Looking to buy a new carb and was thinking about a 650 CFM double pumper with mechanical secondaries. It seems like whenever I talk to anyone they say the motor will make more horsepower with a 750 CFM.

Everything I have ever read says a 600CFM is enough for a 350 ci. Any suggestions ?

 

At this point it depends on the HP of the motor. Since you're making over 400 HP, I'd say go with the 750 vac secondary setup. I am...just bought a 750 Speed Demon

:reddevil

 

I would say go with a HOlley or BG Speed Demon 750cfm mech. secondaries. I believe a rough estimate is a 600-650cfm is good to around 350-375, and a 750 is anything above that. This is based on what another forum told me that BG told him on the phone. Definitely mech. secondaries b/c of the 4spd manual.

 

The generally accepted formula is 1.6 cfm per HP. 400 x 1.6 = 640. I would go with the 650 DP. The 750 might make more peak HP, but the 650 would make more power under the curve, and more torque. Even in Drag racing conditions the 650 would probably be better. :chevy

 

Do what you want it'll work.
:D


[Modified by Wightnite, 1:07 AM 4/5/2002]

 

FWIW and to substantiate your thoughts,..Just bought a '72 LT-1 all original and stock with the original '72 800 CFM Holley DBL Pumper Model 62391, vacuum scondaries carb,..the engine was rebuilt less than 30K miles ago to '70 specs but original '72 Holley had been rebuilt one too many times and was becoming very problematic. I plan to try and get another rebuild but I don't want to have to continue tinkering with it in the meantime. After researching and talking to a few carb building people I was advised that a 650 CFM, Holley Model 04777C, Model 4150 with mechanical secondaries would be more than enough for this engine,...like the other post said up to 400 ponies. Picked one up new for about $290. The engine has just as much torque as before and really screams. I think that u r on the right track with a 650 DBL pumper with mechancial secondaries. Hope this helps!! :chevy

 

The biggest mistake made since the invention of the engine " Over-carburation" Max Carb for a 350, ( 600 to 650). Max carb for a BB ( 700 to 750) In a very few applications a 850cfm.

 

Please explain the differences between mechanical and vacuum secondaries and when one is better to have than the other.

Thanks. :cheers:

 

I gotta go with Rainman69 on this one. Horsepower output is your major consideration here, cubic displacement is second. The http://www.mortec.com site give a mathmatical equation to your question but make sure you follow it closely.
I have a new Holley 750 dual feed with vac secondaries and electric choke. It was on the WeasleVette when I bought it.
Weasle

 

From what Ive heard, cubic inches and max rpm are the determining factors in calculating required CFM. Some others have mentioned HP as a factor. im not sure how HP plays the role. Could someone explain that to me? As I see it, higher rpms usually produces more HP, so long as the rest of the engine is up to the task of higher rpms. So there it is, higher rpms will require more CFM. So where exactly does HP standing alone have a role?

I use the Holley list#1850 600cfm Vac sec. It gets my vette with modified 350ci to 5500rpm no sweat. If I went to a 750cfm, the bigger carb could potentially make more power if I pushed the engine beyond 5500rpm. My max useable rpm with my setup is just below 6000rpm. So, would a 750cfm do anything for me? Why would I need a carb that could take my motor beyond 7500rpm when it will never even see 6000?

If you do elect the 750, which is way more than needed for a sub 6krpm 350, go with vac sec. The vac sec will be more forgiving when over carbed. If you must increase, 650dp would be your best shot.

Im getting mid 13's ET with my 600. And thats with lousey 60' times.

 

Vacuum secondaries: controlled by the vacuum passing through the carb but if you really push the linkage to its max, mine open (but I think the top flap on the Q-jet is vacuum controlled so vacuum is ultimately in control). Mechanical secondaries are driven by the progression of the throttle linkage only so once the lower butterfly opens, you have all 4 barrells. Just my shadetree mechanic knowledge (perhaps more shadetree than mechanic).

