Among a couple of threads I've read as of recently, there seems to be a misconception as to what a power valve is and what it does, as well as where it comes into play on a jet boat application. So I thought I'd post up and give a basic understanding of what they are and what they do in hopes that some can figure out what size to run on a jet boat application.
Before reading this article, I would like to mention that this is a basic reresentation of what a power valve is and what it does as well as apply some basic information regarding how a jet boat engine is loaded to gain an understanding of why you would run a power valve in a lake cruiser.
One thing I'd like to make sure that everyone who reads this understands -
Engine manifold vacuum DECREASES as engine load INCREASES. In this way, engine vacuum is a direct indicator of engine load.
On a typical 4150 Holley carburetor, you will have a 6.5 power valve in it stock. What the 6.5 refers to is the value that manifold vacuum has to fall below before it will open. When manifold vacuum falls below this value, the valve opens, allowing fuel to flow through the "Power Valve Channel Restrictions" (i.e. PVCRs). The PVCRs are basically a second set of jets. They basically allow fuel to bypass the main jets and flow into the main wells. This makes it so that when vacuum is above the valve's rated opening point, thereby depicting a minimal load condition, you're running on a set of smaller jets, allowing the motor to achieve max possible cruise fuel economy. Then when vacuum drops below it's rated opening point, thereby depicting a moderate load condition, it allows more fuel to bypass the main jets and flow into the main wells through the PVCRs. In short, the power valve basically makes the carb think you have 8-10 size bigger jets when engine load reaches a point where it wants a bigger jet size.
The size of the PVCRs is fixed...it cannot be changed without some modification (they'd have to be drilled bigger...they don't make different size ones like they do with jets).
To summarize this, think of the power valve as an "auxiliary fuel switch". It's either open or it's closed...there is no in between. When the engine is loaded to the point where it needs extra fuel flow(depicted by how much engine vacuum drops) that the primary jets cannot supply, the valve opens and allows more fuel to bypass the main jets into the main circuit.
When selecting a power valve, you're deciding at what vacuum level the engine will get that extra fuel. They are available in 1" hg increments and are numbered in half sizes (i.e. 6.5, 5.5, 8.5, etc etc). The bigger the number, the sooner in the throttle plate travel it will open (i.e. a 6.5 power valve will require more of a vacuum drop, IOW more engine load, to open than an 8.5 will).
Now for power valve applications in a jet boat...
Most carburetors come out of the box set up for an automotive application. This works great for cars and some prop driven boats. However, jet boats are in a class of their own. The way the drive loads the engine is completely different from the above two. Cars and prop driven boats unload the engine as they pick up speed. However, forward movement of a jet boat has no effect on engine load. The load on the engine is provided by the water flow through the pump and pump RPM determines how much water flows through the pump. More RPM = more water flow through pump = greater engine load. At a constant RPM, this load is a constant load, much like a dyno. Knowing this, it would make it easier to tune a jet boat engine since you're dealing with a constant load that is completely RPM dependant. This can be seen with a vacuum gauge hooked to manifold on both a car and a jet boat. When you dump the throttle in a car, vacuum drops until the engine overcomes the load and the vehicle starts accelerating. As the vehicle accelerates, vacuum will increase with acceleration (the faster the tires spin, the greater the decrease in engine load in a given gear).
However, on a jet boat, vacuum drops as throttle/RPM is increased and stays constant.
I know some carbs come with a power valve in both the primary and secondary metering blocks on some double pumpers. Although having a secondary power valve would allow finer tuning of the fuel map, IMHO on a jet boat you're splitting hairs at this point since most of the time you're in the secondaries in a jet boat you're at WOT anyway. So on these models I'm in favor of plugging the secondary power valve and increasing jets by 8-10 sizes.
Some racers prefer to remove and plug off all power valves and square jet the carb to the secondaries (i.e. if primaries came with 73s and secondaries came with 81s, you'd plug off the primary power valve and up the primary size to 81). For a boat that sees all of its duty at WOT this is fine since the power valve would be open all the time anyway. But for a cruiser, consider this -
My 18' tahiti copy hits a cruise of between 40-45mph at 3500 RPM. This is typically where I cruise. A vacuum reading at this RPM indicated that I was pulling 11-11.5" of vacuum at 3500 RPM, indicating very minimal load. Since my power valve is currently a 6.5 power valve, at this RPM the power valve is factored out of the equation and I'm running just on the main jets, which are the stock 73s. Imagine if I were to plug off the power valve and up my primary jets 8-10 sizes!? How much fuel would I be wasting by dumping it down the motor while it's at minimal load?
There are different methods for selecting a power valve. Some say to maximize idle vacuum, then select a power valve that is half this value, which I think is how you'd do it on a car. Others have said to check vacuum at cruise, then select one that is rated two numbers below cruise vacuum. One method I have not tried but thought of tonight with a little help from a friend (this applies mainly to carbs with progressive linkage...don't think it will work very well for 1:1s) is to get a vacuum reading with the primaries open as far as they'll open without opening the secondaries, then get a power valve rated for this vacuum rating. If the number is a solid number, select a 1/2 size up (i.e. if vacuum at this point is 6", select a 6.5 power valve). By doing this, I would think that you would be running on the economy jets until the secondaries open, then the carb will basically squarejet itself once the secondaries start to open due to the power valve opening EXACTLY when the secondaries open. Not sure how it will work but will be trying it very soon
Anyway, hope this helps to disspell the mystery behind the power valve and its purpose in a Holley carburetor.
