Rage13

I'm trying to figure out what size carb I'm going to need for the 383 (wild roller cam and a 5000rpm redline)I'm planing on building, and from what I've come up with it looks to need a 500 cfm carb. It sounds kinda small when I hear all these other people talk about 750cfm carbs on their near stock 350s, but the numbers add up to me only needing a 500cfm. Is there something I've missed or is there just alot of people putting over sized carbs on for no reason?

Next topic is 2bbl vs 4bbl. If both are rated for the same cfm, what would be the reason for using one over the other? Only thing I can figure is the 4bbl would have better throttle behaviour.

Daz

iTrader: (1)

raise the VE number

Air_Adam

A 2bbl and a 4bbl both rated at the same CFM, the 4bbl will be more efficient, because all 4 throttle bores are smaller. Smaller = higher air velocity and a stronger 'signal' to the carb from the engine, which makes the carb's fuel metering ability quite a bit more accurate. Thats why Qjets work so well on the street... tiny little primary throttles.

Sonix

iTrader: (1)

A wild roller cam and 5000RPM redline? ?
What's not right here?

A 500cfm carb will just barely do the job for your application, if you've got the volumetric efficiency guessed right. Chances are you don't. Most people over carb, since it's not a very bad thing, especially if it's a vacuum secondary carb.

I don't see many 2bbl carbs for sale anymore, or in high enough CFM ratings. Using a 2bbl on a 383 is just plain silly, there's a reason NO ONE does it.

F-BIRD'88

2 bbl and 4bbl carbs are not rated at the same pressure drop.
3" vs 1.5" . So a "500cfm" 2bbl, when rated at the lesser 1.5" pressure drop 4bbl method will flow .707x its 2bbl rating. Or 353cfm.

The generally accepted formula for sizing a carb to a motor is very generalized. Is good for a stock motor but will under state the max carb cfm size needed to get maximum performance from a high performance motor.
4bbl carbs are rated at 1.5" manifold vacuum. But a motor that has the "right carb" on it to make max hp, when run at WOT at max rpm will not show 1.5" manifold vacuum.
It will show something around .75" to 1" manifold vacuum at WOT. So a 750cfm carb, when on such and engine will actually be flowing around 530cfm when the manifold vacuum is .75" at WOT. or 640cfm if the manifold vaccum at WOT is 1". thats why a 750cfm carb is not too big for a typical performance 350.

But the qualifier comes at the low end of the usable rpm range. If the car has a very low stall speed converter (too low) and low gearing a 750 carb may not have enough booster signal to properly meter the fuel flow at low rpm.

Generally, a dual plane manifold with its split plenum can use a larger carb than a single plane manifold can.

If the car has the right rear gearing and converter stall speed you can use this qualifier in addition to the popular carb sizing formula to get the right carb.
Pick a carb that is 1.1 to 1.2 x what the formula suggests for max hp for a single plane and 1.3 x for a dual plane with full split plenum.

eg 350cix 6500rpm /3456 x..90ve =592cfm. Engine airflow not carb size. Pick a carb 1.3 x bigger for a dual plane =769cfm
Carb size.
A 750 carb is "just right"
A 383 with a "wild roller" will want more than 5000rpm max. Where did you come up with that number?

A motor that has a poor intake manifold design will be better off and easier to tune with a smaller carb.

Rage13

This is why I love this site, thanks guys!
The reason for the low max rpm is because it will be a street motor so its need to run from idle up to max, also cause I'm looking for max torque rather than HP.
Explanation for the "wild roller cam" is that I like the loopy idle sound (kinda childish I know), but the more I research to get the most torque the less likely it seems I'm going to have it. Oh well..

So what I've figured from this is I'd be better off with a 650 cfm (383x5000/3456 x.90ve x1.3=648). Its going to be a Holly Double Pumper(T5 trans in car) sitting atop an edelbrock performer air gap. Sound about right now?

F-BIRD'88

No ... it sounds like you're really confused.
A "Wild cam" indicates a big cam. A big cam has lots of duration and overlap. thus the rough idle. A wild big cam likes lots of rpm. (needs lots of gear). Will make a "lot of torque" but, not at low rpm.

What do you really want the car to do?
Do you want? A "Bad Boy Street Racer, Drive-In Cruiser with the bad boy idle and power to match, or a fuel efficient commuter, towing package that cruises at a low rpm and sips gas, yet can pull a load on the hwyway ( towing)

What other parts are you using (manifold, headers, heads etc)
A radical street 383 will want about 6000-6500rpm.
It will still drive well right from a idle but will accelerate the best if you wind 'er up a bit and then pop the clutch so that the rpm drops to about 3000-3500rpm on the launch with a 5 speed).
It will have a usable best rpm range of about 3000 to 6000+ rpm.
A 750-800 cfm carb is just about right for most "radical street" 383's. A 650 will give up aboout 10-15 hp.
if you're going to buy a carb buy a 750cfm carb.
If you want a 383 that is best from idle to a max of 5000rpm then you do not want a "wild roller". You want a "mild street roller" A little bigger than a stock cam but not bigger than say 216deg at .050" ( or less)
it will not have a rough idle. YA can't have both.

Rage13

I'm building the car for road racing, and was figuring having the most torque I could get would help coming out of the corners and accelerating faster. I don't have the engine yet or any parts at all. I wanted to get it all figured out before I went and spent any money on wrong parts.

What I was planing on using so far is a 383 short block, edelbrock performer air gap manifold, trickflow heads (summit p/n TFS-32400006) and comp cam XR282HR.

F-BIRD'88

Sounds reasonable for a "road racer" I would use no less than a 750 carb. I would suggest less cylinder head port volume. less than 200cc intake port volume. (most overall torque)
Being partial to vortec heads I recommend a Ported, upgraded vortec head. If you want TORQUE These are the babies.