I finally got around to rebuilding the 8ba Holley 94s that will top the 6x2 setup thats going on my 425 nailhead. Physically rebuilding the carbs has been covered a million times on here, so I dont need help there. What I'm looking for is a starting point for the set up. Specifically, I'd like to get an idea of what power valves jets I should be using as a starting point. Here's what I have gathered so far: (Please correct me if I’m wrong!) If the center 2 carbs are the primaries, and the other 4 open as secondaries then only the primary carbs need functioning power valves and chokes. The other 4 power valves will just be plugged. Do I just leave the choke butterflies and linkages out of the secondary units or is there a way to just keep them held open? Or do all six need functioning chokes? I know that when running dual carbs on a flathead, you install power valves that open at ½ half the value of stock, since each carb will now only receive ½ the overall vacuum. But since they’re going on a motor with essentially twice the cubic inches (which I am assuming will make twice the vacuum), do I stick with the stock power valves? Now for jets… the stock carbs came with size 50 or 51 jets, which are (should be) correct for sea level. That’s fine since I’m in Delaware right at sea level. Am I right in thinking that – even with 6 carbs – these would still offer the correct A/F ratio? Any insight would be greatly appreciated. This is my fist run at any multicarb setup and it’s a real challenge. But MAN will it look good! --Jesse
That's a good point. It will be the stock cam initially. Actually the whole engine will be stock with the exception of this intake and the headers. In a year or two it will get pulled and rebuilt.
I guess a running test for manifold vacuum would really be the way to know for sure, but I don't have any way to get that right now. Perhaps someone else out there with a stock(ish) 425 nailhead could help us out with this?
Excellent topic and excellent questions! I will be running into this in the future as well! Will find this useful if we can get these questions answered for BuickinaBucket! BTTT for ya'!
I too have been looking for similar info to setup my 4x2... Below are a few good threads on the subject..... Littleman seems to know quite a bit....good info. http://www.jalopyjournal.com/forum/showthread.php?t=39495&highlight=plugged+94+power+valves http://www.jalopyjournal.com/forum/showthread.php?t=124566&highlight=6x2+94+holley http://www.jalopyjournal.com/forum/showthread.php?t=54768&highlight=holley+94+power+valve http://www.jalopyjournal.com/forum/showthread.php?t=85193&highlight=6x2+intake http://www.jalopyjournal.com/forum/showthread.php?t=123739&highlight=6x2+intake
Hmm... some more good info in those threads, but still nothing very difinitive. Littleman is the first person I've seen in any of the threads I've read who tells you what he is running on an actual car. That's really what I'm after... first hand accounts of setups that have definitely worked. I know I probably won't find a cut-and-dried answer, but I'd like to at least be close with my initial setup. So, to recap, from the threads I have read we know know some things: 1. Keep the choke plates in all six. If you run progressive, rig the secondaries to stay open. 2. One guy has a car that runs well on all six carbs on a straight linkage with blocked power valves. I think I'm going to have to do an all-in-one tech post when I finally get it sorted out.
The reason you cut the PV size in half is because for a given vac, the PV will let a constant amont of fuel into the engine. If you add a second carb, Your Vac stays the same and so does the fuel needs of the engine, but you are now dumping twice the fuel. So you cut the PV size in half to compensate.
Yeah, alot of the info pertains to 94's run on a 8BA flatty.....I've also had trouble finding INFO and linkage pics of what guys are ACUALLY running and DRIVING......ya see plenty of pics of cars at shows and such....but not much set-up info..... The subject has been covered ALOT.....maybe guys are tired of it, I never am.....some subjects cannot be covered too much.... Vintakes has some good info in the tech-o-matic....and a call to Jere Jobe at Vintage Carburation may not be a bad idea as well.... In your situation, I think I'd install both primary carbs with 4.5 PV's in each, block off the other 4 carb mounting surfaces, and tune the motor with just the two.....get 'em in sync and engine idling good....then proceed to install the other four carbs, blocking off their PVs and jetting accordingly... It's going to take time..... That would be great.....
