Tony78_280z

"The opening point of the power valve should be 1 to 1/2 inchess less than the manifold vacuum at idle." Haynes Techbok, Holley Carburetor Manual.

"At idling speed, an engine at sea level should show a steady vacuum reading between 14 in. and 22 in. Hg" Various other web sources.

My 1850-3 (and most Holley Carbs) come with power valve part number 125-65. These power valves open at 6.5 inches.

1. What is the average vacuum of a stock chevy 350 with stock cam? At idle? At cruise 2200 RPM?
2. Why do Holley carbs come with a power valve that doesn't come into operation until so far into the throttle? At that low of a vacuum (6.5") the secondaries should be mostly wide open.
3. Is holley using a different vacuum measurement?
4. What radical cam would one have to be running to run at 7.5 to 8.0 inches of vacuum at idle?

I'm thinkin about tuning my main jets down a little lower, and bringing the power valve in sooner, like around 12".

1991L98G92

The 1 to 1/2" less than manifold vacuum doesn't sound right. At that spec, the power valve would open every time the throttle was opened slightly. The power valve should only open under low vacuum so the 6.5" sounds right. Under WOT there is no vacuum. As the throttle plate is opened under accelleration, the vacuum should drop sufficiently for the power valve to operate.

five7kid

iTrader: (14)

If you'll take the time to read the "Welcome" sticky above, you'll find a link to the Holley tech website. There, you will find information about the power enrichment circuit, which will tell you that the information you quote above applies to race engines. Another technique will tell you how to determine the vacuum for a street engine.

Consider this a fishing lesson. Next time you won't have to ask for a fish. . .

Tony78_280z

Well Holleys webpage has a bunch of downloadable PDFs and I don't have a reader on this computer. (tech problems you see) but I got some other valueable information.

ThisGuy who knows his carbs says "Now that you have the power valve mixture correct, try some different power valves. I like a 10.5 in my Pinto, because it pulls about 14 inches of Mercury going down the road, and the 10.5 comes in pretty quickly when going up slight inclines." A PINTO is not hardly a race engine.

And I'm sure you've heard of Chevy High Performance "Power valve tuning is perhaps most critical with street or race engines using long duration camshafts with very low idle vacuum. These engines can often experience idle vacuum that will drop below 10 or even 9 inches of vacuum. If the carb is fitted with a 10.5-inch power valve, it’s possible for the valve to dump fuel into the engine at idle and create an over-rich condition. The cure is to replace the 10.5 valve with one that opens at a lower vacuum such as 5 or 6 inches." This is a race engine, and it would use the lower vacuum power valve that Holley seems to send standard.

According to these valuable sources, the low vacuum long duration race or aggressive street cam requires the lower number power valve, like a 6.5(What Holley sends standard with most of their carbs) And the street high vacuum short/stock duration cammed motor will function better with the higher number power valve.

Haynes Techbook, "Holley Carburetor Manual" is hardly race engine matirial. It is a simple and strait forward tech book on holley carbs. I'll again quote it to you... "The opening point of the power valve should be 1 to 1/2 inchess less than the manifold vacuum at idle."

I also have a Holley parts catalog (a big thick book that the autozone guy gave me.) This book does go into great detail on race components as well as street components. The back of this book has very detailed technical data/procedures and carb theory/science. It says "A competition or race engine which ha a long duration camshaft will have a lower manifold vacuum at idle speeds...(Basically take vacuum reading when warmed up and at idle and in gear if automatic) the ppower valve selected should hava a vacuum opening point about 2"Hg below the intake manifold vacuum reading taken. A stock engine, or one that is only mildly build for street use, will have a higher manifold vacuum at idle speeds. To determine the correct power valve the vehicle should be driven at various steady speeds and vacuum readings taken. (I read this as at various cruise speeds) The power valve selected should have an opening point of about 2"Hg below the lowest steady speed engine vacuum observed." (Yeah I know that last sentence is kind of rough to read, but it is word for word from technical book.)

But if you want some race engine based info: Mr.Vizard in Vol.2 says "In the carburetor world, the main jet is so named because it's main responsibility is to calibrate the full power fuel demand of the engine. In the case of Holley, it is misnamed. What is commonly known as the main jet is actuallly the principle metering element for cruise mixture. The power valve restriction channel is actually the main jet, and only comes into operation when a vacuum opreated valve opens it... Ideallly, the best way to achieve optimal part and full throttle mixture is to calibrate the main jet so that the cruise mixture before the power valve comes in is best for fuel economy." Unfortunatly he doesn't say exactly at what reading under the cruise vacuum he selects his power valve.

