Question about WOT Vac. / Secondary Vac. spring

[Canadian1968]

So after adding a rear meter block to my 3310 holley carb, to try and bring my WOT , A/F ratio down from 10.5.  I noticed now that I can feel the secondary circuit open. It more noticeable when I am on the street playing around , then when at the track . But I can defiantly feel the secondaries coming in .

I did improve my track times down to a 13.22 from a 13.45 . I fell most of it is coming from an improved launch though ( was able to cut a 1.96 60Ft on the street tire ) . I was able to clock a new personal best of 105.22 compared to a 104.8  in cooler weather.

My question is ,  I have a vacuum gauge installed on the dash, when I hit WOT the thing snaps to 0 as it should . 

Will putting in a lighter spring change anything?  I mean anything past 0 is boost which is impossible with NA.   

I currently have the silver (plain) spring in which according to holley spec, on a 402 is fully open @ 7000RPM   , I am reving to 5300ish  and yes its 446CID not 402. But looking at the chart offered by holley, approx 50 CID is good for 1000 RPM in opening rate, so I feel like I am close to fully opening all the way.

I know only one way to find out is put it in and test.  I just like talking about theory sometimes !!!

Plus I am officially bitten with the drag race bug.    On the list next year is some cheater/ slicks !!

[John_Kunkel]

The secondary vacuum diaphragm doesn't get its source from manifold vacuum, its vacuum source is the venturi where vacuum rises with rpm rather than dropping off like manifold vacuum.

Will a lighter spring help? Maybe, but it might also cause a noticeable bog if the secondaries open too early.

[XH29N0G]

I understand the same way that John suggested.

Also concur that unless you've done something with the diaphragm that opens the secondaries, that a Holley VS carb should not have a big kick in of the secondaries

Further, the reading of manifold vacuum will also depend on RPM.  With mine fully open (I have a mechanical secondary.) I see the vacuum rise slightly, but it stays less than one inch throughout the RPM range.  It always hits the closest to zero as I floor it. 

For my setup, the mechanical secondaries open has the throttle is pushed, and more gas is shoved in with the secondary set of squirters. For you, the vacuum secondary is designed so that the butterflies open as more air is needed and not fast enough to require another squirter.  I haven't thought about it much, but I suppose if you have a heavier spring, then the vacuum secondary would open more slowly and you might see a change in the vacuum reading from the manifold even though the opening was being controlled by the airflow through the venturi.

I also suppose that if you put in a weaker spring he would open early and it might help, but you won't know until you try period if it's too weak you will just open them too quickly and it will be lean and bog.

I did not understand your comment about trying to reduce the a/f below 10.7 I'm assuming that's a typo

Hopefully nothing is garbled here I just dictated this in for my phone

[Canadian1968]

I think I confused some people about the manifold vacuum.  I only mentioned that because if my secondaries were not opening it would be showing more vacuum at WOT a restriction basically. BUT i am not getting that it is responding just like it should. I understand the secondaries are controlled threw ported signal not manifold.

Yes a typo about the A/F , it was running around 10.5 I wanted to bring that to around 12ish , call that up or down 

I doubt will make it out to the track again this year but when I do will have to play with the springs!

[cdr]

I think I confused some people about the manifold vacuum.  I only mentioned that because if my secondaries were not opening it would be showing more vacuum at WOT a restriction basically. BUT i am not getting that it is responding just like it should. I understand the secondaries are controlled threw ported signal not manifold.

Yes a typo about the A/F , it was running around 10.5 I wanted to bring that to around 12ish , call that up or down 

I doubt will make it out to the track again this year but when I do will have to play with the springs!

sounds like you need to change jets or jet plates , the spring wont change the AFR

[XH29N0G]

That clarifies it.  I have some questions about the conversion to the metering block.  Did it come with jet extensions? Others on here can say how important they are and if needed if it did not.

I am all for talking theory.  As you have seen me say before, I am also learning. 

When you change the jets in the secondaries, you are also changing the primaries, right?  Because of the power valve there is a difference between the two of something like 10 jet sizes so you would be at something like 84/74 primary/secondary or 82/72 or .....  If you are doing that, I would get a sense of how the a/f changes and pull it into the range of about 12.5 to start. Then I would use the acceleration (if you have a track - great) to find the best combination. 

