Power Valve Selection: The Definitive Answer

[12many]

There are varying opinions on how one goes about selecting a power valve for Holley based carbs (as anyone who has worked on one well knows) There's the 1/2 of idle vacuum, either with 4-speed in neutral or auto in drive, the more widely accepted 2" less than cruise vacuum, I've also seen people mentioning 1/2 of cruise vacuum, some say split the idle and cruise vacuum reading and 1/2 it, and other ways not widely accepted or worth considering. With the highly regarded knowledge base here, as well as referencing David Vizards Holley tuning book, and just focusing on the 2" less than cruise vacuum theory, all the while keeping in mind it is a way to establish a baseline or ball park selection in order to further fine tune: So I'm using the 2" less than cruise method on my AED 750 dp with an as furnished 4.5" PV (it just so happens I have a best idle in gear vacuum reading of 9", so the current PV is in line with the 1/2 theory) but anyway, all other tuning aside, assuming the other carb circuits are close, timing etc are close with current carb jetting, and you have let's say 17" (and sometimes close to 20"+/- in some instances) cruise on primaries, 50-70 mph, 2000-3000 rpm just as a rough example. I'd be looking at a 15" PV, but as we know they don't make one, so the highest is the difficult to find 12.5", or the readily available 10.5". So I could just slap that in and get to further tuning, or slap a plug in and do afr and vacuum readings to hone in on the ideal PV selection, selecting one that opens in relation to the lean reading and vacuum reading.... but how much above or as close to that point? Hope that makes sense. Who has done the plug method taking readings to identify the going lean point in order to determine ideal PV opening? And for those using the 2" below cruise method, using the closest PV, did you have to make any more valve changes down? And would there really be much difference between 8.5"-10.5" with such high cruise vacuum of 17"? Is it the choice between being lazy and taking any and all time? I'm feeling really lazy right now!!

 

[C130 Chief]

2" below cruise might have the PV opening at idle if you really have 9" of vacuum at idle, and that is what you want to avoid. If it were mine I might try a 6.5 to see if response improves. Since you are running a DP I'll assume response is more important than fuel economy.

 

[mderoy340]

PV open does not flow at idle. A blown PV will will flow fuel thru the vacuum signal port. PVCR fuel flows through the boosters. Most cars start pulling through the booster around 2K rpm. I run a 9.5 in my motor, it's hard to tell a 1" difference with a Mighty Vac hand pump.

 

[RustyRatRod]

Just go to Youtube. Holley has several how to videos on their carburetors. I think they would know.

 

[12many]

The thread was not really meant as a what one to run. More of who's done the two methods and how did they work for you. Had to write a novel only to confuse and make perfect nonsense!

 

[yellow rose]

The thread was not really meant as a what one to run. More of who's done the two methods and how did they work for you. Had to write a novel only to confuse and make perfect nonsense!

I used to do the 1/2 of idle vacuum PV pick. It actually makes tuning harder. You end up with more jet and bigger squirters to cover the hole in the fuel curve from opening the PV way too late.

How the 1/2 idle vacuum got started I do not know. I do know it's wrong.

I have about 15 inches at cruise and I use a 10.5 because that's all I could find. When I get a minute, I'm going to make up a test rig and try either changing springs or mixing and matching what I can to get closer to 12.5 opening.

I now set my cruise A/F ratio with the main jets. You'll be surprised how small the MJ can be if the PV opens at the correct time, and you tune for WOT with the power valve restricter channel.

You'll most likely be able to use a smaller pump nozzle if you get the PV opening sooner/at the correct time.

 

[crackedback]

Yep, the holley recommended method is not a good one for the correct PV for the car.

AF meters are the best way along with a vacuum gauge while cruising is the best method. Roll into the throttle and note the afr spike and vac reading. Rarely is that close to the methods that have been used for years.

A properly set up carb will not care if the PV is open at idle. The carb is nowhere near that circuit through the boosters. The PV adds additional fuel to the main well, no place for it to go once there.

 

[12many]

I now set my cruise A/F ratio with the main jets. You'll be surprised how small the MJ can be if the PV opens at the correct time, and you tune for WOT with the power valve restricter channel.

