Carb talk
Gold Member
12-11-2014, 05:46 AM
#16
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iTrader: (3)
Boost referenced power valves are not necessarily manditory, but I feel they are the better way to go. Blocking the power valve in most setups was strictly a drag racing trick in the day, because strip cars didnt see high vaccuum/low load scenerios. Yes, for the most part, in a heavy boat, most of the time you're on plane, the PV will be open, if its boost referenced. I dont see it as a nuisance, I see it as another fuel supply to the main circuit.
Boost referencing them imo, makes the tuning process easier. For the most part, you are eliminating the lean to rich problem that occurs when the power valve opens, when non referenced. Because loads vary in a boat, based on trim settings, how much fuel or how many people on board, one day your AFR at 3500RPM can be 12.0, another day it might be 11.5 depending on the throttle position at 3500rpm. Its not always identical throttle postition vs rpm. By having the PV referenced, its pretty much certain that your PV is simply gonna be open. Now you simply add/remove jet, or with the quick fuels, change the PV restrictor size to tune the fuel mixture. Carbs can meter fuel well based on the load/rpm when relying on the venturi effect. Throw a random fuel dump into the main circuit, and you start getting unstable fuel mixture. IE, rich/lean condition with a simple 10% change in throttle position.
While yes, blocking them is possible,you may find yourself having to run very large jets in the carb to get an acceptable AFR. Every carb is different. My 850 holley marine carbs came shipped with 88P, 96S with PV front and rear. If i was to plug the PV's, I'd likely have to go to 96P, 104S, to even things out. With that being said, I do think boost referenced PV's are much more important in a street driven blown setup. In marine, we rarely have a scenerio where we are cruising with 10+ inches of vacuum, and gonna mash the throttle to wot needing the instantaneous fuel. For the most part, on plane, they are open anyways. Unless you have a super light cat. But most Vee bottoms, your never really gonna see much MANIFOLD vacuum at cruise. With non referenced PV's, you can see high vacuum under the carbs, at cruise, where the PV can be open.
Boost referencing them imo, makes the tuning process easier. For the most part, you are eliminating the lean to rich problem that occurs when the power valve opens, when non referenced. Because loads vary in a boat, based on trim settings, how much fuel or how many people on board, one day your AFR at 3500RPM can be 12.0, another day it might be 11.5 depending on the throttle position at 3500rpm. Its not always identical throttle postition vs rpm. By having the PV referenced, its pretty much certain that your PV is simply gonna be open. Now you simply add/remove jet, or with the quick fuels, change the PV restrictor size to tune the fuel mixture. Carbs can meter fuel well based on the load/rpm when relying on the venturi effect. Throw a random fuel dump into the main circuit, and you start getting unstable fuel mixture. IE, rich/lean condition with a simple 10% change in throttle position.
While yes, blocking them is possible,you may find yourself having to run very large jets in the carb to get an acceptable AFR. Every carb is different. My 850 holley marine carbs came shipped with 88P, 96S with PV front and rear. If i was to plug the PV's, I'd likely have to go to 96P, 104S, to even things out. With that being said, I do think boost referenced PV's are much more important in a street driven blown setup. In marine, we rarely have a scenerio where we are cruising with 10+ inches of vacuum, and gonna mash the throttle to wot needing the instantaneous fuel. For the most part, on plane, they are open anyways. Unless you have a super light cat. But most Vee bottoms, your never really gonna see much MANIFOLD vacuum at cruise. With non referenced PV's, you can see high vacuum under the carbs, at cruise, where the PV can be open.
12-11-2014, 08:00 AM
#17
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I would run progressive linkage, and power valve in the primary, as far as boost referencing, you need to put a vucuum gauge right under carb base and ck at different RPM's, I have run into some roots type blowers producing so much vacuum that power valves and vacuum controlled meetering rods will stay closed when they need to be open causing a very lean condition...
12-11-2014, 08:07 AM
#18
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I would run progressive linkage, and power valve in the primary, as far as boost referencing, you need to put a vucuum gauge right under carb base and ck at different RPM's, I have run into some roots type blowers producing so much vacuum that power valves and vacuum controlled meetering rods will stay closed when they need to be open causing a very lean condition...
12-11-2014, 03:14 PM
#19
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I've seen that when too small of a carb is chosen for the application. Very common on the baby single carb blowers when guys are trying to make 700hp with a small roots blower and a single 750 carb they yanked off their small block, or some tiny flame arrestor/filter. The right blower/carb combo in a marine setup should never have 6+ inches of mercury under the carb. The old merc 525sc, 600sc, 800sc engines did not have boost referenced pv's nor did they have enough vacuum to pull a 6.5pv shut at wot.
12-11-2014, 03:57 PM
#20
Ok so non referenced you get a fuel dump at 4000rpm with boost referenced you get a fuel dump at 3000..(lets say) No matter where it happens it`s still 8 jet sized dump. What am I missing?
I don`t want to get dump at anytime I want a nice even AFR curve.
Dyno sessions don`t count, AFR is always gonna look pretty because it`s a WOT run from start to finish.
I think I need to talk with a carb engineer no one around here knows anything!
I don`t want to get dump at anytime I want a nice even AFR curve.
Dyno sessions don`t count, AFR is always gonna look pretty because it`s a WOT run from start to finish.
I think I need to talk with a carb engineer no one around here knows anything!
