I,ve installed my new ZZ454 engine in my 69 vert, I need some info on setting up my carb.
The car is a four speed manual trans.
It is a 850 CFM Holley with vac secondarys.
At idle the car has 15-16 'hg, when driving down the road it has around 20" hg, when I really step on it it will drop down to around 3-5"hg for only a few seconds.
It came with a 4.5"hg primary power valve and a 3.5 "hg secondary power valve.
What power valves should I use, the 10.5 & 9.5 power valves?
This is my first real performance project, so I'm a sponge when it comes to learning!
Thanks for the help guys!
Craig

 

Find out what the low point of your vacuum is with just easy acceleration ( slight load not hard load foot to the floor acceleration ), set the power valve just below that vacuum amount, if it were to drop to say 10.5 under normal acceleration before the vacuum begins to rise again use a 9.5 power valve.

 

Pick a valve based on that low vacuum reading. It's supposed to be 1 to 1.5 higher than that. In your case you say 3" to 5" then vacuum starts rising again, so call it 4" of vacuum and put in a 5.5 valve.

 

here a Holley Tech pater from off my Tech Page that will be helpful for you:

http://www.lbfun.com/warehouse/tech_...wer_valves.pdf

 

that's incorrect.
power valve should be one that is about 2"hg BELOW the low steady vacuum reading, not above it.

 

Thanks for the information and advice guys!
I'll be going out for another drive today and see what the vacuum readings are under gentle acceleration, I seem to recall it was around the 16"hg mark.
It's a great sunny day here, perfect for a little sunday drive!
I'll post my results when I get back, thank you again everyone.
Craig

 

The low point would have to be considerably lower then 16hg under normal acceleration, your car is idling at 15/16hg with no load on the engine whatsoever.

Pay the most attension to the drop point when you shift to fourth gear, you don't want the power valve opening under light load but you want the opening point
not much lower then the low point.

 

THAT is incorrect. If it was BELOW the low steady reading how would it ever open? The valve is selected to open 1 to 1.5" ABOVE the lowest reading when the vacuum starts to rise. OP, alternatively, you can take the idle reading and divide it in half, then work from there to see what your motor likes.

 

no, it's below, not above. similar to choosing a correct vacuum advance cannister.
let's say the low steady vacuum level was 9"HG. a level that is about 2" below that would be a 6.5"HG Power Valve making that an appropiate selection.

What happens in this case is that at a level about 2" below the low steady vacuum level (such as the 6.5" PV as exampled above) the PV will open at 6.5" of vacuum. That way you are assured that the PV opens when required.
If the PV were to be selected as one that didn't open until 1.5"-2"HG above the low steady vacuum level (such as a 11"HG PV) than the car wouldn't run or idle correctly because at low steady vacuum the PV wouldn't open yet - the steady vacuum level would be at around 9"HG but the PV wouldn't open until 11" defeating the whole purpose of it.

read the Holley Tech paper I put the link to in my previous post, they explained how to select the proper PV on both a Race motor and a street motor and in both cases they explain to select a PV at a level 2" below, not above, the taken vacuum readings.

 

true,
but all of the original holley designers are retired or worse.
my cars maintain 15" climbing an overpass
so i made a pv open at 13". works great!
http://community.webtv.net/MATTGRU/pv

 

just remembered reading an article a while back, i think it was in the NCRS REstorer magazine, about the Holley's with secondary PV's (my Holley 2818 on my '65 only has the one PV).
A lot of people find issues running the secondary PV since during hard cornering, etc and the fuel sloshes to the back of the fuel bowl the PV is above the jet and if the fuel uncovers the PV you can get fuel starvation.
A remedy mentioned in the article is to remove the secondary PV and replace it with a plug that available for that purpose and only run the primary PV.
If you remove the secondary PV though you need to increase the jetting 4-8 sizes to compensate.

Would be interesting to hear from others that may have done this to see how much of an improvement it was.

 

Here are the results from my drive sunday afternoon.
While just driving along I'll read 18-20"hg, when I accelerate gently I see around 13-15"hg.
I think that means I should be using the 10.5"hg primary power valve and a 9.5"hg secondary power valve.

 

Sorry, but no one has the right explanation on power valve selection.

