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Old 07-16-2007, 03:06 PM
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help on carb choice..

i have 454 bored .30 = 460 ci, have ported 781 heads, roller rocker's, holly dominator single plane intake, cam is 305 comp cam,.575 lift., spin the motor to 6500 rpm's, c/r is about 10.5 to 11.1 to 1.. i have a holly dp 670 cfm,street avenger on it just to here it run, doesnt want to idiel, guy before me said he tryed lot's of carbs, never idiel, had a predator on it ran fine, but it break's down to much.. motor has very low vacum,, wat carb would best sute me..? thank's

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Old 07-19-2007, 08:28 AM
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The 650 is pretty close to what you want on the street/strip. If you are building an all out racer, go to a 750 but then you won't need idle on a WOT engine. Low vacuum, hot engines are tough to get to idle because of the low manifold vacuum. Typically you need to open the idle set screws so wide to get the air flow the engine needs that the butterflys open past the idle gas feed slots and no gas is sucked in with the air. Catch 22 - close the butterflys down and not enough air flow, open the butterflys for more air and fuel flow stops. The typical fix is to drill a hole in each butterfly so air can get past with the butterflys closed down to the point gas flow is reestablished. Start out with 1/8 holes.
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Old 07-19-2007, 09:30 AM
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Assuming that dp means double pumper, the Street Avenger series are not DP's.

Vince
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Old 07-19-2007, 11:23 AM
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Quote:
Originally Posted by willys36@aol.com
The 650 is pretty close to what you want on the street/strip. If you are building an all out racer, go to a 750 but then you won't need idle on a WOT engine. Low vacuum, hot engines are tough to get to idle because of the low manifold vacuum. Typically you need to open the idle set screws so wide to get the air flow the engine needs that the butterflys open past the idle gas feed slots and no gas is sucked in with the air. Catch 22 - close the butterflys down and not enough air flow, open the butterflys for more air and fuel flow stops. The typical fix is to drill a hole in each butterfly so air can get past with the butterflys closed down to the point gas flow is reestablished. Start out with 1/8 holes.
Before i started drilling holes in the butterflys id open up the secondary butterflys about a quater of a turn with the little screw on the underside of the baseplate only on holley's and demon's you can do this.
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Old 07-19-2007, 11:43 AM
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First I think that your carb is too small...and normally I don't say that because people have a tendancy to overcarb. I am not sure that it will help with your idling problems but you may be creating a lean condition in the higher rpm's that could potentially ruin your motor. I read in "how to hotrod big block chevy's" by HPbooks there was a calculation in there that involved cubic inches desired Rpm and a number that came out like this a 427 at 6000 rpm needed a 750 cfm carb. And CFM's are CFM's no matter how you get them there. But I think something with mechanical secondaries would help because the low vacuum may be allowing extra fuel in at idle. Check out this Graphic for the cfm you need.
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Old 07-19-2007, 12:26 PM
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what is your ignition timing like?

initial and total?
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Old 07-19-2007, 01:13 PM
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I think Double V23 has a point.

The "standard" formula for carb sizing is (RPM X Displacement X .5) / 1728. The idea here is that you need air equal to the displacement times the rotating speed. But, since it's a four-cycle, you need only half the air (you're not pulling any in when on the power stroke, only on the intake). Then you divide by 1728, 'cause you calculated everything for cubic inches, and dividing by 1728 will convert cubic inches to cubic feet. This theory all assumes 100% volumetric efficiency. Which with a NA engine, you almost certainly do not have.

So, (6500 X 460 X .5) / 1728 = 865 CFM. I'd say anything over 800 would likely work fine. But, I certainly dunno what to say about the idle. I think the other guys may have posted some good thoughts there.

Pat
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Old 07-19-2007, 03:19 PM
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The calculations were done using a volumetric efficiency of 83% which is pretty standard so the 765 cfm's will get you pretty close. I like the edelbrock mechanical secondary 750cfm for a decent price.
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Old 07-19-2007, 04:35 PM
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Quote:
Originally Posted by Double_v23
The calculations were done using a volumetric efficiency of 83% which is pretty standard so the 765 cfm's will get you pretty close. I like the edelbrock mechanical secondary 750cfm for a decent price.
You're assuming that at WOT a manifold vacuum reading of 1.5" provides maximum performance. It does not. Maximum power with a carb occurs at WOT when the manifold vacuum is between .5 and 1" vacuum. So you have to convert a carbs airflow rating @1.5" to about .75" (it now flows less) to find the right size carb for maximum power on a hi performance motor.

To find what a carb rated at 1.5" flows at .75" vacuum mutiply by .707
eg: 750cfm @1.5" flows 750X.707 =530cfm @.75"

As long as the carb will meter the fuel at the lowest WOT rpm that you'll be using (usually converter stall speed), the carb is not too big.
if the carb will not meter the fuel properly at the lowest rpm, then you either have to improve the venturii signal so that it will, or pick a smaller carb that will.

Selecting a carb based on the rated flow @1.5" is good for most stock and near stock cars that use a stock low stall converter. But if your car is geared for quarter mile and has the right stall converter you can pick a bigger carb and gain some more power than the standard holley formula suggests.

Install a vaccuum guage at go for a ride @ WOT at maximum rpm and read the guage.

Your 460ci BB could use up to a 1050cfm rated carb. A 850 is not too big.
A 850 usually works very well on a motor like yours.

How to get your carb to idle:
A motor with a big 305H magnum cam needs a ton of initial timing at idle to idle decent. Remove the distributor weights and springs and bind the advance mechanism "locked out". I use simple electrics tie wraps. Orient the knots of the tie wraps down so they don;t interfer with the rotor. reinstall the rotor and cap. Now the advance is frozen.
Remove the carb, flip it over and reset the throttle blades opening adjustment (idle speed) on both the primary and secondary throttles so that the idle fuel transfer slot is just slightly exposed under each of the throttle blades (.015" -.030")
The secondary throttle idle opening adjustment is a small set screw on the carb base.
Swap the power valve for a 3.5" rated valve. Reinstall the carb. Do not touch the idle speed screw once it is preset as described. Make sure the PCV hose is connected to the base of the carb and functional (a vent on the other valve cover). Now with the timing "locked out", set it at 38deg BTDC. Let the motor warm up and adjust the idle mixture screws for best idle quality. No throttle blade drilling is nessessary. Should idle roughly but rock steady at about 900rpm. The idle and throttle response will be much better.
If the car is hard to start when hot (typical GM) with the ignition timing locked out install a ignition cutout switch on the distributors power wire to disable the spark during cranking. Crank it over and throw the switch and it will start easily.

Last edited by F-BIRD'88; 07-19-2007 at 04:59 PM.
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