Power Valve Selection: The Definitive Answer

-
image.jpeg
image.jpeg
Ok, finally went out and took these pictures.

Hopefully Mattx can do some work with them and point out all the relevant stuff.

Please notice all the porosity in these block. This is common. I know the history of these blocks from day 1 and ever since. That's why the billet stuff is better for that alone.
 
View attachment 1715392931 View attachment 1715392930 Ok, finally went out and took these pictures.

Hopefully Mattx can do some work with them and point out all the relevant stuff.

Please notice all the porosity in these block. This is common. I know the history of these blocks from day 1 and ever since. That's why the billet stuff is better for that alone.
cool-gif.gif

I'll mark up but if you want something different called out, just say so. .

upload_2019-9-12_15-46-31.png


upload_2019-9-12_16-48-17.png


Maybe too much labelling?
We can go do some other ones just following specific fuel and air flow paths.
 
Just playing with this a little. There are some other programs that are better for drawing transparent colors, maybe I'll play with that some time in the future.

Blue represents fuel. White arrows show air flow from main air bleed.
This just showing how the fuel and air moves to the booster.
Ignoring the idle system flow here, and the power valve is shut.
upload_2019-9-13_11-16-17.png

Low pressure created by air flowing past the booster in a venturi draws fuel through the main jet.
Fuel moves up the main well into the angle channel and then to the booster.
At the same time, air is drawn through the main bleed by the same pressure difference.
The air passing through the upper 'emulsion' hole will pick up some fuel in the air well if its close to the hole.

As air velocity through the venturi increases, pressure difference between the booster and the atmosphere (air cleaner and bowls) increases.
More fuel moves up the main well. Standing fuel in the air well gets pulled out.
upload_2019-9-13_11-46-27.png

When the pressure difference is strong enough, the lower 'emulsion' restriction will be open to allow more into the main well from the main air bleed.
 
Just playing with this a little. There are some other programs that are better for drawing transparent colors, maybe I'll play with that some time in the future.

Blue represents fuel. White arrows show air flow from main air bleed.
This just showing how the fuel and air moves to the booster.
Ignoring the idle system flow here, and the power valve is shut.
View attachment 1715393490
Low pressure created by air flowing past the booster in a venturi draws fuel through the main jet.
Fuel moves up the main well into the angle channel and then to the booster.
At the same time, air is drawn through the main bleed by the same pressure difference.
The air passing through the upper 'emulsion' hole will pick up some fuel in the air well if its close to the hole.

As air velocity through the venturi increases, pressure difference between the booster and the atmosphere (air cleaner and bowls) increases.
More fuel moves up the main well. Standing fuel in the air well gets pulled out.
View attachment 1715393499
When the pressure difference is strong enough, the lower 'emulsion' restriction will be open to allow more into the main well from the main air bleed.



That's awesome Matt. When it's presented like you are doing, it exposes two things. How simple the Holley really is, and how much tuning you can really do if you want to.

Anything you add always makes it better.
 
Maybe this version is a little clearer.
upload_2019-9-13_14-5-51.png


or actually more like this.
upload_2019-9-13_16-28-4.png

In many cases there is no fuel sitting in the idle circuit when the throttles are open enough that the booster is pulling hard. In fact, with very low pressure created at the booster, the IAB at atmospheric, and the transfer port no longer at really low pressure, its possible for air to flow back through the idle restriction into the main well.
This ^^^ is the reason getting the idle circuit figured out first is the best. And if we can't, then its good to do at one more itteration of idle and main circuit tuning. Idle circuit restrictions and bleeds can effect main circuit.
upload_2019-9-13_17-2-25.png
 
Last edited:
Thanks YR and Mattax.
I have had a couple question on this but as i study it more i answer my own questions.
the idle/transfer slot channel is seperate from the main. I couldn't understand how it could pull air from it to the main channel.
Now i can see how and why it can.

still studying it
And Thanks again!
 
the idle/transfer slot channel is seperate from the main. I couldn't understand how it could pull air from it to the main channel.
Now i can see how and why it can.
As you can see, how the idle/transfer flows depends on the pressure at the slots, the air bleed, and the booster. But how much air or fuel moves through them depends on the restrictions. A restricted carb, such as a 2 bbl on a v-8 running WOT high rpm will pull fuel through through the idle/transfer ports. I think someone posted they might be running 6"Hg at WOT.

