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Author Topic: Survey - DZ4053 Holley Carb (vented?)  (Read 5166 times)
Vintage Musclecar
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« Reply #15 on: March 21, 2009, 03:40:54 PM »

Gary;

Honestly, that's still one of life's little mysteries.   Huh

I never have figured out what benefit it really offers over the main vent in the choke tower. The most plausible explanation I've heard yet is that it offers additional ventilation to help alleviate fuel percolation during extended hot-soak conditions.

For now, that's as good an explanation as any to me.  Grin

Eric
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Jrschev
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« Reply #16 on: March 21, 2009, 07:32:43 PM »

The round tube vent in the choke tower will, at high air velocities (W.O.T.), create a low pressure area in the float bowl. (air rushing over the tube creates low pressure in the tube. That's why it's cut at an angle, to reduce this effect) Adding a secondary bowl vent eliminates this possibility and insures that atmospheric pressure acts on the fuel and not something less than that.  Evidently, this was only a problem on higher performance engines capable of high air intake velocity. That's probably why you don't see it on all carburetors.

If you want to witness this effect in action take a look at a Quadrajet on a dyno with the secondaries wide open and you can actually watch fuel pulled out of the float bowl, up the vent tube, and in to the secondary throttle bores. I used to make air dams that disturbed the air flow over the vent tube to eliminate this. It was wild to see it happen.

I know I'm a newbie on here and I hope I'm not boring people to death with my technical ramblings. This just happens to be the only automotive forum I've discovered that has a lot of intelligent people.
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1969 Z11 Pace Car (05A) 350/300 L48 4-Speed
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« Reply #17 on: March 21, 2009, 09:37:47 PM »

The only rub with that scenario is the fact that the aux. ("secondary") bowl vent on the primary float bowl closes when the throttle is opened.
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JohnZ
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« Reply #18 on: March 22, 2009, 10:46:00 AM »

I never have figured out what benefit it really offers over the main vent in the choke tower. The most plausible explanation I've heard yet is that it offers additional ventilation to help alleviate fuel percolation during extended hot-soak conditions.Eric

I agree - that's the rationale I recall from my days at Chevrolet Engineering in the 60's. It's even more important now on our carbureted cars, with our more volatile fuels with lower half-boiling points.
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'69 Z/28
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Jrschev
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« Reply #19 on: March 22, 2009, 08:31:34 PM »

The only rub with that scenario is the fact that the aux. ("secondary") bowl vent on the primary float bowl closes when the throttle is opened.

Yes, you are right about that. I've never seen that vent on a race carb (Holley)  I'm thinking it was to take in cooler air rather than super heated thinner air in the air cleaner assembly that would be present during extended idle periods.

The fuel today is horrible. Front end vollatility diminishes rapidly and leaves behind lots of junk. With all the alcohol and MTBEs present you can't let it sit in your carburetor for very long otherwise it will spoil quickly and make a mess of everything.






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1969 Z11 Pace Car (05A) 350/300 L48 4-Speed
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