Vac. secondaries tuning - I want it all and right now!

General engine tech -- Drag Racing to Circle Track

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427dart
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Vac. secondaries tuning - I want it all and right now!

Post by 427dart »

Is what my engine seems to be telling me. Since we have a couple days of lower humidity 85 degree weather,I was doing more tuning on my vac. secondaries on the dual 650 Holleys.
First test was to remove my .060 sized orifice from the sec. pots and go from the weakest White to purple spring. This gives the full sized .081 hole like having no check ball. Did this on both carbs and but zip tie on secondary diaphragm rod with just enough tension to let it slide.
Result in a 3000 RPM roll in 2nd to 3rd gear up to 5500 RPM was no bog and not quite the strong mid range pull...didn't notice tires breaking loose in middle of 2nd. gear. Zip tie inspection shows about 1/2 open on secondary throttle.

Second test this morning was back to White spring in front carb only since it is easier than the rear carb and same full .081 vac. orifice.
Result was no bog or sense of secondary flopping open just a stronger pull with the rear end of Mustang doing a slight slide to the right at mid point in 2nd gear. Felt real good in 3rd gear.

May switch out the rear carb spring back to the original weak White spring and see how it goes but this seems to be telling me at 2950 lbs and with only a 3.50 gear on 275-50 M/T drag radials the 427 wants all it can get as soon as possible.

I WOULD run a pair of 650 mechanical secondary carbs IF somebody made them with the short 4160 bowls and a passage way for the secondary accelerator pump shot. Doing the race only Center Squirter won't work that well on the street due to excess fuel puddling in secondary barrels everytime you work the primary throttle during part throttle.

Anyway it runs really strong on the street which is where I'm at now days. Note my test area is a industrial park road with fresh new pavement.
Last edited by speedtalk on Sun Jul 15, 2018 12:15 pm, edited 1 time in total.
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turbo2256b
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Re: I want it all and right now!

Post by turbo2256b »

just wondering if you use the equalizer caps for the vacuum secondarys
427dart
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Re: I want it all and right now!

Post by 427dart »

turbo2256b wrote: Sun Jul 15, 2018 11:00 am just wondering if you use the equalizer caps for the vacuum secondarys
If you mean the secondary balance hose...yes.
F-BIRD'88
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Re: Vac. secondaries tuning - I want it all and right now!

Post by F-BIRD'88 »

First: Using "the butt dyno" to evaluate the performance is a bit allusive.
I'd get a $100 baseball radar gun and look for max MPH over that run length to evaluate
+/- perf changes.

....

Some day you will try a set of Edelbrock carbs on it and be all set.
The 650 AVS2 carbs have fully adjustable secondary air doors.
Would be cool on a 2x4 carb adapter, on your racing single plane intake manifold.
pdq67
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Re: Vac. secondaries tuning - I want it all and right now!

Post by pdq67 »

Install a GOOD Q-Jet and then spend some time tuning the rear top air door!

pdq67
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Re: I want it all and right now!

Post by Casper393W »

I can tell you, tuning vacuum Secondary carbs can be fun or a nightmare.... Now on my tunnel ram On the Dyno we couldn't get the Secondaries to open fast enough..even with the lightest springs...

In the truck, it was the complete opposite! I had to put Brown springs in the pots so they wouldn't just slam open open!

It is a monster now! Rolling at 40mph in second gear it will roast the tires 275/60/15's if you stand on it!

I will say this. I have a matched set of Holley 4224 660 center squirters... While they are horrible at low speed driving due to no power valves... The WOT response is unbelievable! It feels like they added another 30-40hp over the vacuum carbs. Im still kicking myself because when we dyno'ed the Engine, my original plan was to take both sets of carbs and do a back to back test... I left them setting on the table at the shop...

I have to do some rear suspension work on my truck ( CalTrac bars) before I can hit the strip... I plan on running both sets at the track so I can see what difference they will make in ET.

I have thought much about vacuum Secondary carbs
People say the engine dictates the airflow opening rate of the Secondaries.. that isn't so... The engine will pull the secondary blades open so fast the engine will bog right? What creates the bog? The huge lean spot by the changing pressure differential, the carbs lack of being to add the additional fuel to cover up this lean spot.
I would love to see a Vacuum Secondary double pumper! Why? Because I believe in certain applications your vehicle would accelerate faster by "giving the engine what it wants". Because with a mechanical carb the Secondaries are going to open regardless if the engine needs the additional airflow or not.." result is reduced mid-range torque". With this setup the Secondaries could open at a faster rate than a vacuum carb could currently thus allowing the engine have the proper amount of air mixture at the right time. Like you have stated 427dart in most dual quad apps there isn't enough room to create this. I am going work on this concept for my setup... Stay tuned!
DanE1
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Re: Vac. secondaries tuning - I want it all and right now!

