cal camara

I had this wrong forum: I was told so here it is again.

I have several carburetors, (as most people), and need a 750 for a build I am working on.

The question are:

1. Is a modern carburetor better than the older 70's technology such as a Holly double Pumper rebuilt correctly?

2. Will the new perform better?

3. Is it worth the extra cost?

Thank you
PS. Already got two FI's, going carb on this one.

7T1vette

Carburetors ALL function in like manner: they attempt to atomize the fuel/air mixture in the proper proportions throughout the entire operational band of the engine.

In that regard, all carbs (that are built correctly and not damaged) are capable of doing a decent job. But, some are a bit better than others on specific features:

Holley carbs are easier to work on than a Q-Jet; Q-Jets are better for providing correct A/F ratios throughout the engine's rpm range AND they are more economical during normal driving; Edelbrock (old AFB design) carbs are simple, readily available, but not particularly efficient or precise; some modern carb manufacturers provide good quality carbs, but they are fairly expensive.

If you have an old Q-Jet [that has not been 'buggered-up'] and a spreadbore carb intake manifold, a good rebuild and re-tune for your engine will provide a good 750 cfm carb that is also more efficient with fuel than the others.

But, if you plan to drag race the car a lot and are interested in fine-tuning the carb between runs, a Holley is probably the better choice.

MakoJoe

carburetors perform on the basis that your engine will take in the fuel being provided to it. A small Block 350 4 Barrel carb is best suited between 500 to 650 CFM depending on the flow through the engine. Big Blocks will consume between 650 to 800 CFM of air flow and fuel mixture.

If you really want the best performance than you could upgrade to an Electronic Fuel Injection carb system they sell kits for these early model engines these days. With EFI you will get more HP and better Fuel economy overall because it is computer controlled for fuel the engine takes in.

Is it worth the extra cost well TCO only tells you this. 20 MPG or 10 to 16 MPG most boast they will get almost or over 20 Miles per Gallon on an older engine. Do you really drive it often enough to feel the difference. Vacuum carbs or Double Pumpers? Vacuum Carbs will only use the fuel you need where as a Double Pumper will waste fuel and if not tuned in properly will run rich and dump more fuel than a Vacuum carb.

If I had the money I would do EFI which is a Throttle Body type Fuel injection system and will provide more power and better fuel economy

427Hotrod

A good 750 Holley DP is about the most universal carb in the world. Works well on just about anything.

That said,,,the newer Holley HP street carbs have a better overall calibration for street use and work very well out of the box.

JIM

garygnu

quick fuel carbs are good .

lars

If you're looking at a square-bore platform, any of the Holley-based designs (Holley, Demon, ProForm, QuickFuel, AED, etc) in the 750 range will run equally well at wide open throttle. There are variations in the metering block designs and calibrations on these carbs, so there are some that may be easier to set up for part-throttle operation. The Holley HP Ultra series carbs have been given a new-design metering block, which works very well for the pump-gas ethanol fuels we are seeing now.

Any carb you choose will need to be set up correctly in order to run right - I've never seen a performance carb run "out-of-the-box without making minor corrections.

Lars

MrJlr

I would guess it's a safe bet that carb technology has advanced quite a bit in 40 years......

7T1vette

Carb technology hasn't really advanced very much. Once fuel injected cars became the 'norm', few folks focused on carbs. Lars mentioned the modern carb makers in his post.

The Q-Jet was probably the most 'developed' carb ever produced. And, it has a "variable" capacity; it only delivers what the engine requires. Can't do that with a Holley....

BOOT77

There are many parts of a car that one can specialize in or on and carbs is one of them. A plain old 750dp is a far cry from a fully customizable on, that you can spend several hundred $ on stuff to just be able to tune.

As with most things, if you want the max your gonna have to learn and spend some $ or find someone who does and pay them.

The nice thing about a holley is you can upgrade it as you go and if your like me you'll end up with two carbs by the time your done.

gerry72

"...And, it has a "variable" capacity; it only delivers what the engine requires. Can't do that with a Holley...."

