I've been messing around with the tuning on a Quick Fuel carb of mine, and while trying to work through various issues, noticed that there was a lot of disagreement on how to select the proper size power valve. Some people claim it's 1/2 the idle vacuum, others claim it needs to be 2"Hg less than cruise vacuum for a street car... or maybe 1/2 of cruise vacuum? There's almost too many opinions to make sense of it. In my particular situation, I have a pretty strong vacuum signal all around. 17"Hg at idle, 21-23"Hg average during sustained cruising. In fact, while watching the vacuum gauge, the only time I EVER see it dip below 10-11" hg is when I'm at WOT or close to it. Regular driving around town, including fast starts from lights, certainly never gets the vacuum low enough to utilize the standard 6.5 power valve at all. The highest they even seem to sell is a 10.5, which would just barely be in the range of what I typically see. So is this right? under normal everyday driving, is the power valve never really used at all? With a much nastier cam, it seems you'd be more likely to get into it. The things I'm reading seem to indicate that it should have a great impact on drivability, but I feel like I could almost eliminate it and never see the difference until I really stabbed the pedal... The motor in question is a built 347 Ford stroker, though it's probably not terribly important for this. I'm mostly interested in the theory behind power valve operation in general, not just for my own car. Ideally, at what point would the power valve come into play?
I am in the process of tuning a Flatty , I am useing a big 5 inch vac gauge , 0-30 in 1/2 increments a wide band race O2 Tac A GoPro , so I can watch vac,& Rpm's , O2 gauge all at the same time . Interesting thing I can see going on , Hear is a link that may help , myself I am at a crossroad that I am good at Ideal ,& wot (13.5 afr) but still a little rich at cruise, I personly want my AFR to be around 14.9 to 16.1 at Cruise I am playing with 94's , if I was using a typical Holley , I would know what to drill ,restrict A nice read on carb's http://www.bob2000.com/carb.htm
From what I've read and understand about power valves is that they add fuel at WOT, so there will be less vacuum than at idle. So you should be good with a 6.5.
From what I've read the contradictory recommendations stem from whether one has a stock motor with a high vacuum versus a modified engine with a rumpety rump cam. The latter do not often idle well and don't pull much vacuum to begin with - and it fluctuates. Therefore the standard selection sizing advice will cause the PV to "flutter". For all that often times the PV is omitted and a plug installed, and jetting increased several sizes. Your method is best, determine what your particular engine draws at idle and steady cruise at your location/altitude. This means you can tailor the PV very closely. Guys that get good at tuning get cars that start easy, run smooth, smoke the tires and get really good fuel mileage. What's not to like?
6.5 is about right for you, having it in there lets you run with smaller main jet and if you eliminate it you'll find yourself running lean because the 6.5 rating is ambiguous - however that word is spelled. The PV isn't a gate that remains closed until it hits a value and then becomes fully open, it is partially open much of the time and constantly changing adding a dribble of fuel here and there.
You are way lean, you should be somewhere around 12.6ish. Quit looking at gauges, they'll worry you to death. Read your plugs and 'feel' the engine - they'll tell you what it wants. The engine will hurt itself running at 14.9 - 16.1, the Holley 94 is a typical Holley if you know what you're looking at and please don't drill or restrict any of the holes in it.
The 10% blend will give you false plug reading, Makes plugs look & smells rich , Over yr way is their ethanol free on pump ? I do not know if Ethanol Free will give the same readings of 10 % blend , Some time's you have to drill & Restrict , On my other car , Carb's are completely adjustable. Race car , Enderle Buzzard . I am looking for gas mpg, Not performance , I put 300-500 miles a week
I have to disagree here on the operation of the PV. I run Holley carbs. I also test each valve before installation. In my case, the valves are fully closed @ 5" vacuum. Below that they begin to open and are fully open @ 4". Above 5" vacuum, they are never open. My engine idle vacuum is about +/- 12 inches
Holley says it's half the idle vacuum in neutral with a stick or IN GEAR with an automatic. Most people miss that if they have an automatic. That usually gets me pretty close . . . on stockish motors it's usually right on. With big overlap, it's a great starting place but usually needs to be changed as per A/F meter
6.5 with stock/small cams 4.5 with everything else and done! do it! Been at this a looong time AED has been selling hot Holleys for decades--if it goes out the door they guess you have a cam so they always use a 4.5 PV Trust me If a 6.5 ain't getting it done use a 4.5 these are the two choices toss the others
If it bogs on acceleration, its opening too soon (too rich) , if it hesitates , the PV is opening to late (not enough fuel). NOTE: the 1/2 idle vacuum setting for the PV is just a starting point.......some times you have to "seat of the pants" tuning to get it right.
