Mechanical Secondaries

[Stallion]

Do we have mechanical secondaries on our C3s?

 

[MaineShark]

No, the Quadrajet carb has a vacuum-operated secondary.

Actually, it has something that goes beyond the typical vacuum-operated secondary. Rather than the secondaries having butterflies and venturies like the primaries, and a vacuum actuator to move them in response to manifold vacuum, the Q-Jet has an "air valve secondary."

Basically, there is a spring-loaded door that covers two holes. Instead of relying on manifold vacuum to operate an actuator, the manifold draws directly against this door, pulling it open.

In a traditional carb, there is a venturi (narrow area) that speeds up the air, creating a low-pressure zone that draws fuel in through the jets. Works well, but the carb's airflow potential is limited by the narrowest point. The Q-Jet has two wide-open throats, with no narrowing. Instead, the angle of opening of the air door is used to meter the fuel flow. This allows the incredible airflow potential (750-795 cfm) of the Q-Jet, without requiring large primaries, which would cause street driving to suffer.

A mechanical-secondary carb has a linkage to the throttle cable which causes the secondaries to start to open after the cable moves a certain distance, after which point they open in response to the movement of the throttle cable, just like the primaries. This gives the driver absolute control over the opening of all four throats, but requires that all four have venturies (since the secondaries may be open more than the engine actually needs, you could end up running rich if the fuel was metered in response to the butterfly opening angle), as well as requiring the opening point and rate of the secondaries to be properly setup. Direct control over the carb is nice, but it requires more setup, or that control is wasted.

Other than not being able to compensate for altitude, the Q-Jet rivals many EFI setups. No, it's not the same as having fuel injection, but it comes a lot closer than most carbs.

Joe

 

[matchframe]

Yeah! What he said.

Seriously, that is a great description Joe.


Bill

 

[MaineShark]

I'd like to take credit for it, but almost everything I know about Q-Jets, I learned right here on the CAC. This place is great

Joe

 

[Stallion]

Joe, I didn't think I had a Q-Jet in my Vette. '78 L48. Do I? :confused I thought this was a carb for early C3s. Is this stock for us?

 

[MoeJr]

Joe not to disagree with you but QJets are not a true vacuum secondary carb. The throttle linkage opens the secondaries on a QJet all the vacuum does is pull the lock away from the lower secondary throttle blade lock when the choke is fully off. I have been around QJets all my life so I have seen how they operate and it is quite a nice piece of engineering for its time.Manifold vacuum does regulate the airflow from the back half of the carb by only opening the top secondary blades as much as the engine is calling for. They are a great carb when set up right but technically they are more of a mechanical secondary carb. I prefer a QJet to a Holley or Edelbrock/Carter.

Stallion yes your 78 came from the factory with a QJet.

 

[MaineShark]

Yup, the Q-Jet was used up through '81, until it was replaced by the CrossFire system in '82.

Actually, though, the Q-Jet, at its worst, was an equal match for the CrossFire fuel injection, which is why I say that it's a pretty dang advanced carb. It wasn't until TPI that the Q-Jet actually became obsolete.

If you take off your air cleaner lid, you should be able to see the top of the carb, and the large air door that covers the secondaries.

[edit: Moe posted while I was typing]
Moe, if I recall correctly, the throttle linkage adjust the preload on the spring that holds the air door closed, making it easier and easier for the engine to pull the door open. At least, that's how it was explained to me...

Joe

 

[MoeJr]

Hey Hey easy on ole' CFI LOL. My 84 runs like a watch and never embarrases itself. That air door tension is a spring and set screw set up. Only vacuum piece on that carb is the choke pull off and what tha does is pull the choke all the way off so the lock comes off the secondaries. Linkage does open the secondaries on that carb. If ya ever in the Manchester Nh area look me up and I'll show ya on my old Pontiac.

 

[MaineShark]

I just mentioned the CFI to point out that there is overlap between EFI and carbs. It ain't the "carbs are always inferior to EFI" that some people tend to think. There is a lot of overlap between the high-end carbs and the low-end EFI systems.

I do think I may have been mis-informed on the Q-Jet. Looking mine over, I think you may very well be correct, about the secondaries being controlled by the throttle, which makes me wonder why people always refer to the Q-Jet as being a vacuum-secondary carb?

The Shark isn't exactly running at the moment, so I can't watch the carb to see how it acts when exposed to operating conditions, but there are clearly secondary butterflies, and some sort of link (containing a few springs) between the throttle linkage and the secondary shaft. I can't tell if those springs simply allow the butterflies to remain close when the choke lock is active, or if they serve some other purpose in partially isolating the butterflies from the throttle.