EDIT: All vacuum readings obtained for power valve selection MUST be done on the water under load (thanks Tittyman )
Before reading this article, I would like to mention that this is a basic reresentation of what a power valve is and what it does as well as apply some basic information regarding how a jet boat engine is loaded to gain an understanding of why you would run a power valve in a lake cruiser.
One thing I'd like to make sure that everyone who reads this understands -
Engine manifold vacuum DECREASES as engine load INCREASES. In this way, engine vacuum is a direct indicator of engine load.
On a typical 4150 Holley carburetor, you will have a 6.5 power valve in it stock. What the 6.5 refers to is the value that manifold vacuum has to fall below before it will open. When manifold vacuum falls below this value, the valve opens, allowing fuel to flow through the "Power Valve Channel Restrictions" (i.e. PVCRs). The PVCRs are basically a second set of jets. They basically allow fuel to bypass the main jets and flow into the main wells. This makes it so that when vacuum is above the valve's rated opening point, thereby depicting a minimal load condition, you're running on a set of smaller jets, allowing the motor to achieve max possible cruise fuel economy. Then when vacuum drops below it's rated opening point, thereby depicting a moderate load condition, it allows more fuel to bypass the main jets and flow into the main wells through the PVCRs. In short, the power valve basically makes the carb think you have 8-10 size bigger jets when engine load reaches a point where it wants a bigger jet size.
The size of the PVCRs is fixed...it cannot be changed without some modification (they'd have to be drilled bigger...they don't make different size ones like they do with jets).
To summarize this, think of the power valve as an "auxiliary fuel switch". It's either open or it's closed...there is no in between. When the engine is loaded to the point where it needs extra fuel flow(depicted by how much engine vacuum drops) that the primary jets cannot supply, the valve opens and allows more fuel to bypass the main jets into the main circuit.
When selecting a power valve, you're deciding at what vacuum level the engine will get that extra fuel. They are available in 1" hg increments and are numbered in half sizes (i.e. 6.5, 5.5, 8.5, etc etc). The bigger the number, the sooner in the throttle plate travel it will open (i.e. a 6.5 power valve will require more of a vacuum drop, IOW more engine load, to open than an 8.5 will).
Now for power valve applications in a jet boat...
Most carburetors come out of the box set up for an automotive application. This works great for cars and some prop driven boats. However, jet boats are in a class of their own. The way the drive loads the engine is completely different from the above two. Cars and prop driven boats unload the engine as they pick up speed. However, forward movement of a jet boat has no effect on engine load. The load on the engine is provided by the water flow through the pump and pump RPM determines how much water flows through the pump. More RPM = more water flow through pump = greater engine load. At a constant RPM, this load is a constant load, much like a dyno. Knowing this, it would make it easier to tune a jet boat engine since you're dealing with a constant load that is completely RPM dependant. This can be seen with a vacuum gauge hooked to manifold on both a car and a jet boat. When you dump the throttle in a car, vacuum drops until the engine overcomes the load and the vehicle starts accelerating. As the vehicle accelerates, vacuum will increase with acceleration (the faster the tires spin, the greater the decrease in engine load in a given gear).
However, on a jet boat, vacuum drops as throttle/RPM is increased and stays constant.
I know some carbs come with a power valve in both the primary and secondary metering blocks on some double pumpers. Although having a secondary power valve would allow finer tuning of the fuel map, IMHO on a jet boat you're splitting hairs at this point since most of the time you're in the secondaries in a jet boat you're at WOT anyway. So on these models I'm in favor of plugging the secondary power valve and increasing jets by 8-10 sizes.
Some racers prefer to remove and plug off all power valves and square jet the carb to the secondaries (i.e. if primaries came with 73s and secondaries came with 81s, you'd plug off the primary power valve and up the primary size to 81). For a boat that sees all of its duty at WOT this is fine since the power valve would be open all the time anyway. But for a cruiser, consider this -
My 18' tahiti copy hits a cruise of between 40-45mph at 3500 RPM. This is typically where I cruise. A vacuum reading at this RPM indicated that I was pulling 11-11.5" of vacuum at 3500 RPM, indicating very minimal load. Since my power valve is currently a 6.5 power valve, at this RPM the power valve is factored out of the equation and I'm running just on the main jets, which are the stock 73s. Imagine if I were to plug off the power valve and up my primary jets 8-10 sizes!? How much fuel would I be wasting by dumping it down the motor while it's at minimal load?
There are different methods for selecting a power valve. Some say to maximize idle vacuum, then select a power valve that is half this value, which I think is how you'd do it on a car. Others have said to check vacuum at cruise, then select one that is rated two numbers below cruise vacuum. One method I have not tried but thought of tonight with a little help from a friend (this applies mainly to carbs with progressive linkage...don't think it will work very well for 1:1s) is to get a vacuum reading with the primaries open as far as they'll open without opening the secondaries, then get a power valve rated for this vacuum rating. If the number is a solid number, select a 1/2 size up (i.e. if vacuum at this point is 6", select a 6.5 power valve). By doing this, I would think that you would be running on the economy jets until the secondaries open, then the carb will basically squarejet itself once the secondaries start to open due to the power valve opening EXACTLY when the secondaries open. Not sure how it will work but will be trying it very soon
Anyway, hope this helps to disspell the mystery behind the power valve and its purpose in a Holley carburetor.
EDIT: All vacuum readings obtained for power valve selection MUST be done on the water under load (thanks Tittyman )