See, that's not how I understood it at all. From what I've read, the different power valves flow the same amount of fuel. The difference is in the strength of the spring holding them closed. In other words, the difference isn't how much fuel is delivered, but when. You are right in that overall vacuum and fuel needs are constant. However, with two carbs sharing the vacuum equally, (think of sucking through two straws versus one) it stands to reason that they'd require a lighter valve that will open with less vacuum. If they were getting their vacuum signal from the manifold via a hose, they would all need the same higher vacuum valve. But, since it is taken from the carb base, the airflow is split between the carbs.
Yeah, there have been many posts, but most are filled with fluff and little actual info. So I'd say the subject has been broght up alot, but covered very little. The setup you suggested is about right according to most of what I've read so far, but the info search continues!
Agreed!!!! I was going to use 94's, but found a few 97's and will be using those instead (still need 2 more though).....seems to be less confusion on setting them up......
I've never run all my carbs at the same time on the same motor...but some thoughts: I think a point of excess will be the pumps: This is the only place where 6 carbs will actually put in 6 times the fuel as one. I also don't think you can simply eliminate half the pumps...I believe all the carbs will be running essentially separate air paths when stomped, and so will need the cover the pumps give in that moment when airflow suddenly goes to wide open but the venturis are't yet into their zone...I think summer setting will still be too much, so likely you will need a drilled passage bypassing some shot, as Navarro did on multi Strombergs but harder on Fords. Power valves...do NOT meter amount of fuel, only on-off for the enrich circuit. Metering is at drilled restrictions in carb body, just as on Holley 4 barrels. Number refers to vac level at which they open--this will doubtless need to be a very small number, as vac will drop off very fast when 12 throats open and main circuit will end up enriched all the time. Power valves do NOT dump fuel in magically--go home and tear that page out of Tex Smith's book and flush it. They work entirel as supplementtary extra main jets--they add two small extra openings into passage fed by the two larger openings of the main jets, enriching the mixture BUT supplying only as determined by air flow through venturis! They are necessary because engine needs richer mix at full throttle than at steady cruise. If you eliminate them, main jets must be enlarged to flow equivalent to enriched, and mix will be too rich at any time those carbs are open at cruise. Carbs spend a lot of low RPM time running only on the (relatively poorly controlled) off-idle circuit til enough air is moving to actuate the main circuit via venturis, and this may well be the fundamental problem to focus on--when everything opens up, you have air flowing through 12 throats, and there won't be enough in any one passage to bring in gas via the main and power jets until RPM comes up.
Bruce is right, what I meant was that the PV controls a constant amount of fuel, not that it lets it in itself. The PV opens at a set vac point... If you don't change anything internal to the motor, the idle vac signal will be approximately the same no matter what sorta carburation you put on it (if the carbs were letting in more air, the engine would idle higher). the PV opens when vac drops below a certain point (indicating throttle opening), and allows fuel in VIA the mode Bruce stated. Cutting the PV value in half will have them opening at a slightly higher throttle % but they will still dump the same amount of fuel. I mis-spoke before. I've never tuned a multi-carb setup before, so I can't offer too much beyond just the simple mechanics of how the system works, but I can say that people I know who have all started with the primary carbs only and it got it idling and running ok on those before adding the others.
