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Old 02-14-2013, 11:20 AM
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CFM of Primaries

My turn-key ZZ4 crate motor came with a Holley 750cfm dual-feed, with vac secondaries. Some guys on another forum think that is too big for a ZZ4 on the street, and recommend a 600cfm instead, claiming it will be more responsive and still deliver power. I believe they are thinking about a double pumper. When I suggest that I am mostly running on the primaries, and the vac secondaries will open as required at higher rpms, I don't get any answers.

Does anybody here know what the cfm flow of just the primaries might be? I have searched all sorts of Holley specs, but can't find this particular spec. The engine seems very responsive to me, with no lags when I put my foot in it. The newer ZZ4s now come with a 770cfm Holley.
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Old 02-14-2013, 11:39 AM
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carb flow has very little to do with throttle response, you can run a 1050cfm carb and have better response tahn a 500cfm carb- if it is tuned better. THE TUNE IS WHAT COUNTS. And getting it right involves a hell of a lot more than just swapping jets around.
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Old 02-14-2013, 01:49 PM
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Quote:
Originally Posted by sedanbob View Post
My turn-key ZZ4 crate motor came with a Holley 750cfm dual-feed, with vac secondaries. Some guys on another forum think that is too big for a ZZ4 on the street, and recommend a 600cfm instead, claiming it will be more responsive and still deliver power. I believe they are thinking about a double pumper. When I suggest that I am mostly running on the primaries, and the vac secondaries will open as required at higher rpms, I don't get any answers.

Does anybody here know what the cfm flow of just the primaries might be? I have searched all sorts of Holley specs, but can't find this particular spec. The engine seems very responsive to me, with no lags when I put my foot in it. The newer ZZ4s now come with a 770cfm Holley.
It depend on the final gear ratio you have and how big the cam is. If you have 4:10 a 750 will work. If you got 2:43 gears. You may have some bog or slow response. Then you need a 600 CFM carb to make it work. The bigger the cam, the less torque at the low RPM range resulting sluggish throttle response. Every component must work together as a whole, not just the carb.
Chevy used 780 CFM on 302 and 350 Z/28 Camaro's. I would not worry about it.
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Old 02-14-2013, 02:15 PM
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Originally Posted by ap72 View Post
carb flow has very little to do with throttle response, you can run a 1050cfm carb and have better response tahn a 500cfm carb- if it is tuned better. THE TUNE IS WHAT COUNTS. And getting it right involves a hell of a lot more than just swapping jets around.
ya ok.. if that was the case, holley/etc would make one sized carb and jet/air bleed/etc it to c.i.d.
have you ever run a carb , just because you can drop a 1000 cfm t/b on an efi intake doesn't mean it works with a carb..
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Old 02-14-2013, 02:39 PM
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Air door carbs are forgiving. The carbs with more vacuum pods can cover different driving conditions. The variablity in the transitional circuit is important too. Some holleys have like 3 holes. That is fine, for the app it is all you need. Holley is all open jets. A springed rod and jet system can offer some driveability improvement. Untill it causes a lean in the carb circuit.

The Qjet on my old car 3.8 231 v6 was 750cfm . Not sure if the secondaries ever opened up much. The secondary pod also wont unlock them until the condition is right. It did not bog at wot. 750cfm on a stock 1976 231, is overcarburetion in my book.
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Old 02-14-2013, 04:28 PM
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Chevy put all this together (turn key), and I have been real happy with it. I don't really plan to change anything, it just seems to me that a vacuum secondary carb gives you small when you want it, and big when you need it! Just for information, here's the specs according to Chevy:
Cam 208intake/221exhaust (intake lift .474, exhaust lift .510)
10:1 CR
58cc aluminum heads
1.94 intake valves / 1.50 exhaust valves
HEI ignition
hydraulic roller cam
dual plane intake
355hp / 405 tq

I have a 700-R4 and a ford 9-inch 3.50 gears.
28" tall rear tire. 70 mph is just over 2000rpm in o/d.
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Old 02-14-2013, 05:42 PM
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Quote:
Originally Posted by sedanbob View Post
My turn-key ZZ4 crate motor came with a Holley 750cfm dual-feed, with vac secondaries. Some guys on another forum think that is too big for a ZZ4 on the street, and recommend a 600cfm instead, claiming it will be more responsive and still deliver power. I believe they are thinking about a double pumper. When I suggest that I am mostly running on the primaries, and the vac secondaries will open as required at higher rpms, I don't get any answers.

Does anybody here know what the cfm flow of just the primaries might be? I have searched all sorts of Holley specs, but can't find this particular spec. The engine seems very responsive to me, with no lags when I put my foot in it. The newer ZZ4s now come with a 770cfm Holley.
How this averages out between the primary and secondary depends on which 750 body you've got. One uses a square design with 1-3/8ths venturies in each location. Another uses a 1-1/4 primary venturi with a 1-9/16ths secondary.

