How do Holley Power Valves affect idle AFR?

k-star · Post by **k-star** » Mon Jan 19, 2009 6:21 pm

[/quote]
Troy, Help me here. How does changing the size of the MJ change the atmospheric pressure the fuel is exposed to? Also, isn't the weight of the fuel in the main well the same regardless of the size of the MJ? We haven't increased the main well size. The pressure difference between the manifold vacuum and atmospheric seems unchanged to me. What am I missing?[/quote]

Also add some number to it. what is the pressure in the main well at the PV closed and what does it rise to when open???

Also what did you use to measure the AF when you did the test. I used a lm1 and thought they were about as sensitive as you can get?? By how much did the A/F change???

Keith

Ron Gusack · Post by **Ron Gusack** » Mon Jan 19, 2009 7:55 pm

Keith, if you are asking me what we used to measure the AF ratio when we tested, the answer is nothing. We went by the characteristics of the 2 cars and there was no apparent difference.
I can't see how an open or closed PV would change the pressure in the main well at idle.

Drag Chevette, If an engine uses less fuel at an idle than the MJ's flow, why would increasing the flow change the idle AFR? Would a bigger float bowl change the idle AFR? Would a bigger jet change it? Would a larger PVCR change it? You should do your own test to see if an open PV changes the idle characteristics. I gotta go with k-star's test results. If he couldn't measure it with O2's, then I feel even more confident about the theory I've had for 30 plus years.

Wolfplace · Post by **Wolfplace** » Mon Jan 19, 2009 8:16 pm

Drag Chevette wrote:
Wolfplace wrote: If there is no fuel flowing from the circuit the power valve enriches,,,,
How does the engine become richer
Oh, but there is fuel flowing from the curcuit the powervalve enriches....the pv enriches the mainwell, which enriches the idlewell...

however, the demand of the idlewell does not exceed the restriction of the PMJ and there for will only slightly draw from the open PCVR if at all.

=
I understand that the pv enriches the main well
What I am asking is how can fuel be flowing from the power valve circuit until you have booster demand if all it is in essence is a bigger main jet?
I can see having slightly more "head" before the idle restrictions but there is no flow thru the main circuit so,,,,
I guess I should have said there is no more flow than the idle restriction allows

So to be correct in your terminology my question is,,,
How is there more fuel flowing through the circuit that will effect idle mixture?
You are flowing through a fixed idle orifice & saying that the slight amount of "head" you may get before it due to the power valve opening will make it richer?
I suppose this is possible but I have not seen it or it was so slight I missed it.

Troy Patterson · Post by **Troy Patterson** » Mon Jan 19, 2009 8:29 pm

Ron Gusack wrote:
For example, if your PVCRs flow ten jets and you remove the power valve and go all jet, theoretically you are going to increase jet size 10 jets. Increasing main jet size 10 sizes has a measurable effect on wellhead pressure throughout the entire metering circuitry, or how much atmospheric pressure the fuel in the float bowl and metering circuity is exposed to. This affects the idle air / fuel ratio to some degree.
Troy, Help me here. How does changing the size of the MJ change the atmospheric pressure the fuel is exposed to? Also, isn't the weight of the fuel in the main well the same regardless of the size of the MJ? We haven't increased the main well size. The pressure difference between the manifold vacuum and atmospheric seems unchanged to me. What am I missing?

Ya, I was not real clear. I shouldn't have included fuel in the float bowl as the amount of atmospheric pressure it is exposed to is unchanged by / when changing jets and / or power valve channel restrictions.

What does change by changing jets and power valve restrictions is the degree to which atmospheric pressure can act downstream of jets and restrictions to push fuel out the discharge orifices into the air stream - how easily and how quickly.

The weight of the fuel does not change, but consider that the fuel does not exist in a vacuum. Consider a drinking straw. If you have a straw in a cup of water and place your finger over the upstream end of it, then lift the straw out of the water. The water contained in the straw will not run out - until you remove your finger from the upstream end.

Now, think of the metering circuitry in the carburetor. The jet or pvcr is the equivalent of your finger over the upstream end of the straw, but with a small bleed or restriction. The fuel can only be discharged into the air stream at the rate the restriction (finger) on the upstream end of the circuit allows refilling of that circuit. This is how / why jet size affects air / fuel ratio.

You say, well at part throttle the volume of fuel drawn is only a small percentage of the jets capacity so how can jet size be a factor at part throttle, but nevertheless it still affects "wellhead pressure" and therefore fuel delivery.

