Theoretical Vacuum Secondary Question

[patrickt]

Well, maybe not so theoretical. Let's say I have my Holley Vacuum Secondary spring picked perfectly for my engine, but the engine has a restrictive air cleaner. The secondaries open as quickly as possible without the presence of any hesitation or a bog.

I then replace the restrictive air cleaner with a much less restrictive one, so the engine now "breathes" much easier.

How does "breathing easier" affect the venturi vacuum and will the secondaries now open sooner, later, or with no change?

[FWB]

i never noticed a change with mine....run aircleaner on the street but not at the dragstrip.......air moving over the venturies shouldn't change the internal vacuum...if it does it's so infinitesimally small it wouldn't make any difference.
are you having drivability issues after a aircleaner swap?

[patrickt]

Well I just found the answer in my Holley technical books. Charts too. I'll post them if anyone is interested. The presence of an air cleaner restriction increases venturi vacuum and causes the secondaries to open sooner in all cases. What caused me to ask the question was the fact that I monitor how much my secondaries open via a zip tie on the shaft. After putting in my low restriction X-treme top, my car runs better, but my secondaries open less. I was quite surprised at this. What it also means is that I need to change the spring that controls the secondaries now so I get the full benefit. I can't remember what I have in there either. I think I'm going to try the Quick Fuel adjustable secondary vacuum housing. It just bolts on and then adjusts when the secondaries open with the turn of a screw.

[Gaz64]

Patrick,

Your secondaries shouldn't open all the way unless the carburettor is sized EXACTLY correct for the engine.

Example would be 850 VAC Secondary on a 300 inch engine.
The engine would have to spin to about 9800rpm to have the secondaries even close to fully open.
This doesn't take typical volumetric efficiency into account.

Your car goes harder because it is pulling on the boosters harder.

Don't worry about wondering whether they open fully, if they do then at some point the engine is not receiving the fuel that the carb was designed to deliver - fuel delivery curve.
Hook up a vacuum gauge and roadtest again.
Observe WOT vacuum, should be between .5 and 1.5 inches. Less than .5 - carb is too big, or secondary spring too light. More than 1.5 - carb too small or spring too heavy.

Changing from a restrictive air filter proves your own point of the air cleaner was restricting power. The secondaries open sooner as a consequence of a higher venturi vacuum signal across ALL 4 barrels.

Thoughts?

[patrickt]

Originally Posted by Gaz64

Changing from a restrictive air filter proves your own point of the air cleaner was restricting power. The secondaries open sooner as a consequence of a higher venturi vacuum signal across ALL 4 barrels.

Thoughts?

We're in total agreement. My observations on the opening of my secondaries, both before and after the change out to the low restriction Xtreme filter top, were based on "lightly spirited" romps, and how much the maximum opening of the secondaries were during those light romps -- but the car ran as well, or better, than it ever did so I thought nothing of it. This was not WOT, by any means. In fact, back with the small S&H filter, my secondaries would open a bit even on easy, around-town driving. Now with the Xtreme top, they hardly opened at all on the "light romp," but, again, the engine ran beautifully. This weekend I had pushed the zip tie over, to mark the fully closed position, took her for a nice ride, and then looked at the zip tie upon return home and was surprised to see that, instead of having moved the customary half inch or so, it had only moved maybe a third, or maybe even less, from what it has always done. I had had to readjust the mixture when I switched to the new filter top, and I don't know why it didn't occur to me that I would have to eyeball the secondary spring as well... but it didn't. I don't have the quick change unit on the Holley. And I don't remember what spring I put in there years ago, but I do recall it was on the stiff side, say the plain one or maybe the brown one. Changing them out is a bit of a pain and the QF Adjustable Housing looks pretty trick, so I've ordered one. Right now, with the Xtreme filter top and the existing spring, I am pretty much running a two-barrel carb! But what's even more interesting is that there are a bunch of threads on different forums where guys have switched out to the Xtreme and then complained that performance decreased. Nowhere in any of those threads does anyone recommend that you double check your secondaries because you've just decreased venturi vacuum by increasing overall flow. It's actually very cool.