Post by DanE1 »

I had a 735 cfm vs Holly carb professionally prepared.
I wasn't happy with the rate of deployment of the secondaries.
I don't remember what spring I used, however, I removed the check ball, put a phillips screwdriver against the ball seat, and hit the screwdriver a solid wack, leaving 4 dents in the ball seat. Reinstalled the ball.

Then on the drivers side of the carb, I put a screw in the linkage in such a manner that when the primaries were about 2/3 open, the secondaries were opened by the screw.

Worked great and throttle response was instantaneous.
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Re: Vac. secondaries tuning - I want it all and right now!

Post by NormS »

I figured out why most Holley vacuum secondary carbs have the reputation for making less power than the equally sized double pumpers. Most of the vacuum sec carbs do not open the secondaries all the way, and some don't open them at all. I figured out how to fix this issue, and YES, you can open the secondaries real early without a bog. One of the first vacuum sec carbs I built after figuring out the fix, was a modified 600 with a 780 baseplate(840 cfm) for a 400hp 355 street small block. The secondaries began opening at 120 cfm, and were fully open by 290cfm. You could stand on it from barely above idle in first gear, and it would come alive with no bog. The feeling in the car was that of 4 barrels being snapped open simultaneously.
On a dual quad set-up, opening the secondaries that early might have an issue, but I think they could all be fully open by 400-450 cfm, particularly on
your larger engine.
Competition Fuel Systems Birch Run,MI. www.compfuelsystems.com/index.html 520-241-2787
treyrags
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Re: Vac. secondaries tuning - I want it all and right now!

Post by treyrags »

Do the carbs have the brass "signal tube" in the primary venturi that helps open the secobaries?
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Re: Vac. secondaries tuning - I want it all and right now!

Post by F-BIRD'88 »

I also did the trick with the Phillips screw driver under the check ball. It helped.

But you really need to evaluate perf+\- changes based on et and or mph +/- gains to find the best sweet spot.

The air cleaner has an effect on the secondary opening rate and quality of.....
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Re: Vac. secondaries tuning - I want it all and right now!

Post by F-BIRD'88 »

pdq67 wrote: Sun Jul 15, 2018 12:33 pm Install a GOOD Q-Jet and then spend some time tuning the rear top air door!

pdq67
Or better a pair of qjets run in a Six pack configuration.
(Front carbs primaries are not used, nore exposed to the plenum.)
4 big secondaries with central small responsive primaries.

A pair of edelbrock afb's could also be run in this Six Pack configuration (6 venturi) on a 4 bbl intake.
Last edited by F-BIRD'88 on Sun Jul 15, 2018 1:40 pm, edited 1 time in total.
427dart
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Re: Vac. secondaries tuning - I want it all and right now!

Post by 427dart »

I think a lot of it has to do with the whole carb/manifold and cam combo. The air velocity thru the intake and carb will determine how fast the boosters deliver the fuel mix. Now my carbs still have the old style straight leg booster. Swapping in the dog leg style which is supposed to be a click more responsive could be a benefit for faster booster action maybe.

I looked at the Eddy 650 AVS 2 carbs but didn't care for the different sized throttle bore/venturi from primary and secondary. I like same sized throttle bores all around.

A short bowl Holley with double accel. pumps COULD be made but no big company will do it...not enough demand.
The challenge is just mapping out a accel pump passage into the secondary side of a 4160 style carb body without running into another passage and of course have the squirtter nozzle boss in the main body to.
427dart
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Re: Vac. secondaries tuning - I want it all and right now!

Post by 427dart »

treyrags wrote: Sun Jul 15, 2018 1:19 pm Do the carbs have the brass "signal tube" in the primary venturi that helps open the secobaries?
No these don't. Just the hole.
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Re: Vac. secondaries tuning - I want it all and right now!

Post by F-BIRD'88 »

I looked at the Eddy 650 AVS 2 carbs but didn't care for the different sized throttle bore/venturi from primary and secondary.

The running engine sees it differently.
427dart
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Re: Vac. secondaries tuning - I want it all and right now!

Post by 427dart »

Now back in the day....1970 my brother had his new Hemicuda with of course the dual Carter 625 AFB's.
With the Torquflite auto and 4.10 Dana we would be cruising at 50 MPH and pull the shifter down to 2nd and hammer it at same time and there would a be a slight lag then a whomp of a power surge as you were pushed into the seat....hang on!!
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