[James408]

On a Holley 3310, when using the different springs, I know that the lighter the spring the sooner the secondaries come in. I've seen the spring chart which shows, the rpm that the secondaries come in and are fully open.

My question is do the different springs automatically mean they will be fully open at a certain rpm according to the chart or does it depend on the air flow needs of the motor?

for example, lets say the purple spring(according to the chart) is not fully open until around 6000 rpm. Depending on the motor, could the secondaries upon all the way prior to 6000 rpm?

[gtome]

Thats a good question! I never really thought about it before. I always just went lighter on the spring until there was a bog, then went back one spring.

[Traker82]

Is there an advantage to mechanical secondaries as apposed to vaccum ones? Just trying to get a little education.

[gtome]

Maybe mechanical would have an advantage for drag racing, but for street driveability, vac would be better.

[rolling money pits]

Originally Posted by Traker82

Is there an advantage to mechanical secondaries as apposed to vaccum ones? Just trying to get a little education.

yes, to race cars and gas stations.

i had a vac secondary holley way back when, and hated it. i felt i could never get the secondaries tuned in right.

to your original question, i would not take holleys numbers as gospel, more as a guide.

you'll have to kind of tune it with your butt dyno, put the lightest one in and do a blast. then the next stiffest one and repeat. keep going until you get the smoothest acceleration said butt dyno feels. it may never be "perfect", but you should be able to get close.

if your carb isnt already so equipped, i would invest in a "quick change" kit for the secondary spring diaphragm.

[77 TRASHCAN]

Maybe Senior Holley, (Tom V) will give us all the perfect advice, here soon...

[ericwy]

Originally Posted by gtome

Maybe mechanical would have an advantage for drag racing, but for street driveability, vac would be better.

I gotta disagree with you on this one. I ran a VS Holley on my daily driver TA for 9 years. I swapped it for a DP in 2008. Improved the mid-range rpm performance due to eliminating the delay in opening of the secondaries. The VS just can't open as quick even when dialed in perfectly because it requires venturi signal to operate them(built in delay). Also, Unless you can make your own secondary springs to open at the exact right time/rate you end up using one spring stiffer than the bog spring, thus they open later than optimal.

Mechanical open to WOT instantly. Only thing you have to do is tune the pump shot.

For reference both my Holley carbs are 4165/4175 spreadbore models nearly identical in design and flow (650cfm). The main difference is the secondary operation so its a very good comparison.

[James408]

Yes Tom where are you?

[77 TRASHCAN]

He's probably towards the end of a "normal" 12 hour day......

[gtome]

I would think with vac secondaries, it would take fuel as it needs it, instead of being forced fuel when it didnt.

[Tom Vaught]

Originally Posted by 77 TRASHCAN

Maybe Senior Holley, (Tom V) will give us all the perfect advice, here soon...

Did a test one time on a friend's 65 GTO clone. It was built with a 455 engine and turbo 400. It would run 11.20s very, very consistently.

We tested a 4781 Holley 850 Double Pumper (his normal carburetor) against a 850 vacuum secondary carb designed for a BB Chevy Corvette that I had tuned. We did the testing at Milan Dragway, in Michigan.

The 850 Vacuum Secondary carburetor had identical 60 ft times and ran with-in .04 seconds of the 850 DP carburetor over several runs. Deadly repeatable.

That being said, given some effort, I believe that you can make a vacuum secondary carburetor as quick and fast as a double pumper carb. The carb has to be sized properly for the engine and the pump shot and diaphragm springs tuned properly.

Tom Vaught

The car mentioned above has won either the Saturday or Sunday street car race at Norwalk several times.

[PMDRACER]

Originally Posted by James408

On a Holley 3310, when using the different springs, I know that the lighter the spring the sooner the secondaries come in. I've seen the spring chart which shows, the rpm that the secondaries come in and are fully open.

My question is do the different springs automatically mean they will be fully open at a certain rpm according to the chart or does it depend on the air flow needs of the motor?

for example, lets say the purple spring(according to the chart) is not fully open until around 6000 rpm. Depending on the motor, could the secondaries upon all the way prior to 6000 rpm?

