Need some carb opinions

justbob

justbob

Sick-Speed play toy
So I sent out my 461 BBC to get a mild rebuild and the engine builder is insisting my Edelbrock 750 is not going to be enough, he is suggesting anything in the 850, maybe even the 900 range. Now I have never owned anything outside of the typical quadrajet so I’m trying to research and most everything I find is streetability issues on much more radical engine builds on carbs of that size and many guys dropping in size all the way to a 650, even quite a few just going with a new quadrajet? Confused..

Truck will never be raced, this is strictly a look the part, sound the part Rat rod cruiser.

Basic specs:

454 30 over
Mostly stock bottom end, standard bore and stroke
GM Performance Roller rockers
Double valve spring
Large oval heads casting #336781 supposedly one of the best flowing cast iron heads, still waiting word on if the valve size was previously increased.
COMP XE268 cam 224/230
Billet MSD distributor

700R4 / Unknown stall but 12” so probably quite low.

9” with somewhere around a 3.50 gear

Roughly 3200lbs


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I'm no expert but I had a 396 BB Chevy and it ran best with a 650cfm carb. I tried a 750 and an 850 but those were way too much cfm.
 
How many RPMs you gonna spin it? I would just throw the 750 on it and see how it acts. Throw a vacuum gauge on it and see how much of a restriction it is when you get it running and at the max rpm you will take it.
 
That’s my thought guys. It just doesn’t make sense to blindly throw a different carb at it completely untested. This guy specializes in BBC engines primarily for racing and very well known locally for his head work and I can’t seem to get the point across of “been there done that, not interested.” I’m quite sure he knows his stuff by now and I try to respect that, I just feel his thought patterns are more toward performance or maybe the fact that over fueled means zero chance of a lean condition and getting a possible bad rap? You know, the last minute “fuel tweak” after a tune..

He said I should have no problem hitting 7,000 RPMs. I honestly can’t ever see hitting 5,500, probably more like 4,000..

I forgot to add we are targeting the compression for around 10.5 to 11.1 and 93 friendly. He projects easily hitting 500 crank HP and I told him try and keep it under. Hell 400 is enough for my application. I want dependability first and foremost, and a nice big displacement cammed sound second, HP is dead last. We all know how long things last when we max them.. [emoji6]


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1HP per cubic inch is easily obtained and very reliable and streetable. You finally have displacement on your side :P
 
I’m no mechanic, but what about one of the EFI units that bolt onto the manifold and replace the carburetor?
Or do you want to keep it 100% old school?


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A bigger carb almost always provides more HP.....at wide open throttle on the dyno
They never talk about driveability at partial throttle (95% of most street driving)
If you are strictly racing the car/engine, then of course, who cares about part throttle operation and driveability.

Here's a calculator to try.
I came up with @ 650 CFM for your combo on the street....5,800 rpm and .85 VE
You can play with a higher Volumetric Efficiency 'VE' along with higher maximum RPM and get an answer closer to 750 cfm....but anything larger than 750 cfm is too big.


How to calculate Carburetor CFM

The formula for calculating how much CFM (cubic feet per minute) your engine requires is:

CFM = Cubic Inches x maximum RPM x Volumetric Efficiency ÷ 3456

Any ordinary stock engine will have a volumetric efficiency of about 80%. Most rebuilt street engines with average bolt-ons have a volumetric efficiency of about 85%, while race engines can range from 95% up to 110%.

Example: Using a 461 CID engine x 5,800 max rpm = 2,673,800
Take 2,673,800 x .85 VE = 2,272,730
Then 2,272,730 ÷ 3456 = 658 CFM


Even with about a 10% cushion, a 700 CFM carburetor will handle this engine great. If you try this formula yourself, be honest with how much rpm the engine will see.
For street driving, you’d be better off with a carb smaller than needed rather than something oversized that can lead to poor drivability and performance.


Here's a good read on different types of Carbs:

https://www.motortrend.com/how-to/much-carburetor-need-application/
 
Last edited:
Joe Walsh said:
A bigger carb almost always provides more HP.....at wide open throttle on the dyno
They never talk about driveability at partial throttle (95% of most street driving)
If you are strictly racing the car/engine, then of course, who cares about part throttle operation and driveability.
Click to expand...

My 406 6V is 920 CFM. Makes lots of noise and puts on a great show, but if I really want to do fast it requires throttle control and slow opening. Too much air at once doesn't get you there.
 
MM03 said:
I’m no mechanic, but what about one of the EFI units that bolt onto the manifold and replace the carburetor?
Or do you want to keep it 100% old school?


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Click to expand...
ZERO interest. In fact after I went return style on the Marauder I was close to slapping a carb on that too! [emoji16]

I’m definitely old school. Never once had issues with any carb, cars, bikes, mowers, no matter, Normally I’ve got great hands at tuning them, even multiples.

Joe Walsh said:
A bigger carb almost always provides more HP.....at wide open throttle on the dyno
They never talk about driveability at partial throttle (95% of most street driving)
If you are strictly racing the car/engine, then of course, who cares about part throttle operation and driveability.

Here's a calculator to try.
I came up with @ 650 CFM for your combo on the street....5,800 rpm and .85 VE
You can play with a higher Volumetric Efficiency 'VE' along with higher maximum RPM and get an answer closer to 750 cfm....but anything larger than 750 cfm is too big.


How to calculate Carburetor CFM

The formula for calculating how much CFM (cubic feet per minute) your engine requires is:

CFM = Cubic Inches x maximum RPM x Volumetric Efficiency ÷ 3456

Any ordinary stock engine will have a volumetric efficiency of about 80%. Most rebuilt street engines with average bolt-ons have a volumetric efficiency of about 85%, while race engines can range from 95% up to 110%.

Example: Using a 461 CID engine x 5,800 max rpm = 2,673,800
Take 2,673,800 x .85 VE = 2,272,730
Then 2,272,730 ÷ 3456 = 658 CFM


Even with about a 10% cushion, a 700 CFM carburetor will handle this engine great. If you try this formula yourself, be honest with how much rpm the engine will see.
For street driving, you’d be better off with a carb smaller than needed rather than something oversized that can lead to poor drivability and performance.


Here's a good read on different types of Carbs:

https://www.motortrend.com/how-to/much-carburetor-need-application/
Click to expand...
Nice calculator you found Joe! You are all confirming my thoughts in trying a go at this one. I only drove it 1 mile at most like a grandma with some SERIOUSLY BAD noises coming from the bottom end! Just confirming that it shifted, stopped, ETC to gauge a budget so certainly no spirited driving. I can say it idled and drove just fine considering it had at minimum 3 spun bearings.. I can also say they certainly didn’t know how to tune as my nostrils were on fire and my clothes stunk to hi hell!!


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the 750 cfm should be adequate . EDELBROCK [carter] replacement is a reliable brand and trouble free. for your 72 up vintage block should be fine
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