Author Topic: The 427 Thunderbolt and vacuum secondary carbs?  (Read 1273 times)

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427LX

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The 427 Thunderbolt and vacuum secondary carbs?
« on: July 08, 2018, 08:46:45 AM »
In today's Super Stock racing how are the vacuum secondaries tuned for quick positive opening?
Are they using the short yellow or white spring or the a slightly stiffer spring without the check ball?

I would think given the high RPM they run at they don't need the check ball.

Drew Pojedinec

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Re: The 427 Thunderbolt and vacuum secondary carbs?
« Reply #1 on: July 08, 2018, 09:42:08 AM »
I have no knowledge of what SS racers do. 

That said, what I do is this:
-Use the ball, but drill a .040 or so hole at an angle as an air bypass.

-I use the light spring, typically the short yellow.

-If more signal is needed, or if a stiffer spring is needed, I install a brass tube through the VS channel into the primary.  This signal amplifier greatly increases the force available to open the secondaries.  If used on a 600cfm carb the secondaries will dang near be 1:1 with the primaries with a light spring.

Barry_R

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Re: The 427 Thunderbolt and vacuum secondary carbs?
« Reply #2 on: July 08, 2018, 09:46:15 AM »
x2
eyebrow tube works really well

Drew Pojedinec

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Re: The 427 Thunderbolt and vacuum secondary carbs?
« Reply #3 on: July 08, 2018, 10:18:40 AM »
People love photos.....

Drilled hole to bypass the check ball:


Signal amplifier tube:




To install, you remove the lead plug, being that it's soft, it's easy to catch with a drill bit. 
The passage should be about .120, so I drill an entrance into it at .128, the middle section I drill to .125, and the last little bit I leave.
This is ever so smaller than 1/8 brass tubing so it'll be hard to knock in, but it isn't going anywhere.
The brass tube is 1/8.  I cut it about 3/4 inch and file the end for the 45degree end.
Hammer it in with a small punch, making sure it's aligned correctly.
Make sure the VS passage is fully clear to the secondaries, as once they crack open they help pull the VS's open.
When done, I drill and tap the entrances at 8-32 and plug with a set screw.

Best to use a marker on a small drill bit to gauge depth when doing any of this.

FLikr isn't working right, so I pulled these pictures from my fb page, hope they work.

427LX

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Re: The 427 Thunderbolt and vacuum secondary carbs?
« Reply #4 on: July 08, 2018, 10:25:15 AM »
Thanks for that info!  That tube rang a bell as I remember some of the old 780's having that on the primary.

My thoughts are that with a pair of vac. secondary carbs would not the vac. signal be weaker than with same sized carb in a single 4 bbl. setup?

I setup my GoPro cam on top of my clear lexan air filter lid,dual 650 Holleys on my 427 Windsor, and recorded a 2nd gear WOT from a low speed roll.  Midway thru the quick run the tires started to break loose so made 3rd gear shift at around 5800 RPM. Video showed the rear carb secondary opening maybe 1/4 or less opening...wasn't much.

Car weight is 2970 with me and 1/2 tank of gas and HP and Torque 500+ along with a 3.50 gear and T-600 trans.

427LX

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Re: The 427 Thunderbolt and vacuum secondary carbs?
« Reply #5 on: July 08, 2018, 10:32:20 AM »
I am running the short White spring and have the check balls removed with a drilled aluminum restrictor with .058 hole sitting in check ball hole. No bog and it seems to want all you can give it.
Would LOVE to try sideways mounted 650 Double Pumpers but is the cost really worth it.

427LX

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Re: The 427 Thunderbolt and vacuum secondary carbs?
« Reply #6 on: July 08, 2018, 10:36:58 AM »
We have an original untouched pair of the 1968 426 Race Hemi Holley 770 carbs used on my brother's cross ram Hemi.
Might examine those and see what they used as those seemed to work VERY well!

hwoods

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Re: The 427 Thunderbolt and vacuum secondary carbs?
« Reply #7 on: July 08, 2018, 01:17:54 PM »
why not just run the 660's mechanical secondaries?
it is hard to balance your check book with your testoserone level

WerbyFord

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Re: The 427 Thunderbolt and vacuum secondary carbs?
« Reply #8 on: July 08, 2018, 02:15:06 PM »
Thanks for that info!  That tube rang a bell as I remember some of the old 780's having that on the primary.

My thoughts are that with a pair of vac. secondary carbs would not the vac. signal be weaker than with same sized carb in a single 4 bbl. setup?