All of this seems to back up the theory that I posted in this thread starter.

I have done considerable research. Most of my earlier questions revolved around the question "Why?" And to get some feedback from living people. Perhaps opening the discussion "Why?" and "How come everyone tunes their carbs wrong?"

Angelis83LT

ok. So basically what you are saying. is that if you have a vacuum reading of 12 inches of vac, at idle in gear (auto) would be a 10 power valve. in abouts? (Since idle not in gear is 22-23 inches of vac, and in gear is drops in half almost)

Tony78_280z

If you got 12 inches of vac at idle in gear (auto) then yes, you would want a power valve rated for 10, part number 125-100. This would come in alot sooner than the 125-65 that is standard with most carbs, and probably give better performance, AND when the main jets are properly set (tuned for cruise econamy, not WOT) it'll deliver better gas milage too.

I'm definatly gonna linky this thread to my webpage.

five7kid

iTrader: (14)

You get best cruise economy when the power valve is closed at cruise. The Holley tech site will tell you to get several steady-speed cruise vacuum readings and pick a number 2 below the lowest observed cruise vacuum reading.

Again, you only use the idle vacuum for more radical camshafts. Haynes ain't got it right for that call.

Tony78_280z

This isn't what the haynes or the holley catalog book says. But the pinto guy suggests setting up the power valve at cruise. So does Mr.Vizard. Holley does not! The pinto guy says he has 14" at cruise so a 10" valve is still coming in a little slow for him. It is true that you don't want the power valve flopping open at cruise But perhaps with a street engine. Cruise vac won't be 2 under it's idle vac. I don't know yet. I haven't bought a vac guage yet. But I think I'd set power valve up during cruise too.

Whether Haynes has it right or not, this is the powervalve that holley sends with most of their carbs is too weak excepting for the bigger cammed motors.

Perhaps Holley put in this very low vac power valve to protect the "Bigger is Better" mindset from its self.

So, to stear this conversation into a new direction..

1.> Does anyone know what power valve they are using, and at what vac the motor idles, and at what vac it is at cruise?

2.> How about those two stage power valves? Anyone got some data on them, and how to set them up.

five7kid

iTrader: (14)

Okay, Holley has changed their on-line instructions since I last looked (they've been modifying their website in the last year or so). So, I'll quote the actual words from Holley's tech site for 1850 carbs ( http://www.holley.com/data/Products/...199R7948-5.pdf , page 14):
The number stamped on a power valve, such as 65, indicates the manifold vacuum below which the power valve is operational. In this case, all manifold vacuums below 6.5” Hg, the power valve is operating. Generally a 65 power valve is sufficient for most high performance applications that have a manifold vacuum of 12” Hg or higher. However, some problems can result with radically cammed machines equipped with automatic transmissions. These vehicles often “idle” at 2000 rpm, approx. 6.0” Hg. At this point the main nozzles are starting to feed and richen the mixture (supplied by the power valve) and the engine will probably “load up”. To correct this problem, install a 45 or 35 power valve. If the engine has a manifold vacuum of 12” Hg or less, a good way to determine power valve size is take the manifold vacuum at idle and divide that number by two. The answer is the power valve size. This will provide idling and proper fuel flow under wide open throttle conditions when manifold vacuums seldom rise above 1” Hg.
They've dropped the cruise vacuum instructions. And, are indicating an engine with 14" Hg will be fine with a 6.5 PV.

That will get you in the ballpark. What I've observed as the best way to nail down the right PV is the off-idle response. If, with a low number PV installed, you get a dead spot coming off idle, put in a higher number until it goes away - this is assuming you have cruise jetting correct. On the other hand, if you need a high number PV to prevent lean cruise, you're jetted too lean. I will agree that Holley tends to error on the rich side as-delivered, but using a higher number PV would only make that worse.

The 2-stage PV's were for emissions applications. I haven't tried one, nor have I heard especially good things about them.