The math would say for the 4 individual 1-3/8ths venturies the primary and secondary both flow half or 187.5 cfm per bore or 375 on the primary with another 375 on the secondary. The other model with 1-1/4 primary and 1-9/16th secondary venturies would flow 333 primary and 417 on the secondary.

The 350 seems to respond very well to the Holley 750 where 600 CFM carbs always leave the engine hunting for more when you start getting into larger cams, not that the ZZ4 is a monster but it's a lot more than what comes in the truck engines. In some ways the ZZ4 cam times in the neighborhood of the old 300 horse 327 cam.

If this engine is running good I certainly wouldn't mess with it and I'd certainly not go to a smaller carb.

There's a lot more to tuning a carb than meets the eye. If you know how to get beyond just changing the mainjets there's a lot that can be done to make a really big carb function extremely well on a smaller engine than you'd expect.

Bogie
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Old 02-14-2013, 07:45 PM
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I agree with you Bob...if it ain't broke and your happy, don't fix it.

Ray
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Old 02-14-2013, 07:58 PM
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doesnt matter what the carb flows on the primary side as the secondaries start to open before the primaries finish opening. A 750 is a standard size for a 350 cubic inch. If you have a 750,keep it.A 600 might be better for you but not the engines potential.Some people you could put a 2 barrel on their car and they would never notice,,,,If you dont drive it like you stole it on occasion,then put a 2 barrel on it
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Old 02-14-2013, 08:24 PM
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Originally Posted by vinniekq2 View Post
doesnt matter what the carb flows on the primary side as the secondaries start to open before the primaries finish opening. A 750 is a standard size for a 350 cubic inch. If you have a 750,keep it.A 600 might be better for you but not the engines potential.Some people you could put a 2 barrel on their car and they would never notice,,,,If you dont drive it like you stole it on occasion,then put a 2 barrel on it
I don't have any plans to go smaller. I have taken it pretty easy on this one so far (have just over 1000 miles on the odo), but when I put the pedal down, I want every pony I paid for!
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Old 02-14-2013, 08:50 PM
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It's 600 cfm at 1.5 inches of mercury which is nearly 20 inches of water. Pretty restrictive if you think about it. It means that in order to pull 600 cfm through it your vacuum guage would have to be on 20. What really gets me about these small carb fans is that they don't understand who controls the opening rate of the throttle blades.
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Old 02-15-2013, 07:53 AM
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Originally Posted by hpete View Post
It's 600 cfm at 1.5 inches of mercury which is nearly 20 inches of water. Pretty restrictive if you think about it. It means that in order to pull 600 cfm through it your vacuum guage would have to be on 20. What really gets me about these small carb fans is that they don't understand who controls the opening rate of the throttle blades.
there isn't a single carb out there that flows what it is rated at, most are under, a few are over.

And as for what controls the throttle blades, on a vacuum secondary carb its the spring, which can be easily swapped out- if it needs to be.

A quick test is with your pedal to the floor what is your vacuum gauge reading? You can lower that number (to a point) by adjusting your vacuum secondary spring. A 600 carb will be more than enough for most 350's up to about 5,000 RPM (with a proper spring of course) most people never rev that high, if you don't then a 600 would be fine BUT that's not to say it would be better than a 750. A 750 carb can also work very well but again may just need some spring adjustment.

That is of course assuming your fuel metering is spot on- which it may or may not be.
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Old 02-15-2013, 11:45 AM
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Quote:
Originally Posted by sedanbob View Post
I don't have any plans to go smaller. I have taken it pretty easy on this one so far (have just over 1000 miles on the odo), but when I put the pedal down, I want every pony I paid for!
Well stepping down to a 600cfm carb would be a step back.
The 750 carb usually is the best for a high performance 350SBC.
Remember the 3310 750VS carb is a "universal" carb. You can improve it
by upgrading to the Holley Proform 750HP VS center body.
You get more flow, more /better venturrii velocity thru improved flow and shape. down leg style boosters
You get swappable air bleeds to allow dialing it in much sharper than out of the box.
You can further custom dial in the action of the vacuum secondaries with
a modified vac sec housing such as sold BY Quick Fuel.
You can mod your own to allow screw adjustment of the vacuum metering bleed
instead of a universal fixed orrifice. you combine this with vac sec spring selection to get it just right for your car. Every car is different.
Plan on doing some drive testing to get it just right.
The out of the box 3310 750 is good but you can make it even better thru custom tuning. It wil do every thing better.
No 600 cfm carb will compete in either power or throttle responce
to this custom tuned 750cfm+ VS carb.
The proform HP carb body does not have a choke.
Its a great upgrade to a stock 3310 carb.
Proform #67101C
The included jets and idle and high speed air bleeds are a baseline. Fine tuning is required.
The result is worth the effort.
The shape of the base of the air cleaner is critical too. They are not all created equal.

you can upgrade the secondary metering plate with a secondary metering block kit or modify it to allow
swappable main jet changes.

Last edited by F-BIRD'88; 02-15-2013 at 11:57 AM.
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