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Power Valves (long read)

18K views 45 replies 11 participants last post by  Performance Jet 
#1 · (Edited)
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 :D)
 
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#2 ·
Nice read. I've been fighting a lean pop right at 3500, so I'm wondering about the 6.5 PV being a little small too. This could also be my primary bowl running dry, so between a fuel pressure gauge and some vacuum readings it shouldn't be too hard to figure out.
 
#3 ·
Thanks. Yeah that could be a few things but I wouldn't think the power valve since vacuum is so high at that RPM. My motor is a BBC 468 with a Holley 750dp, Edelbrock RPM Air Gap intake and Performer RPM heads running 10:1. Carb is jetted stock (73 primary, 81 secondary) and pump has an A impeller in it. I also still have the stock 6.5 power valve in it and at 3500 mine runs fine. Pulling 11-11.5" vacuum at 3500 as well. I plan on fattening up the primaries on the next run and checking vacuum again at 3500 to see whether vacuum went up or down.

Could be a stuck float/needle/seat assembly in the primary bowl if the bowl is running dry. You may also be jetted a bit small on the primary side as well. Which carb and what jets do you have installed?
 
#4 ·
30 over olds 455

my olds has fresh heads with the exaust crossover tubes blocked off, an edlebrock performer intake, 10-1 compression, 280 comp cam, and a holley spred bore 800 cfm dp.
my carb from holley came stock with 62 primaries with an 8.5 pv and 85 seconds with a 6.5 pv.
i trailer tested this @3000 and had white plugs.
so i took the 76 jets out of the secondairys in my 650dp and put them in my primes!
i took it out saturday, did all kind of plug tests, at idle, at 3000, at 4000, and at wot.
all tests were the same looking plug?? i dont know plugs very well?? if you looked at the porcilin and electrode, the plug looked brand new(not altered at all)
(dont know the tech name for-bear with me) if you look at the peice of metal that bends up and around the electrode, if you look straight down on it, it looks light brown.
if you look at the side of it(metal) it has a hint of white
if you look at the base metal that would be flush with my combustion chamber, it is black?????????????

other than being woried about plugs??????

my boat runs way better and faster than it ever has!!
from 0 to 70 in seconds, i have never felt this before in my boat!!
no hesitation, no pops, nothin but thrust!!

p.s. can anyone tell me what cubes i have???? (455 30 over)

any help or thoughts on plugs and or carb would be awesome!!
 
#5 ·
Yours is the same as mine '75, 461. ;)

Yep, 30 over Olds, iron heads with big valves, 10.25 compression, 280 comp, 750 hp DP with 72/84 front pv 6.5, rear plugged.

I ordered a jet kit, another PV, and a new fuel pressure gauge for testing this weekend. I have a feeling my old glass fuel tanks are the issue...
 
#7 · (Edited)
Nice read Jon. I agree with most of what you write. I still prefer secondry power valves because without them you may be setting your self up for a momentary lean condition when you dump the throttle from cruise. Remember, as you said the jet drive is loading the engine continuosly , even at cruise. So when you dump your foot into it from cruise the jets may need some help for a split second. Same as the primary. Especially if you have a tall manifold like a tunnel ram with one carb . Or an air gap with spacers. The 4160 series , vacume secondary Holleys don't have a secondary power valve because the secondarys open on a velocity signal and are slower to respond. I have seen lean conditions before, in the secondary, especially on motors that have a very high volumetric efficiency, with large cams and high flowing heads. Thats why the engineeers at Holley designed in the secondary power valve. (or should I say Smokey Yunick, he invented the Holley in 1956) If your carb is set up right for your application everything should work great. I don't understand why you want to block this off, unless you are running a dragster.. One of the reasons I prefer the Holley double pumper is because it has a secondary power valve. In all of the years I have worked on Jet boats for the lake boater I have never removed a secondary power valve. Never needed to.

Jim :D:D
 
#8 ·
Actually, not all double pumpers have a secondary power valve. My secondary power valve is not only blocked off, but the metering block does not have any power valve channel restrictions in it so if I were to install one, it would do absolutely nothing unless I get a different metering block.

I think one of the reasons power valves got a bad rap was due to the fact that the older Holleys with no anti-backfire valve would blow them out in the event of an intake backfire. However, it's my understanding that all Holleys made after '92 or so come with this power valve backfire protection valve. For the ones that don't have it, they make an inexpensive kit to add it so that this is no longer an issue.
 
#10 ·
That's probably why you'll notice that the secondary jets are 8-10 sizes bigger than the primary jets. Secondary power valves are a tuning luxury rather than a necessity.

As for your spark plug question...

The porcelain should be tan/light brown in color when the mixture is right.
 