It is a common myth that the p.v. opening point should be selected to be 1 or 2 inches of Hg lower than idle vacuum to avoid really rich idle mixtures, especially when a big cam is used, however, it is just a myth, and here's why...
1. At idle, all the fuel comes from the curb idle and transfer circuits - there is no fuel that flows up the main well to the main venturi boosters at idle! Recall that these boosters receive fuel through the main jets, which are always open. Since air flow at idle is not producing enough of a vacuum at the booster to pull fuel up that well to start the main circuit, it doesn't matter one bit whether the power valve is open or not at idle because fuel from the p.v. joins the main circuit through the power valve channel restrictions at the main well- these are the same wells from the main jets that feed the boosters, but there just isn't enough booster vacuum at idle to pull the fuel up regardless of p.v. opening.
2. The purpose of the p.v. is to enrich the mixture when the ENGINE needs it for best power at any particular vacuum. For example, if it is found during test that there is a flat spot starting at 9 inches of vacuum as the throttle is opened, the power valve should be selected to open at ~9.5 inches to enrich the mixture to eliminate the flat spot. Alternately, if a power valve opens too soon, it will cause the mixture to be too rich, which will also cause a bit of a power loss. Either too soon, or too late will cause some loss power and response.

I recommend that one drive with a vacuum gauge and open the throttle at a moderate rate from high vacuum to the lowest vacuum (full throttle). Do this starting at successively higher rpms to get a feel for any flat spots across the vacuum and rpm range, and then change the a power valve to enrich or lean the mixture to eliminate the flat spot. Of course, the stock power valve is a good place to start, but testing and tuning will give you that extra edge.

Other than an engine dyno, a good spot to test would be a long uphill grade where speed can be semi-controlled while checking for smooth response across the usable range.

I do agree that on the secondary side, it makes some sense to remove the power valve IF the car launches very hard, like 11 second quarters or better, because the power valve opening is higher than the those of the secondary jets, and fuel slosh may actually uncover the p.v. opening during very strong acceleration.

In short, test first, analyze, and then tune!

 

myth? than I suppose that Holley who designed and built these carbs is wrong, along with people like Lars, and pretty much everyone else that works on and sets them up? EVERY single article and paper on setting up Holley carbs lists setting the PV approx 2"HG below the vacuum level and every person that I've talked to says the same thing.

on a street use motor (compared to setting up a race motor) idle doesn't matter (no load, always a high vacuum level), you want to take vacuum readings at varying steady speeds and select a PV that is 2"Hg below the lowest reported vacuum reading. Since you are driving the car and it's under load you are past the point of replying on the idle circuit only for fuel.
Besides, you mention venturi boosters...... the PV depends on manifold vacuum to open up the PV, not venturi vacuum. Since you want to set the PV based on the lowest steady vacuum levels under load while driving, once the PV opens up from manifold vacuum the venturi vacuum will than carry the fuel up.

if there is a flat spot at 9"HG what good is a 9.5 PV, it wouldn't open until AFTER you passed the flat spot...... kinda defeats the entire purpose.

 

Aha! I figured I'd get some lively discussion on this topic...

Without intending to gore anyone's ox, when one disassembles and inspects where the passages actually go in a carburetor, some of the conventional wisdom does not hold true.

As a retired pilot, engineer, and professional tester, I have learned to analyze and test first, and then to form conclusions based on fact.

To this end, I have a 400 SBC in my '78 and have an air-fuel ratio indicator (Howell Engine Developments), and a vacuum gauge, both of which I use to set up my carburetor. I have set up both Quadrajets and Holleys; I've been using a vacuum gauge for over 40 years in all my cars, and know the value of this valuable tool.

On the topic of the power valve having to be 2 inches less than the idle vacuum of the carburetor to avoid ruining idle quality, it doesn't matter - it's a non-player. To illustrate, imagine trying to suck a glass of water up an ever longer straw. At some point (it would probably be over 10 feet), the water can't be raised any farther because the vacuum is not sufficient to raise the water high enough (suction pressure required is equal to density of water times the height times the gravitational constant). This is analogous to the vacuum generated by air flow through the venturi booster pulling the fuel up the main well through the main jets. At idle, there is just not enough air velocity through the booster to pull fuel up the main well! (The equation for venturi vacuum is air density divided by 2 times air velocity squared.)

When the power valve opens, it is analogous to teeing in a second straw to the first straw, but the water still won't go any higher in the straw because of the physics involved - there just isn't enough suction pressure regardless of how big the straws (jets or PVCRs) are.

