Holley 750 vac secondary spring change - Chevelle Tech

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post #1 of 36 (permalink) Old Sep 9th, 10, 10:50 AM Thread Starter
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Holley 750 vac secondary spring change

My Nova has a 750 vac on the 350 in the car. The engine is pretty mild, with a .480 cam, performer rpm intake and 1 5/8 headers. First question is, what is the stock spring for the secondaries, and does anyone really see a gain by opening the secondaries sooner. I have a feeling the carb may be too big already, but setting wide open to be around 6500 on a 5-5200 engine may give it a little more power. If I'm right, the secondaries don't fully open now until like 8000 rpm.

Ray

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post #2 of 36 (permalink) Old Sep 9th, 10, 11:12 AM
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Re: Holley 750 vac secondary spring change

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Originally Posted by bracketchev1221 View Post

My Nova has a 750 vac on the 350 in the car. The engine is pretty mild, with a .480 cam, performer rpm intake and 1 5/8 headers.

First question is, what is the stock spring for the secondaries, and does anyone really see a gain by opening the secondaries sooner.

I have a feeling the carb may be too big already, but setting wide open to be around 6500 on a 5-5200 engine may give it a little more power.

If I'm right, the secondaries don't fully open now until like 8000 rpm.
What's the RPM limit of your engine? Hyd. FT cam & std. springs?

How are you assessing when your secondaries open (or don't open)?
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post #3 of 36 (permalink) Old Sep 9th, 10, 11:20 AM Thread Starter
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Re: Holley 750 vac secondary spring change

The car doesn't have a tach in it, but based on driving it, I would say it's dead by 5500. It's a small cam, you really can't even hear a lope in the idle. They are small springs and press in studs, so the engine isn't much. Stock, this was a 350/300 L-48 car. The engine was just freshened, so i don't expect it to be more than 325 hp at best. I know the vac secondary carb will open as much as the engine needs, to a point. Just wondering if letting them open a little more will make a difference.

Ray

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post #4 of 36 (permalink) Old Sep 9th, 10, 11:23 AM
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Re: Holley 750 vac secondary spring change

factory is either the plain or black springs, i cant remember.
black doesnt open until IIRC 7000+ on a SB and 6000+ on BB
and plain doesnt open fully until 5600 IIRC.

these are all by holleys specs. i will check when i get home.

i can feel a seat of the pants difference going from my 3310 to 4777. it hits the tires so much harder. i used to never have traction issues in 2nd, now i do.

but yes there is a differance in when secondarys open. i cant tell you how much, but when it bogs, put the next heaviest one in.

aaron

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post #5 of 36 (permalink) Old Sep 9th, 10, 11:25 AM Thread Starter
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Re: Holley 750 vac secondary spring change

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Originally Posted by BowtieAaron View Post
factory is either the plain or black springs, i cant remember.
black doesnt open until IIRC 7000+ on a SB and 6000+ on BB
and plain doesnt open fully until 5600 IIRC.

these are all by holleys specs. i will check when i get home.

i can feel a seat of the pants difference going from my 3310 to 4777. it hits the tires so much harder. i used to never have traction issues in 2nd, now i do.

but yes there is a differance in when secondarys open. i cant tell you how much, but when it bogs, put the next heaviest one in.

aaron

This was basically what I remembered about these carbs. Haven't used one in 20 years But that was another idea I had that maybe the car would run BETTER with a 650 dp on it instead of a 750 vac carb. I think it's the plain spring which doesn't really open to quickly, but I know over carbing the thing will slow it down too.

Ray

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Best pass so far 9.034 at 148.10, 1.242 60'
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post #6 of 36 (permalink) Old Sep 9th, 10, 11:37 AM
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Re: Holley 750 vac secondary spring change

It's a 4 speed car with a fair amount of gear out back? If so, doubt the motor can load the carb enough to make it work right. My experience with vac sec carbs is auto with low stall or stick with high gears, they work well. Higher stall converter (3000+) or stick with 3.73+ gears, it will be hard to get a vac sec to work right. Regardless of cam, 350-300 motor, I'd be looking at no more than a 650DP.
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post #7 of 36 (permalink) Old Sep 9th, 10, 11:43 AM
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Re: Holley 750 vac secondary spring change

quick change springs with chart..

