Hello

I've been a lot of reading but didn't find all the answers.
I'm building a 3x2 setup with 3 holley 94.
At this moment i'm running a single EBU with vacuum advance hooked up on the back of it, and must say it has a snappy respons and works fine. So this will be my primairy carb. The other 2 will be ECG-5 with progressive linkage.
Am i right when I say, the powervalve won't work at the moment the secondaries come to it because vacuum divides over all 3 carbs?
But with a lower value powervalve, lets say 3,5 it will be open all the time?

Or am i wrong here?

Thanks

 

The lower the power valve number, the more delayed will be its opening. Not putting in a lower rated power valve increases the chance for the power valve to be open at idle or light cruise with the three carbs as you are correct in the vacuum signal being split between the three carbs. A power valve rated at 3½ inHg will stay closed until vacuum drops to 3½ inHG.

 

So if i hit the trottle rpm starting to go up, vacuum goes down, then the powervalve opens at 7,5 or even later with the 3,5 valve. As engine speed approaches the throttle setting, vacuum goes up and the valve closes again.
So if i would like the powervalve to open at about 2500 rpm for example, i should cruise around at a constant 2500 rpm and then mesure the vacuum in the primary carb.
The mesured vacuum will be my powervalve "size" at that point. Lets say this is 7,5
Now coms the tricky part.
Asuming i drive mountain up at 2500rpm, engine speed drops due to its load, vacuum goes down and powervalve will open again. But at that moment i hit full trottle and secondaries are open to keep my crusin sppeed. Right at that moment is my lowest vacuum due to its high load and 3 wide open carbs. Lets asume vacuum is 5 at this moment. If I wan't the powervalve to open only with primairy carb i should use a 4,5 valve maximum.
Right??

I think i got it now.

What are you're opinions?

 

The pwr valve vacuum is supplied by manifold vacuum below the butterflies in the intake track itself. Unless you have a really unique 3X2 intake, there is no single little port that supplies manifold vacuum to all carbs. Therefore, 3,4 or 6, 2bbl carbs will not be affected directly from each other. Actually, the pwr valve will work when the 2 extra carbs open. Pwr valves are "working" when they open up----thats what they are for.

When you fire the engine up, with the butterflies at idle, that is the approximate most manifold vacuum, unless you start getting into non stock cams. That means your pwr valve is closed. It (pwr valve is on primary carb only) will start to open at the value printed on the side. For most economy/street you (this was covered by someone in a recent post on this board) want a pwr valve that will stay closed (higher number)at most of your normal driving. As you start to kick it, the pwr valve will open and supply extra fuel through the same nozzle as the normal jetting.

When you kick it, almost any pwr valve will open because the manifold vacuum drops to almost nothing, as the butterflies open. Thats why racers dont use power valves, they use the plugs to block that port off because they "aint" got any vacuum! Use a vacuum guage to determine what vacuum your engine runs with under normal conditions, then pick a pwr valve that will open slightly under that range. You can also get a pwr valve that is progressive and opens at different values.

 

Unless you have a bigger cam installed, the 7.5 might be OK for the primary carb, since as long as you don't get into it enough to open the secondaries, it's like running a single carb. And a 4.5 or 3.5 might be good for the outside carbs. You don't want those PV's opening until the carbs are in use too. If you have a bigger cam, it'll lower the max vac and therefore you would want lower rated PV's.

If you had all PV's at the same ratting, it would dump way too much gas when they all opened at the same time. Some don't run PV's in the secondaries, but I think it usually causes flat spots. It's kind of a trial and error thing. Don't forget about adjusting the accelerator pump linkages too.

 

Just a little something else for you to consider. I had a 427 vette years ago with tri-power (3 2bbl hollys) It came from the factory with a vacuum controlled linkage for the end carbs.

After reading a magazine, I decided I had to have a "Segal Ball Bearing linkage" for the secondaries it was a mechanical, progressive system that you could control to opening of the secondary carbs at any point you wanted. Being young and even more foolish, I bought the d### thing. Looking back, I wish I had kept the vacuum control of the secondaries, much more streetable. There are times when you want to run on the primary carb as much as you can. Vacuum control is infinately more adjustable and responds to the engine demands much better (in my opinion ), especially with all the springs etc. made for them now.

I cant see a lot of benefit of a PV on the secondaries (dosnt mean someone else cant) because when the secondaries come in, 9 times out of 10, the vacuum is too low for most of them anyway----whatever you want! You should prob jet the secondary carbs larger than the primary because when you kick them in, the engine will be under a load and cannot utilize a PV very much anyway. An average size primary carb will be sufficient for most engines up to a ~mid RPM---unless you want the motor screaming when you dump the clutch, then your worry is traction---a real double edged sword. LOL--$.02 more

You didnt say is vacuum was available for the secondaries on your setup, but if it is, you might want to really look at it closely

 

My engine gets a rebuilt with a stroker kit 310cu
A little bigger cam 264 degree instead of 256
And some headers or ram horns, depends on dipstick clearence
My idea is running the center as primary with vacuum port to distributor, 7.5 PV and 65 jets like i have now. After driving and tuning the primery i will bold on the secondaries. Then progresive link the secundaries at 65% after they are carefully syncronized,setup with plugged powervalve, 70 jets and vacuum port plugged.

Im only worried about the vacuum to distributer, when opening the secundaries

 

With the secondary carbs opening at 65% of the primary opening, you wont have to worry about vacuum advance to the distributor------You probably "aint" got much-- vacuum that is when the end carbs come in!! You will be too far into the throttle to worry about any kind of "economy" mixture. At that point you need to be at the power setting of timing which can vary, but generally 36 -38 deg. You may even have to play with that. Some people run more power timing in the lower gears (Bear said he did it in one of his posts). You arent going to be done with timing in a few posts, but sounds like you have a plan LOL--now get er done

 

You are right about that, who cares about fuel economy when flooring it ? I don't.
Hopefully my MSD distributor i got this week will make it a little easier.
Thanks for info