Nordicflame

iTrader: (4)

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

tomcat

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.

articfriends

iTrader: (1)

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 when I try a 32 pitch labbed at hardy pond,Smitty

Nordicflame

iTrader: (4)

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

thundercat1001

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.

ezstriper

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 ??

Rookie17

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.

GPM

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.

mcollinstn

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

articfriends

iTrader: (1)

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