Chickenman35

First of all...do not bother with two stage Power Valves. They are meant for Heavy, heavy vehicles such as Trucks and RV's. They will not flow enough fuel ( Restricted opening) on a Performance engine. Stick with a single stage. Vizard, Doug Roe and Dave Emanual all confirm this.

You ask some very good questions. PV tuning seems to be the least understood, yet one of the single most important aspects to carburator tuning....particularily on Holley's. PV tuning in most imortant for Road Racing and Street driving.

Drag Racers seldom need to do much with a PV....since all of there time is at WOT. The rear PV is often plugged because the PV inlet will be uncovered by fuel slosh during a " wheels up" launch. Not the case with Road Racing or Street Driving. Proper tuning of the " Cruise" circuit" ( main Circuit ) and the " Power Circuit" ( Power Valve and Power Valve Channel restiction ) can give big returns in both driveabilty and ECONOMY.

I see that you have some material by David Vizard. Excellant. I am a big fan of his. "Performance with Economy" and " How to build Horsepower Volume 2 " really get into tuning Power Valves and their proper function.

The problem with selecting Power Valves is that all the information is generic. Holley errs on the cautious side because they don't want people to burn holes in there pistons.....they have no idea what you're putting the carb on.

Take for instance the umbiqeuos 3310-1 780 VS series. These carbs came with both front and rear PV's and came with a rather high stock setting of 10.5". Why so high? Well it turns out that many of these carbs were popular on big motors homes. Low RPM, Big Block motors that may be pulling a ton of weight. I'll use the VS as an example because they better illustrate the function of the Power Valve circuit. DP's are really only HiPerf and Race carburators.

Vehicle. Motor Home with BB 454, 8.2 to 1 compression and some nice tall " flat lander gears".

Scenario A: Lightly loaded Motor Home going up slight to medium grade, medium throttle setting. High load on motor...but still relatively high manifold vacuum because:

1: Big block 454 wasn't pulling many rpm, thus not a lot of airflow through boosters.. A BB 454 at 14\ to 1\2 throttle may still be pulling 8 to 9 inches of vaccuum. Because you're only on the Primaries. However, because of the load on the engine ( all that Motor Home weight ) you need some extra enrichment. Thus the PV has to have a fairly high setting.

Scenario B: Steeper longer hill. Motor Home filled to the nuts with pots and pans, rock collections, 500lbs of fresh water and 300 lbs of putrid crap in the holding tanks.

2: Motor Home ( moving chicane) pulls out to pass Logging truck in passing zone ( Two days later he gets by ). GrandPa puts pedal to the metal. 454 gets up to a screaming 3,500 rpm to 4,000 rpm. At this point the secondaries are just starting to open....but the load on the motor is huge....so the Secondary PV ( again a 10.5 ) comes in early to prevent a Chernobyl situation.

Scenario C: Nice light 3rd Gen car with 350, 3.73 gears, T5...good heads and a mild cam. 350hp at the rear wheels.

1: Car goes up same hill as Motor Home. Engine not even breathing hard. Very little throttle to maintain speed. Vacuum 9 to 10" but engine under very little load. No extra enrichment required so PV'(s) ( 6.5" selected ) not even required.

2: Decides to pass Motor Home. Lay into the throttle a bit more, but driver, being lazy leaves car in 4th gear. Vacuum drops to 4 to 5" and Power valve circuit cuts in to provide necessary enrichment to prevent detonation. Still on Primaries only....but motor under mild load.

3: Driver decides to pass Motor Home....but sbicks it down to 3rd and gives her about 1\2 throttle. With 3.73 gears in 3rd motor is happlily hunmming at 3,500 to 4,000. Still on Primaries only as load is light and manifold vacuum may be 8" or higher . With a 6.5" PV , PV circuit not needed. Main boosters are pulling strong and can easily provide all the fuel car needs with no risk of detonation.

4: Driver says..." Screw this. That freakin' Motor Home is about to pull out and pass that Logging truck. I don't want to be stuck behind that moving chicane for 2 days!!! . Drivers shifts down to 3rd and mashes it. Now we have a happy motor. Everything wide open. Engine is singing at 5,000rpm plus. Boosters are flowing like mad, manifold vacuum is down to 1" and PV circuits are providing all the extra fuel the engine needs when operating at WOT.