The hard situation to be know whether the idle circuit is contributing or fuel or air to the delivery is part throttle higher speed.
Lets say a highway geared vehicle running steady 3000 rpm, 60 mph has 15"hg manifold vacuum.
15"Hg vacuum is 7.3 psi below atmospheric (7.4 psi absolute).
So does the booster create a strong enough drop in pressure to pull fuel out of the idle circuit?
My guess is normally not. There's less restriction pulling fuel at atmospheric pressure (14.7 psia) through the main jet.
But this is an example why its difficult to make blanket statements about carb behavior. And then when it comes to restrictions and flow, there are choking flows and changes in flow behaviour when it goes from laminar to turbulent, etc, etc.
We can take specific situations and try to understand them even when they don't behave as expected. Maybe that should be especially when they don't behave as expected.

Here's another see-thru version of a idle circuit from the bowl side.
Blue represents fuel just sitting at same level as in the fuel bowl.
H, P, L are the three possible location for idle feed restrictions.
upload_2019-9-16_8-20-2.png


upload_2019-9-16_7-38-0.png
 
I've never seen an IFR in the P position. Is there a benefit to that?
There must have been, but I don't know if it was performance related or manufacturing related.
I'm pretty sure I have a block at the garage with the IFR in the well. Just haven't removed the top plug to check.

Both Mark and Tuner have mentioned the somewhat high IFR on a secondary metering plate can cause fuel in the idle well to get blown into the main well when the throttles open. It acts like a pump shot. It seems like that could also happen with a metering block that has a IFR around the same height as fuel level in the bowl.
Just guessing here.

lemme see if I was smart enough to save any photos. Apparently not. Hopefully I saved some links in a word document.
I'll have to search ST and more likely RFS forum.

Edit:
Here's the references to the meter plates - mostly on vac secondary carbs.
Secondary metering plate vs Secondary Metering Block?
and this one referencing down the well location in blocks
2 Corner Idle Question
As always with links to Racing Fuel Systems, just click the X to read the thread. There is no need join the evil Tapatalk service.
 
Last edited:
I've never seen an IFR in the P position. Is there a benefit to that?
Well check this out!
After months away, I logged into RFS while finishing some coffee after lunch. Just going through the threads I had posted in at one time and....
Here's a picture of an IFR in the well. 4328 block from a 3 barrel
upload_2019-9-20_14-46-11.png


upload_2019-9-20_14-48-25.png


This block also has emulsion tubes in the main well. The clue is the flat top main well plug and the air hole in the dog leg of the air well.
2154_5887fb62a245438f8a7f8103bed5d5ed_t.jpg


apparently used on NASCAR 426 Hemi, L-88 and ZL-1.
cool-gif.gif


Tuner writes:
"The idle jet is in the well under the cup plug. The IJ in that location gives more rapid acceleration enrichment when the throttle is opened suddenly. "
So that confirms the purpose of this location is similar to the placement in secondary meter plates with the little horizontal connecting passage.

Related to Power Valve Tuning, these blocks have PVCRs around .093" dia.
Tuner writes:
"The PVCR size is affected by the emulsion tube in the main well. The lower end of the E-tube is just slightly above the PVCR orifice, you may be able to see it.

The PVCR is larger than similar size 1-9/16" venturi carbs which have no E-tube, such as 850 DP which come with .070", because the length and diameter (therefor surface area) of the tube adds friction and increases the restriction of the well, requiring larger metering area to result in same fuel flow as less metering area with no E-tube and less restriction in the well. "

Holley 950 3 Barrel
 
Last edited:
Well check this out!
After months away, I logged into RFS while finishing some coffee after lunch. Just going through the threads I had posted in at one time and....
Here's a picture of an IFR in the well. 4328 block from a 3 barrel
View attachment 1715396657

View attachment 1715396659

This block also has emulsion tubes in the main well. The clue is the flat top main well plug and the air hole in the dog leg of the air well.
View attachment 1715396642

apparently used on NASCAR 426 Hemi, L-88 and ZL-1.
View attachment 1715396643

Tuner writes:
"The idle jet is in the well under the cup plug. The IJ in that location gives more rapid acceleration enrichment when the throttle is opened suddenly. "
So that confirms the purpose of this location is similar to the placement in secondary meter plates with the little horizontal connecting passage.