Pulled from Motec, inc...http://www.mortec.com/carbtip1.htm good stuff....it applies to the newer type PVs, but the concepts still apply here.... "POWER VALVES and ENGINE VACUUM There is a lot of misunderstanding concerning power valves in Holley carbs. Many 4-barrels come with a particular power valve depending on the carb list# and application. Some carbs have two power valves, while others only have one. The power valves are numbered by the amount of engine vacuum in inches at which they will open and add additional fuel to the power circuit. In other words a 6.5 power valve will open when the vacuum signal on the engine drops below 6.5" and will remain closed above that amount. One of the misconceptions is that they can't be trusted to work because an engine backfire or "belch" can "blow out" the power valve. Many Holley performance carbs models and list#'s now come with built in power valve "blow out" protection which eliminates this problem. If you have an older model carb you can purchase a small, inexpensive, easy to install kit from Holley (PN - 125-500) that will also protect the power valves in case of an engine blowback thru the carb. CENTEK in Redmond, Oregon, (see their website at www.powervalveshield.com ) also sells an inexpensive Holley power valve blow out protector, "Power Valve Shield", which takes about two minutes to install and does not require any drilling. Many tuners will automatically remove the power valves and use a "plug" thinking this is the "hot" ticket. However, if the power valve is removed and plugged, the main jet size must be increased 6-10 jet sizes to make up the required fuel amount lost by the removal of the power valve. In addition, when the power valve circuit is plugged, the part throttle fuel economy is worsened and may become overly rich. Plug fouling may become a problem at part throttle. Stock engines can have high vacuum readings (10-18 inches at idle) and the Holley power valves with higher readings like 6.5 to 10.5 will work correctly. Long duration non-stock camshafts and other performance related parts can cause a problem, because engine manifold vacuum may be lowered with these performance parts and the power valve, if incorrect, will always be open, even at part throttle, leading to an overly rich air/fuel mixture. The solution is to choose the correct power valve and to do that you need a vacuum gauge. On a manual transmission vehicle, hook up the vacuum gauge and take the reading with the engine at idle. Then use a power valve that is rated 1-2 inches below that amount. For example, a motor that shows 7" of vacuum at idle should use a 6.5 or 5.5 rated power valve. If you have an automatic transmission, take the vacuum reading at idle in "Drive" (with the emergency brake on and the wheels blocked) and chose the power valve 1-2 inches below that figure. You can get a little more detailed information by driving the car with a vacuum gauge hooked up with a longer hose so you can read it while driving. Drive the car at medium loads and while cruising and note the various vacuum readings. Then chose the appropriate power valve rating. Holley makes performance style "standard" flow or a "high" flow power valve which has a large opening. "Single stage" power valves are available in 1" increment sizes from 2.5" thru 10.5". There are "two stage" power valves available that are more for "economy" minded users rather than "performance" enthusiasts."
Getting the engine running and tuned on 2 carbs only is an EXCELLENT idea! Especially with a new engine, you want any extraneous problems sorted out and a general idea of how the engime feels and runs before you throw in a rat's nest of new issues!
Since I only have two carbs at the moment, once I rebuild 'em, I may install them on a 4 bbl adaptor and tune 'em......mainly so I don't have to pull the intake, as my engine is broken in and runs great. Once those two carbs are dialed in and running good, I'll swap to the 4x2 intake, install the 2 carbs and try it with the other two carb spots blocked..........all the while looking for two more to complete the setup...... Haven't decided on progressive or straight linkage.....I may try both....
First, thanks to Bruce, Sinister and Bort for the info... good stuff. From this, I have a better understanding of what I'm looking at. I have formed a hypothesis that goes like this... First I will finish rebuilding the carbs. I will install blocked power valves in all initially, to take them out of the equation so I can tune for idle. Once I get it idling, I can get a vacuum reading, which will allow me to select the correct power valves for the center two carbs. With the progressive linkage, only the center two will operate from low to mid range. So, I will keep the outer four blocked, and tune the center two until they work great from idle until just after the secondaries begin to open. Then I will unblock the secondaries. The secondaries will retain the blocked power valves, and will just be jetted accordingly. Since they will basically only be open near WOT, they should work just fine that way, since there should be plenty of RPM to keep it humming at that point. Thoughts?
Man! killer info, I am in the same place with my 6x2 401 nailhead for the 32 5w. this will get me started. You need it, its on the HAMB! Great stuff.
Sounds good to me, should work well......I'll be doing similar setup..... Be sure to post the progress!
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