I setup my GoPro cam on top of my clear lexan air filter lid,dual 650 Holleys on my 427 Windsor, and recorded a 2nd gear WOT from a low speed roll.  Midway thru the quick run the tires started to break loose so made 3rd gear shift at around 5800 RPM. Video showed the rear carb secondary opening maybe 1/4 or less opening...wasn't much.

Car weight is 2970 with me and 1/2 tank of gas and HP and Torque 500+ along with a 3.50 gear and T-600 trans.

If the front secondary is opening, plus 1/4 of the rear one, you probably have about 950cfm there. "Double your horsepower" would be about 1000cfm, so maybe that's all the air it wants!

If you block the front secondary SHUT, does the rear one then open up fully?

What about a balance tube? (Mine ran better WITHOUT one on the GTECH)

Good idea to try the other carbs though, it might just "like" them better.

Barry_R

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Re: The 427 Thunderbolt and vacuum secondary carbs?
« Reply #9 on: July 08, 2018, 07:49:02 PM »
The eyebrow tube was found on several Holley carbs back in the day before cost savings took over (they were often just straight ended - probably not as effective as Drew's angle cut one).  Despite the name, its not really vacuum in the traditional sense that opens them - its airflow through the (mainly...) primary side going past an orifice.  Increasing the signal strength will open them more positively and you can then control the rate with spring and/or orifice.

Drew Pojedinec

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Re: The 427 Thunderbolt and vacuum secondary carbs?
« Reply #10 on: July 08, 2018, 08:50:51 PM »
Yup, airflow.... I test all of my VS carbs to assure opening.  Plus after you've done a few you can get a good feel for the right spring opening rate, or at least can make a pretty decent educated guess before tossing it on an engine.

https://www.facebook.com/AirFuelSParkTech/videos/221842335221172/

CaptCobrajet

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Re: The 427 Thunderbolt and vacuum secondary carbs?
« Reply #11 on: July 08, 2018, 08:55:51 PM »
On the Stock/Super Stock carbs, a smaller orifice with no ball seems to be more positive, and on bad air days, more consistent.  The lightest springs in the kit (you'll have to have two), and also a short diaphragm.  The long diaphragms will really squeeze the spring near the top, and won't open all the way with two carbs.  It is a reduced signal.
Blair Patrick

BattlestarGalactic

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Re: The 427 Thunderbolt and vacuum secondary carbs?
« Reply #12 on: July 09, 2018, 08:10:28 AM »
I'm surely not SS material, but here's my set up.

Lightest springs, balance tube.  They do not really do much til high gear(5th).   If I lock the secondaries, the 1/8 miles does not change much at all.

I did find out one day, if you leave the balance tube unhooked from one carb it really doesn't run well in high gear.  It pulls, falls off, pulls, falls off, etc.  Lost that round and wondered WTH?  I get back and open hood and realize I left that tube unhooked after doing some tinkering.

Have heard how I need to have DP carbs, as VS carbs are in consistent.  Ya, okay.  Last Saturday's race, first round and forth round I was dead on with a 4 and dean on with a 3.  Not too shabby for a heavy, small tire, stick shift car with TWO vac. carbs on it.

427LX

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Re: The 427 Thunderbolt and vacuum secondary carbs?
« Reply #13 on: July 09, 2018, 07:00:15 PM »
I'm running the weakest spring and using an aluminum plug which is a slip fit in the ball hole. This has an .055 hole if I remember and I can drill it larger if needed. The passage is about .080 under the check ball.

A shame Holley didn't try the spring loaded air door since it's much simpler in function and adjustment plus you know the throttle plates will be full open.

Another thing I have found is you need to have that primary/secondary link right close to the end of the slot on the secondary shaft when running the weak spring. Sometimes the airflow under the throttle plates will try to crack it open during cruise and can mess with the idle speed.

Now I did try a little test with my own version of a Holley AVS a  couple years ago on a single 4bbl. setup!

Barry_R

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Re: The 427 Thunderbolt and vacuum secondary carbs?
« Reply #14 on: July 09, 2018, 10:17:55 PM »
Thats a neat little concoction there!  How did it work?

Not sure anymore - but it used to be that the purple spring was the only non-linear spring in Holley's vacuum secondary assortment.  It was stiff at opening but once moving it opened fairly fast.  A personal favorite back when I ran a lot of those deals.  The initial resistance prevented the floaty idle that you can get with the white spring if the slip linkage is not snug.

The check ball is/was in there so that when the barrels are snapped shut the secondary diaphragm will empty quickly.  Having the slip link adjusted well reduces the need for the check ball.  Together they are sorta like having a belt and suspenders...