Carb Sizing/vacuum or Mechanical

[Horder]

Well, the '66 L-79 Chevy2 Nova had a single inlet 585 VS Holley 2818 and ran really great.
That was the carb the Grump used in his record winning 66 Nova Grumpy's Toy.

On the previous engine in my car, 327, 461 iron heads and a Comp Magnum 280 cam, I could never get a Holley 600 vacuum to run as good as the 600 Double Pumper. Tried all I could and it just loved the mechanical secondary.

This engine likely makes 75HP more. At least thats the waynit feels in the car.

[lefty o]

id be looking at a 650dp, or 750vs.

[Tom68]

Well, the '66 L-79 Chevy2 Nova had a single inlet 585 VS Holley 2818 and ran really great.
That was the carb the Grump used in his record winning 66 Nova Grumpy's Toy.

On the previous engine in my car, 327, 461 iron heads and a Comp Magnum 280 cam, I could never get a Holley 600 vacuum to run as good as the 600 Double Pumper. Tried all I could and it just loved the mechanical secondary.

This engine likely makes 75HP more. At least thats the waynit feels in the car.

Yer, I bought the Holley vac sec spring kit for the 600, ended up grinding the soft spring and gouging a bigger trough under the check ball, snapped the secondaries in then.
Only ran the Vac because I started with a Powerglide with kick down, once I went manual shift valve body I put the 850 on.

[Horder]

I always felt a 650 Double Pumper was the perfect carb for this car. Not because Im an expert, just my gut feeling. Then I wondered if there was much to be gained with a 650 Dp versus my 600 DP.

We have a highly respected carb guy in our city. He always told me a 750 was the answer.

[Tom68]

750, depends what you want, 650 is fine, a 950 will work to.

I'd put 1 to 1 linkage on your existing carb and see what you think of it, that'll give you better throttle control as long as it isn't too chirpy off idle then.

Don't forget the smaller Quadrajets were something like 750cfm.

[HQM383]

We have a highly respected carb guy in our city. He always told me a 750 was the answer.

Maybe you could work something out with him to loan a 750 to see how you like it? In his interest if you are to be a potential customer.

I agree with Colaradoracer’s first post that a lot has changed over the years that can allow carb size beyond the old formulas to work effectively. It has to be remembered that carburetors are not cfm devices but air/fuel density regulators. If a larger size can supply a higher air/fuel density at any given rpm while still providing a satisfactory homogeneous mix with minimal delay in response to throttle angle change (does not need excessive acceleration enrichment) then it will always do better performance wise than a carb with a smaller cfm rating. There are going to be upper limits where that becomes difficult to accomplish with diminishing returns, eg the old 850cfm dimensions of 1.56” venturi X 1.75” throttle bore would be finicky to tune on your engine to out do a 750 on the street.

Different carb dimensions bring on the change of air/fuel density at different rates, especially with throttle bore sizes. In this respect on the street when the throttle is modulated a lot, pinpointing a size through theoretical methods should take a backseat to driver preference.

So see if you can borrow various sizes to feel the difference (yes, I know they won’t be dialed in exactly to your engine) and when settled on get yourself something in the preferred size and fine tune. In the meantime Toms suggestion of trialing 1:1 linkage of your current carb should give you informative feedback.

[KnightEngines]

If it was mine I'd put a good ol plain Jane 4779 750 square bore double pumper on it.
If I felt like getting fancy I'd drill & tap for screw in bleeds.

4779 has 4 corner idle & downleg boosters, underrated carbs with the classic look you want.

Motor *should* be in the 410-430hp region, with a dual plane that's smack bang where a 750 does its best work.

[Horder]

Really appreciate all of the advice and information. I have access to a 750 VS that I could borrow and tune. That wont be until the warmer months here in Canada. Where I am it will be April before I get the car out again.

As well, A friend has a new 4777 650 Double pumper that I can buy for a real good price. My fun is always to make my car run as good as it can for what it is. Just figured Id start with the right carb before putting in the time.

The “right carb” is up for debate obviously.

[rgalajda]

I looked at the picture of your intake manifold. Not much of the centre wall is cut down, so what I said still applies.
Things have not changed much in most Holley carbs within the cfm range we are talking about unless you include annular booster carbs.
For a street car ,like yours, I am not sure why anyone would consider 1:1 linkage.
When people say they don’t care about gas mileage, consider this. A carb that is tuned to correct A/F ratio for street driving , throughout the entire operating range , will always perform better and return better fuel mileage .
Just because successful racers use mechanical secondaries , many street performance enthusiasts tend to regard vacuum secondaries as something of a downgrade. Whereas this is far from the truth.
On the opposite side of the coin, a vacuum secondary is of little or no advantage when the stall speed of the converter is above the rpm at which a vacuum secondary comes in.
If torque output for a street engine ( aprox below 4000 rpm for smaller engines and 3000 rpm for 400+ inch engines ) is part of the engines operating range , then a vacuum secondary carb is worth looking at.
You can choose a slightly larger vacuum secondary carb without any penalty at the low end.

As far as tuning, the internet is full of issues tuning ( Holley ) mechanical secondary carbs.
I know the whole subject of vacuum secondary vs mechanical secondary is full of opinions .
For those who are dead set on mechanical secondaries there will be no reasoning that will change there mind and that is fine with me.

[rfoll]

"there will be no reasoning that will change their mind"...Is there anything this doesn't apply to?

[Horder]

Good info!

The 600DP that is on presently works well. No bogging. Does use some fuel but I drive the car with some pedal so I expect that. Its one issue, maybe not solely the carb, is that its a touch weak just off idle on clutch engagement. I have a Holley 108-12 .325” open gasket under the carb that I may take out to see if that helps.

This is my first double pumper. And honestly it just felt right vs the vacuum secondary that I had used on the previous engine. I did test and tune both the Holley 1850-2 and the 4776 DP. I tried different secondary springs, etc. the secondaries were opening fully. But, my rolling burnouts were always better with the DP. Seat of the pants dyno was improved as well.

I have no issues going to a 750 VS if I can benefit with a stronger engine package.

[lefty o]

"there will be no reasoning that will change their mind"...Is there anything this doesn't apply to?

religion, politics, carbs.. nope it fits 'em all!

[Alaskaracer]

Don't forget the smaller Quadrajets were something like 750cfm.

Early ones were. Later versions were 800 cfm and used on all sizes of engines when equipped with a four barrel carb.

[Horder]

Anyone try these Edelbrock VRS-4150 carbs? Taking some decals off and toning the looks down a bit might work for me. They seem to have all of the bells and whistles.

[Tom68]

Edelbrock looks good, the 750 ?