Bear I wasn't disagreeing with you, not sure what you want me to explain all I did was add that all of the major american car manufacturers (I lumped all teh gm together, and all the chrylser together as one each) used a holley from the factory on some vehicles at one point or another as a response to PatsPOS rhetorical question about how many were actually used and to supplement your answer.

 

I was adding info to what you said was all. If it came across as disagreeing with you, that was not my intent. Sorry.

 

SO,,,, if you have an 87 model 460 truck engine, it has a 4180 carb on it, right ?, 600 cfm.

Where does the carb get its vacume from to open the secondarys & how much vacume will you have at WOT.
Neil

 

Yes.
It's a port inside the carb.
A lot of variables: depends on engine rpm's, load, even how dirty the aircleaner is...

(edit)
There are a host of parts to tune it, Secondary springs, powervalves, etc...
Are you asking what the vacuum has to drop to for the secondarys open up?

 

The vacuum from the primary throttle bores controls the secondary openings, it is all internal in the carb body. As to vacuum at WOT, hard to tell as CID, RPM, efficiency of the engine, etc. will make it vary. Probably close to 0 thought.

 

Power valve has nothing to do with the secondary opening. The only things controlling it are the spring in the vacuum unit and the orifice size in the vacuum unit and air flow thru the primary throttle bores.

 

No it doesn't *have a thing to do with the secondarys*, I was just trying to point out that it is another carburator tuning part affected by vacuum.
But it will affect how quickly and smoothly the engine gains RPM's, and therefore regains vacuum (given the somewhat restrictive intake tract and exhaust on a stock truck)

Am I right on that?

*Edit:* perhaps I misinterpreted the OP, I was thinking of the first sentence where he asked about it being a 4180...

 

Bear, You DO have some cool signature pictures!

I've rigged an indicator light to show when my secondaries are coming in. With Clyde (stock o/t straight up timing, purple spring IIRC) the light comes on usually in the area of ~3" of vacuum ... at this point I usually "lighten up" to make the light go out. I'm now running ~7.5 mpg average towing the 10,000# fifth wheel.

 

Last night when I tried to answer the site kicked off on me. So a little late, but you get your answer, after I reread what you wrote.

How would the power valve have anything to do with regaining vacuum at WOT? When regaining vacuum at WOT is the last thing you want, because the increase in vacuum would start closing the secondaries. Which is undesirable at WOT. And the power valve will close long before the engine reaches idle if you close the throttle. So no it does not. I think you have an misguided idea of what a power valve does.

 

actually bear the secondaries open WITH vac not when you have a lack of it, proof of this is leave your vehicle turned off and open the throttle wide open do the vac secondaries open? no but there is no vacuum at this point is there?
Remember vacuum is caused by a restriction of the air coming in, at WOT you have no vac because you have no restriction of the air. If you begin to get a vacuum reading you don't have enough carb, vacuum secondaries work on this principle, if you have zero vacuum with just the primaries then you are feeding the engine enough air for the demand, as the engines rpm's increase and the demand goes up the primaries can't keep up and it becomes restricted enough to begin to build a small amount of vacuum, this vacuum is what begins to pull the secondaries open against a very light spring pressure. As the rpms further increase the engine demand keeps climbing the vacuum keeps staying high enough to keep them open and to open them further to meet the demand. This is why especially on a heavy car a DP mechanical secondary carb with have a stumble or dead spot, the secondaries are open but the engine doesn't have enough demand to be able to pull the air and you get dead air in the intake and no fuel pulled through the venturies.

 

Actually you are right and wrong. The secondaries are controlled by vacuum signal from the primary throttle bores, not the manifold. The reason the secondaies open is to keep the manifold vacuum low. Other than that you are correct.

 

Your saying yes, the 4180 was what came on the 87 model 460.

Bear, ?
A port in the primarys supply the vacume to open the secondarys, right, so if I'm flat footing up a hill & that engines pulling hard, there still enough vacume generated in the primary wide open to keep those secondarys open to feed the engine.

I didnt know vacume was what made them stay closed, that be the case, if for some reason the engine lost vacume to the diphram they would open up & no way to close them.
Thanks, Neil

 

Well, they can't because mechanical linkage at idle holds the secondaries closed. It is the spring in the vacuum (vacuum is spelled with 2 "u"s and no "e") unit that holds the secondaries closed. It takes a vacuum signal to open them. That's why a lighter spring allows the secondaries to open sooner and a heavy on later. The signal pick up is in the narrowest part of th venturi which changes how a person would think it works. Go here and read how it works. The vacuum secondaries are the last item on the page. The rest of the page is also great information on Holley carbs and how they work.

http://www.holley.com/data/TechServi...ech%20Info.pdf

 

Hey guys, let me throw in $.02 here Yep, bears right the 390GT motors had hollys, 735 I think just like the CJ's. We both can attest to that!

The secondaries pull up from vacuum created in the primary venturi's, thats between the "neck down" at the air horns and the butterflies. It works just like one of those vacuum "pumps" in the air conditioning catalogs that use an air compressor blowing over a small port thereby creating a vacuum in that port. The faster the air (venturi) moves over the hole, the more vacuum it creats. Thats why with even "0" manifold vacuum you have vacuum at the secondary opening diaphram. I have never measured it, but I assume (yea, I know what that means) its not nearly as much as full manifold vacuum would be.

Now I am going to throw this in and run like He## . Not to disagree with anyone----chances are I will deny saying any of it--

 

390GT had a 600 CFM Holley, same size the standard 428 and the PI had. The 428CJ had the 735 with the "Booster Venturies" to cut the CFM of the carb from 780 to 735.

You are so correct about the vacuum source for the secondaries.