Can you log your a/f and do you have a way to log anything else?  I made a very crude logger for a/f, rpm and vacuum.  What I found was I used the a/f logging with RPM to see that the carburetor was doing what it was supposed to.  Then I used the RPM with time (acceleration) to optimize the Jets.  I found that what I thought was optimal from reading was too lean for my car and was losing me some on acceleration.  When it is close it isn't a lot, but when it is far, it can be.

[cdr]

That clarifies it.  I have some questions about the conversion to the metering block.  Did it come with jet extensions? Others on here can say how important they are and if needed if it did not.

I am all for talking theory.  As you have seen me say before, I am also learning. 

When you change the jets in the secondaries, you are also changing the primaries, right?  Because of the power valve there is a difference between the two of something like 10 jet sizes so you would be at something like 84/74 primary/secondary or 82/72 or .....  If you are doing that, I would get a sense of how the a/f changes and pull it into the range of about 12.5 to start. Then I would use the acceleration (if you have a track - great) to find the best combination. 

Can you log your a/f and do you have a way to log anything else?  I made a very crude logger for a/f, rpm and vacuum.  What I found was I used the a/f logging with RPM to see that the carburetor was doing what it was supposed to.  Then I used the RPM with time (acceleration) to optimize the Jets.  I found that what I thought was optimal from reading was too lean for my car and was losing me some on acceleration.  When it is close it isn't a lot, but when it is far, it can be.

In my experience the jet extensions are VERY important, BUT you need a float that is made for use with the extensions 

[Canadian1968]

The rear block i put in has power valve block off.  I'm simply doing the exact same thing that the meter PLATE did , excepts it's not adjustable.

The current carb set up is 72/72 with 35 nozzel and 6.5 power valve .The choke horn has been shaved and inlets radiused , knife edge butter Flys  and shaved screws .

The thing works amazing, throttle response is instant no matter what rpm and A/F  is  consistent other than this small surge in power I can feel sometimes. I don't think it's a problem  I am simply looking for every last bit when I am at the track .

[XH29N0G]

I see the secondary venturi diameters are larger than the primaries on that carburetor https://documents.holley.com/techlibrary_carb_numerical_listing.pdf and this will also affect the jets and offset what the power valve does.


Since  you said you like to talk theory, take a look at the power circuit diagram on this page https://www.cartechbooks.com/techtips/holley-carburetor-operating-principles/ to see why I wrote what I wrote before.


My understanding, and reading of the circuit on that page, is that the power valve opens to provide an additional flow of fuel into the manifold and therefore smaller jets are used when it is there (as in the primaries) AND larger jets are used when it is not there (as in the secondaries).  The difference depends on the size of the power valve restrictors (PVCRs).

The issue is that the a/f of the primaries COULD end up being different than that of the primaries, AND if it were different, the mixture to the cylinders fed more by one or the other COULD be leaner or richer than the others.  This might give you a reasonable overall a/f but still cause problems.  Look at the video at about the 3 minute mark to see an illustration https://www.youtube.com/watch?v=cnz4Q2vhGHg

[cdr]

I see the secondary venturi diameters are larger than the primaries on that carburetor https://documents.holley.com/techlibrary_carb_numerical_listing.pdf and this will also affect the jets and offset what the power valve does.


Since  you said you like to talk theory, take a look at the power circuit diagram on this page https://www.cartechbooks.com/techtips/holley-carburetor-operating-principles/ to see why I wrote what I wrote before.


My understanding, and reading of the circuit on that page, is that the power valve opens to provide an additional flow of fuel into the manifold and therefore smaller jets are used when it is there (as in the primaries) AND smaller jets are used when it is not there (as in the secondaries).  The difference depends on the size of the power valve restrictors (PVCRs).

The issue is that the a/f of the primaries COULD end up being different than that of the primaries, AND if it were different, the mixture to the cylinders fed more by one or the other COULD be leaner or richer than the others.  This might give you a reasonable overall a/f but still cause problems.  Look at the video at about the 3 minute mark to see an illustration https://www.youtube.com/watch?v=cnz4Q2vhGHg

YES This is correct, ^^^^^^
When I tune a carb I disable the secondaries & get the tune good with just the primaries, then make it a working 4 bbl again & retune it by making jet changes in the secondaries , I use a Wideband O2 set up.

[c00nhunterjoe]

I would suspect stepping up to 78s in the rear would yield results