I'm confused as to what "rule of thumb" to use for a power valve in a Holley.
#1 2" value below highest vacuum reading at idle.
#2 1/2 of the highest vacuum reading at idle.
The car is a 4 sp with 10" at idle.
Which way do I go?

About 7 and 1/2 to 8"Hg by the rule of thumb. The standard 6.5"Hg should work pretty well.

-dnult

Holley says minimum 2 below idle vacuum. You would be fine with a 6.5 unless you are trying to tune the last drop of power out of your engine and if that's the case you would most likely plug the of the PV enrichment and jet up a bunch.

The PV is a WOT enrichment so when you put your foot to the floor and vacuum drops it opens. An 8.5 will open sooner than a 6.5 and so on...

The "Rule of Thumb" I've heard is this:

Four automatic trans
Idle vac in gear minus 2 = Power Valve

For manual trans
Idle vac divided by two = Power Valve

Originally posted by BPOS:
The "Rule of Thumb" I've heard is this:

Four automatic trans
Idle vac in gear minus 2 = Power Valve

For manual trans
Idle vac divided by two = Power Valve Interesting thought on the manual and auto. I've heard the divide by two before but not tied to a manual trans. Holley publishes:

A competition or race engine which has a long duration high overlap camshaft will have low manifold vacuum at idle speeds. If the vehicle has a manual transmission, take the vacuum reading with the engine thoroughly warmed up and at idle. If the vehicle is equipped with an automatic transmission, take the vacuum reading with the engine thoroughly warmed up and idling in gear. In either case, the power valve selected should have a vacuum opening point about 2" Hg below the intake manifold vacuum reading taken. and
A stock engine, or one that is only mildly built for street use, will have high manifold vacuum at idle speeds. To determine the correct power valve the vehicle should be driven at various steady speeds and vacuum readings
taken. The power valve selected should have an opening point about 2" Hg below the lowest steady speed engine vacuum observed.I'm pulling about 12"-14" in gear idling and have 8.5's in mine...

Here's the page I quoted from...

http://holley.com/HiOctn/TechServ/TechInfo/TI-222A.html

I don't know where I got that rule of thumb...but I'd go with what Holley says. There is obviously a little room for error on the low side...none on the top side. From what I know, if your accelerator pump can adequately cover the transition from "idle" to "power" you probably won't notice a stumble.

There's another misconception BPOS. The purpose of the power enrichment circuit has nothing to do with the off idle transition. That's what the accelerator pump is for. They claim an engine runs most efficiently at an air to fuel ratio of about 14:1. They also claim an engine makes its best power at an air to fuel ratio of about 12:1. When you're cruising with a street engine you're pulling fuel through the jets which should be set up to achieve the 14:1 ratio. When you floor it the vacuum drops and the power valve opens, adding a little fuel to achieve the 12:1 ratio. At least that's how it's supposed to work.

There's another misconception BPOS. The purpose of the power enrichment circuit has nothing to do with the off idle transition. I'm curious then, why Holley recommends the PV be selected to open 2" below idle vacuum. I understand that you don't want the PV open at idle... You'd think that if it should only open when you floor it that a power valve with a very low opening point would be right for every application.

BPOS, The PV can open at throttle positions other than WOT. If you drive your car up a hill, the load on the engine will increase and engine vacuum will decrease. As the hill gets steeper, you have a bigger vacuum drop. The purpose of the power valve is to provide extra fuel when the engine faces heavier loads which just so happen to correspond to a vaccum drop. Without the PV circuit, the engine would go lean during those times.

gheatly - thanks - I understand that. That's why I'm not understanding what 1 2run meant when he said that "the purpose of the power enrichment circuit has nothing to do with the off idle transition." Isn't coming off idle and accelerating the vehicle from a stop a "heavy load" situation? I'm not talking about flooring it either, just driving away from a stop light. It is my understanding that the PV is "open" during this situation, and that is why the PV should be rated only slightly lower than idle vac - so that it opens as you begin to put your foot in it. If I'm wrong...let me know! Certainly wouldn't be the first time!

I can see how the power valve choice could effect the transition off idle. Rev your engine with a vacuum guage hooked up and you will see vacuum drops before it increases. You could have a power valve start to open depending on how much the vaccum drops and how long it takes to pick back up. I don't think the power enrichment ckt is intended to be used to augment the accel pump's squirt though. Refer back to the holley info for stock street engines in my post above.

Since the line between when to use which recomendation is so grey with the engines we have and the idea of the transition ckt is to provide extra fuel to make up for the rush of air when the throttle is cracked, a tuner may find results in having the power valve momentatilary add a little to the transition mix. I personally wouldn't look at a power valve to fix an off idle stumble though. I also wouldn't look at jet changes to solve transition problems but I know guys that have found results in doing so.

The bottom line is each circuit interacts so close with the next that you can see off idle transition results from things like power valve and jet changes. I strongly recomend keeping each ckt seperate when trouble shooting and setting up a carb. I think it makes things easier, more predictable and your carb over all performance will be at it's best...

If you want a real eye-opener about vacuum, connect a vacuum gauge to a manifold vacuum source so you can read it while driving, and see how little throttle opening and load it takes to drop it almost to zero - it takes very little - that will give you an idea of how often your power valve is adding more fuel and how often it's involved in determining your air-fuel ratio.

