Carb Sizing/vacuum or Mechanical

[Horder]

Put a new engine in my 66 Chevy II last season. Got all the bugs out and its been running very well. The car is an all street car, never been to the track and likely wont. Its a fun car that a drive a couple times a week in our summer months, April- October. It gets driven hard for fun on some quiet safe roads.

Before I dig too deep into tuning the existing carb I wanted to get opinions on the carb Im using presently and weather its worth while to change to something else. Here are the details

66 Chevy II Nova 2 door hardtop
Small journal Chevy II recessed oil filter block
.040” 327, forged and balanced rotating assembly
Trickflow DHC aluminum heads 258cfm int 208ex @ .500”
10.2-10.3:1 with .038 quench
Retro Roller 273/278 adv 223/225 @.050” 110 LSA installed at 105.5 ICL .544/.525 lift
Scorpion 1.6/1.5 rollers. Trend Pushrods
1967 Z28 Winters aluminum dual plane
Holley 4776 600 CFM DP
MSD distributor. 18 initial 18 mech 36 total, 10 deg vac adv
1 5/8” headers, 2 1/2” exhaust
Muncie M21
12 Bolt 3.73 Posi
26” street tire

Engine revs well to slightly over 6000 Rpm. I have not taken it much over yet. Pulls hard and roasts the street tires easily

Some tell me the 600 is too small. Some say a vacuum sec carb world be better. I dont really care about gas milage. Its my hobby. I enjoy making it run a bit better/ a bit faster.

Lots of smart guys here and I appreciate all the help Ive gotten and would love to hear opinions on what you would run if you had this combo…

Thanks in advance…

[Alaskaracer]

If it's running well with what it has, leave it. You might gain some going to a bigger carb, but considerable tuning may be required for little gain...Here's how to tell if you have the right carb on it, and most will disagree with this, but this is coming from people that build carbs for a living, not me:

Hook up a vacuum gauge and make a wot pass. Note the peak vac reading....any measurable vacuum below the carb at wot is power left on the table...Some people are still stuck in the past on carb sizing/function. Carbs to not work based on depression below them, they work based on airflow THROUGH them....the boosters create the needed depression to pull fuel. Carbs aren't the same now that they were many years ago, despite "looking" the same....changes to metering block design, booster design, etc. are very different now than in the past, even though you can't "see" the differences in most carbs....You can actually run a much larger carb than is generally accepted, and maintain the same response, drivability, and fuel mileage that you have with a smaller carb. But that also doesn't mean it's always needed.

Like I said, if it's running well, there isn't really any reason to change it, as the cost of a new carb plus tuning it isn't worth the gain you're likely to see....I'd leave it as is and have fun.....but that's just me

[Bigchief632]

A Quick Fuel Slayer 750 vacuum secoddary carb would wake it up. Those work very well out of the box, and usually don't need any jetting. You can fine tune it obviously, but they work well otb. You can dial the secondary opening rate to your liking, and set idle and it'll run good. Pretty inexpensive in the carb world these days. They've gone up in price lately, like anything, but are still reasonable. Or if you prefer the mechanical secondary, get the Brawler. A little more money but work well also. I wouldn't go any bigger than a 750 though, it's not needed on your combo.

[Horder]

Thanks for the suggestions. A couple of things I should have mentioned. I really would like a carb that keeps that classic holley look/ colour. Not looking for shiny or chrome. This is to keep the factory exterior appearance of my engine. The hump heads are painted orange and the goodies inside cant be seen. The other is, I don't mind spending some money on a new carb. Especially if there are some gains without much loss in street manners.

As well, if my current 4776 Holley Double Pumper is the right one, Ill just spend my time tuning it to the best of my abilities. Its fun for me, not a chore. But, if Im gonna go through the effort Id rather start with the best carb for my application. I do like a car that gets after it quickly once the pedal is stepped on. Thus the smallish 175cc heads and 275ish cam with fair lift.

[rgalajda]

Those 302 z28s used a 780 VS holley from the factory.

The main reason you can use the bigger carb is the intake manifold . That intake manifold is a true dual plane , fully divided. One plenum is completed separated from the other.
This means each cylinder only sees about half the carb cfm. ( in simple terms )

I have used the Quick Fuel 770 cfm VS Brawler and was very impressed. https://www.holley.com/products/fuel_sy ... s/BR-67258

Expect to reduce the main jet from #72 to # 70 . This is normal with any aftermarket carb as they are all a little too rich especially at cruise mode.

[rgalajda]

"I really would like a carb that keeps that classic holley look/ colour. Not looking for shiny or chrome"

In that case the 3310-1 780 cfm ( if you can find a good one )
or this https://www.holley.com/products/fuel_sy ... ts/0-80531

[Tom68]

Manual so stick with DP.

600 is a bit small for a good 327.

[rfoll]

"I really would like a carb that keeps that classic holley look/ colour. Not looking for shiny or chrome"

In that case the 3310-1 780 cfm ( if you can find a good one )
or this https://www.holley.com/products/fuel_sy ... ts/0-80531

I totally agree. The early 3310 carbs had down-leg boosters, and they are vastly superior to the booster on the 4776 and 4777 carbs. With those vacuum secondary carbs you can easily dial in opening rate to suit the needs of the motor. The larger primaries could easily be a benefit Nothing to lose, everything to gain.

[Horder]

Those 302 z28s used a 780 VS holley from the factory.

The main reason you can use the bigger carb is the intake manifold . That intake manifold is a true dual plane , fully divided. One plenum is completed separated from the other.
This means each cylinder only sees about half the carb cfm. ( in simple terms )

I have used the Quick Fuel 770 cfm VS Brawler and was very impressed. https://www.holley.com/products/fuel_sy ... s/BR-67258

Expect to reduce the main jet from #72 to # 70 . This is normal with any aftermarket carb as they are all a little too rich especially at cruise mode.

The Z28 isnt fully divided as they have about a 1/8” section milled out of the divider. As well, mine has a small notch cut into it.

821B2829-238E-4747-9E62-BB0FF8B50C98.png

[Tom68]

The main reason you can use the bigger carb is the intake manifold . That intake manifold is a true dual plane , fully divided. One plenum is completed separated from the other.
This means each cylinder only sees about half the carb cfm. ( in simple terms )

I went from a C3bx 600 vac sec on a 327 to a tarantula with an 850 DP.

The single plane wasn't ideal but the carby didn't pose any problems.

[ClassAct]

"I really would like a carb that keeps that classic holley look/ colour. Not looking for shiny or chrome"

In that case the 3310-1 780 cfm ( if you can find a good one )
or this https://www.holley.com/products/fuel_sy ... ts/0-80531

I totally agree. The early 3310 carbs had down-leg boosters, and they are vastly superior to the booster on the 4776 and 4777 carbs. With those vacuum secondary carbs you can easily dial in opening rate to suit the needs of the motor. The larger primaries could easily be a benefit Nothing to lose, everything to gain.

I don’t know about your booster comments. If you are using a heated intake manifold the boosters used in the 4776 and 4777 work very well.

[rfoll]

I'm not suggesting the straight leg boosters don't work, but there is a reason most of the aftermarket HP stuff uses the other type.

[amc fan]

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.

[Horder]

The FelPro 1256 gasket I use has metal plates with small restrictor holes for exhaust crossover.

[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.

But of course his recommendation for sub 4000rpm 325 to 350 small blocks was a C3-B with a 715 Holley.

In fact he points out that having to use a small carb for class racing creates lots of problems with fuel fallout because of the high air speed through the carby.