1968CutlassSupreme

Hello All!

What are the pros and cons to a mechanical versus a vacuum secondary on four barrel carburetor applications?

Thanks!

oldcutlass

Mechanical secondary carbs are generally used with manual transmissions. They are a more performance based carb to give positive operation of the secondaries based on throttle position.


The cons for the street, less than stellar fuel economy and are a bit harder to setup. Most Holleys have a 4 corner idle adjustment.

cjsdad

With mechanical secondaries it is possible to have the secondaries cracked open slightly while driving at higher RPMs. Like cruising at 45MPH in a lower gear waiting for your nemesis in the Mustang to hammer down. The vacuum operated secondaries will only open when the engine vacuum drops such as when you floor the throttle in that lower gear when that Mustang driver accepts your challenge.

Mechanical carbs typically have accelerator pumps for the secondaries to compensate for the lean condition when all that air is introduced into the intake stream and that is where you lose your fuel economy. It is too much fun feeling that power as all that fuel gets squirted down the intake.

jag1886

Mechanical carbs are more oriented to real HP use, you need a stick or good deep gear to be able to use them without causing a bog when you hammer down on it plus then tend to be real gas hogs (I'm not saying you can't use them with an automatic, you just have to have the right combo for them to work smoothly) .
Vacuum carbs are more forgiving and you can taylor the secondary to open as fast as you want, (that is as fast as your engine-drivetrain can tolerate without a bog or hesitation) they can be used in most any application.

1968CutlassSupreme

I know this is not talking about vacuum vs. mechanical carbs, but I figured it would be good to put this question here rather than starting a new thread.

I am looking at the edelbrock carbs and they have the electric option or the manual option. The rochester 2bbl I have is neither, it is a vacuum choke. I do not want to manually control the choke with a cable. Is the only other option to have the electric choke or can I set up the carburetor to operate like my current vacuum setup? I really do not want to have to put wires where they were not originally ran (kinda ugly and cheap looking to me).

olds 307 and 403

For the carb, buy the Street Demon instead, better design. If you mean hot air, all Olds Qjets up to the end in 1990 had hot air choke. The hot air choke works but not as well as the electric choke. I dislike the hot air choke, not as adjustable as the electric. I always convert mine to electric. I think the manual choke blows, may as well run no choke.

MDchanic

That's an odd thing to say. The manual choke gives you complete control of the choke at all times. I have trouble seeing how that equates to having "no choke."

- Eric

joe_padavano

The electric choke is a perfectly fine way to go, and that's really what you want. By the way, your "vacuum" choke is actually a hot-air choke. There's a bimetallic coil inside the black plastic choke cover, which is heated by hot air as the engine warms up and expands, opening the choke plate. The electric choke works exactly the same way, but uses an electric heater instead of hot air. Wire it this way.

By the way, the Ebrock carbs (and the Qjet as well) are air valve carbs. People confuse these with vacuum secondary carbs, but they are not. The secondary throttle blades are mechanically linked to the primaries and always open. The air valve above the secondaries is weighted and opens based on mass air flow through the secondaries. This limits airflow to only what the engine needs for a given RPM and load, making the carb more efficient.

cutlassefi

Lots of confusing info here. Here are the facts;


Joe is right in that there are two types of vacuum operated secondaries. One in which the throttle blades themselves open gradually via opposing spring pressure.
The other in which the secondary flap is opened only after the corresponding throttle blades are opened as well.

Some Holley carbs have 4 corner idle but most of their baseline Street carbs i.e. Street Avenger", 3310, 1850 and so on do not. However their competition "DP" carbs do but those do not have chokes of any sort. The newer/upgraded model Avengers now have 4 corner idle, but they're more expensive than most other carbs around the same size.
You can get crappy mileage with vacuum secondary carb just about as much as you can with a mechanical one.
Any carb, mechanical or vacuum secondary, will bog if the transition fueling isn't correct.