http://www.holley.com/data/Products/...199R8219-2.pdf

3310
http://www.holley.com/data/Products/...199R9934-3.pdf

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post #8 of 36 (permalink) Old Sep 9th, 10, 11:48 AM Thread Starter
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Re: Holley 750 vac secondary spring change

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Originally Posted by GRN69CHV View Post
It's a 4 speed car with a fair amount of gear out back? If so, doubt the motor can load the carb enough to make it work right. My experience with vac sec carbs is auto with low stall or stick with high gears, they work well. Higher stall converter (3000+) or stick with 3.73+ gears, it will be hard to get a vac sec to work right. Regardless of cam, 350-300 motor, I'd be looking at no more than a 650DP.
Yup, 4 speed, and 4.11 gears with a 28" tall tire. That's why I think the car is running on just primaries. I thought a 650 dp would be good, but like any other typical "project" it needs a lot of other stuff before a new carb. I've already spent $650 in parts just to go through the brakes and add frame connectors.

Ray

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Best pass so far 9.034 at 148.10, 1.242 60'
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post #9 of 36 (permalink) Old Sep 9th, 10, 11:59 AM
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Re: Holley 750 vac secondary spring change

They come with a plain (silver) spring from factory.
If you have the quick change cover changing the springs is easy.
For your car keep going lighter until it bogs.
I have had best luck running the purple spring, its the next lightest from stock.

Dave
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post #10 of 36 (permalink) Old Sep 9th, 10, 12:19 PM
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Re: Holley 750 vac secondary spring change

Pretty sure the 3310's come with the black spring, the 770 SA's come with the silver/plain. My 454 went faster and faster as I used the next lighter spring. The white spring produced the best et/mph with a stick car no less. Went faster when I got a DP

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post #11 of 36 (permalink) Old Sep 9th, 10, 12:53 PM
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Re: Holley 750 vac secondary spring change

carb is already on the big side for the engine/car combination. Opening the secs faster is not likely to be helpful. I'd be tempted to try the next stiffer spring, get better mid-range.

Tom
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post #12 of 36 (permalink) Old Sep 9th, 10, 1:30 PM
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Re: Holley 750 vac secondary spring change

Ray I've had several cars with combo's very close to that. All of them ran much better with something in the 600-650 area (especially on the street). My auto's worked best with a simple 1850 but a stick would probably work fine with the 650 dp...3310 is too big for that combo...JMO...Dave

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post #13 of 36 (permalink) Old Sep 9th, 10, 1:34 PM Thread Starter
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Re: Holley 750 vac secondary spring change

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Ray I've had several cars with combo's very close to that. All of them ran much better with something in the 600-650 area (especially on the street). My auto's worked best with a simple 1850 but a stick would probably work fine with the 650 dp...3310 is too big for that combo...JMO...Dave
Yeah, that was my guess as well. Just figured if there was anything I could gain from a $12 spring kit for now I would do it. I even had the springs on the shelf and threw them out when I moved, because I didn't own a vac secondary carb.

Ray

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Best pass so far 9.034 at 148.10, 1.242 60'
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post #14 of 36 (permalink) Old Sep 9th, 10, 1:46 PM
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Re: Holley 750 vac secondary spring change

yeah I just gave away all my springs too....that combo will be a lot more fun and responsive with the smaller carb. I tried many time to get the 3310 to work and it's just is too much carb....

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post #15 of 36 (permalink) Old Sep 9th, 10, 4:32 PM
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Re: Holley 750 vac secondary spring change

All the 3310 carbs I used on 350 motors worked just fine. This is Chevy,Oldsmobile and Ford 351W
I usually leave the box stock jetting and always swapped the secondary spring for one of the yellow ones.Stock spring the secondaries will never open.
Last one I used was on an Olds 350 with the Performer cam and it ran mid/low 13's with a 3000 stall and 3.73 gears.
A 3310 is not too big for a 350..the LT1 motors used them from the factory (780 cfm) and all the other motors came with 750 cfm Qjets.
Now a 750 cfm double pumper might be a bit much for a mild 350 with highway gears and a tight convertor.
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