the bonnet does allow much better access to the idle screws and such..

I keep seeing people mention backfire and blowoff valves in this thread and Arctic was originally asking about a method for unloading the blower impeller when the additional boost wasn't needed.

There are OEM auto blower applications that do what he's talking about. In all of those applications, there is a valve that opens between the blower discharge and the blower inlet. The valve is controlled by a vacuum diaphragm that measures the pressure delta between the intake manifold and the blower discharge (above and below the throttle plates).

Typically, the delta is around 2psi. If the motor is running in vacuum, then the blower only has to pump "against" 2 psi of boost. The remainder is constantly recirculated to the blower inlet. When the throttle is opened, and the intake pressure rises, the valve closes and full boost is available.

It's a great idea, but I don't know of any system that has been approved for marine usage that incorporates this ability.

Using an Eaton blower for a BuickV6 or a side-mount Ford 5.0 Windsor blower (Kenne Bell I believe made the one I'm thinking about), and you get this ability built into the blower case.

To do this with a centrifugal blower, you would have to use something like a wastegate device with a "differential" pressure can on it. Not difficult to make, though.

MC

This is a old thread but I never did get a blow off valve or unloader valve, I was worried at the time (and it still makes me nervous) when I pull throttles back too fast that I get a big blower surge and that it might destroy the impeller (I think it was right after this thread 5 years ago that I DID have a supercharger fail but it was from bearing failure accelerated by turning blower higher than it's rating), Smitty

I had the impeller of a M-3 fail. I was not spinning it too high but mine lost a seal then I think it cooked a bearing. It made a nice mess. Thank goodness for the cooler. I found lots of blades in there.

I lost an impeller too and they fixed it for free the new unit has less than 1 hour on it and the whole system is for sale cheep 419 473 2628 bonnet blow thru dean nickerson carb pop off valve the works ................get it off my shelf.

I missed this post first time around, did you make 968 hp at 7 psi of boost or did they just call the pulley a "7 psi " pulley? Your speed is right inline with what mine was with almost exact power level I had at the time, I'm glad to see your post and numbers so I have sometghing to compare my boat to. I made 945 hp back in 04 or 05 but I was running more like 13 psi of boost but at the time my boat would run 89.5 or so with a full tank of gas and 4 250lb guys in it and saw around 92 running light. I have since upped the power and see around 95-96 mph w/my 1050 hp pulley or real close to a hundred with my 1115 hp pulley. I have been told boat should be much faster with that much hp but for Christ sake its so far past the hull limitations who really knows how fast it should go with that kind of power, Smitty

I make 942 hp with my 540 (similar setup as yours). It gets my 26ZX to 103. I have been there once and may do it again. It is kind of like Mardis Gras, you don't want to go there every weekend.

I am surprise that yours does not break 100. You are making some HUGE power.