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post #1 of 17 (permalink) Old May 9th, 15, 10:58 PM Thread Starter
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Holley vac. sec. question

I was adjusting the floats in my holley carb the other day. They were set too high at first. It was easy doing the primary because you could just rev it a bit to use up the extra gas in the bowl after adjusting the valve. For the secondaries, I adjusted the valve then manually opened the secondaries. It would bog at first and then catch up and clear out the secondary bowl. The question I have is that it was fairly easy to overcome the vacuum canister to open the throttle a bit. The link from the primaries was out of the way. Is it normal to be able to overcome the canister so easily? I was thinking maybe there was a vacuum leak or missing check valve or something. I figured that at idle with full vacuum it would be harder to overcome the canister. I know its real hard to overcome a choke pull-off canister or distributor vac advance canister with full vacuum applied.
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post #2 of 17 (permalink) Old May 9th, 15, 11:31 PM
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Re: Holley vac. sec. question

The secondary's use a venturi vacuum to open them. As the air rushes through the primary throttle bores it creates vacuum in a small passage from the primary bore to the vacuum diaphragm. The secondary should open very much like the primary does when you work the associated linkage.

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post #3 of 17 (permalink) Old May 10th, 15, 07:14 AM Thread Starter
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Re: Holley vac. sec. question

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The secondary's use a venturi vacuum to open them. As the air rushes through the primary throttle bores it creates vacuum in a small passage from the primary bore to the vacuum diaphragm. The secondary should open very much like the primary does when you work the associated linkage.
Ok, that makes sense then. So its vacuum that opens them and not vacuum that keeps them shut. Kind of opposite of how the quadrajet air valve works. So then at idle its mainly the spring in the diaphragm that keeps it shut?
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post #4 of 17 (permalink) Old May 10th, 15, 07:25 AM
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Re: Holley vac. sec. question

That is correct and you can tune the tip in point by changing the springs. Holley sells a collection of springs and a quick change top to the vacuum pot that allows you to change the spring without unbolting and risk tearing the vacuum diaphragm inside. the pot. It is fairly expensive but worth it if you think you are going to experiment with the tune by trying different springs, squirters and cams to get a smooth bog free acceleration.

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post #5 of 17 (permalink) Old May 10th, 15, 12:42 PM Thread Starter
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Re: Holley vac. sec. question

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Originally Posted by Larger Dave View Post
That is correct and you can tune the tip in point by changing the springs. Holley sells a collection of springs and a quick change top to the vacuum pot that allows you to change the spring without unbolting and risk tearing the vacuum diaphragm inside. the pot. It is fairly expensive but worth it if you think you are going to experiment with the tune by trying different springs, squirters and cams to get a smooth bog free acceleration.

Big Dave
Thats awesome. All these years will holley carbs and Ive just run them stock. Never really had a problem that wasnt just a broken or misadjusted part. Then again, its been on mostly stockish street driven motors.
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post #6 of 17 (permalink) Old May 10th, 15, 01:06 PM
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Re: Holley vac. sec. question

I show on my site at what RPM the various springs open the secondaries. See http://www.nastyz28.com/~copo/vacuum...ring_chart.htm

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post #7 of 17 (permalink) Old May 10th, 15, 06:09 PM Thread Starter
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Re: Holley vac. sec. question

Good info.
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post #8 of 17 (permalink) Old May 10th, 15, 07:31 PM
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Re: Holley vac. sec. question

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I show on my site at what RPM the various springs open the secondaries. See http://www.nastyz28.com/~copo/vacuum...ring_chart.htm
Mark, the data you recorded on your two charts are only valid for those two engines you recorded the data from. The secondaries don't open with each of the different springs based on RPM's, they open based on vacuum levels which will be different for most every engine out there. Each engine needs to be tuned with what secondary spring works best for it.

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post #9 of 17 (permalink) Old May 10th, 15, 08:14 PM
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Re: Holley vac. sec. question

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Originally Posted by NH69Z28RS View Post
Mark, the data you recorded on your two charts are only valid for those two engines you recorded the data from. The secondaries don't open with each of the different springs based on RPM's, they open based on vacuum levels which will be different for most every engine out there. Each engine needs to be tuned with what secondary spring works best for it.
The chart is from Holley. I'm just the messenger.