Curiouser and curiouser. And learning something new each day

Joe

 

[MoeJr]

Yeah there is alot of linkage on a QJet. The book Rochester Carburetors by Doug Roe is an excellent source of info on them and nice tuning tips and charts. I got mine at Barnes and Noble. You'll get an idea how they work when ya start the shark and have it "on" choke and kick it off. Then u will see the lock will be off and the secondaries only linkage activated. Like I said only vacuum piece on the carb is the choke pull off. Have fun.

Moe

 

[Stallion]

WOW!! I didn't know that I had a QJet under my hood. Everybody is always raving about this QJet and I just assumed that it was earlier Sharks that wielded this magnificient carb. You just made my day by telling me I have a QJet. Thanks, guys.

 

[MaineShark]

Moe: I think I'm going to try and find a Q-Jet on a wreck somewhere, and buy it so I can tinker around with one, without potentially damaging my car

Stallion: The Quadrajets have a sort of "love it or hate it" following. Being one of the most technologically-advanced carbs ever produced, they can be extremely tricky to get set-up correctly for performance applications. From air-door tension and opening angle, to a complex jet system and a host of other tricky adjustments, they require a lot of patience in getting the right tune for a particular car. This turns many people away. Those who are willing to put the time in (or those driving nearly-stock cars) wil often learn to love the Q-Jet for those same settings, which make for a great balance between diveability on the street, and excellent performance when the pedal heads south

Joe

 

[MoeJr]

Good idea Joe. They are a bit complex however they are fairly easy to tune once you do it a couple times. I love them as they have small primaries for good throttle response off the mark and they have the biggest secondaries of any carb that makes them good for some strip bashing. Enjoy.

Moe

 

[MaineShark]

I love them as they have small primaries for good throttle response off the mark and they have the biggest secondaries of any carb that makes them good for some strip bashing.

I'm thinking about EFI eventually, but I'm wondering if I can create a throttlebody with a similar spreadbore arrangement. I think it might keep more manifold vaccum at low-throttle, which would help to prevent the problems that EFI systems typically have when dealing with cams that don't produce much vacuum.

Joe

 

[Stallion]

Yeah there is alot of linkage on a QJet. The book Rochester Carburetors by Doug Roe is an excellent source of info on them and nice tuning tips and charts. I got mine at Barnes and Noble. You'll get an idea how they work when ya start the shark and have it "on" choke and kick it off. Then u will see the lock will be off and the secondaries only linkage activated. Like I said only vacuum piece on the carb is the choke pull off. Have fun.

Moe

I want to get really delved into carbs, especially the QJet, so is this the best piece of literature to fully explain the parts and components as well as functionality? I would also like to pick up a QJet at a junkyard, fix it up real nice, and the sort. Carbs have always interested me.

So you would recommend this book over anything for learning about the QJet and the parts, components, and functionality?

 

[MoeJr]

I tell ya Joe EFI is nice and all but a properly set up carb is much easier to tune IMHO. Also there is nothing like the sound a QJet makes when all 4 bbls are open. EFI cannot recreate that sound.

Stallion yes I reccommend that book for you. It is well worth the price and has lots of good info.

 

[Stallion]

I tell ya Joe EFI is nice and all but a properly set up carb is much easier to tune IMHO. Also there is nothing like the sound a QJet makes when all 4 bbls are open. EFI cannot recreate that sound.

Stallion yes I reccommend that book for you. It is well worth the price and has lots of good info.

Okay, thanks a lot. Is that what makes our old muscle cars rumble like we love? The carb? I've always wondered that. Maybe it's the whole package. A kick@$$ engine, the exaust (even if it doesnt have headers), and the carb.

So EFI is quiet then? I don't think I'll be putting that in. I love the roar of my Shark.

TR

 

[MaineShark]

I tell ya Joe EFI is nice and all but a properly set up carb is much easier to tune IMHO. Also there is nothing like the sound a QJet makes when all 4 bbls are open. EFI cannot recreate that sound.

I dunno... my first prototype will probably just be a gutted Q-Jet, so it may sound very similar

Joe

 

[igasper10]

Other than not being able to compensate for altitude, the Q-Jet rivals many EFI setups. No, it's not the same as having fuel injection, but it comes a lot closer than most carbs.

Joe

Hey Joe,

currently I live in Michigan but my wife and I are considering moving to colorado near aspen. I made the mistake of replacing the q-jet in my 74 with a Holley carb I had. I didn't know how much of a better carb for fuel economy the Rochester was. Needless to say, I rebuilt the q-jet and will put in back on very soon. My question is regarding your altitude comment. What do I have to be aware of if we decide to move up to the mountains as to q-jet adjustments???

ian

 

[MaineShark]

See here

Joe