#22 ·
intake leaks

intake is new. heads are new. carb i bought of craigslist. all i did to the carb was cleaned it up, checked the power valves to see if they worked(leaked), put new bowl and metering block gaskets on it and changed the primes to 76.
oh yeh and blew compressed air through all the EVERYTHING!!

on my trailer testing i sprayed srarting fluid all around the carb and the intake. the idle did not change. i dont beleive i have an intake leak??

i brought a plug to my machinist who is also a drag racer guy. he said my plugs still look a little lean??
he also said i should try a less hot plug( one step down)???

yeh it is a 6213

what next????
i am almost there
 
#23 ·
intake is new. heads are new. carb i bought of craigslist. all i did to the carb was cleaned it up, checked the power valves to see if they worked(leaked), put new bowl and metering block gaskets on it and changed the primes to 76.
oh yeh and blew compressed air through all the EVERYTHING!!

on my trailer testing i sprayed srarting fluid all around the carb and the intake. the idle did not change. i dont beleive i have an intake leak??

i brought a plug to my machinist who is also a drag racer guy. he said my plugs still look a little lean??
he also said i should try a less hot plug( one step down)???

yeh it is a 6213

what next????
i am almost there
I agree with your machinist. IF your plug's heat range is too hot, it will burn all deposits off of the porcelain, giving you a false lean indication and also creating a hot spot where detonation can occur. I'd go 2 steps colder. When the heat range is correct, a short reach plug will discolor on the first 1 or 2 threads while a long reach will discolor on 3-4 threads.

If all your threads are discolored this means the plug is way too hot.
 
#24 ·
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.

Consider this...... the load on the engine will change as the load in the boat changes IE one day you have one 160 lbs friend in the boat and another day you have three friends from 115 to 205 lbs. If you're going to split hairs at .5" vac or even 1" vac the opening point will change as your load changes. I think you'd be better off with a 5.5-3.5" pv and call it a day.

You didn't mention that Holley makes Hi-flow power valves and standard ones. Anyone running a high hp engine should find the hi-flow power valves.

For the most part I've run 4.5 or 3.5 hi-flow PV's over the years. The engine creates enough vac at 3,800-4,000 cruise rpm that the top of the intake and carb spacer frost over. The secondary transition with the old carb with progressive linkage was at 4,000. The new carb has soft progressive linkage and I'm a bit worried about it but Patrick James the carb builder told me it will be a better combo. We'll see, who am I to argue with the guy who builds carbs for Billy Glidden, Tony Bishoff and Jon Kaase. ( But they aren't driving jet boats)

Just my .02

S CP :D

Jon
 
#27 · (Edited)
Hi-flow power valves are only for use on metering blocks with large power valve channel restrictions (PVCRs). These metering blocks are only used on large CFM carbs and are designed to be used with metering blocks which have 0.097"-0.128" size PVCRs.

Remember, the PVCRs are what determine the amount of fuel that can flow from the bowl to the main well (they are your second set of jets), not the power valve itself. With PVCRs smaller than the above listed size range, a regular flow power valve is all you need. Just like you would need to use the hollow screw only when using a size 40 or bigger squirter on the accelerator pump.
 
#28 ·
Jet extensions extend the inlets of the jets into the bowl more. They're made for vehicles that accelerate hard off the line and have a problem with fuel sloshing away from the main jet inlets.
 
#30 · (Edited)
Sleeper...weight of the boat has nothing to do with engine load, but everything to do with thrust:weight ratio. Having said that, this would mean that the pump would have to spin faster to generate more thrust to get the same mph than it would with minimal weight in the boat.

Yes spinning it faster will increase engine load. But actual engine load at your normal cruise RPM will not change with boat weight or movement. However, MPH for a given RPM will.

To sum it up...yes there's a natural tendency to drive a jet boat via the speedometer and spin the pump faster to achieve the same mph fully weighted that you cruise at unweighted. But actual engine load at what your NORMAL cruise RPM is remains the same regardless of boat weight or forward speed.
 
#31 ·
To sum it up...yes there's a natural tendency to drive a jet boat via the speedometer and spin the pump faster to achieve the same mph fully weighted that you cruise at unweighted. But actual engine load at what your NORMAL cruise RPM is remains the same regardless of boat weight or forward speed.
Nice edit :)bulb

I was about to write the same thing :) The engine load maybe the same, you just won't be moving very fast. But as a think about it it takes more throttle effort to get a fully loaded boat on plane vs a nearly empty boat. So if you cruise at 3,500 regardless of speed with people in the boat vs one person is the load on the engine the same ? I'm not sure it is, but maybe?

Sleeper CP :D
 
#32 · (Edited)
Yep. The load placed on the engine is provided by the amount of water flowing through the pump, not the weight or forward speed of the boat that it's attached to. The amount of water flowing through the pump is directly related to pump RPM. More RPM = more water flowing though pump = greater engine load. This is why a jet drive is a completely different load on the engine than what an automotive powertrain, v-drive prop, or an outdrive unit would place on the engine. With a jet drive you're trying to compress water. The faster you spin it, the more water gets pumped and pressurized in the pump, which poses a greater load on the engine the faster it spins. This should also explain why a transmission would not be practical in a jet boat application.