.​

I'm seeing that. While this carb I have is race oriented, as well as test run on an unknown spec engine, I'm working on as best a happy medium strip/street deal. Was jetted with 76/87 jets with 4.5 PV, etc. Baby steps at the moment, at 75 on primaries, just did a good test run and cruise up to 3000 I've got a good 17"-18" afr hovering about 13, and think 72-73's will put me at about 14 or so. But with bigger cruise/steady load on primaries (no pump in play) it climbs up to 15+ and vacuum is near to 10-12" region. So I know the PV is still in REM mode at that point and it'll take tweaking on, if I'm reading the good book and other resources correctly​

 

[qkcuda]

I agree, I believed the 2" below idle vacuum method and used a 6.5 PV. I have an A/F gauge and noticed a pretty severe lean condition at part throttle. I run about 17-18 inches of vacuum at cruise, so I will probably switch to something like a 10.5 PV so I will get some enrichment to help the part throttle stumble. Once I sort that out i may be able to lower the primary jetting.

 

[Mattax]

I agree, I believed the 2" below idle vacuum method and used a 6.5 PV. I have an A/F gauge and noticed a pretty severe lean condition at part throttle. I run about 17-18 inches of vacuum at cruise, so I will probably switch to something like a 10.5 PV so I will get some enrichment to help the part throttle stumble. Once I sort that out i may be able to lower the primary jetting.

It should be lean at part throttle. It should get leaner with more throttle. Somehow all the magazine experts somehow missed this and misled most of us for years.*

The power valve, or step up, is needed when the engine gets close to putting out maximum power for the rpm.

A somewhat practical method on a modified engine is to note the vacuum when increasing throttle doesn't bring a gain in acceleration. Then try a PV that opens at or before that vacuum.

Mike Urich, at the time a VP at Holley, discusses something like that method in Holley Carburetors and Manifolds. Back then, it had to be done on a track with a stop watch while "crowding" a vacuum.
I've tried it using the vacuum gage and datalogger with acclerometers and its hard to do. A flat spot will show up though.

If you happened to be so lucky as to know a place with NOx gas measurement and a load dyno then it can be done by observing the combustion.
"The purpose of the power valve selection is to enrichen the mixture prior to the combustion temperatures going too high. To set a powervalve correctly requires a NOx sensor. Different engine combinations need fuel enrichment at different power levels."
Bruce "Shrinker" Robertson. post #4 "Another Power Valve Question" archived Innovate Motorsports forum

*

In this engine, they found it needed relatively early enrichment. That would be something like a 10.5 Power valve on an engine that cruised at 17 or 18"Hg. An engine that didn't need to go rich until 80 % load might be more like a 8.5 PV, and so forth. Wideband

 

[12many]

It should be lean at part throttle. It should get leaner with more throttle. Somehow all the magazine experts somehow missed this and misled most of us for years.*
The power valve, or step up, is needed when the engine gets close to putting out maximum power for the rpm.

A somewhat practical method on a modified engine is to note the vacuum when increasing throttle doesn't bring a gain in acceleration. Then try a PV that opens at or before that vacuum.

Mike Urich, at the time a VP at Holley, discusses something like that method in Holley Carburetors and Manifolds. Back then, it had to be done on a track with a stop watch while "crowding" a vacuum.
I've tried it using the vacuum gage and datalogger with acclerometers and its hard to do. A flat spot will show up though.

If you happened to be so lucky as to know a place with NOx gas measurement and a load dyno then it can be done by observing the combustion.
"The purpose of the power valve selection is to enrichen the mixture prior to the combustion temperatures going too high. To set a powervalve correctly requires a NOx sensor. Different engine combinations need fuel enrichment at different power levels."
Bruce "Shrinker" Robertson. post #4 "Another Power Valve Question" archived Innovate Motorsports forum

*
View attachment 1715325658
In this engine, they found it needed relatively early enrichment. That would be something like a 10.5 Power valve on an engine that cruised at 17 or 18"Hg. An engine that didn't need to go rich until 80 % load might be more like a 8.5 PV, and so forth. Wideband

Very informative, and the examples above and in link (posts #8 and up) are pretty much what I'm dealing with while I await the arrival of a 10.5" PV. I have my jetting set pretty close for cruise, and wot with a 6.5" PV (all I have atm) but it needs to open sooner, and I also now have a slight tip-in hesitation that wasn't there before. Holding off on any further tuning until I get the 10.5" , then I'll settle on jets, then accelerator pump cams, and/or squirters, and looking at the primary blocks various restricters etc

 

[famous bob]

Very informative, and the examples above and in link (posts #8 and up) are pretty much what I'm dealing with while I await the arrival of a 10.5" PV. I have my jetting set pretty close for cruise, and wot with a 6.5" PV (all I have atm) but it needs to open sooner, and I also now have a slight tip-in hesitation that wasn't there before. Holding off on any further tuning until I get the 10.5" , then I'll settle on jets, then accelerator pump cams, and/or squirters, and looking at the primary blocks various restricters etc