Yes.

Before I was enlightened to the wonders of the Q-Jet, I ran VS Holleys.

The VS carb basically works that when you mash the gas, only the primaries open mechanically, and vacuum drops to nearly zero. As rpms climb, the amount of air (air/fuel) needed rises, which is shown as vacuum. A port in the primaries 'sees' this vacuum and feeds the secondary dashpot. It is this vacuum that works against the spring. Lighter spring = less vacuum needed to start opening the secondaries.

So, generally, a bigger motor will pull vacuum sooner, and open the secondaries at a lower rpm.

IIRC, that chart is based off a Chevy 400. The chart should say so.

EDIT:
Here is the chart for the 4150/4160. It gives the rpm numbers for a Chevy 350 and 402. Notice the rpm differences while using the same spring color.
www.sportcompactonly.com/manuals/Brand/Earls/199R8219-2.pdf

[gtome]

Would an engine that normally makes say 20" of vac pull the secondaries open sooner than an engine that normally makes say 7"?

[screamingchief]

Air cleaner restriction will also play into how the secondaries open on the holley VS carbs.

They (holley) note that in their tuning info for the secondary spring kits.

The more restrictive the air cleaner is,the earlier the secondaries will open as SOP.

And yes,larger engines tend to open the secondaries earlier as SOP too.

Info included below vvvv.

HTH

Bret P.

[BruceWilkie]

Quick-Fuel makes an excellent adjustable vacuum secondary canister for dialing in the opening rate to exactly what you want/need. Adjusts when it opens and rate it opens.

I've seen plenty of dyno tests over the past 40+ years showing equal hp size for size between mechanical and vacuum secondary carbs. My old trusty 3310-1(780) has been on alot of combos(not just mine). Its been very consistant run to run and it has more often than not shown better 60 ft times and et as it didnt shock the tires as hard at launch.

[Cliff R]

"i had a vac secondary holley way back when, and hated it. i felt i could never get the secondaries tuned in right."

+1, 2 and 3!

I ran one on my 440 engine that powered my 1970 Roadrunner. I raced it nearly every weekend.

I had the spring kit, and tried every single spring in there, and clipped them a half coil or so at a time, making countless runs with it searching for best performance.

One day at the track I ripped the diaphram. Not having a spare on hand, I went ahead and made the next round.

The car ran the fastest it ever had by quite a margin!

Soon after, I dumped that carb and ran a 4781-2 850 DP. It was just a tad faster, and more consistant than the vacuum unit.

A few years later I discovered that you can run just as quick with a Q-jet, and have FULL control of the secondary opening rate, with a small allen wrench and screwdriver.

I never messed much with Holley's afterwards, only to confirm to myself that they are no better on the dyno or at the track to a well prepared Q-jet......Cliff

[James408]

[QUOTE=BruceWilkie;4214490]Quick-Fuel makes an excellent adjustable vacuum secondary canister for dialing in the opening rate to exactly what you want/need. Adjusts when it opens and rate it opens.

Very interesting, I'm going to have to look into that one.

[rolling money pits]

Originally Posted by BruceWilkie

Quick-Fuel makes an excellent adjustable vacuum secondary canister for dialing in the opening rate to exactly what you want/need. Adjusts when it opens and rate it opens.

I've seen plenty of dyno tests over the past 40+ years showing equal hp size for size between mechanical and vacuum secondary carbs. My old trusty 3310-1(780) has been on alot of combos(not just mine). Its been very consistant run to run and it has more often than not shown better 60 ft times and et as it didnt shock the tires as hard at launch.

is this what youre talking about?

http://www.quickfueltechnology.com/m...c-housing.html

thats an interesting item, does it function as a controlled vacuum leak? i see that little round "port", and that makes me think it's a vent, in some way tied into the adjustment screw.

[gtome]

I have never needed anything extra to get my Holleys to work well on the street.

[James408]

I guess my next question is what if your car needs a stiffer spring on take off but a lighter one to allow secondaries to open all the way within your rpm range. How do you tune a vs holley for that?