Is a 600cfm carb enough for a 383??
03-27-2001, 10:36 PM
#51
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Alright ODB, of course there is a pressure diffential in the intake but at zero vacuum GAUGE the atmosphere is pushing in at a theoretical 14.7 PSI at sea level. More simply put, there may be no vacuum at the gauge but that just means full atmospheric pressure in the intake.
Really a vacuum in the intake just means pressure that is less than atmospheric. Just throwing numbers but as an example if you pull 7 inches of HG then intake pressure could be at 5PSI or 9.7PSI lower than the atmosphere outside the intake.
I'm gonna throw a monkey wrench into this one. I just read at work, according to Holley, that peak volumetric efficiency is at the RPM where peak torque occurs. That means VE decreases beyond the torque peak but more air is still needed because air demand due to engine speed surpasses the drop off in engine efficiency.
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84 Camaro ZZ4 with HOT cam. 1.88 60' (12.98 @ 105MPH E.T.)
Recently Ported Heads, Installed Larger Race-Flo Valves and RPM Air Gap Intake (No new times)
Other Mods: You name it and I have probably changed it.
2001 Dodge Ram Quad Cab 5.9L 4*2
The Bowtie
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Really a vacuum in the intake just means pressure that is less than atmospheric. Just throwing numbers but as an example if you pull 7 inches of HG then intake pressure could be at 5PSI or 9.7PSI lower than the atmosphere outside the intake.
I'm gonna throw a monkey wrench into this one. I just read at work, according to Holley, that peak volumetric efficiency is at the RPM where peak torque occurs. That means VE decreases beyond the torque peak but more air is still needed because air demand due to engine speed surpasses the drop off in engine efficiency.
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84 Camaro ZZ4 with HOT cam. 1.88 60' (12.98 @ 105MPH E.T.)
Recently Ported Heads, Installed Larger Race-Flo Valves and RPM Air Gap Intake (No new times)
Other Mods: You name it and I have probably changed it.
2001 Dodge Ram Quad Cab 5.9L 4*2
The Bowtie
ASE Certified Auto Tech
LIVE AND DIE BY THE ALMIGHTY BOWTIE!
03-28-2001, 12:06 AM
#52
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Car: 1984 Z28 Hardtop
Engine: 383 Carb
Transmission: 4L60
Axle/Gears: 3.54 Dana 44
I think I oversimplified with my "zero vacuum" comment above. ODB is right, air won't pass through the carb without a vacuum in the intake. Measurable vacuum in the intake is produced when the intake stroke of the pistons removes air from the manifold faster than it can easily be replaced through the carb.
The intake stroke of the pistons removes air from the manifold, causing a vacuum in the manifold. Air is drawn in through the carb to fill the vacuum. As the vacuum increases, the velocity and volume of the air drawn through the throttle plates of the carb increase as well. Because the airflow and vacuum are dependent on each other, they establish an equilibrium. When the throttle is only partially open, a large vacuum is produced in the manifold before the airflow through the throttle is sufficient to replace the air lost. At WOT when air flows easiest through the throttle, only a very small vacuum is required to produce a sufficient flow. If the carb is not overly small, the manifold vacuum may be NEARLY zero, but not quite, at WOT.
This has implications on the mixture as well since it's the air velocity through the venturi (not the same as the velocity through the throttle) which produces the vacuum to draw the fuel into the mixture. Add to this the change in the engine's demand for air with varying RPM and a thousand other details and you find out that smart people design carbs and I don't.
The intake stroke of the pistons removes air from the manifold, causing a vacuum in the manifold. Air is drawn in through the carb to fill the vacuum. As the vacuum increases, the velocity and volume of the air drawn through the throttle plates of the carb increase as well. Because the airflow and vacuum are dependent on each other, they establish an equilibrium. When the throttle is only partially open, a large vacuum is produced in the manifold before the airflow through the throttle is sufficient to replace the air lost. At WOT when air flows easiest through the throttle, only a very small vacuum is required to produce a sufficient flow. If the carb is not overly small, the manifold vacuum may be NEARLY zero, but not quite, at WOT.
This has implications on the mixture as well since it's the air velocity through the venturi (not the same as the velocity through the throttle) which produces the vacuum to draw the fuel into the mixture. Add to this the change in the engine's demand for air with varying RPM and a thousand other details and you find out that smart people design carbs and I don't.
03-28-2001, 07:36 AM
#53
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I guess I could continue to argue this, but I'm not. I'm right, I know I'm right and I proved it. CamaroMike asked for hard numbers, I gave them to him. He then ignored them, because they didn't bear truth to his arguement. Everyone can believe what they want. You can believe in theory and mathematical equations or you can believe thousands of racers who have made millions of passes down the quarter mile. Both have their merits, both sides can be argued. But when a Mustang pulls up alongside of you, all you want to do is smoke him! And that is what it's all about, performance, so I'll take the 750, every time! 
03-28-2001, 07:48 AM
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Just read the thread on the K&N Extreme lid and stub stacks that CamaroMike posted. I have to admit it was pretty funny. I would have liked to see him stomping on that poor old stub stack. Sounds similar to when I took a 600 Holley off my 70 Mustang and threw it on the ground, then into the garbage can where it belonged. 
03-28-2001, 08:43 AM
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Guys I'm locking this thread, simply because it's just getting too stupid. 50 something replies over the cfm of carb needed is absurd.
Here is the breakdown, believe it or else. You need at least a 750cfm carb to match that engine well. If both carbs are dialed in, you won't gain any mileage with a 600 over a 750, you will just choke off performance. Especially don't use a 600 cfm Edelcrock, those things are jokes on an engine they are properly sized for.
That is all
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"American made baby. 100% American iron. The muscle among the masses. My hero. Yep, you can take your ergonomically designed, space age, computer controlled, 4 door, cup holding map lighted split double wishbone split fold down retractable cargo covered moon roof piece of transportation and keep it. For I have felt the thunder. And I know the difference!"
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Here is the breakdown, believe it or else. You need at least a 750cfm carb to match that engine well. If both carbs are dialed in, you won't gain any mileage with a 600 over a 750, you will just choke off performance. Especially don't use a 600 cfm Edelcrock, those things are jokes on an engine they are properly sized for.
That is all
------------------
"American made baby. 100% American iron. The muscle among the masses. My hero. Yep, you can take your ergonomically designed, space age, computer controlled, 4 door, cup holding map lighted split double wishbone split fold down retractable cargo covered moon roof piece of transportation and keep it. For I have felt the thunder. And I know the difference!"
JSP Motorsports
ICON Motorsports
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