Related to Power Valve Tuning, these blocks have PVCRs around .093" dia.
Tuner writes:
"The PVCR size is affected by the emulsion tube in the main well. The lower end of the E-tube is just slightly above the PVCR orifice, you may be able to see it.

The PVCR is larger than similar size 1-9/16" venturi carbs which have no E-tube, such as 850 DP which come with .070", because the length and diameter (therefor surface area) of the tube adds friction and increases the restriction of the well, requiring larger metering area to result in same fuel flow as less metering area with no E-tube and less restriction in the well. "

Holley 950 3 Barrel



I'll say it again, Tuner needs to write a book. So does mark whitener. I'd buy them both.
 
A More recent article: Carb Science Series: Holley Power Valves — Explanation And Tuning And Just a little addendum to this topic. I recently swapped to Trick Flows, about .5 bump up in compression, an AG to a single plane, and a much bigger cam (from a [email protected]” 106 LSA 68 degrees overlap hyd to a [email protected]” 106 LSA 84 degrees overlap solid) using the same 750 DP and to date have only fiddled with both idle speed and four corner idle mixture screws. My cruise vacuum has dropped from the original 17-20 with the previous combo to 15-17 with this current combo. And idle vacuum has plummeted to around 5 or so in gear (from approx 11 previously) But that 10.5 PV I have been running, that had really given a good kick in the rear previously still kicks in for an even bigger kick in the *** when riding on the primaries. Power Valve selection based on cruise vacuum:thumbsup:
 
Last edited:
Good stuff yall. I am fixin to do some PV tuning on my slant in a few days.
 
I will never understand all of the info in the previous 4 pages. Changing power valves from one extreme to another have done nothing for my six pack. 10 ins of vac at idle, 19 at 2500 rpm driving 55mph. Promax rear throttle plate, have tried both the factory metering plates and the ProMax replacements. Found one big mistake that I had made with the ProMax replacements, which started two more problems, start over. I believe that my phasing is off too, stock type mopar dist with FBO module. Testing last nite, engine is backfiring under wot now, and all that I had changed were the vac pod springs. So that is new.
It gets tiring when you think that you know or understand what is happening, but I don't, and I am not totally new to this. I haven't quit, but I am getting tired, and I didn't really want to go 4 barrell or FI TECH injection. I will continue to read and test, keep up with your results!
 
I will never understand all of the info in the previous 4 pages. Changing power valves from one extreme to another have done nothing for my six pack. 10 ins of vac at idle, 19 at 2500 rpm driving 55mph. Promax rear throttle plate, have tried both the factory metering plates and the ProMax replacements. Found one big mistake that I had made with the ProMax replacements, which started two more problems, start over. I believe that my phasing is off too, stock type mopar dist with FBO module. Testing last nite, engine is backfiring under wot now, and all that I had changed were the vac pod springs. So that is new.
It gets tiring when you think that you know or understand what is happening, but I don't, and I am not totally new to this. I haven't quit, but I am getting tired, and I didn't really want to go 4 barrell or FI TECH injection. I will continue to read and test, keep up with your results!

I plan on trying both ways, myself just for experimentation's sake. I'm going to try both a 9.5 and a 3.5 PV, primarily because I want to LEARN more about how the systems work. My cruise vacuum is 12 hg @ about 60 MPH and about 2650 RPM with my big toe on the gas just keeping it going on a level road. Using that theory, I should use a 9.5 "or so" PV.

However, my idle vacuum is 6hg @ right at 1000 RPM. So with the 6.5hg PV, it's open at idle. With no signal across the boosters though, that fuel has nowhere to go, so it cannot flood, like some people say .....and it's not.

With the 9.5hg PV, it will certainly be open at idle. But the cruise vacuum theory is the closer you can get the PV opening to the cruise RPM vacuum reading, the leaner you can go with the pump shot and main jets.

