Alright work with me here. I did not build the engine. Question Holley 900 or 950 Cfm (not sure whice one it is yet) Is it too big for my 396 with I belive 10 to 1 compression, slight cam, Hi-rise intake, Msd iginition and coil, hooker headers.
Will it run alright at street speeds (oh yea, I have 411 gears) and yet I'll still know it's there when I want it? I have a 650 now but it is somewhat lacking,
what-cha think?
Donny

A 900-950CFM IS too large for a mild 396CID engine, a real basic formula: CID X Max RPM / 3456 = CFM 396CID X 5500RPM / 3456 = 630.20 CFM, your 650CFM sounds about right.

I would run a 750 HP series carb. Lots of tune ability with the HP series. Oh, mechanical secondaries too.

You people with your simple math tear me up! a carb guy said it was to big too, but I just wanted to check with the experts.
I swear if the doctor said I needed to have an operation i think I would check with you guys first:You_Rock:
Thanks
Donny

Both 72 & Tom have given you GOOD advice, and a question very similar to yours was asked just a few days ago by another member, so I thought I'd give a better explanation here. Hope it helps.

#1 Q. Mechanical secondary or vacuum secondary, which style of carburetor
should I use and why?
A. Generally, a mechanical
secondary carburetor is
preferred on vehicles with
manual transmissions and on
automatic transmissions with
3000+ RPM stall-speed
converters. For automatics
with less than 3000-RPM
stall-speed converters, the
vacuum-secondary carburetor
is usually the better choice.

#2 Q. Does a bigger carb make more
power? What’s the limit?
A. A larger carburetor can
produce more power on a
dynamometer, but under
normal operating conditions
can result in slower
acceleration and lower
efficiency of fuel
atomization. Select the
smaller carburetor,
especially if you’re
undecided about sizes. The
smaller diameter venturi
increases the velocity of the
air-fuel mixture. As a consequence, it
usually provides better acceleration
and proves to be more efficient.

#3 Not having the proper size of carburetor for the application
Having the proper venturi sizes for a given application ensures the carburetor
generates sufficient air speed. Air speed creates the necessary depression (low
pressure) in the constricted area of the boost venturi to draw fuel through the
metering systems and into the air stream to be atomized.

The optimum air/fuel ratio varies from one engine to another. This ratio is often stated in terms of its Stoichiometric value (pronounced Stoy-kyo-metric)—that is, the ideal ratio of air and fuel required to provide the complete burn. For unleaded fuels, the Stoichiometric ratio is stated as 14.7:1, while the values for 100-octane race fuels and methanol are 15.1:1 and 6.45:1, respectively. Also, air/fuel ratios vary with load conditions (those that are ideal for fuel economy are not ideal for maximum performance). Because of small losses during the vaporization process and because other losses that occur when the residual exhaust gases in the cylinder dilute the fresh incoming charge, a more realistic air/fuel ratio for maximum horsepower on gasoline will probably be a richer 12.5:1 to13.5:1.
When the mixture ratio strays from the optimum, either excessive air or fuel will be present in the cylinders and the power output of the engine will deteriorate. Whatever the optimum ratio, maximum power is reached when the air/fuel mixture is distributed evenly to the cylinders and when that mixture contains just sufficient fuel to consume the air in the cylinders. Optimum air/fuel ratios are usually determined by an engine dynamometer via BSFC numbers (Brake Specific Fuel Consumption—the rate at which an engine converts fuel to power) or by a chassis dynamometer using the highest torque and horsepower values. Once established, an air/fuel ratio meter, like the hand held LM1 produced by Innovate, could help monitor the figures.

#4 Further fuel factors

Fuel distribution is influenced not only by the carburetor but also by intake manifold design and cylinder head port placement. When fuel is not atomized well, the larger droplets of fuel will fall from suspension and collide with the walls of the intake runners or cylinder head ports and begin to puddle. "Puddling" adversely affects the air/fuel ratios of the cylinders because of the subsequent disparity in cylinder filling: Some cylinders are rich while others are lean. Consequently, richer air/fuel ratios will produce more power in good
air and, therefore, increase the speed of the car; in poor air conditions, the rich mixture (improper air/fuel ratio) will lose power, and the speed of the car will falter.
But hay, what do I know, :dontknow:

Nice info Coyote. Thanks.

I have a friend with a 402/TH350 in a 68 with 3.73 gearing. I believe he's running a classic Holley 750 CFM carb with no problems. Nice throttle response. Never bogs. I'm running a Street Avenger 770 CFM on a 454 and it's fine around town, but a little small for WOT HP.

950 is way too much for even a modified 396 in a heavy car on the street. Based on the post above, I think you'd be happier with either the slightly undersized 650 or slightly oversized 750 depending how "mildly modified" your 396 is.

Nice info Coyote. Thanks.

I have a friend with a 402/TH350 in a 68 with 3.73 gearing. I believe he's running a classic Holley 750 CFM carb with no problems. Nice throttle response. Never bogs. I'm running a Street Avenger 770 CFM on a 454 and it's fine around town, but a little small for WOT HP.

950 is way too much for even a modified 396 in a heavy car on the street. Based on the post above, I think you'd be happier with either the slightly undersized 650 or slightly oversized 750 depending how "mildly modified" your 396 is.

I went through Holley's, Eddies, Q Jets, Quick Fuel, and finally stayed with a Speed Demon 750 on my 454 / kitted 350 / 373's. For me, I found it had the BEST ALL AROUND performance of all the rest. Performance you could feel in the seat of your pants I might add.

I took this from the TA Performance catalogue on how to size your carb to your particular setup.

"One method of measuring for the proper size carburetor is to take a vacuum reading at full throttle. Less than 1” of vacuum is recommended and between .2 and .5” of vacuum indicates the proper size carburetor is being utilized."

I would run a 750 HP series carb. Lots of tune ability with the HP series. Oh, mechanical secondaries too.

thats what id say also

Good info. Thanks.

Does engine RPM have any bearing on the reading? Seems to me that WOT at 3500 RPM would hold less vacuum than WOT at 6500 RPM. Anyone with real world experience out there that has watched their vacuum gage and can say one way or another?