I do not profess to being as knowledgable as a couple of u guys, but on 600 horse/(approx.) , sbc , w/ an 850 double pumper , I had to go down from a 10.5 , to a 4.5 , the thing ran and wrapped like a top .
Of course we all know its way easier and cheaper to make a sbc run----

 

[yellow rose]

Very informative, and the examples above and in link (posts #8 and up) are pretty much what I'm dealing with while I await the arrival of a 10.5" PV. I have my jetting set pretty close for cruise, and wot with a 6.5" PV (all I have atm) but it needs to open sooner, and I also now have a slight tip-in hesitation that wasn't there before. Holding off on any further tuning until I get the 10.5" , then I'll settle on jets, then accelerator pump cams, and/or squirters, and looking at the primary blocks various restricters etc

Good plan. Keep us posted. This is a real learning/teaching thread. Hopefully lots of guys are watching you and learning from it.

Of course, you are paying to educate others but so it goes. We all learn better from others who are actually doing stuff than reading it in a book.

 

[12many]

Got the 10.5 PV in, ready to run, just haven't had a chance to road test it. In the meantime, seems there's a bit of chatter about power valves and it got me wondering if there was any feasibility in an adjustable preload power valve? Adjustable within a certain range, especially above 10.5" up to 14"? Would be an engineering marvel!

 

[Mattax]

An enrichment circuit like Carters use is another way to skin the cat.
As far as enrichment at 14" - the question becomes why.
If the engine has even half decent fuel distribution, half decent mid rpm combustion, etc etc then it should make more power with leaner mixes until at least 50% load.
If the engine cruises at 16"Hg, 65 mph, 3000 rpm on a flat interstate, then it should not need enrichment with a hair more throttle (14"Hg) to go up a slight incline.
Does that make sense?

The engine described above should accelerate without enrichment at 14, 12 and maybe even 10". But seeing it has somewhat poor vacuum at cruise, its probably not a great part throttle combo, so that's why 10" is a maybe.

Just looking throiugh some of the 60's Chrysler FSM, an 8.5 PV was used on many of the Chrysler Holleys.
The well known 3310 Holleys used on some of the Chebby hi perfromance engines had 10.5 PV in the primary side and 8.5 in the secondary.
The 2bbl Motorcraft 2150 on my Wagoneer's AMC's 360 has a PV that staged to partially open at 9"Hg and fully open at 5"Hg.
So there's some real life examples of what some different engine combos need.

 

[AJ/FormS]

My 367 only has about 425 hp, (currently with a Hughes 230/110 cam in it), as indicated by a 93 mph Eighth mile.
Previously it had a bigger 292/292/108 cam in it. I never tracked that combo.
I did the power tuning to establish the MJs, and power timing.
Then I took the PV out and roadtested it with a vacuum gauge on the windshield. I have a 4-speed. I got her into second gear, and took it up to ~30/35 mph and played with the throttle up to ~50 mph. It was easy to spot the lean-hole developing at 10" vacuum. I screwed the 4.5 (half-idle vacuum) back in, and the hole was still there. I had bought a Morosso PV tester and learned how to use it, organizing my valves according to the tester which in very few cases matched the stampings. I worked my way up to the 10.5, and saw a marked improvement in throttle response and torque delivery, and the hole was just gone. I had snuck up on it. Then I worked on my timing curve, for a couple of weeks. Later, I dropped two jet numbers, with no change in tip-in performance, and the hole did not return. But the engine had become an animal.
Happy HotRodding

Oh yeah this was on a 292/292/108 cam that idled around 8 to 9 inches depending on the idlespeed.and idle-timing (usually down at 14*). Best vacuum was around 2400 rpm (kindof guessing, that was year 2000). And It cruised at 56=~2600 with 3.55s and 245/60-14s, still at same vacuum.( that part I remember, from the hours and hours it took to go the 20 miles to work as my DD).

EDIT: that cam was only in there until midsummer, when I had enough of filling the tank every couple of hours,lol. I sold it and moved on. But I just gotta say; from 5000rpm on, that thing was a beast. The brand new 245s never stood a chance; they were done before the cam came out.

 

[12many]

An enrichment circuit like Carters use is another way to skin the cat.
As far as enrichment at 14" - the question becomes why.
If the engine has even half decent fuel distribution, half decent mid rpm combustion, etc etc then it should make more power with leaner mixes until at least 50% load.
If the engine cruises at 16"Hg, 65 mph, 3000 rpm on a flat interstate, then it should not need enrichment with a hair more throttle (14"Hg) to go up a slight incline.
Does that make sense?

