Hi all,

I'm trying to identify three spare Holley power valves that I found, and I have some questions:

1.) Are all Holley power valves the same? That is, aside from the valve's rating, will a given power valve fit any Holley carb?

2.) How do I identify the rating of a given power valve?

3.) Does the number of openings determine whether it's "high flow" or "standard flow"?

This one has what appears to be "25" stamped on the outside edge:



It also has 4 openings on the bottom side (high flow?):



This one has nothing on the outer edge, but a "6" on top:



This one has "7", and "5" below it:



Both of these only have two openings on the other side (standard flow?):



Any help would be appreciated, thanks!

Roger

(1) they interchange (2) the stamped numbers you see (refers to in hg opening point) (3) I believe so

The first one is a high flow 2.5

Second I would assume is a 6.0 standard flow (have never seen just 1 number)

3rd 7.5 standard flow

4 window power valves are generally high flow, 2 window are standard flow.

From everything I've experienced all Holley power valves are interchangeable among the common model numbers, 2300, 4150, 4160, 4165 and 4500. With the exception of the obscure Holley 4360 that used a 2 stage power valve.

I'm no expert on all the Holley's but hope this helps

Thanks, guys. I'm asking because I need a 6.5 high-flow valve. Since none of these are it, I've just ordered one. I appreciate the help!

Roger

If you have a MityVac then you can buy a little PV adapter and test them yourself. Then you'll know exactly when they open.

The stampings on the Holley power valves is often obscure. Sometimes if you know what the value is you can figure out the stamping, other times you can't.

Interesting, I hadn't thought of that. Which side do you pull from, the top, or bottom (where the openings are)?

You would pull from the large diameter side.

If you look at power valves enough you'll eventually get good enough that you can tell what value it is by seeing how thick the wire is in the spring. The wire size determines how strong the spring is and when it will open.

I thought the hi flow PV. is a alki or e85 prepped carb thing, and doesn't flow any more than a standard PV and a "regular" carb.

I would think more windows would be more flow but do you need larger PVCR passages to take adv of it/make em work. I dont know Holleys so I am just thinking out loud

I dunno, this is my first foray into six packs. I read in this "Six Pack Parts post" that the correct PV was 125-165, which is the high-flow 6.5 valve (standard flow valve is 125-65).

Just about everything I've read says to start with the 6.5 power valve, but I don't recall reading anything about standard vs. high flow. What do most guys run?

I'm assuming you have noted what your idle in hg vac number is (in drive/hot curb idle if auto as that is what the system will be dealing with stoplight to stoplight)

Unfortunately, I did not. Vacuum is between 11-12" at idle, measured in neutral, but not in Drive (it's an automatic). Unless I missed something, Tom_Quad's six pack tuning guide stated that vacuum should be measured at idle and in neutral (he says nothing about being in Drive). However, I completely understand what you're saying, and I agree (I've never been very good with carbs, and I'm still trying to learn some of this stuff). According to the PO, this engine made about 6" vacuum with parking brake on and in Drive. Of course, this means a 6.5 power valve would be open most/all of the time under normal street driving conditions. That's probably why it's currently running a 2.5 power valve (that's according to the spec sheet the PO gave me; I have not visually verified this).

I will have to take some additional vacuum readings, and research this some more. Thanks for the heads up, Robert.

Roger

If the carb is set up right and idles on the idle circuit, the power valve sees no vacuum. So even if you have a 9.5 PV and you idle with 6" of vacuum, the PV plays no role.

Now if your carb is so screwed up that the throttle plates are open too far, then the PV could see vacuum and start sending fuel. But the idle would be extremely rich, as in difficult to keep running and black smoking bad.

For a street car, I always set the PV opening off of the cruising vacuum. And the leaner the cruise, the quicker I set the opening in order to prevent a lean tip-in under light load. Many of the engines I see, vacuum at cruise is in the mid-teens (14"-17"). If the cruise A/F is in the mid to upper 14s, then I will usually install a 9.5 PV.