Hey. Ask Lars! He will know the answer!

I had a 600cfm, dual feed, dual pump, mechanical secondaries on a 283. That was one sweet Holley!

 

Wayne, talk to your engine builder, these guys know the stuff, they have done some very high end mods, given that I would listen to them they may even have a 750 lying around, try it. BTW Santos has a vacuum sec. one at his shop - try it. See you soon. :cheers:

 

You can get on the holley website and call their technical experts. Tell them what you have and what you want to do and they will do the math and come up with some choices for you. Bigger isn't always better when it comes to carbs.

 

this is the tech response to my question when I asked them for mine:

CFM and Carburetors:
Carburetors are rated by CFM (cubic feet per minute) capacity. 4V carburetors are rated at 1.5 inches (Hg) of pressure drop (manifold vacuum) and 2V carburetors at 3 inches (Hg). Rule: For maximum performance, select a carburetor that is rated higher than the engine CFM requirement. Use 110% to 130% higher on single-plane manifolds . Example: If the engine needs 590 CFM, select a carburetor rated in the range of 650 to 770 CFM for a single-plane manifold. A 750 would be right. An 850 probably would cause driveability problems at lower RPM. A 1050 probably would cause actual loss of HP below 4500 RPM. For dual-plane manifolds use 120% to 150 % higher.
:seeya

 

Quote:
"Everything I have ever read says a 600CFM is enough for a 350 ci. Any suggestions ?"

Those formulas are not accurate in the real world.Actual manifold design and cam used ect is how I arrive at a particular cfm carb to be used.In your case,you need a Holley 4779 750cfm

 

Looks great, but you are a braver man that me running those filters. I've seen WAY too many carb backfires and they go up in flames. My friend had it on his 340 Charger and the carb sucked the foam into the butterflies.
Becareful bro!! :eek: :eek:

 

I second you respones. Around here we know them as the flamethrower air cleaners.

Mark B.

 

Motorhead,
I agree with 69N.O.X. that a 650 would suit your needs best.

I have a Holley 4777 and a 650 Demon, both mech. secondaries. I like the Demon the best!

If you do decide on a 750, don't even consider mech. secondaries!

When I dump the 650 on the Chassis Dyno it dropped the a/r to 12.2 at 2,000. before it recovered.
Bud

 

Not for me!!!!!!!!!!! :smash:

 

MotorHead, whats the list# on yur 600? Is it 1850? If so its vac sec with a secondary metering plate making it a 4160 series carb. Some things can be done to make this carb better. With mine ive done these: Put the lightest secondary diaphram spring in. Holley makes the kits. This will allow your secondaries to come in sooner. I could notice the diff when I made this mod to my 600. The 4barrel kicked in much sooner and did not cause any bog at all. With your modified 350 engine using the 600, im sure it too would not have a bog problem comming in sooner. Next, The secondary metering plate on all 4160 series carbs does not contain removable jets. So basically when it comes to secondary jetting, you really cant change it. Some people mentioned drilling the plate out to increase the fuel delivery but theres another way yet. Holley also makes a kit that comes with a new secondary metering "block" that has provisions for removable jets. You replace your secondary metering "plate" with the metering "block" from the kit. This converts the 4160 to a 4150 with changable jets for primaries and secondaries. I did this mod with my 600 also. Firtst, I incresased the front jets two sizes. Then I removed the original metering plate and noted the # on it. The # will indicate what size jets the plate would mimmic. Then you can get a starting point for expierementing with different secondary jets. I think i remember increasing the secondary jetting an effective two sizes with this kit. Again the modified 350 had no trouble gobbling up the extra fuel on the 600 carb.

To everyone, Honestly I would like to try a 750 vac sec on my engine. I dont however, want to pay 300 some odd bucks to find out that it mat not make that much difference. Anyone close by got one laying around that I could borrow for testing?