Look for a carb with multiple tunability (Quick Fuel, Proform, Demon etc), with changeable air bleeds, power valve circuitry etc. That will give you the most bang for your buck.

As mentioned, more often than not a mechanical secondary carb is best suited for larger cubic inch engines and/or manual transmissions. Of course there are always exceptions

Jmo.

olds 307 and 403

It does give complete control but is a step backwards. Kind of like installing a hand crank for the starter. Not for this guy.

joe_padavano

The big difference between a mechanical secondary carb and a vacuum secondary or air valve carb is that the mechanical carb MUST be calibrated for your specific engine combo. The other two are much more forgiving due to the "air on demand" nature of these carbs. The mechanical carb always opens the secondaries when you mash the pedal, no matter what the engine load or RPM. If you have it dialed in properly (jetting and accelerator pump), it will make more HP. If you don't want to spend a lot of time dialing it in, avoid it.

my71olds

Lots of good information at the time in which I want to replace the 4b quad on my olds 350. So is a vacum one the same term as Climatic Choke? I am looking at sites now. Just going to get a new rebuilt one any sites to recommend.

Thanks

MDchanic

Climatic choke???

Are you talking about the optional Climatic Combustion Control air cleaner on '67s (and maybe '66s)?

That's a (really cool) air cleaner system that regulated the air intake temperature, in order to improve carburetor operation.
Its basic function was performed by all air cleaners starting in 1968 (though it didn't look as cool) in response to new emission control standards.

It has nothing to do with any of the previous questions about vacuum vs air valve vs mechanical secondaries.

- Eric

my71olds

Oldsmobile
$189.95
SKU: 41-781
Year: 1971
Trans: All
Engine: 8 Cylinder Engine
Description:
Rohcester Quadrajet 4 barrel w/Climatic Choke on Carburetor

MDchanic

Ummmmm... What are you trying to say?

It looks like you have posted some sort of a sale listing for some object, from some unknown source, with no background information.

Perhaps you could flesh that out a bit?

- Eric

my71olds

http://www.carb-x.com/domestic-carbu...s.-olds/41-781

MDchanic

Ah. It's listing for a rebuilt QuadraJet from CarburetorExchange.

Their site does not instill great confidence. "Remanufactured Carburetors from $99"

Why not check with Cliff Ruggles or Sparky if you're looking to buy a carb, or to get yours set up right?
Or just rebuild it yourself for $30 to $40?

As for the term "Climatic Choke," I've never heard it before. I suspect it's a neologism from someone at that company, but perhaps others here can enlighten me.

- Eric

oldcutlass

Click on the link below. A lot of folks on this site have used him in the past and no one has been disappointed yet.


http://www.sparkyscarbs.com/

joe_padavano

"Climatic Choke" is a term made up by this vendor and is not a term used by Oldsmobile. I suspect the vendor is referring to the integral choke (the round housing on the side of the carb) as opposed to the divorced choke (choke coil is separate and mounts in a pocket in the manifold). In any case, neither of those has anything to do with vacuum vs mechanical secondaries. And again, all Qjets are air valve carbs, where the secondary throttle plates are mechanically operated but the air valve above the secondaries opens based on the mass flow rate of the air through the carb. Oldsmobile NEVER used a carb with mechanical secondaries from the factory, period. Any rebuilt factory-replacement carb for your car will be a Qjet with the air valve, period.

MDchanic

Thanks for confirming that, Joe.

- Eric

olds 307 and 403

The Edelbrock Thunder series AVS and Street Demon are also both air valve carbs that can be adjusted like a Qjet. The regular Edelbrock carb has a not so easily adjustable secondary set up. The Edelbrock carbs are Carter AFB and AVS carbs with a few refinements. The Street Demon is heavily based off mostly the Carter Thermoquad, some of the AVS with all new castings to fit square and spreadbore manifolds. It also eliminated the O rings between the body, has aluminum and Aerospace polymer body options instead of the Thermoquad's Bakelite body and a much better choke than the Thermoquad factory carbs.