Note that Booster design plays a huge part in all this . The more efficient the booster...the lower the setting of the power valve can be. Straight leg boosters are the worst ( often used on trucks ), Dog Leg Boosters are better....but the Annular Booster is a quantum leap above those in terms of efficinecy. With Annular boosters the Power Valve can come in much later, because under low airflow situations, the Annular boosters start flowing much sooner and atomise the fuel much more efficiently. The PVCR can also be smaller with Annular boosters and the Pump shot ( Pump cam and nozzle ) can often be smaller.

On a Holley, the Power Valve circuit and the Acceleratotr Pump circuit is used as a crutch since ( With the exception of the 4500 Dominator series ) all 2300\4150\4160 series are a 2 circuit carb....which is very inefficient as far low speed, cruise and transition circuits go.

So proper Power Valve selection ( and Power Valve Channel Restriction tuning ) depends on many variables. Engine size, RPM, compression ratio, gearing, transmission type and of course vehicle weight.

There is really no " Right" answer. Best to read what you can and test and tune. Vacuum guages, a Wide Band A\F meter ( Narrow band will work as well....but be prepared tp learn how to read plug coloring ) and a nice long hill are the best ways. Unless you can find a constant load Chassis dyno.....and have lots of time and money. The great " Rolling Dyno" is cheaper....and more fun.

Just my rather lenghty.....02c ( Lord I do babble on sometimes )

Edit: Basically five7Kid said the same thing....but in less words. Beat me to it while I was typing fiendishly...

Chickenman35

Oh yeah....to answer the whole danged question. On a 3rd gen, 305 or 350, mild cam, decent gears.....a 6.5" PV should work just dandy, and will provide better economy than a 10." for example. YMMV

Edit: Note the revision to Holley's recommendation on Power Valves that five 7kid mentioned. Seems to be much more in line with what is happening in the real world.....IE: Fuel economy is becoming much more critical.

Sure would be nice if Holley would design more 4150 carbs with Annular boosters AND a proper 3 circuit metering circuit.

BTW, I'm building a new carb . 750DP with HP mainbody, thin throttle plates, Annular boosters and DUAL Weber Power Plates. Should be as close to Fuel Injection as you can get at 1\4 the cost

Tony78_280z

I'm assuming the 12" is at idle for this demonstartion.
If they suggest a 6.5 for a 12" motor, then I don't think a 6.5 would be as well tuned on a 14". And As I quoted above the average stock motor would run at 14-22" at idle. Once again Holley says that their 65 (standard jet) is tunned for a high performance motor with low vacuum. Why send these standard? So a stock motor with much higher vacuum will perform better with a quicker opening power valve. Not only that, but the motor is less likely to go lean just before the power valve opens, and this would be less likely to lead to detonation. The properly tuned power valve would be more economical and more piston friendly.

Chickenman, good info. I plan to digest it further.

Me, I should be running up near 20" at idle. I have a light car with a big motor and an automatic transmission. I should have a quicker opening power valve. I think I'll try an 8.5 or 9. And see how much quicker it runs. My rearend suspension is torn apart right now. So I'll update when I test it.

Chickenman35

You missed a very important fact. The actual statement from Holley is:

Generally a 6.5" Power Valve is sufficient for most high performance applications that have a manifold vacuum of 12” Hg or higher. Note the last two words...."Or higher"

If you're indeed running 20" at idle then an 8" or 9" should work.

However, don't be afraid to pull it down some if you can for street use. This is only to gain some fuel economy at cruise mind you. Test on that big, long hill with vacuum guages and and an A\F meter. Creep up on the Power Valve actuation point with medium throttle opening. You should be able to tell when you go too low with the Poer Valve as a lean spot will show on the A\F meter.


At the strip, Power Valve size should make little difference to the performance and ET unless you are miles off with your tune-up. Auto's using a brake hold or a Trans Brake will probably be helped a bit with a higher Power Valve as they are loaded up against the convertor need the extra enrichment.

At the Strip I would be testing with a buddy strapped in ( Or a video camera focused on Vacuum guage and A\F meter ) to record just what your vacuum readings and A\F readings are at launch RPM. I "think" I would tune the PV ( For Drag Strip only ) to be on when you're holding it against the brake.

This is completely different from how I would set up Power Valve Tuning for Road racing or Street use. Please bear in mind that I am a Road Racer....so I could be completely out to lunch ( launch? ) with my recommendations.....but I think I've got a fairly good practical knowledge of these things by now.