Back in the days of the old "Mobil Economy Run" in the 50's and 60's, the name of that game was to mount a vacuum gauge right on the column or the dash and drive almost entirely based on minimal deflection of the needle for the absolute optimum fuel economy (you didn't want to be behind one of those guys in traffic) :D

I have a vac/boost gauge in my car and an A/F ratio meter purchased from the guy that built my carb. My car idles at 13 or so inches of vacuum and rises to 15 or so at cruise RPM. In normal driving, vacuum rarely falls enought to open the PV (mine's an 8.5). I need a good romp on the throttle to fall much below 10 inches.

A lot of great info.
Please correct me, but when a backfire or "pop" occurs when accelerator is floored and comes from the carb, does that mean a lean condition?
Does the power circut then have anything to do with that situation IE changes in power valve compared to idle vacuum?
Presently, I'm trying to get rid of the "stumble" and sometimes backfire through carb.
I've played with pump squirters and pump cams using a .31 squirter with the brown cam in the 2 position.
Am using a 5.5 power valve based on the 1/2 vacuum reading theory.
Would it benefit me to use a higher valve rating and would it help the backfire and stumble?
I also have a vacuum gage in the car and as John Z states, it really moves around with very little throttle movement.
Again, thanks for all the info.
AJ

I had a very similar problem and posted it here:

http://www.camaros.net/cgi-bin/forum/ultimatebb.cgi?ubb=get_topic;f=4;t=006236

Read the response by Eric68. I followed his advice to the letter and it corrected my problems.

Hi AJ, my camaro has a 4 speed with 10" of vaccuum also. I have a 3310-4 carb and I currently have a #5 installed and it seems to just right, although I had a 6.5 in before and It ran fine then too. What does your car look I may have seen you around, my camaro is gun metal grey with a 4" cowl hood and auto drag wheels,only one like that Ive seen here in Salem. Do you plan on taking your car to the local cruisin? I think they moved it to the chemeketa area.

Nitrous. My car is '69 X77 Z28, frost green with black strips,all # matching except for motor (don't I wish).
Yeah been to Scholtzsky's a few times. Just heard that they will move to Winema place @ Chemeketa this year. ALOT more parking and probably less congestion. Should be a lot nicer, maybe even some burn-out contests this year again.
Will be looking for your car this spring.
I see that your 383 is internally balanced. Did Hatch do the work?
Looks like its back to pump cams and squirters for me to get rid of the carb back fires.
See you this spring.
AJ

AJ

any pictures of your car mines frost green
x-33 with dark green int

AJ, yes my work was done at Hatch, they do nice work. I did hear there is going to be burnout contests and thats a big plus, see you around.

I have another question on power valves and vacuum.
At WOT. Vacuum drops, opening the power valve eventually, as speed and load increase does vacuum increase, thus closing the power valve? and causing the air/fuel ratio to go slightly lean?

any thoughts?


TTFN
Mat

Matt,

Vacuum really has VERY little to do with A/F ratio. Here's my best explanation of the five circuits and how they work.

1. Main metering circuit. Air Fuel ratio is controlled by a combination of air bleed, main jet size, and PVCR size. Air velocity across the venturi pulls fuel from the bowl, through the jets and PVCR (power valve channel restriction). The fuel is pre-mixed with air in the emulsion tube (right below the air bleeds - they control how much air enters the emulsion tubes) then then it is pulled out through a hole in the venturi where it further mixes with air in the carb and it remains suspended (hopefully) all the way into the cylinders.

2. Idle circuit. Idle adjust screws control how much fuel gets pulled through the idle slot (there are some Holleys that are "backwards" and actually control how much air). Anyway, the engine remains on the idle circuit as long as the idle transfer slot is partially covered by the throttle blades.

3. Transition circuit. This is your accelerator pump. The pump cam and squirter (on a Holley) control how much and the duration of the accelerator pump squirt. The transition circuit is designed to "cover" a temporary lean condition that occurs when the throttle is quickly opened. When the engine comes off the idle circuit there is not enough air flow (velocity) across the venturi to pull fuel through the main metering circuit yet --- so mechanically squirting fuel into the carb maintains the correct mix until the main circuit is flowing fuel.

4. Power enrichment circuit. The power valve (Holley) or metering rods (Edelbrock, Q-jet, Carter) add additional fuel (a small amount - about 6 - 10 jet sizes depending on the carb) at part throttle based on vacuum level. Engine effeciency is best when the A/F mix is closest to stoichiometric (sp?) roughly 14.6:1 if I recall. But at part throttle when the engine is loaded (pulling up a hill, etc) the engine needs a little richer mix to make power and keep combustion temps down.

5. Secondary circuit. Two basic types - vacuum and mechanical.

On a mechanical secondary carb the secondaries have a duplicate main metering circuit, a transition circuit and sometimes a duplicate power enrichment circuit --- all independent of the primary except that they are opened by a mechanical linkage that is connected to the throttle linkage. The secondary accelerator pump is needed because when the throttle is snapped open quickly, air velocity drops to nil on the secondary side just like the primary.

On a vacuum secondary carb, the secondaries are opened more slowly by linkage connected to a diaphram. the diaphram is slowly pulled by a vacuum signal from an opening near one of the primary venturi. A spring opposes the diaphram so you can fine tune secondary opening characteristics by changing springs. Since the secondaries are opened more slowly than on a mechanical secondary carb, velocity across the secondary venturi is already high enough to pull fuel through the secondary main metering circuit so a second accelerator pump isn't needed.

Hope this helps.