The engine described above should accelerate without enrichment at 14, 12 and maybe even 10". But seeing it has somewhat poor vacuum at cruise, its probably not a great part throttle combo, so that's why 10" is a maybe.

Just looking throiugh some of the 60's Chrysler FSM, an 8.5 PV was used on many of the Chrysler Holleys.
The well known 3310 Holleys used on some of the Chebby hi perfromance engines had 10.5 PV in the primary side and 8.5 in the secondary.
The 2bbl Motorcraft 2150 on my Wagoneer's AMC's 360 has a PV that staged to partially open at 9"Hg and fully open at 5"Hg.
So there's some real life examples of what some different engine combos need.

Makes perfect sense, the 14" was just an arbitrary number. I guess maybe I've spent too much idle time thinking about power valves and tuning with this new to me AED Holley DP, and after extensive tuning on a Street Demon 750 with the metering rod step-up springs I previously was running. With the Demon, the tuning kit comes with 3-4-5 and 8" springs, the carb comes with 6" springs ootb. In the beginning it seemed I needed 7" to keep the rods down, and come up when i needed them to, so unable to find springs, I decided to do some improvising. Ended up using brass oil line ferrules that just so happens to position perfectly in the springs.I then used a file to whitle down the ferrule to a thickness that with it installed on the spring and checking the pressure (using a scale) to compress found it was exactly in between a 6" and and 8" spring, viola a 7"!. And it worked. Problem was, I did further tuning (and got better at it and went faster and quicker) and was last using an 8", but may have needed to go to a 9" as I was "so close" Basically, I've been racing with a street carb and tuning on it to the inth degree. Not the usual for carbs like that. Its a great carb, but it doesn't have the tunability of a Holley obviously. Id need to get into the idle bleeds etc and im just not gonna start drilling and tapping etc. So anyway, thats what got me thinking about pv's and the springs, likely too much anal obssession when one of the available valves will be all I'll need for my little van, Too much free time!! I'll see what happens with the current PV tuning, and see how much more I'll need to do.

 

[12many]

My 367 only has about 425 hp, (currently with a Hughes 230/110 cam in it), as indicated by a 93 mph Eighth mile.
Previously it had a bigger 292/292/108 cam in it. I never tracked that combo.
I did the power tuning to establish the MJs, and power timing.
Then I took the PV out and roadtested it with a vacuum gauge on the windshield. I have a 4-speed. I got her into second gear, and took it up to ~30/35 mph and played with the throttle up to ~50 mph. It was easy to spot the lean-hole developing at 10" vacuum. I screwed the 4.5 (half-idle vacuum) back in, and the hole was still there. I had bought a Morosso PV tester and learned how to use it, organizing my valves according to the tester which in very few cases matched the stampings. I worked my way up to the 10.5, and saw a marked improvement in throttle response and torque delivery, and the hole was just gone. I had snuck up on it. Then I worked on my timing curve, for a couple of weeks. Later, I dropped two jet numbers, with no change in tip-in performance, and the hole did not return. But the engine had become an animal.
Happy HotRodding

Oh yeah this was on a 292/292/108 cam that idled around 8 to 9 inches depending on the idlespeed.and idle-timing (usually down at 14*). Best vacuum was around 2400 rpm (kindof guessing, that was year 2000). And It cruised at 56=~2600 with 3.55s and 245/60-14s, still at same vacuum.( that part I remember, from the hours and hours it took to go the 20 miles to work as my DD).

EDIT: that cam was only in there until midsummer, when I had enough of filling the tank every couple of hours,lol. I sold it and moved on. But I just gotta say; from 5000rpm on, that thing was a beast. The brand new 245s never stood a chance; they were done before the cam came out.

I believe I read of your process on tuning these carbs in an older post somewhere here, what it took to get there. Im believe I am on my way also to sneaking up on it. Eager to see where things end up

 

[yellow rose]

Keep us posted on your results. If you are dats logging the results it would be nice if you could post the logs. Graphically it's easier to see.

I've played with dual stage PV's way back in the day. Never got much with them, as they are flow limited. I also made up a tool to actually test PV opening. I even started changing springs to increase the opening time. I found if you can get the PV to open at about 60% of cruise vacuum you had it by the short hairs.

 

[AJ/FormS]

I believe I read of your process on tuning these carbs in an older post somewhere here, what it took to get there. Im believe I am on my way also to sneaking up on it. Eager to see where things end up

I don't think my process is any different from any of the other knowledgeable guys on here, in fact I am sure they are better at it than am I. I just type a lot.....
You got at least 4 or 5 of the best guys around, chiming in on this thread; me, I'm just a newbe.

