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

[427dart]

Great info here on working with the vac. secondaries...Thanks!

My carbs are the 1850/ 600 CFM modified for the 650/750 sized throttle plates which is basically the Holley 650.
I would think the signal should be a bit stronger with the large throttle bore?

I will look into adding the tube after doing some road test with the White spring in both with the full sized .081 vac. hole which is like removing the ball. As said before the venturi signal is weaker when running 2 carbs.

[Tuner]

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.

Is somebody really used such a configuration? Qjet primary fuel metering is ok(well, for US standards), secondaries are just a bucket with holes on it. At stock configuration primary accelerator pump and primary venturi support quite a lot secondaries, and still it can't be dialed in perfectly - secondaries acceleration shot is from bathtub which has tiny fill holes - when you really drive car and don't just floor it mixture become way leaner than at first WOT.

So why use bad parts from two carbs and leave good parts unused? For low-vacuum accurate metering and emulsioning dual Qjet secondaries are probably worst configuration I could imagine.....

You have no clue. Comparing metering precision, a QJet is to a Holley as a micrometer is to a yard stick. What you (have been lead to) believe is "acceleration shot" is the compensation jet circuit that allows precision mixture correction over the extremely wide range of air flow provided by the large secondary throttle bores.

[Tuner]

here you go

I installed that nipple and took that picture.

[rq375]

I installed that nipple and took that picture.

Do you think the nipples would be of a benefit on 1850s since we are drawing through two, or only the 3310s that it was omitted from in later revisions?

[Tuner]

I installed that nipple and took that picture.

Do you think the nipples would be of a benefit on 1850s since we are drawing through two, or only the 3310s that it was omitted from in later revisions?

Yes, in some cases with 2x4 setups.

An important but mostly overlooked feature of the vacuum secondary function is the offset of the upstream side of the secondary throttle butterflies. The area of the upstream side is larger than the downstream side so manifold vacuum provides a static closing force, the intention of which is to provide a more consistent idle speed and I suppose for safety should the closing link fall off or be forgotten in assembly.

In the dynamic function of opening the secondary, whatever manifold vacuum closing force there is must be overcome to initiate opening. Subsequent to opening and establishment of airflow through the partially open butterflies, the air flow impact against the larger upstream side provides a strong closing force, against which the vacuum servo must have a larger force to fully open the throttles. The airflow impact closing force on the larger area upstream side is greater than the opening force on the smaller area downstream side by the difference in areas of the up and down stream sides of the center of the throttle shaft. You with maths skills and measuring tools can figure out how much is this ratio.

The catch 22 is as the carbs get bigger the larger throttle area has more impact closing force and the larger venturi provides less vacuum signal opening force. This is why the larger carbs came with the booster nipple.

The reason the secondary will not open fully in the 750-780-850 carbs is, as the secondary opens the airflow through the primary diminishes when the flow is divided between P & S, so then with reduced primary flow there is less vacuum signal for the secondary servo, and without the nipple the larger closing force vs. smaller opening force balances at about 1/3 to 1/2 opening of the secondary.

Adding the boost nipple to 600s in a 2x4 setup provides more signal and allows using stronger springs and gives more control of the range from secondary opening to full open. A simple trick is to increase the coil diameter of the heaver springs. This reduces the overall spring rate and the rate of progressive rate increase resulting from the tapered (or beehive) shape. Look at the way the coils stack as the spring compresses and play with a coil spring calculator some and you will get a glimpse. If you have a small electronic scale you weigh floats with you can compare springs and compress springs on the scale pad and watch how the rate changes.

Blow through the primary with a shop air blow tip, throttle open, across the vacuum port or nipple, while you try to hold the secondary closed. Ha Ha.

[427dart]

Well took the Lil' Beast out this evening to my test area and setting up the rear carb like I did the front...ALL IS WELL!
It all comes together now and does pull harder quicker than before.
From a good low speed roll and around 3000 RPM in 2nd gear you nail it hard to the floor, the engine revs up than a harder pull as the whole car comes up on the suspension...the drag radials start to let go with a very slight slide to the right and the revs shoot up due to traction loss...time for 3rd gear! Now I'm only running a 3.50 rear gear which is perfect for the car weight and engine torque. Anything lower and just earlier tire spin.

Perfect for me for the little bit of fun I have!

[pamotorman]

would adding a angle cut nipple to the secondary side venture increase the vacuum pull when the secondary starts to open since it is there to increase the vacuum pull on the diaphragm as it opens ??

[427dart]

Would be interesting to hook a vacuum gauge up maybe Tee into the balance hose and see how much it is pulling!

Another thought....have a vacuum storage can with switch operation at throttle linkage to dump in as an assist?

[cjperformance]

Great info here on working with the vac. secondaries...Thanks!

My carbs are the 1850/ 600 CFM modified for the 650/750 sized throttle plates which is basically the Holley 650.
I would think the signal should be a bit stronger with the large throttle bore?

I will look into adding the tube after doing some road test with the White spring in both with the full sized .081 vac. hole which is like removing the ball. As said before the venturi signal is weaker when running 2 carbs.

