Carb CFM recommendations

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thecatsfan

thecatsfan

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Recently ordered a 318 long block assembly and waiting for it to ship. Getting the rest of the parts together to complete this build to drop into a 67 Cuda. The 318 is a standard LA block with stock heads. Upgraded to 9.5 to 1 flat top pistons, a mild performance cam (.450 lift with 272 duration and 110 separation), high performance valve springs and push rods and double row timing set. Everything else is pretty much stock. With a 4 barrel and headers, this engine should be around 275 to 285 crank hp, with a cam hp range from 1500 to 5000 rpms. This is not a race car. This is a convertible being rebuilt as a sentimental family restoration and we simply are looking for a daily cruiser with moderate performance and responsive drivability. I am doing all of this as a favor to a sister who bought this car in the 70s as a teenager. She is paying for parts only.

I am putting an Edelbrock 4 barrel performer intake and planning for an Edelbrock carb. I would love an LD4B intake, but everyone wants more for a beat up used intake than a new Edelbrock. For carb, the best options I can buy from Edelbrock are the 1406 which is a classic style 600 cfm square bore, or the new AVS2, which only comes in 500 or 650 cfm. 500 seems small to me, but 650 seems like too much. The sweet spot for this engine seems to be around 550, but that is not an option Edelbrock. I like the new design with annular boosters, but do not want to undersize the carb.

I should mention that we live at 5K feet elevation.

Any recommendations from experience would be appreciated.
 
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well, if your limiting yourself to the carbs you have, then that makes it easy. Try 'em all out and see which one you like best. I run a 770 cfm on my 318.
 
Go with the 650 AVS2 and don’t look back. The performer is a spread bore pattern so you will need the adapter. I’ll just say it, find an LD4B and you will be happy
 
318willrun said:
well, if your limiting yourself to the carbs you have, then that makes it easy. Try 'em all out and see which one you like best. I run a 770 cfm on my 318.
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So you want me to buy every carb option and throw away the ones that do not work well? Really? 770 cfm is 30% more air than this engine can possibly consume.
 
For the uses you describe, the primary barrels will be the only ones flowing air 99.9% of the time. You'll get into the secondaries only when making a hard acceleration at mid rpms (3000 up). So don't worry about the marketing size so much. None of those mentioned will limit the fun because they are too small. At the drag strip, odds are you'll mph better in the 1/4 with the slightly larger ones. But since thats not in the plans, its irrelevant.

IMO, use one that matches the intake. Speadbore if its a speadbore intake like the performer or LD4B.
 
DesertRat said:
Go with the 650 AVS2 and don’t look back. The performer is a spread bore pattern so you will need the adapter. I’ll just say it, find an LD4B and you will be happy
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Thanks. I would if I could. But an LD4B is simply not available unless ridiculously overpriced. This is a car that will be cruised and almost never driven at WOT. So to overpay for a used intake just does not make sense.

I know the performer carb needs an adapter plate to fit that intake, but they provide one for free. Thanks for the advice. I do really like the AVS2 design.
 
thecatsfan said:
So you want me to buy every carb option and throw away the ones that do not work well? Really? 770 cfm is 30% more air than this engine can possibly consume.
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Relax dude, trying to help. The way you wrote it is "the best options I have" I took to mean you already have the carbs. That's why I said "try them all". So if you haven't bought the carb, then I don't think you listed the best options you have. I have a 770 on my 318 because it outperformed the 600 eddy and 670 I had on it, ET proven. I have less of an engine than you. Good luck :)
 
thecatsfan said:
So you want me to buy every carb option and throw away the ones that do not work well? Really? 770 cfm is 30% more air than this engine can possibly consume.
Click to expand...


Do NOT get caught up in the CFM numbers. The carb will flow the air the engine needs. With the adjustable air valve on the secondaries you can tune that up.


Again, like many things carb related, the math to calculate CFM usage is at best, silly. It's really pretty much worthless. It doesn't take into account what I said above, and it also doesn't account for pumping losses and things like that.

Drop the 650 on there and work on your tune up. Ignore the math.

In fact, my carb is considerably larger CFM rating wise than the math calls for. Think about tunnel rams with dual carbs.
 
Mattax said:
For the uses you describe, the primary barrels will be the only ones flowing air 99.9% of the time. You'll get into the secondaries only when making a hard acceleration at mid rpms (3000 up). So don't worry about the marketing size so much. None of those mentioned will limit the fun because they are too small. At the drag strip, odds are you'll mph better in the 1/4 with the slightly larger ones. But since thats not in the plans, its irrelevant.

IMO, use one that matches the intake. Speadbore if its a speadbore intake like the performer or LD4B.
Click to expand...


The reason I like the square bore carb is the larger primaries - and you are right, that's all this car will use 99% of the time. Edelbrock provides the adapter plate with the carb for a direct bolt on since they don't offer a square bore intake for this engine. But I can see your point. Thanks!
 
yellow rose said:
Do NOT get caught up in the CFM numbers. The carb will flow the air the engine needs. With the adjustable air valve on the secondaries you can tune that up.