There are other considerations other than the cruise vacuum when choosing a PV, but rather than try to take all things into consideration and get complicated, it is easiest to just start with the cruise tip-in point and try it. If there is a problem, then you have guidance on which way to go. But most of the time, when you get the cruise tip-in right, it's all good.

There is rarely any down side to a quicker opening PV, but there are real driving issues if the PV opening point is too late.

That's what I thought, I have the tip in stumble also but all I ever read is vacuum at idle in drive and half that for the PV so if idle is 8" you want a 4.5" PV

I thought that the hi flow PVs had a larger plunger opening. And weren't necessary until about .080" PVCRs, which is huge.

Here's a link to a good discussion on this:

http://speedtalk.com/forum/viewtopic.php?f=1&t=20492

Don't the standard and high flow versions use different gaskets?

There isn't really a problem with the power valve open at idle. If you look at how the carb works you'll see that when the power valve is open it just acts like a bigger main jet. The PV circuit is in parallel with the main circuit so it adds fuel flow to the main circuit. But at idle, the main circuit isn't working since there isn't enough airflow thru the venturi to pull any fuel. So having the power valve open at idle doesn't really matter.

If the power valve has a leak in it then the fuel will flow thru it and into the intake manifold and that is bad. But that is a ruptured or "blown" power valve which is different than an open one.

The biggest issue with a power valve that opens at idle is that it basically negates the purpose of the power valve. If it is open at idle then it is probably open at part throttle and then you will have an issue. I think this is why Holley recommends setting the power valve according to the idle vacuum level but it has confused people for years. I don't think the Holley engineers care about PV operation at idle but they were probably very concerned about the PV opening at part throttle.

The best advice is to just mount a vacuum gauge in your car and watch it as you drive around. You'll quickly figure out when you want a PV open and then you can select that number and install it.

that & make sure the rest of the carb subsystems are spot on incl the AP cuz it it ain't right then it might confuse you with a bog etc. Want IMMEDIATE tip in/correct nozzle

Let's say that your car cruises with 15" vac +/-. Now if you mash the throttle pretty good, you get a shot of fuel from the acc. pump and you generally get down fairly low on vac which should open most any PV. So, for that situation, the PV opening point is not as critical as when you need to just throttle up a little bit.

Now let's say you are cruising at 15" vac +/- and you want to accelerate just a bit. Or come to a hill that requires a little bit more throttle opening to maintain your speed. As you gradually increase the throttle opening, the vac will drop off due to the fact that more air is beginning to flow. And if your cruise A/F ratio is where it should, you will get to the point that the mains alone will not be able to supply enough fuel for the load and the RPM by themselves. That is when you need the enrichment of the PVCRs. The size of the PVCRs determine the amount of fuel that augments the flow of the main jets. The PV controls when the additional fuel flows.

When you need the additional fuel from the PVCRs via the PV depends on a number of factors. Some engine combos can go from 15" cruise to 6" load and be okay but many, if not most, others will begin to stumble before that. So you give the engine the additional fuel when it wants it, or before. There really is very little problem with the fuel coming in early. But if the additional fuel comes in late, there will be a stumble or hesitation.

There is a lot of discussion about Power Valves and idle. IF things are as they should be, that is usually a non-issue. In my humble opinion, on the typical street car, most of us should be setting our PV opening point based on our cruise vacuum and response to load changes, not the idle. Remember that the PV's only job is to control when the PVCRs get to do their job. And nobody talks about PVCR flow in regards to idle. They refer to PVCR flow in regards to load changes. Those two items work hand-in-hand and should be dialed in for the combo the same way.

When I am asked around the shop about this subject, I tell most guys (with milder street cars) that don't want to become carb tuners and don't want to buy a lot of tuning tools and parts to just throw a 9.5 in and go. If that brings the fuel in a little bit soon, so what? It will usually only be a for second or so at most. But the alternative is to experiment with several different PVs and multiple dis-assemblies. To gain what? Also, if the 9.5 doesn't cut it, then the PV opening point isn't the issue. Move on the the next step.

I hope this helps somebody 'cause I got writer's cramp.