Re: 302 SBF stumble
Post Fri Jun 29, 2018 9:35 am

427dart - with some mucking around maybe you could mount 2 carbs sideways but yes you look very tight for room.
But, What you can do is use a 45 or thereabouts squirter, a 50cc acel pump kit and a cam to come in gently but pump to or almost to WOT then modify the secondary linkage to operate mechanically OR if a class states VS must be used (enter cheat here!) with some well disguised vac lines , elec over vac switch and a vac tank you can have Vac operated secondaries that will follow the primaries at whatever rate you like!
Craig.

^^

[Tuner]

would adding a angle cut nipple to the secondary side venture increase the vacuum pull when the secondary starts to open since it is there to increase the vacuum pull on the diaphragm as it opens ??

Seems reasonable, I don't see why not. I've never done so or felt a need to, but it would be interesting to see in an application like the OP's. A nipple in the primary of the smaller (than 750+) 600 venturi will speed it up plenty.

The bleed in the secondary is there because without it the secondary would be open every time you crowd the throttle a little bit, whereas it doesn't really need to open until you are beyond about 1/2 throttle.

Also, without the secondary bleed the secondary opens with too little primary opening, which will reduce the airflow in the primary and the secondary opening signal with it, so the secondary will close, which will increase the primary airflow and signal so the secondary will re-open, repeat ad infinitum or until the primary is opened further or closed.

[Tuner]

Would be interesting to hook a vacuum gauge up maybe Tee into the balance hose and see how much it is pulling!

Do so, report what you find. This will be more illuminating if you have a gauge calibrated in inches H2O.

Another thought....have a vacuum storage can with switch operation at throttle linkage to dump in as an assist?

The venturi vacuum is more than enough. Have you tried blowing shop air across the primary orifice to see how hard it pulls on the secondary? I think you will be surprised.

A vacuum reservoir and throttle valve is how the GM Tri-Power systems function on the 50's and 60's on 348 Chevy, Olds, Pontiac and Cadillac. They have a vacuum reservoir tank and a vacuum switch on the center carb with a large single diaphragm pulling open the linked together end carbs. When a system aged and hoses got soft and started having slow leaks so the vacuum bled off while at WOT for a few seconds or (a while) you would need to let off and close the carbs to get a shot of intake vacuum to reload the vacuum tank. This frustrated people who would then remove and sell the intake and carbs and fail to include the vacuum reservoir, so the next guy couldn't get the end carbs to open at all and so the systems were scrapped by idiots or converted to manual linkage which required changing the throttle shafts to accommodate the necessary levers. All such nonsense did increase the value of the survivors.

[427dart]

Whenever I rebuild a vac. secondary carb I check for secondary operation by taking the suction hose of my shop vac. and placing it under the primary bore with the signal hole. Works really well!
As of last evenings road test seems I may have a good combo on the secondary operation.

This is all about fun on the street as I haven't run it at the track in several years.

[F-BIRD'88]

You should get yourself a radar gun or a Vericom (VC200) ...or later
to evaluate your street blast perf tuning. The butt dyno can be very deceiving. You can measure perf gains/// losses over any test length
Either standing start or rolling start test between speed a and speed b etc.
Or distance a to distance b. Cheap used....

[pamotorman]

Would be interesting to hook a vacuum gauge up maybe Tee into the balance hose and see how much it is pulling!

Do so, report what you find. This will be more illuminating if you have a gauge calibrated in inches H2O.

Another thought....have a vacuum storage can with switch operation at throttle linkage to dump in as an assist?

The venturi vacuum is more than enough. Have you tried blowing shop air across the primary orifice to see how hard it pulls on the secondary? I think you will be surprised.

A vacuum reservoir and throttle valve is how the GM Tri-Power systems function on the 50's and 60's on 348 Chevy, Olds, Pontiac and Cadillac. They have a vacuum reservoir tank and a vacuum switch on the center carb with a large single diaphragm pulling open the linked together end carbs. When a system aged and hoses got soft and started having slow leaks so the vacuum bled off while at WOT for a few seconds or (a while) you would need to let off and close the carbs to get a shot of intake vacuum to reload the vacuum tank. This frustrated people who would then remove and sell the intake and carbs and fail to include the vacuum reservoir, so the next guy couldn't get the end carbs to open at all and so the systems were scrapped by idiots or converted to manual linkage which required changing the throttle shafts to accommodate the necessary levers. All such nonsense did increase the value of the survivors.

i converted my 1958 348 impala to the progressive mechaincal linkage with out even taking them off the engine. i used the sliding rod type like GM used on the 2 X 4 carb corvette setup. i had to fabricate a few parts.

[pdq67]

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.

F-BIRD'88,

Have you actually ran this configuration before?

Just the back barrels of the front Q-Jet and all of the rear Q-Jet??

How did it run if you have?

Very interesting design concept in my book.

Kinda like running four 2GC two barrels on a flat cross-ram intake with each one of them set 90 degrees off so that a barrel is over each cross-ram throat and not set like a regular in-line 3x2 set-up. Think an old modified, "Man-A-Fre", 4 two barrel carb'd intake....

pdq67