Again, like many things carb related, the math to calculate CFM usage is at best, silly. It's really pretty much worthless. It doesn't take into account what I said above, and it also doesn't account for pumping losses and things like that.

Drop the 650 on there and work on your tune up. Ignore the math.

In fact, my carb is considerably larger CFM rating wise than the math calls for. Think about tunnel rams with dual carbs.
Click to expand...
yeah, what he said.... ^^^
 
yellow rose said:
Do NOT get caught up in the CFM numbers. The carb will flow the air the engine needs. With the adjustable air valve on the secondaries you can tune that up.


Again, like many things carb related, the math to calculate CFM usage is at best, silly. It's really pretty much worthless. It doesn't take into account what I said above, and it also doesn't account for pumping losses and things like that.

Drop the 650 on there and work on your tune up. Ignore the math.

In fact, my carb is considerably larger CFM rating wise than the math calls for. Think about tunnel rams with dual carbs.
Click to expand...

Yeah, I see what you are saying. I understand the math is sketchy to a point. But overcarbed is overcarbed. I bult many small blocks back in the 80s and they all ran at their best with carbs well under 700 cfm. I had a 327 in a Chevelle with 202 heads that put out almost 350hp with a Carter Comp AFB 625 and it ran really strong. Not saying you are wrong. You make some good points. Just hard to kill old habits. And I have seen so many people flood cars with oversized carbs.
 
thecatsfan said:
Yeah, I see what you are saying. I understand the math is sketchy to a point. But overcarbed is overcarbed. I bult many small blocks back in the 80s and they all ran at their best with carbs well under 700 cfm. I had a 327 in a Chevelle with 202 heads that put out almost 350hp with a Carter Comp AFB 625 and it ran really strong. Not saying you are wrong. You make some good points. Just hard to kill old habits. And I have seen so many people flood cars with oversized carbs.
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Carbs aren't superchargers or forced induction. They do not FORCE anything into an engine. IF your engine will only pull 440 cfms, then that's all a 750 cfm carb will flow on THAT particular engine. Factory thermoquad on a 318 was 800 cfm's, and that was with single exhaust.
 
318willrun said:
Relax dude, trying to help. The way you wrote it is "the best options I have" I took to mean you already have the carbs. That's why I said "try them all". So if you haven't bought the carb, then I don't think you listed the best options you have. I have a 770 on my 318 because it outperformed the 600 eddy and 670 I had on it, ET proven. I have less of an engine than you. Good luck :)
Click to expand...

Yes, you are right. That was poor wording. Best options is like favorite flavor. I doubt we will agree on what is best. And in carb world, sometimes it comes down to knowing what you are comfortable working on. I am not a carb expert by any means, but have always had good success with Carter, and Eddy's are basically a new version of them. I appreciate you sharing your experience.
 
thecatsfan said:
The reason I like the square bore carb is the larger primaries - and you are right, that's all this car will use 99% of the time. Edelbrock provides the adapter plate with the carb for a direct bolt on since they don't offer a square bore intake for this engine. But I can see your point. Thanks!
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Welllllll...........
The big advantage of a 4 bbl over a 2 bbl is that the primary barrels will not cause a restriction at high rpm.
Soooo
Smaller primaries should not be of concern because when more air is needed, the secondaries will open and insure no restriction of concern.

The advantages of smaller primaries is the potential for better effeciency and throttle response.
With bigger primaries, the throttles need to open less to get the same rpm.
This means the engine is getting fuel is mixed in from the 'idle circuits' at higher rpms compared to an exactly the same carb with smaller primaries.

In reality they all work reasonably well.
My only point was that the small primaries are not a disadvantage.

Sticking with a same 'pattern' means less chance of issues, and better chance the cylinder to cylinder fuel distribution will be better.
 
thecatsfan said:
Recently ordered a 318 long block assembly and waiting for it to ship. Getting the rest of the parts together to complete this build to drop into a 67 Cuda. The 318 is a standard LA block with stock heads. Upgraded to 9.5 to 1 flat top pistons, a mild performance cam (.450 lift with 272 duration and 110 separation), high performance valve springs and push rods and double row timing set. Everything else is pretty much stock. With a 4 barrel and headers, this engine should be around 275 to 285 crank hp, with a cam hp range from 1500 to 5000 rpms. This is not a race car. This is a convertible being rebuilt as a sentimental family restoration and we simply are looking for a daily cruiser with moderate performance and responsive drivability. I am doing all of this as a favor to a sister who bought this car in the 70s as a teenager. She is paying for parts only.