Crapped if I know any of it, but I plan to try it both ways when my power valves get here tomorrow. I am also going to jet it up a jet size or two on the primaries, as I am experiencing what I think it s lean miss at part throttle cruise. It kinda "bucks" a little at part throttle going down the road. If I put my foot on into it, it cleans right up.
 
I too have a lean miss, have traced it to the frt carb, I switched the end carbs, my cool cyl went from #1 to #7. Something is in there, have disassembled and blown out with air, not much change. Getting an ultra sonic cleaner to see if it will dislodge that issue.
 
I think you have to keep in mind that Primary WOT fuel is delivered by all four systems, which include the mains and the PV channel restriction. Sooooo, you gotta get that right first.
After that is right........then you can start delaying the PV, until it goes lean when the mains get to full capacity.
I like to bring the PV in as late as possible, without getting into a sag.
If you wanna know where your sag is, just install a vacuum gauge on the intake, swap out the PV for a plug, and go for a roadtest. Get up into second gear and about 2500rpm or stall speed, then start accelerating to a certain vacuum reading. Say 12 inches. Roll into the throttle until you are accellerating at 12 inches and then just hold it there until you get to about whatever rpm your ignition timing is all in, or 3600.
No sag?
Repeat, but, dropping the vacuum to 11 inches.
Sooner or later, the engine is gonna go lean and it will complain about it. Make note of the vacuum reading, and go find a PV with a 1 inch higher rating.
Here's the thing;
If your PMJ is not right, then your PV rating will be Not right either.
If your PMJ is too rich, then your PV can come in later.
If your PMJ is too lean, then the PV will have to come in earlier.
Some guys like to run the PMJ lean for fuel-economy, but that usually creates other problems. If you are concerned with fuel economy, then get the ignition timing worked out instead; you will be way further ahead.
Your best bet is to disable the secondaries, throw a boatload of Vacuum advance timing at it,then get the PMJ figured out. And finally delay the PV, until the engine sags and then back up a hair.

For example;
My 367 runs 32/34 Power Timing at 3200/3400, with alloy heads. More timing does not make it any quicker. The Idle timing is 14* and I run a two-step centrifugal curve that switches at 2800, where the timing starts at 28*. And this timing allows me to burn 87E10 gas at 11/1 Scr and up to 185 psi, jus saying.
But my Vcan brings in 22*, and it is all in by 12.5inches vacuum. So at 2800rpm, my Cruise timing could be as high as 50* or as low as 28* depending on the manifold vacuum.
Ok so suppose at 2800 and cruising, my vacuum is 18 inches, and timing is 50*. So I can slam the pedal down until the vacuum drops to 12.5 inches with NO Loss of timing.
Now, my PV is timed to start delivering fuel at 10.5. . But the Vcan has only dropped out a couple of degrees, so the engine is still pulling hard. The harder I step on it, the lower the vacuum drops which causes the Vcan to smoothly and rapidly drop timing out to prevent detonation, while simultaneously the PV begins delivering ever more fuel.
This all happens automatically, and the car surges smoothly ahead.
I like it a lot, and I think everybody should experience this because I gotta tell ya, when you get your 360 working like this, you will love it, and it will be hard to wipe the grin off your face. Now add a Thermoquad, and you will get addicted to the roar,lol.
 
I think you have to keep in mind that Primary WOT fuel is delivered by all four systems, which include the mains and the PV channel restriction. Sooooo, you gotta get that right first.
After that is right........then you can start delaying the PV, until it goes lean when the mains get to full capacity.
I like to bring the PV in as late as possible, without getting into a sag.
If you wanna know where your sag is, just install a vacuum gauge on the intake, swap out the PV for a plug, and go for a roadtest. Get up into second gear and about 2500rpm or stall speed, then start accelerating to a certain vacuum reading. Say 12 inches. Roll into the throttle until you are accellerating at 12 inches and then just hold it there until you get to about whatever rpm your ignition timing is all in, or 3600.
No sag?
Repeat, but, dropping the vacuum to 11 inches.
Sooner or later, the engine is gonna go lean and it will complain about it. Make note of the vacuum reading, and go find a PV with a 1 inch higher rating.
Here's the thing;
If your PMJ is not right, then your PV rating will be Not right either.
If your PMJ is too rich, then your PV can come in later.
If your PMJ is too lean, then the PV will have to come in earlier.
Some guys like to run the PMJ lean for fuel-economy, but that usually creates other problems. If you are concerned with fuel economy, then get the ignition timing worked out instead; you will be way further ahead.
Your best bet is to disable the secondaries, throw a boatload of Vacuum advance timing at it,then get the PMJ figured out. And finally delay the PV, until the engine sags and then back up a hair.