Don't doubt you, believe me - just pointing out they make PVs from (I think) 2.5 all the way up to 11.5 and maybe more?
This subject drives me crazy every time it comes up because there are so many misconceptions, half truths and flat out lies revolving around it, starting with the instructions directly from Holley. Let's start with some history. The power valve was first introduced as the economizer valve, because it allowed engines that had previously had to run full rich all the time to run leaner and, therefore get better fuel mileage. Later, after the sales dept got ahold of the concept it got labelled power valve. It's purpose is to add fuel while under heavy load, or power. Holley's formula prescribes half of idle vacuum for an opening point. That's ok for a stock engine that pulls 18" of vacuum. Not that it can open at idle anyway. It can't. If it is blown it can flood the idle circuit and cause it to run rich, but that's trouble diagnostics, not tuning a good engine as we are here. So you built this rowdy engine that idles at 8" of vacuum. So the appropriate pv would be 4, right? Except that at cruise, where it really matters, that 4 ( or actually 4.5) would come in way too late. As you're at cruise speed running down the highway, even with the most radically cammed engine you're going to be pulling decent vacuum, let's say 15". At this point you're under light load and can run a lean mixture. But, from this point under heavy acceleration, you will need additional fuel. As you roll into the throttle, the jets can only supply so much fuel at that rpm and air flow, so when vacuum gets down low enough, the power valve starts to open. If you're cruising at 14.5:1 afr at 15" of vacuum and have a 4.5" power valve you're going to go considerably lean on the way there. As previously stated, they don't open like a flood gate, but begin opening at the rated number. In this situation, I would recommend and most often use a 10.5 pv. This is just part of the total tune on a good street car that can run like stink when the pedal is to the metal but still pull decent mileage while cruising. Watching a wideband afr gauge and vacuum gauge can teach you a lot if you're willing to learn. Your engine will respond if you listen to it. Every one is different, so there is no be-all end-all answers. Devin
Devin you are right, good stuff right there but...... 80% of all carb issues are not carb issues at all-- but are distributor curve issues when the distributor is done right then the carbs get better real fast! Example: MSD sends every unit out the door with two of the stiffest springs known to the industry--totally wrong for everyone but MSD Take any MSD out the box add the black stop bushing, one lite silver, one lite blue spring and Wow! your cammed up beast suddenly will idle--it will now idle at 15-18 degrees initial and mechanical is limited by the black bushing so you set your total where you want it at the RPM where mechanical is all in and now...you have become a tuner. You have now made the most important step you can before moving on to what I call driveability tuning like PV, vacuum advance ( for MPG only not needed on a fast car IMO) etc If you guys mess with any carb before distributor is correct then you will never ever get it even close--back in the day a SUN machine operator spun the distributor first, got it right for your application , then you were ready to tinker with that Carb. Over the years I have found that a correct curve and a 4.5 PV will work wonders in most any hot engine and street testing. When you get one on a dyno you find that ehh..maybe that 6.5 is OK after all but then a dyno ain't going down the street under same conditions either I agree...a vacuum gauge is one of the best tools and should be one of the FIRST tools ever- in any mechanics box. I have the one my dad gave me when I was 14 years old--he said this will help you more than anything else in your box --you...and dad, were dead on! Thanks!
I have been tuning *that type of carb for over 40 years. I have always used the 1/2 idle vac as a starting place. it is not an exact science the .5(X) with X being idle vacuum is just a started place, it ball parks you. about 70% of the time it is right on and the rest of the time you have to play with it a little bit. *a Quick Fuel Carb is just a Holley Copy. They are good carbs but pretty much everything you ever learned bout tuning a Holley is applicable to the Quick Fuel.
There's a method to their madness, betcha they don't want anyone grenading the engine as a result of installing their product.
True enough so.... they send out the product as if it will be installed in a bone stock slant 6 Chrysler--guess if you just run slow and crappy then that is better than ????? They can't prove that folks would hurt engines anymore than you could prove it if it did--the POOR explanation of what is actually going on and needed by most engines and how to get there is a joke-- talking about the MSD instructions.... Oh well
Somebody said a vacuum gauge works OK for carb jetting, they said start out real fat and jet down in size for highest vacuum at steady cruise. Sounds reasonable, tho that's why I asked.
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