I am putting an Edelbrock 4 barrel performer intake and planning for an Edelbrock carb. I would love an LD4B intake, but everyone wants more for a beat up used intake than a new Edelbrock. For carb, the best options I have are the 1406 which is a classic style 600 cfm square bore, or the new AVS2, which only comes in 500 or 650 cfm. 500 seems small to me, but 650 seems like too much. The sweet spot for this engine seems to be around 550, but that is not an option. I like the new design with annular venturis, but do not want to undersize the carb.

I should mention that we live at 5K feet elevation, so airflow is critical because air density is low up here. We lose about 13% power due to lack of air density. You definitely do not want to limit airflow with thin air.

Any recommendations from experience would be appreciated.
Click to expand...

In the "How to" articles...

PICKING THE RIGHT CARBURETOR SIZE FOR YOUR MOTOR
 
IMO skip LD4B and don't look back.
It would only be of use at the strip.
Study what happened when an LD4B replaced a factory intake on a stock 340.
Edelbrock LD4B
Below 3500, the gains from installing headers were practically lost.
Something to think about.
 
thecatsfan said:
Yes, I get that. But those carbs had to be jetted down at the factory to match the limited airflow of the 318. Just because they made it work in limited production does not mean it was the best option. They simply had them on the shelf so they used them. But again, good point.
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I don't want to beat a dead horse, but I find this discussion interesting. So I want to cover the jet issue.

Again, getting caught up in the math or the numbers can get you way into the weeds. I learned this as a kid tuning 2 strokes and I still see it today with 2 strokes and cars.

Never get excited about jet numbers, CFM number and such. IOW's, don't let a jet number or CFM number throw your thinking.

Any time you change a jet, all you are doing is altering the fuel curve to match engine air demand. The number on the jet is essentially inconsequential.

When the carb was designed, the engineer had to adapt the jetting, emulsion, air bleed, idle restriction (in this discussion) metering rod diameters and plunger spring ratings all have to be set to some generic numbers. You could be buying the carb to go on a 289 Ford or on a 480 inch BBC. Since the carb is self adjusting to a fairly large degree, the engineer will err on the rich side of damn near every jet/bleed whatever. Because being slightly rich to rich is better than being lean and cooking part.

So adjusting jetting to Mach your air flow curve has nothing to do with the CFM rating of the carb. That's one reason why Holley took (and still takes) such a beating about low speed driveablity, fuel mileage and such. Most of those carbs cam calibrated on the far right side of rich for most everything. Bleed area, emulsion and such were calibrated so if you had more compression and lots of cam and gear you wouldn't bolt on a Holley and cook the engine.

Carter didn't do that nearly as much. They were looking at a different market. The fact still remains that the fuel curve for any given carb as it comes out of the box is most likely never going to be correct.

So the upshot is just because someone has to reduce the jet area of a carb to correct the fuel curve doesn't mean the carb is too big. In fact, it's probably a good thing. The engineers sweat bullets about getting lean issues with carbs. So they over jet everything.

Again, just trying to paint a picture about NOT getting caught up in the numbers game.
 
KISS...
Get the as you described classic Edelbrock 1406 and leave all the troubles behind. It will fit right on your Edelbrock manifold no problem. A couple simple adjustments and you'll be off with no problem. Actually you may have to jet down or quite possibly just tell Edelbrock your altitude they may send it pre-calibrated Jet and metering rod wise and leave just a little fine-tuning for you which is easier than falling out of your bed....
 
You are overthinking it. Happens all the time to the best of us. All those trees are preventing you from seeing the forest. You said "moderate performance and responsive drivability" so, you don't have to worry about dyno numbers or ET slips, it's an occasional use cruiser for fun. You could put a two barrel on it and it would still cruise around just fine.
I'd just use the 650, tune it properly, and drive the car. Have fun.
 
thecatsfan said:
Yes, you are right. That was poor wording. Best options is like favorite flavor. I doubt we will agree on what is best. And in carb world, sometimes it comes down to knowing what you are comfortable working on. I am not a carb expert by any means, but have always had good success with Carter, and Eddy's are basically a new version of them. I appreciate you sharing your experience.
Click to expand...

I absolutely agree with you and Jason both here.
Put something on there that is easy to deal with, set it and drive the heck out of it.
600 is plenty for a driver 318.

I got my Edlebrock 600 electric choke out of the pick and pull for 40 bucks and it's been flawless on my 5.9 Daily driver now for 3-4 years.
They are also on craigslist for 100 bucks or less all day long.
 
yellow rose said:
So the upshot is just because someone has to reduce the jet area of a carb to correct the fuel curve doesn't mean the carb is too big. In fact,
Click to expand...
In fact reducing jet area could mean that it has a strong signal. An 'oversized' carb would have weak signal.

Since we're discussing it, its probably worth describing why.

When the velocity is higher than expected through the booster(s), the pressure drop created in the booster is larger than expect.
A larger difference in pressure between the booster and pressure in the bowl will draw more fuel.
upload_2019-8-14_15-32-36.png


To reduce the amount of fuel relative to the air, a more restrictive jet is used.
 
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