Hey, My Vacuum Secondaries Don't Open .........

[Joe C.]

.................fully at WOT, with my engine on the dyno, turning 6000 RPM!!!! I have heard that question many times from people who have had their engines dyno tested, and noticed that the vacuum operated secondaries are not opening fully. Consider the following:

CARB CFM Requirement = [Engine Displacement(cu. in.) x Max.Engine Speed (Rev/min) / 3456] x V.E.(%)
Where V.E. = Volumetric eficiency, usually between 80-90% for normally aspirated engines.
Assuming (generously) a VE of 85% for a 327 cu in Corvette engine, running to maximum RPM of 6000, we get 482.55 CFM. A typical Holley 2118 carb is rated at 600 CFM (max), but the engine only needs 482, so the carb has plenty of margin. This engine can either be modified to rev higher, and/or breathe with a higher VE, and still utilize the stock carburetor.
This discussion does NOT factor in jet sizing, whose determination can only be analyzed by accurate A/F mixture measurements at various engine speeds.
I would think that, if the secondary butterflies on their VACUUM CONTROLLED SECONDARY ("single pumper")open fully, then the carburetor is probably undersized.

Is this a valid analysis?

Many people blame the secondary diaphragm spring for this, but, AFAIAC, the spring only affects the TIMING of the opening, because the spring sets the resistance against which engine vacuum must work in order to actuate the secondary plates.

Does the secondary diaphragm spring affect the "full open" point, as well as the timing, or is the "full open" point determined by air density, flow area, and VELOCITY, as expressed by:

Mass flow rate = Rho . V . A (for subsonic flow)
Where Rho = density
V = flow velocity
A = flow area

Joe

[John H.]

Re: Hey, My Vacuum Secondaries Don't Open ........

Joe -

Two things affect the opening of the secondaries - the strength of the venturi vacuum signal from the primary venturi pickup tube (assisted by a similar pickup tube in the secondary venturi once the secondary begins to open), and the strength of the spring acting on the diaphragm in the housing. Anything that degrades the venturi vacuum signal to the diaphragm will prevent the secondaries from opening all the way; make sure the little cork gasket is in place between the housing and the main body, and make sure the venturi vacuum passage in the main body that feeds the diaphragm housing is open.

[Clem Z.]

i have a holley chart that list springs vs opening

RPMs for different displacement engines. i can scan it to some one who can post it on the website. i would leave out the ball check valve for dyno testing if one is in the housing

[Duke W.]

Re: Hey, My Vacuum Secondaries Don't Open ........

For a SHP engine with massaged heads I use 90 percent VE at6500, 80 percent on an OE engine. So for massaged heads, the peak airflow at 6500 is:

.9(327/1728)(6500/2) = 553 or 545 using 80 percent VE at 7200, and these calcuations jibe with Engine Analyzer's prediction.

Maybe the Holley experts can explain it to me, but with the AFB when you got to WOT the secondary butterflies open fully, but air flow is controlled by an "secondary air valve" that has an offset shaft and counterweight so it doesn't open until there is sufficient vacuum to offset the counterweight, however, it is full open at about 2500.

It seems to me that the Holley secondaries should also be full open by about 2500 to 3000 as at that point there is sufficient airflow to provide a good metering signal at all four venturis, and the spring in the secondary opening diaphragm controls the rate that they open and, ultimately, the point where they are full open, which should be below the rated 585 CFM air flow.

What am I missing?

Duke

[Michael W.]

Re: Hey, My Vacuum Secondaries Don't Open ........

A bunch of us over at that website with the funny logo -the one with the strange looking shark mascot- discussed this and came to a pretty good conclusion.

Thanks for asking.

[Clem Z.]

even with the lightest spring,yellow short spring

with a 350 engine the secondary is not fully opened till 5680 RMP

[Clem Z.]

i must have missed that session. was it before or

after the adult beverages were served ?

[Phil P.]

Re: even with the lightest spring,yellow short spr

there is more to the eye than a set of numbers produced in an exact enviroment---following the above formulas why would gm put a 780/800 cfm carb on a 302---with factory cross flow exhaust muffler---the 302 will rpm or ("rmp clem" joke) but really does'nt pull until you put headers on it---std spring on a 350 will wot at 8160 according to holley and 7130 rpm on a 454---what motor configuration are they using ?---this ofcourse without an aircleaner,but they don't mention anything about air density etc..
another thing to think about is the fact that a medium to light weight car has different airflow needs based on trans & rear gears---i.e 4500rpm 2:20 first gear on 3:73 rear gear will wing real quick on a 2bbl when wot---shift to third with a 3:31 or 3:08 in the same car add a 4bbl give it wot and the secondaries will get with it---now factor in headers,cam,intake,timing recurve,altitude and on and on, the game changes as far as the carb goes---it's all tuning for your application beit stock/hotrod/race---

[Joe C.]

Re: even with the lightest spring,yellow short spr

Clem,

Please! If you can find a way, send me the chart! I wanta convert my friend's 2818 into a Dominator 1450 by changing the secondary diaphragm spring.
Actually, no. I think that the chart you mentioned earlier might help answer my question.

Thanks in advance.

Joe

[Clem Z.]

joe i need your e mail addy to sent the scan *NM*

[Clem Z.]

the secondary opening can be controlled

by using different size "jet" screwed into the vacuum hole drilled into the secondary venturi side. this way you can use a light spring to make sure you get WOT on the secondary side without a light spring causing it to open too soon. we did that on race cars were the track rules said you had to use a vacuum secondary carb,no double pumpers

[Joe C.]

Careful! Big Brother Is Watching!

Because my previous (irreverent? wiseacre! truthful?!)post was banished to the "great beyond" I'll clean it up and try again:

"Hey, it's a slow news week! Why, there have been no "homicide"(sic:@FOX News) bombings..........MSNBC's Keith Olbermann has milked the Libby "pardon" for all it's worth...........and last but not least, the Democratic candidate, whose initials are B.O.(yeah, I know what you're thinking, but his name actually means "blessed")has not been accused of experimenting with controlled substances while entertaining multiple se*ual partners."

Joe

[Duke W.]

Re: Careful! Big Brother Is Watching!

Hey, Libby didn't get "pardoned". His sentence was "commuted", which means "changed" i.e. "reduced". It was a witch hunt anyway. One juror was quoted as saying he shouldn't go to jail immediately after the trial, which leads to the obvious question: Why the hell did you vote to convict him?

Compare that to the 140 actual pardons Clinton ordered his last week in office including fugitive financier Marc Rich, whose wife - coincidentally - gave a big contribution to the Clinton Library, not to mention the convicted terrorists that Clinton pardoned.

A BO Whitehouse would probabably be hotter than a BC Whitehouse. Matters of state would have to wait until the party is over.

Oh, and on the subject of carburetors: What's a Holley.

Duke

[Joe C.]

Re: Careful! Big Brother Is Watching!

"pardoned": figure a speech.......at least thats how tha commie Olbermann sees it. Anyways, he still gets to pay his $250,000.00 fine. Fine.....he already payed 'er. Besides, he's on probation, and a convicted melon.

Joe

Holley(sic):
1. any of a genus (Ilex of the family Aquifoliaceae, the holly family) of trees and shrubs; especially : either of two (I. opaca of the eastern United States and I. aquifolium of Eurasia) with spiny-margined evergreen leaves and usually red berries often used for Christmas decorations.
2.They did "Carrie Ann", "Long Cool Woman (In a Black Dress)", "Bus Stop", and "He Ain't Heavy, He's My Brother".
3. Buddy
4. Berry