As an example, in one of my 327s, the idle vacuum is only 5 inches due to a big cam and lots of overlap, but the motor needs power valve enrichment below 8 inches of vacuum to avoid going lean (flat) between 8 and 5 inches when I open the throttle, dropping the vacuum lower than 8 inches. Yet idle is clean.

BarryK stated that "you want to set the PV based on the lowest steady vacuum levels under load while driving". If I did this based on cruise vacuum levels, my 406 pulls 19.5" at steady state cruise, and my 327 pulls 17", I'd have to use PVs at 17.5 and 15 inches - power valves aren't even made beyond 12.5"! Besides, the main jets cover mid-range vacuum conditions down to where the power valve starts to open, and then the additional fuel from the power valve through the power valve channel restriction holes supplies additional fuel to that of the main jets - motors typically need this enrichment to start somewhere in the range of 10 inches to 5 inches of vacuum.

BarryK also stated, "if there is a flat spot at 9"HG what good is a 9.5 PV, it wouldn't open until AFTER you passed the flat spot...... kinda defeats the entire purpose." Barry, recall that as you add throttle, the vacuum DECREASES. A power valve opening at 9.5" will be open BELOW 9.5" adding fuel as you add throttle.

I hope this helps

 

Wow guys!
You've made things as clear as mud for me, I know from reading the carb manual and the above postings that I should be 2'hg below in the powervalve selection from the vaccum reading.
My question is with the readings my car has, are the 10.5 & 9.5 powervalves a good starting point in the tuning of my car?
I think it is myself, I just wanted to hear from you fellows who have experience if I'm on the right track?
It just seems "funny" that a mild engine as I have, would require the largest powervalves Holley makes in a standard flow.
Am I correct in thinking that I have to get the powervalves correct before I think of rejetting?
So far the jets seem to be in the ballpark, the plugs are a light tan color.
Thanks again to all!

 

Dont think of the numbers on power valves as having anything to do with their size.
It just tells you when they open relative to engine vacuum.

 

a power valve is used to enrichen the air/fuel mixture during acceleration and when you want max power from your engine. a light car can use a power valve that does not open until the engine vacuum goes below let's say 10 inches but a heavy truck (gasoline dump truck)may need a 2.5 inch power valve so the power valve is not open during normal driving.

most of the time we use a 6.5 inch power valve in a application such as a corvette unless the cam is so big that the vac is below 8.5 inches at idle (the 2 inches below manifold vacuum at idle rule). if you have a lean off-idle problem you may be able to help it by using a power valve that opens sooner (higher #) so the power circuit opens sooner. in these days of high gas prices a 6.5 power valve may give you better fuel mileage tan a 10.5 power valve.

take care when using a power valve lower than a 4.5 because many high high performance engine can see 2 or more inches of vacuum at wide open throttle @ peak power engine rpm's.

i hope this helps henry @ olescarb

 

There are several circuits working together to give you your total fuel load at most throttle openings. If you get your idle mixture, part throttle cruise (very hard) and main jet right the power valve only has a roll under high load. The power valve is like a light switch - on or off. It is used to supply fuel beyond the capacity of the main jets under low vacuum/high load situations (heavy throttle applications). You can also tune the amount of fuel when the PV opens by altering the PV channel restrictors. You are setting when it opens not what comes out with the vacuum rating on the valve.

Once you have your ignition timing all worked out and idle where you want it take a vacuum gage reading. Divide this by 2 and add .5. For example I have 12" of vacuum. 12/2 = 6, add on .5 to get a 6.5" power valve. Using this formula your PV will be almost perfect. Using this I have never been off but I have never worked on something with vacuum lower than 5-6"

 

Thanks guys, the light bulb has come on!
I was thinking of jet size and then that is when I made that stupid comment on largest powervalve size, really..I do know that it is a statment of when the valve opens!
I will start with the 10.5 & 9.5 and see what happens.
Last couple of questions, is this a typical vaccum reading for one of these engines, I was surprised when I had a reading of 20"hg when crusing?
The more vaccum you see on a engine, no matter what the RPM, is it running at it best, what I mean by that is if at a certain RPM, by tuning the carb and you can have different Vac readings, at that certain RPM, you have a high vac reading, is that the best state of tune you can have for that engine?