You must never have heard of a Holley vacuum secondary.

And here's my input to your questions:

1&2. No. A brand new Holley works exactly the same as a 50-year-old Holley. A new carb just has completely clean metering channels. The problem with an old carburetor is many don't respond fully to a typical cleaning and rebuild. Sedimentary deposits can build up in the emulsion wells and a dip or spray may not remove them. An ultrasonic dip may be in order for a rebuild that's not running right.

3. The cost issue really depends. If you already have a core carburetor, then you are out only the cost of the rebuild kit if it doesn't work out. If you are buying a carburetor core to freshen up and use; that's a real crap shoot since many of the carbs out there have had some fiddlin' done to 'em and who knows what mayhem the previous owner created while "super tuning the carb with their mad skills". You see that a lot in old carbs.

7t9l82

oh my, I'm gonna give up my age again. back in the fuel crisis of the 70's
in an effort to improve mileage in my LT-1 corvette i tried a qjet set up by an extremely qualified friend . the mileage was the same as the double pumper i had been running and was less responsive. with that said , i will note it was easier to get optimum economy with a spread bore because you had to push down on the pedal further to get to the same speed. that was my experience and very shortly i went back to the double pumper as the response was better. thats why i run a quick fuel double pumper now. but any carb needs some adjustment to run right

v2racing

The problem with all carburetors is they are no better than the person building/rebuilding and/or tuning them! I think Lars will agree with this. Most carbs I have seen that didn't work properly was because of the person who built/rebuilt it or tuned it. This is not to mention how many carbs get blamed for ignition problems! This extends to boat, motorcycles, lawn mowers, and anything else that has a carb on it.

7T1vette

Holley vacuum secondary system: When the engine goes WOT, the secondaries open via a vacuum diaphragm.

Q-Jet secondary system: The more airflow the engine requires (as rpm's increase) the more the secondaries open to feed it. Thus, the same basic carb can work on about any engine up to about 400 hp. They were installed on some 'performance' Pontiac 6 cylinder engines. as well as Chevy 454 engines.

marc9889

marc9889

I recently installed an Edelbrock Torker II intake manifold (polished finish) and an Edelbrock carb on my LS5. It took very little tuning, and idles and runs smooth as glass. The electric choke is just a way better setup than the factory divorced choke IMHO.

scottyp99

The Holley vacuum secondary system effectively senses engine demand and reacts to it, just like a Q-jet. The actual method used is just different. On a Holley, there is a small passage that goes from the diaphragm to one of the primary venturis. As air velocity increases in the venture, a low pressure area, or "vacuum" (not to be confused with manifold vacuum, two different things) is created over the orifice to this small passage, causing the diaphragm to move, which, in turn, opens the secondary throttle blades. The rate of opening is controlled by a spring on top of the diaphragm that gives resistance to the throttle blade opening. Six of one, a half-dozen of the other.

Scott

v2racing

Same end result with a different method!

7T1vette

And, I'm sure every Holley comes with a bunch of springs and different sized orifices that can tune that "secondary regulator" to match your specific engine...

At best, it's a crude approximation that opens the secondaries more gradually than just going WOT instantly. An improvement, perhaps, but not very precise; and not driven by engine demand.

MrJlr

It IS worth remembering that GM spent MILLIONS developing the Q-Jet.
It is a well thought out, if not complicated, carburetor and very capable for street and race.

I personally have no problem with the mighty Q-Jet.

gerry72

You don't seem to understand how the vacuum secondary on a Holley works and can be tuned. It, like the Q-jet, is entirely operated by engine demand. If one wants to tune the secondary to suit their application, they spend the hefty $10 and get the spring kit or buy a QF diaphragm that adjusts opening rate with a simple screw (even easier than a Q-jet). I really don't care what carburetor anyone uses or likes. Q-jets are just fine. GM used many varieties of Holley carburetors on Corvettes...all of them vacuum secondary. Ask anyone with an LT-1 how pathetic their Holley is.