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post #10 of 17 (permalink) Old May 11th, 15, 08:27 AM
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Re: Holley vac. sec. question

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The chart is from Holley. I'm just the messenger.
Holley also states that the vacuum advance for the distributor be connected to ported vacuum rather than manifold vacuum but that is also bad information. I'm just trying to help others get correct information. If you're going to be a messenger, best to have the message be accurate

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post #11 of 17 (permalink) Old May 11th, 15, 09:03 AM
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Re: Holley vac. sec. question

When we would get cars with vacuum secondary Holleys into our shop for dyno testing, I always made a point of forcing the secondaries wide open at about 5000 rpm on one of the runs to demonstrate how much power they were leaving on the table. It's almost impossible to get a vacuum secondary spring that offers a smooth transition to the secondaries and still allows the secondaries to open fully at the max power point.

One side effect that most folks don't think about is the effect that opening the secondaries has on the vacuum in the primary venturi that controls the opening of the secondaries. The secondaries don't open smoothly from a closed position, but rather they open a bit, then close off some, then re-open at a slow rate.

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post #12 of 17 (permalink) Old May 11th, 15, 09:12 AM
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Re: Holley vac. sec. question

Well truth sometime depends upon certain point of view, if you can believe Obi Wan Kenobi. In the case of the RPM tip in points it depends upon the cam, heads and carb size as well as the displacement (hence the two reference points) as all will affect the vacuum in the mainfold.

With the Quick Change kit:



all it takes is one screw to change a vacuum secondary spring.

You can then buy a Holley 20-13 spring kit:



and tune the tip in point to match your car's weight and gearing and it's cam and torque converter combination.

Of course as you change one thing it affects another so I suspect you will be changing the power valves next to keep up with the added air (think of it as a self adjusting main metering jet).



And as I have already mentioned the accelerator pump circuit will also need to be tweeked to cover the sudden opening of the rear barrels when you stomp on the gas. Principle will be the squirters as that controls how long a period of time that the accelerator pump squirt lasts.



With the other half of the equation being the accelerator pump cam selection.





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post #13 of 17 (permalink) Old May 11th, 15, 09:16 AM
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Re: Holley vac. sec. question

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Originally Posted by NH69Z28RS View Post
Holley also states that the vacuum advance for the distributor be connected to ported vacuum rather than manifold vacuum but that is also bad information. I'm just trying to help others get correct information. If you're going to be a messenger, best to have the message be accurate
You are splitting hairs, and also passing on bad info because you don't clarify what you say! Most cars of the '60s - '70s came from the factory with ported vacuum advance! Switching to a manifold source is a performance trick but not necessarily accurate information depending on who is getting the info!

The Holley diaphragm spring charts show a small block and a big block just to show the springs and secondary open different depending on the cubic inches and engine configuration. Nobody is making a hard fast statement!

...Dennis

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post #14 of 17 (permalink) Old May 11th, 15, 09:29 AM
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Re: Holley vac. sec. question

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Originally Posted by DjD View Post
You are splitting hairs, and also passing on bad info because you don't clarify what you say! Most cars of the '60s - '70s came from the factory with ported vacuum advance! Switching to a manifold source is a performance trick but not necessarily accurate information depending on who is getting the info!

The Holley diaphragm spring charts show a small block and a big block just to show the springs and secondary open different depending on the cubic inches and engine configuration. Nobody is making a hard fast statement!
Dennis, your response sounds as though I have ruffled your feathers which was not my intent. I'm not splitting hairs, I'm rather trying to keep the information as accurate and useful as possible. While it is true that some cars of the 60's and 70's did come from the factory using ported vacuum advance, it was being done primarily for emissions reasons, and as you stated, it was only some cars that it was done on. Using manifold vacuum advance isn't just about performance, it's about fuel economy and keeping the engine temperature cooler, performance may well be an added benefit as well.

As Larger Dave pointed out, there is far more to secondary vacuum spring sizing than small block vs big block and/or cubic inch differences. I stand by what I responded with previously but you are of course welcome to respectfully disagree with my opinion just I I am with yours.

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post #15 of 17 (permalink) Old May 11th, 15, 10:50 AM
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Re: Holley vac. sec. question

Honestly Scott it just sounds like you wanted to argue with another member! If not, my apology.

That spring chart has been published in Holley Tuning books for decades and I've never heard anyone picking it apart before for not being complete info or being mis-informative. It's a guide not a bible!

As for the vacuum advance, what 1st gen came from the factory with a manifold source? We are in the 1st gen engine forum after all!

...Dennis

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