For example;
My 367 runs 32/34 Power Timing at 3200/3400, with alloy heads. More timing does not make it any quicker. The Idle timing is 14* and I run a two-step centrifugal curve that switches at 2800, where the timing starts at 28*. And this timing allows me to burn 87E10 gas at 11/1 Scr and up to 185 psi, jus saying.
But my Vcan brings in 22*, and it is all in by 12.5inches vacuum. So at 2800rpm, my Cruise timing could be as high as 50* or as low as 28* depending on the manifold vacuum.
Ok so suppose at 2800 and cruising, my vacuum is 18 inches, and timing is 50*. So I can slam the pedal down until the vacuum drops to 12.5 inches with NO Loss of timing.
Now, my PV is timed to start delivering fuel at 10.5. . But the Vcan has only dropped out a couple of degrees, so the engine is still pulling hard. The harder I step on it, the lower the vacuum drops which causes the Vcan to smoothly and rapidly drop timing out to prevent detonation, while simultaneously the PV begins delivering ever more fuel.
This all happens automatically, and the car surges smoothly ahead.
I like it a lot, and I think everybody should experience this because I gotta tell ya, when you get your 360 working like this, you will love it, and it will be hard to wipe the grin off your face. Now add a Thermoquad, and you will get addicted to the roar,lol.


The later you bring in the PV system, the fatter you need to be on the T slots and the quicker you’ve got to get it on the booster.
 
I will never understand all of the info in the previous 4 pages. Changing power valves from one extreme to another have done nothing for my six pack. 10 ins of vac at idle, 19 at 2500 rpm driving 55mph. Promax rear throttle plate, have tried both the factory metering plates and the ProMax replacements. Found one big mistake that I had made with the ProMax replacements, which started two more problems, start over. I believe that my phasing is off too, stock type mopar dist with FBO module. Testing last nite, engine is backfiring under wot now, and all that I had changed were the vac pod springs. So that is new.
It gets tiring when you think that you know or understand what is happening, but I don't, and I am not totally new to this. I haven't quit, but I am getting tired, and I didn't really want to go 4 barrell or FI TECH injection. I will continue to read and test, keep up with your results!


One of the issues with PV timing (well two actually) is that some carbs are so rich that changing PV timing doesn’t affect anything. It is so rich that bringing it in later won’t make a difference.

The other is the opposite. You are lean enough that opening the PV sooner still doesn’t add enough fuel to make a difference.

The way I do it (and there are other ways but I’m pretty old and lazy) is I block the power valve off. Then I drive the car at a cruise so I’m on the main jet. Then I start taking main jet out of it until I get a slight lean surge at cruise. The I go up two sizes on the primary main jet and leave it alone.

Now you can put the power valve back in. Then I disconnect the secondary throttle blades and I wire them shut so they can’t open.

The I go out and mash on it real hard and I start looking at the plugs. If it’s lean, I open up the power valve channel restricters.003-.004 and go test it again. I keep opening up the PVCR’s until it’s correct at WOT.

Of course, of you are fat at WOT, then you would close up the PVCR’s a few thou until it’s cleaned up.

Then you can hook up the secondaries and test for WOT. Only change the secondary main jet to correct any issues you have.

Once you get the cruise right, you’ll find out how important it is to get the power valve open on time. The later you open it, the bigger the hole in the fuel curve you have to fill with accelerator pump shot.

It takes time to do it, but IMO it’s worth doing because the carb will reward you with incredible performance and drivability.
 
-
Back
Top