Quadrajet on a Slant Six

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fjr

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While it might seem that a 750 CFM Rochester Quadrajet is way too much carburetor for a slant six, I would have to disagree. Since it has air-valve secondaries, the Quadrajet actually performs like a small 2bbl carburetor. The primaries on a Quadrajet are rated at 161 CFM @ 1.5” Hg. For comparison, the primary side of a Holley 0-8007 (390 CFM) is rated at 195 CFM and the slant six Carter BBD is rated at about 202 CFM @ 1.5” Hg (~285 CFM @ 3.0” Hg). This is only slightly bigger than the original BBS which is rated at about 141 CFM @ 1.5” Hg (~200 CFM @ 3.0” Hg).

http://cars.rasoenterprises.com/Carburetion.htm

The air valves are spring-loaded so that the secondaries stay shut at wide open throttle until manifold vacuum pulls them open. On a 225 CID engine, they will never open fully, although I can hear them starting to open when I’m accelerating at highway speeds.

Besides the small primary barrels, the other reason I like the Quadrajet is its small central fuel bowl. I have my carburetor mounted sideways so as to have a symmetrical fuel distribution. Whether this is really of any practical benefit is still up for debate. I found that the AFB’s much larger fuel bowls caused a lean-out when making hard left turns. The Quadrajet works perfectly in this orientation. I believe off-roaders like the Quadrajet for similar reasons.

http://cars.rasoenterprises.com/Carb-Quadrajet.htm

DutraDualsHeaterJPG.jpg
 
fjr said:
The air valves are spring-loaded so that the secondaries stay shut at wide open throttle until manifold vacuum pulls them open. On a 225 CID engine, they will never open fully, although I can hear them starting to open when I’m accelerating at highway speeds.

But that is true of any vacumn secondary carb including the Holley 8007 (390 cfm) you mention.

FWIW, I ran a 390 Holley on the slant 6 in my Barracuda for almost 30k miles (until I swapped in a 360). The car ran great with this carb. The throttle reponse was great and the car still got 19-20mpg around town and 23-24 on the hiway. It was a fun set-up and gathered a lot of attention with the headers, 4bbl and dual exhaust but still was not what anyone would call a fast car.
 
Not quite sure why you would want a 4bbl carb that the secondaries would only partially open?On a 225 engine the secondaries will fully open if you have the proper carb.The reason you secondaries wont fully open is because your 750cfm carb was designed for the vacuum of a 400cid-450cid engine.

Bigger isnt always better,but if it is working for you more power to ya !!!!
 
Technically air flow opens the secondary air doors not vacuum. There is nothing on the Q-jet air doors that reads manifold vacuum except for the dashpot that keeps them closed when there is manifold vacuum. But your example is right that the air doors will open only as much as the motor can pull.


Chuck
 
Well, Dave, you're right. A big carb doesn't make a small engine powerful. My goal has always been to have a fun-to-drive, fuel efficient car as I do most of my driving on the highway. If I really wanted more power, I would have done the same as you and dropped in a 360 a long time ago. The power I have with my engine is plenty for what I need. Like you said about your slant six getting attention at car shows, I find V8 cars to be a bit too common. To each his own.

As for why I didn't use the conventional 390 CFM Holley, it's because I liked the design (and price) of the Quadrajet better than the Holley. The (oversized but free) AFB I was using previously worked OK for the most part but it didn't handle the fuel sloshing well enough on left turns. The fact that it can properly meter fuel on a small engine like mine is what I think is important. Incidentally, the Quadrajet would also have never fully opened on the Chev 350 that it originally sat on either.

The part about air flow opening the secondary doors is something I never quite understood. When the doors are closed, there is no air flow. Therefore, how can no airflow cause the doors to open? However, when the secondary butterfly valves open, the pressure in the secondary barrel below the door is less than the atmospheric pressure above. This difference in pressure is what I thought caused the doors to open. The greater the pressure difference, the greater the amount of opening.
 
Your explination of how the Q-jet flows makes a lot of sense to me(small primaries). The only reason I am using the 465cfm Holley is because it was free. You are right, 4wheelers like q-jets because they are slosh resistant and work well at low rpm's for crawling, then really kick in with the huge secondaries when you need the top end for mud bogging.
 
fjr said:
Well, The part about air flow opening the secondary doors is something I never quite understood. When the doors are closed, there is no air flow. Therefore, how can no airflow cause the doors to open? However, when the secondary butterfly valves open, the pressure in the secondary barrel below the door is less than the atmospheric pressure above. This difference in pressure is what I thought caused the doors to open. The greater the pressure difference, the greater the amount of opening.

It's the air flow through the primaries that creates a vacuum signal in the primary venturi. It's this vacuum that opens the secondaries.

When people talk about feeling the secondaries open what they are actually feeling is a momentary bog when the secondary opens to quickly/much. The car will actually accelerate faster if the secondary spring is stiffened up a bit to delay the opening.
 
Thanks for explanation. Doug Roe of "Rochester Carburetors" (page 50) wasn't all that clear about the influence of primary venturi air flow on the operation of the secondary air valves:
Doug Roe said:
Secondary System
As engine speed increases, the primaries can't meet the engine's airflow demands and the secondaries begin to operate. The secondary section contains:
  • Throttle valves.
  • Spring-loaded air valves.
  • Metering orifice plates.
  • Secondary metering rods.
  • Main fuel wells with air-bleed tubes.
  • Fuel-discharge nozzles.
  • Accelerating wells and tubes.
The secondary side operates as follows.
Fuel Flow -- When engine demand requires more A/F that the primary bores can supply, the primary-throttle lever opens the secondary throttle valves with its connecting linkage to the secondary throttle-shaft lever. As air flow through the secondary bores creates a low pressure (vacuum) beneath the air valve, atmospheric pressure on top of the valve forces it open against spring tension. The required air for increased engine speed then flows past the air valve. ...
It still looks to me that vacuum transmitted up from intake manifold is what initially causes the air valves to open. While it may seem that air flow is what causes the air valve to open, the difference in pressure is always the cause of air flow.
 
I am not familiar with Rochester carbs but the description you listed indicates that the secondary throttle blades are mechanically connected to the primaries and that manifold vacuum indeed opens the air door at the top of the secondary venturi.

A holley vacuum secondary carb has not direct connection between the primary & secondaries. The linkage that is there only prevents the secondaries from opening when they shouldn't.
 
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