Someone said the power valve doesn't deliver fuel at idle so it's irrelevant to idle mixture. This may be true if the power valve is closed, but the existence and purpose of a power valve is allow the main jet to be downsized - thereby reducing "wellhead pressure" in the circuit as a result as I explained above. Since the idle jet or idle fuel restriction or IFR is a part of the main circuit, drawing it's fuel from downstream of the main jet in the main well, a change in main jet affects it the amount of atmospheric pressure acting on the IFR.

If the power valve is open or when it is open, it serves as a another flow path from float the bowl (where atmospheric pressure is pressing on the fuel - think hydraulics here), thereby exposing the fuel within the metering circuit to flow more freely (as the circuit can be refilled more quickly) - even at idle and again, increasing "wellhead pressure" as a result.

Troy Patterson TMPCarbs.net TMP Carbs

Troy Patterson

Eric68 · Post by **Eric68** » Mon Jan 19, 2009 9:58 pm

The only thing the main circuit and idle circuit have in common is that the idle well gets its fuel from the main well. It doesn't matter if the fuel in the main well is a few CC's or an entire ocean of fuel, the fuel height in the main well and atmospheric pressure don't change just because the power valve is open WHEN THE MAIN CIRCUIT IS NOT ACTIVE. The IFR is the same, the idle bleed is the same, and the pressure in the idle and main well are the same -- the volume of available fuel in the main well doesn't matter to the idle circuit!

By the way I run a 10.5 power valve and idle at 7". According to my wide band, switching from a 4.5 to a 6.5 to an 8.5 to a 10.5" power valve made ZERO difference in idle mixture.

Maybe . . . just maybe . . . there could be some odd combination of very large IFR and very small main jet where the power valve could have an effect at idle, but I don't see how personally.

Drag Chevette · Post by **Drag Chevette** » Mon Jan 19, 2009 10:09 pm

but would not an open powervalve at idle allow more fuel into the mainwell?... I think yes but no...

yes it would, but no it will never happen.

it would allow more fuel but the PMJ restriction is such a large restriction that it will never effect the small amount of fuel being drawn off by the idlewell curcuits...and therefor making the PCVR a mute point.

if the idle well curcuit is drawing from the mainwell curcuit it would make sense to me that there would be less restriction and more barometric pressure on the fuels flow and could there for feed the idlewell, by way of the mainwell, easier if the PCVR would be open.

but this is where area and flow take effect....the supply from the PMJ far exceeds the idle curcuits demand.

until the flow exceeds the PMJ ability to feed the idle curcuits it may never draw off the PCVR...at this point your well into transition if not into the power and pulling through the mainwell to feed the engine and the fuel flow into the idle well becomes secondary.

feeding the idlewell easier could then in effect cause an ever so slight rich effect due to the added air pressures, or lack of "-" fuel pressures within the curcuits...making the Idle well richer at High rpm...??

BUT...this being said the orfaces that allow the fuel into the carb at idle would be the restricting factor here that would not allow an overly rich condition...as the primary fuel flow at this point is being pulled from the boosters.

so my thoughts stand as such....the PV and PCVR do not effect the idle, and cannot effect the idle due to the size and flow requirements of the idlewell in compairison the the PMJ.

MaxFlow · Post by **MaxFlow** » Tue Jan 20, 2009 12:31 am

I can see how it may effect venting............in very extreme circumstances. I thought they used the 2 below rule to meet carburetor needs during differing pressures between manifold and atmosphere. I never thought of the rule as its effect on idle AFR ect. because I'm like everyone here on PVCR effects on idle AFR......Powervalve open or closed at idle doesn't matter to idle AFR.

olescarb · Post by **olescarb** » Tue Jan 20, 2009 11:05 am

I do think the power valve opening and closing at idle CAN change the a/f mixture at idle. my example is on a supercharged engine that has a supercharger roll at idle (no boost referenced power valve) but when no other change is made other than boost referencing the power valve to the vacuum under the supercharger the rich then lean roll goes away.

Next time i get a engine with a boost referenced power valve carb on it i will get exhaust gas readings with no vacuum and full vacuum to the power valve at idle to confirm my view but for now...

Henry @ olescarb

andyf · Post by **andyf** » Tue Jan 20, 2009 12:35 pm

Tuner, you didn't tell us what the idle speed was! I've run a few motors where the PV has an effect on the AF ratio at idle. A lot of drag race motors with large by huge cams that idle in the 1500 rpm range will be pulling some fuel thru the main circuit at idle.

banjo · Post by **banjo** » Tue Jan 20, 2009 12:40 pm

I have run with and without a powervalve, and there was no decernable differance in the afr at idle on the 02 sensors. Without the pv, the engine would be to rich at cruise. With the pv it would lean it out till the pv vacuum rating was hit and go rich.