The pump has no idea that it's even moving anything. It will still pump the same amount of water under the same amount of pressure at the same RPM settings whether it's attached to a moving boat or a stationary platform. The pump being able to move the boat is a byproduct due to the pump being attached to a moveable platform (for every action there's an equal and opposite reaction). Berkeley has even done the test of running one WOT with the boat moving and WOT with the boat tied to a dock and both tests achieved the same top RPM.

Notice how when you stab the throttle from one RPM to the next how the RPMs instantaneously swings from one RPM to the next regardless of how much weight there is in the boat? That's cause the pump doesn't see the weight of the boat.

Jet engines work exactly the same way. At full throttle the engines increase to 100% RPM with the jet aircraft at a stand still. As the jet accelerates down the runway, the RPMs stay at 100%. Load that same jet aircraft down with ordinance and a couple of external fuel tanks and the RPMs will still hit 100%...the jet will just have a slower acceleration rate and won't be able to achieve its max unloaded top speed in flight. This is because the engines do not see the weight of the aircraft, however in both scenarios the thrust:weight ratio is different.

So if you were pulling 7" of vacuum @ 4000 RPM with just you in the boat, then loaded it down with booze and chicks, you would still pull 7" of vacuum @ 4000 RPM. However, you would just be going slower at 4000 RPM with all those chicks and booze. :D

To summarize, jets are all about thrust:weight ratio. The load the engine sees is purely supplied by the drive and has nothing to do with the boat itself.
 
#34 ·
Now that you mention it, I haven't seen Rexone on much. I know he's a moderator...wonder if he could make it a sticky?
 
#36 · (Edited)
It would make sense that the power valve would assist the main jets under hard acceleration. Under hard acceleration, the power valve opens to allow fuel into the main wells as well as fuel being pulled through the jets into the main wells. This would create 2 seperate paths for fuel to flow into each main well, thereby allowing fuel to be available quicker and minimize the "hole" in the fuel curve. This would make it so that less pump shot duration would be needed since fuel is able to get into the main well quicker, and allow you to be able to run a bigger squirter size for even more fuel to cover up the hole.
 
#39 ·
Selecting Correct PV Size

So what you guys are saying is that to select the correct PV you should take your vacuum reading at cruising speed, say between 3500-4500 rpm. Not the vacuum reading at idle..is that what you are saying. I always thought it was at idle and then 1/2 the vacuum reading to size the PV...and in addition to that you want to take your reading while under load not at cruising rpm in your driveway..
 
#40 · (Edited)
That's how you'd do it on a car. But a car places a completely different type of load on the engine than a jet drive does.

Vacuum reading at cruise is what you use to jet the primaries on the carb, not for power valve selection.

A jet drive's load curve is not linear. As you increase RPM, vacuum will slowly decrease until it hits the RPM where the pump starts really loading the engine, then it will start dropping by a large amount for a small change in RPM (you'll see on the tach that it will take more throttle movement to obtain a noticeable increase the RPMs at this point). On a double pumper carb (the only way to go on a jet boat IMHO...easier to set up than a vac secondary) I prefer two different ways -

1) Take a vacuum reading with the primaries open as far as they can open without opening the secondaries and take a vacuum reading, then select the power valve that opens at this vacuum reading. This way it's open just before the secondaries open and you know for sure that it's not opening at your cruise RPM setting when it's not needed.

2) Slowly increase RPM while watching the vacuum gauge. You will see vacuum start decreasing by a small amount. Once you reach the point where the jet drive starts applying considerable load to the engine, you'll notice vacuum starts decreasing by a large amount for a small change in RPM. Note the vacuum reading where it starts to do this and select the power valve rated to open at that vacuum reading. This way the pv is opening when the pump starts applying heavy load to the engine.
 
#42 ·
No. As stated in the article, all vacuum readings must be done on the water with the engine under load.
 
#44 · (Edited)
Thanks



Great info for a semi-hacker like myself..should be a sticky...I was under the impression that you took your vacuum reading while on the trailer, at idle. Never thought about the secondary plate issue..
 
#46 · (Edited)
Sleeper CP, Man you have some nice stuff there. Your last post proves my point...you know what you are doing and you have specific needs for your application. You are not the average jet boater that I see most of the time. If the average jet boater that I deal with made radical changes to a carb design, more problems would be created than solved. That is where pro carb shops can really make a difference for the novice. Theory is good, and nice to know, but its not absolute, as you have shown. Your needs are much more exotic than most , and you make it work. I respect that....Jim :D:D:D

By the way, let us know how your latest creation performs...you've peaked my curiosity...
 
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