I am looking to find out what CFM rating carb is best for doing the carb swap.

My previous experience with LA motors says 600-650 CFM should be plenty for a 5.2 or 5.9.

However on the crate 5.9 Magumns sold with the carb, they are using a 750-800 CFM range. These are not race motors, they are just carbed 5.9s running a stock cam and head. Why such a large CFM carb on these?
Do the Magnums just flow so much better that they can and will use the additional? I think they had around 80 or so more horsepower than their LA counterparts IIRC maybe that too has something to do with it.

I just don't want to buy more than I need, but don't want to short myself.

Looking to run a 5.2 with a whiplash cam, using the current 2200 stall, 518, and 4.30 gears.

The carb they used was on an engine dyno, not a street car. They tested at WOT, no thought given to throttle response or anything else.

On my LA 360, mild setup, I tested a 600 and a 750 cfm carb in my daily driver. The 750 could peg the 125 mph speedo. The 600 could not but in all other areas, again on a daily driver, was superior. Better throttle response, better get up and go, better mileage. But anything running 4.30 gears isn't a daily driver even with an A518, imo.

It really depends on how you are going to use the vehicle, imo. Daily driver or weekend cruiser, 600-650. Race car 750, weekend warrior, well I'd probably go with the 750 myself.

Oh and saying whiplash cam doesn't really help, there are four possible whiplash cams that can be run, two specified for the Magnum and two for the LA, all of which will work, all with different specs.

It would be the 5.2 Magnum Whiplash camshaft

I have been using the 4.30 gears for over 6 years in the truck as a daily driver, grocery getter, and just a fun driver. 70mph at 2300rpm.

A lot of people forget the 340 6 packs had 1340 cfm. More than most will need for a DD/weekend drag vehicle. Most of my LA motors i ran 600-700 CFM carbs. As stated it depend on how you are going to be using it. The stock Throttle bodies are 500 CFM.

Can’t go wrong with a TQ

there are two of those cams listed, different specs.

I drove around with 4.10 gears and no OD for years too. Drove it from Memphis to San Diego like that too. Would I do it today? No.

You really need to give ALL the specs on your setup when you want something like a carb recommendation.

^^^ Yup.
It's something of a generalization, but for street use, a smaller carb will work better than a big carb, especially on a small engine like a 318.
Velocity vs volume, and better vacuum signal as a result.
Airflow of the heads also figures in.
Note that the 6-bbl/Six-Pack cars generally ran with the small center carb for most street duty.
Agree with the T-quad, kinda best of both worlds on the street, been there.
2nd best I think would be an Eddy AVS2, I really like mine.
For most 318s, 600-650 CFM is fine.

I believe you are mistaken. There is only ONE 5.2 Magnum Whiplash camshaft listed on Hughes site. Not a 5.9,, not an LA 318, or LA 360, a 5.2 Magnum.

And just because you dont care to drive with steep gears, how is that relevant to carb size?

I gave you the 5.2 Magnum, I gave you the stall, I gave you the transmission, I gave you the gear ratio, I listed the cam (believing that a reasonable person would understand, that when stating they were using a 5.2 Magnum engine, and a whiplash camshaft that said camshaft would be for said application, not a mix and match situation) If you would like to know my tire size, it is 295/60R15 and it is a Hoosier Quick Time DOT radial. I am sorry I do not know my curb weight, or the weight at each corner, and I cannot tell you where the exact center of gravity is on the truck.

I like the TQ but it wont work with the available intakes for the magnum. I will be using an AVS-2 which is a nice carb I have one on a 360 in my 77. I was just trying to understand the reasoning behind the 800 cfm being used on a stock magnum. I've had a 383 with dual 750s on an M1 tunnel ram, but it wasn't a stock 383. So I was just looking for the reasoning. I thought possibly it was due to the better flow, the magnum breathes better. I'm not trying to argue for a bigger, or smaller.

The responses have been in the 600 to 750 range which most would agree is appropriate for a 318. Which you would choose would probably depend on whether you want slightly better street manners or whether max output was the end goal. At least that is the way I see it.

But what I think is the most important issue in carb selection is the calibrations. In any popular size there will be quite a few choices on that. So not every 600 will run the same on your particular combo. Nor will every 750. Your happiness with your final selection may have more to do with the calibrations than the actual size in a primarily street application.

I am not familiar with that cam. Does it have a bumpy idle? If so, I sure wouldn't use one of the CarterBrocks. Their idle and transition circuits are antiquated and second best at the least. And adjusting them is a nightmare. If it is a bumpy idle, I would suggest one of the 4150 Holley style platform carbs that have the 4 corner idle adjustments. And if you need to go inside to tighten the tune, they are much easier to do and to get the parts for.

If the idle is not bumpy, then the best street carb out there today is the Street Demon. Available in 625 and 750 and with different center sections to choose from. They have the best street manners of any carb I have installed. But do not like a bumpy idle. The Street Demon was designed with the help of a retired Carter engineer and take off from where the old ThermoQuads left off. And every carb has both intake patterns, so it will fit your intake. The reason for my preference for the Street Demon is the throttle response. It is just crisper than most any other carb I have ran out of the box.

But let me say again that these carbs work best on mild street engines. They would not be my first choice for hotter or more rodded combos.

That's my

www.youtube.com/watch?v=DPOuEOkazVc

www.youtube.com/watch?v=jN2xbt6Mzdc&t=70s

Drill the heads to use any LA intake or Magnum intake

Many 318s came from the factory with 800 CFM carbs and a few with the 850 so I would use an 800 CFM eddy AVS and not look back. The AVS works very similar to those factory 800s by running on smallish primaries for cruise and kicking in big secondaries when you floor it. The 750 eddy seems to be a tuning nightmare and the 600 is too small for WOT but pretty close in primary size.

And as far as actually running a TQ, I would. There is the M1 single plane (don't worry about it being a single plane it will work great with that stall, cruising RPM and gears) that it bolts right up to or the re-drilling the intake bolt pattern option opens up a ton of options.

Have you considered a Holley 670 Street Avenger. Ran that on two 383 cars with great experience.

https://www.summitracing.com/parts/hly-0-80670?utm_source=bing&utm_medium=cpc&utm_term=%2bholley+%2b0-80670&utm_content=Holley&utm_campaign=Part+Number+Ad+Groups

Summit link to Holley SA 670 cfm

My fuzzy memory is that the OEM throttle body rating on a Magnum 5.9 was 600 cfm at 1.5 inches of Hg drop.

Several people sold ported and “blade blended” Magnum throttle bodies but power gains were minimal.

My 1995 Magnum 5.9 V8 MAP sensor would read 1.6 inches Hg at wide open throttle at 4000 rpm.

One of the crazier things I did was check the accuracy of my MAP sensor by driving up Mt Mitchel in NC, stopping several times to compare MAP to barometer.

I discovered that the 2 EGR outlets just below the butterflies caked up with carbon deposits that must have reduced flow.

Do not forget that pre-Magnum there was an OEM TBI fuel injection throttle body.

I have the Eddy Air Gap RPM 4bbl magnum intake ready to go, so the intake is set. I really like the 650 AVS2 on my 77 LA, and was looking at the 800 AVS2 for this if it needed it (or would be able to use). while it will be mostly driver use, it will also go to our 1/8th mile track now and then.

i have used the T-quad a bunch of times using common spread bore to square bore adapters.
it took me a long time to figure those carbs out, especially tuning the air door and doing the adjustments in the proper order.
after learning that, i grab all of those i can when others toss 'em in the trash bin.

625 with a 5.2, 750 with a 5.9