I belive the smallest orifice is going to dictate the fuel flow/afr at idle.

Drag Chevette · Post by **Drag Chevette** » Tue Jan 20, 2009 12:48 pm

andyf wrote:Tuner, you didn't tell us what the idle speed was! I've run a few motors where the PV has an effect on the AF ratio at idle. A lot of drag race motors with large by huge cams that idle in the 1500 rpm range will be pulling some fuel thru the main circuit at idle.

BINGO...

it is dependant on the throttle blade opening....if the blades are closed to .020 on transfer, I doubt it will make a difference, but open the transfer slot and the PCVR will effect Idle AFR if the PCVR is open.

thats what Im saying.....

Post by **jmarkaudio** » Tue Jan 20, 2009 1:54 pm

Drag Chevette wrote:
andyf wrote:Tuner, you didn't tell us what the idle speed was! I've run a few motors where the PV has an effect on the AF ratio at idle. A lot of drag race motors with large by huge cams that idle in the 1500 rpm range will be pulling some fuel thru the main circuit at idle.

BINGO...

it is dependant on the throttle blade opening....if the blades are closed to .020 on transfer, I doubt it will make a difference, but open the transfer slot and the PCVR will effect Idle AFR if the PCVR is open.

thats what Im saying.....

If you don't run the throttle blade closed at idle exposing too much t-slot the carb is not set up correctly. This is why you drill holes in the throttle blade, or in the case of the Demon use idle-eze. Also one more thing to remember, the IFR is not the final restriction in the idle circuit, the mixture screw is. If the carb is set up correctly the PV should have no effect on idle enrichment as the actual fuel pulled from the mains is only a small percentage of the main jet area.

andyf · Post by **andyf** » Tue Jan 20, 2009 2:16 pm

I agree that most of the time the PV should not have any measurable effect on the idle AF ratio. But, I also know why Holley provides the recommendations that they do. Holley sells carbs to the performance market and there are some guys out there who stick a 850 DB carb on a 9:1 compression 350 smallblock that has a 280@050 cam in it and then they wonder why they can't get it to idle.

If the carb is dialed in and the motor will idle with the throttle blades closed down then the PV isn't going to be play. But when a guy jacks the throttle blades way open to get his monster cam to idle then you might see an issue.

On the same subject, I saw another thread on here today where a guy is saying that his plugs are fouled and the idle mixture screws on his 750 Holley don't have any effect at idle. That is a sure sign that the throttle blades are in the wrong spot which tells you right away that the transition slot is probably exposed and maybe the PV is working at idle and all other kinds of bad things are happening. So yah, it happens.

RayJE Carburetors · Post by **RayJE Carburetors** » Tue Jan 20, 2009 6:19 pm

Even if the powervalve was open at idle it will and has no effect on idle metering of any significance......the level of fuel in main well at idle should be the same as the fuel in the float bowls .... the fuel level has already settlled in the main well.... fed via PVCR or main jet.

Biggest influence on idle quality all things being equal is fuel level

Whenever you loweror raise fuel pressure or lower or raise floats.. you will need to adjust idle quality.

Eric68 · Post by **Eric68** » Wed Jan 21, 2009 2:17 pm

jmarkaudio wrote:
Drag Chevette wrote:
andyf wrote:Tuner, you didn't tell us what the idle speed was! I've run a few motors where the PV has an effect on the AF ratio at idle. A lot of drag race motors with large by huge cams that idle in the 1500 rpm range will be pulling some fuel thru the main circuit at idle.

BINGO...

it is dependant on the throttle blade opening....if the blades are closed to .020 on transfer, I doubt it will make a difference, but open the transfer slot and the PCVR will effect Idle AFR if the PCVR is open.

thats what Im saying.....
If you don't run the throttle blade closed at idle exposing too much t-slot the carb is not set up correctly. This is why you drill holes in the throttle blade, or in the case of the Demon use idle-eze. Also one more thing to remember, the IFR is not the final restriction in the idle circuit, the mixture screw is. If the carb is set up correctly the PV should have no effect on idle enrichment as the actual fuel pulled from the mains is only a small percentage of the main jet area.

EXACTLY. You can get all kinds of weird stuff to happen with a carb if it is setup wrong -- and idling on the main circuit under any circumstance that I can think of is wrong.

Even a drag car with a HUGE cam can and should idle on the idle circuit. Might have to drill holes in the throttle blades and use a pretty darn big IFR, but it can be done.