 

[12many]

No road test yet, weekend rain and Momma's day, but did get it running in garage, warmed up good and just doing some quick checks with the a/f and vacuum gauge. With the 10.5" PV the idle readings didn't change, but when just doing some quick rev's (on primaries only, but includes the accelerator pump shot) it is now into low 11's on the a/f gauge vs. in the mid 12's I saw previously doing the same quick checks. Need to get it out and do proper testing but I can already see I'll need to go down on the primary jets (pvcr is fixed at .052, will deal with what to do with it being fixed later) and look at everything else...oh boy. Here's a link to the Holley forum where someone posted asking about selecting Power Valves, the Holley rep gives the standard line, but the op obviously knows a thing or two and the Holley rep then agrees that using cruise is the more accurate way, but that because most don't have a wideband, they advise using the 1/2 of idle vacuum. Just one thread I found as an example. https://forums.holley.com/showthread.php?169-Power-valve-selection

 

[yellow rose]

No road test yet, weekend rain and Momma's day, but did get it running in garage, warmed up good and just doing some quick checks with the a/f and vacuum gauge. With the 10.5" PV the idle readings didn't change, but when just doing some quick rev's (on primaries only, but includes the accelerator pump shot) it is now into low 11's on the a/f gauge vs. in the mid 12's I saw previously doing the same quick checks. Need to get it out and do proper testing but I can already see I'll need to go down on the primary jets (pvcr is fixed at .052, will deal with what to do with it being fixed later) and look at everything else...oh boy. Here's a link to the Holley forum where someone posted asking about selecting Power Valves, the Holley rep gives the standard line, but the op obviously knows a thing or two and the Holley rep then agrees that using cruise is the more accurate way, but that because most don't have a wideband, they advise using the 1/2 of idle vacuum. Just one thread I found as an example. https://forums.holley.com/showthread.php?169-Power-valve-selection

You can make the PVCR adjustable with some 6/32 brass set screws. You just tap the existing hole (after you drill it out to the correct size for a 6/32 tap) and then just drill the brass set screws to whatever size you need.

I think you'll find when the power valve opens sooner you can use a smaller pump shot and you may be able to lean out the mains further than you think as long as you can open up the PVCR.

 

[yellow rose]

I have to LOL at that link. You damn sure don't need a wide band to get the jetting and power valve timing correct.

Evidently the Holley guy can't read a plug and thinks the rest of us are too stupid to do it either.

Regardless, he was still telling people to do it wrong. Just amazes me it's still being taught like that.

 

[12many]

Quick (long version!) update on things. With the 10.5" PV installed, doing calculations and settling on jet selection (73 primary, 83 secondary) with ootb 31/35 clusters and 1st hole pink cams, fixed .052" PVCR) I now have it in the ballpark better than previously. Idles good in park, gear, revs crisp and quick, vacuum 14"/10" respectively, a/f readings low/mid 13's (which I can tweak further) with easy primary only running up the rpm's with no pump shot and cruising up to over 3000 rpm readings from upper 13's up to a few ticks over 15. Heavy primary acceleration with the PV opening drops it to upper 12's (and it pulls stronger much better than before, WOW would be the best description . And then just doing some WOT runs the response etc is just as good, Upper 12's on gauge. Only niggle is the slight lean hesitation at tip-in, will try pump shot, look at the feed/bleed tuning. Still more to tweak, but I "could" live with it as is! Running the 10.5" with my 18" +/-cruise vacuum readings....WOW.

 

[qkcuda]

Quick update on things. With the 10.5" PV installed, doing calculations and settling on jet selection (73 primary, 83 secondary) with ootb 31/35 clusters and 1st hole pink cams, fixed .052" PVCR) I now have it in the ballpark better than previously. Idles good in park, gear, revs crisp and quick, vacuum 14"/10" respectively, a/f readings low/mid 13's (which I can tweak further) with easy primary only running up the rpm's with no pump shot and cruising up to over 3000 rpm readings from upper 13's up to a few ticks over 15. Heavy primary acceleration with the PV opening drops it to upper 12's (and it pulls stronger much better than before, WOW would be the best description . And then just doing some WOT runs the response etc is just as good, Upper 12's on gauge. Only niggle is the slight lean hesitation at tip-in, will try pump shot, look at the feed/bleed tuning. Still more to tweak, but I "could" live with it as is! Running the 10.5" with my 18" +/-cruise vacuum readings....WOW.

Great to hear, I am going to pick up a 10.5 for mine to try.