[FWB]

Pat,
i do think you are wasting precious life......to dwell on when they open now is a waste of time and to start changing things you will defeat the addition of the aircleaner to get back to where is was before.

drive the car damnit....

[patrickt]

The Quick Fuel adjustable vacuum secondary housing came today, so I put it in this evening. It's pouring cats and dogs, otherwise I would take her for a late night run to test her out. You adjust the opening of the secondaries with the little screw that I've circled in yellow.

[patrickt]

Yay! Road tested and this $20 piece works very well indeed. You just unscrew it until you have a bog on heavy acceleration, then turn the screw back in until the bog goes away. The zip tie test shows the secondaries are now opening nicely. What a pleasant surprise from a relatively inexpensive little item (it was only three dollars more than the Holley spring kit itself).

[Gaz64]

Patrick,

If you are ever keen enough to pull it apart, I'd be keen to know how it works.

[patrickt]

Originally Posted by Gaz64

Patrick,

If you are ever keen enough to pull it apart, I'd be keen to know how it works.

When I was six years old, I got a drum for Christmas. I banged on it incessantly. My older brother, who was more clever than I, said "Patrick, have you ever wondered what's inside that drum that makes that wonderful sound when you beat it? Why don't you cut it open and see?" That was the end of the drum.

[ERA Chas]

Originally Posted by Gaz64

Patrick,

If you are ever keen enough to pull it apart, I'd be keen to know how it works.

It's probably a tapered needle and seat in the vacuum passage. Adjusting it changes the vacuum on the diaphragm.

[trularin]

The parents of patrickt are still looking for the bloody bastard that gave him those drums. The older brother was much older...and wiser.

[Paul F]

Patrick,

Is that a universal kit - one size fits all carbs? How does it attache to the carb?

[patrickt]

Originally Posted by Paul F

Patrick,

Is that a universal kit - one size fits all carbs? How does it attache to the carb?

No, it is not universal. It is made to fit the 4150/4160 line of Holley carbs. You remove the three screws that hold the choke plate, then remove the tiny C-clip that holds the stem of the vacuum secondary housing to the secondary shaft (if you didn't know it was there, you would over look it at first, but you wouldn't be able to remove the stem so you would figure it out). Then remove the three screws that hold the housing to the side of the carb. Six screws and one C-clip -- that's it. If you have an electric choke, then there would be a couple of addtional screws. Blow out the passage with compressed air while you have it all off, make sure the little cork gasket is properly aligned with the hole, put the screws back in and you're done. Even RodKnock could do this one... well, maybe. But I'll tell you one thing he could do is adjust the screw that controls when the secondaries open. You just road test your car, stop it, back the screw out, stomp on the gas, do it all again until you create a bog off the line, then turn the screw back in until the bog goes away. On the one to ten Easy Scale, that's a 9.8, maybe higher.

[Gaz64]

Originally Posted by ERA Chas

It's probably a tapered needle and seat in the vacuum passage. Adjusting it changes the vacuum on the diaphragm.

That' true, but it must be a bleed in parallel with the diaphragm. The venturi signal strength then gets altered since it doesn't require a spring change to alter the opening rate.

Here you can see the bleed off:

Quick Fuel Adjustable Vacuum ADV Secondary Housing 63 1 | eBay

[ERA Chas]

Thanks Gaz. So no more spring involved?

[rodneym]

Nice find, patrickt! Thanks for sharing.
A 9.8 on the 'easy scale'.
No pictures of well manicured nails.
We're on to something...

[patrickt]

Originally Posted by rodneym

No pictures of well manicured nails.
We're on to something...

Well, there was nothing to hold... and I don't let Ellie play in the engine compartment so no shot of her either.

[Gaz64]

Originally Posted by ERA Chas

Thanks Gaz. So no more spring involved?

I'd say it has a white or short yellow internally, then with the screw seated you'll get a bog like running a very light spring.
Back the bleed screw out and you could end up with equivalent of a brown or even black spring.

So yes, no more spring changes.