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Proper size carb

so I put togther a mild 440. Stock bore, edlebrock rpm heads and manifold, lunati cam.
According to the math my carb should be 649CFM. Now that seems a little small to me but thats what the numbers say. I have an old 1412 edlebrock, 800CFM, I could install that but I think I won't be happy because the lower carb signal with that size carb will be constant idle issue and tuning problems.
So the next choices are 670 street avenger or a 750 avs or just go with the 650 avs. Thoughts on this?
I'm sure this has been covered ad nauseum, just seeing what you guys did
You don’t need to worry about poor booster signal if using your old Edelbrock carb. It is an on-demand secondary, meaning secondary barrels do not really open unless needed. So really you’d be cruising around on 400 cfm and should have pretty crisp throttle response. If you stay in the throttle and engine demand is high enough the secondary air valve will open and then you get more air and fuel.
 
Keep in mind also that Holley carbs in particular always flow less than their rating..
I've always had much better results with a minimum 800 even on very mild builds. Properly tuned the low-end response is just as good as a smaller carb and they do everything else better.
 
Consider that Ma Mopar took the factory stock 4bbl motor, and put a 950 cfm six pack on it. It performed fine in every day driving, but kicked the 4 bbl motor's butt. And of course the factory Carter 4bbl was rated at 750 cfm.

The best running 4bbl on my 440 (smaller cam and heads than yours, and factory exhaust manifolds) was a Holley 800 dp.
 
Keep in mind also that Holley carbs in particular always flow less than their rating..
I've always had much better results with a minimum 800 even on very mild builds. Properly tuned the low-end response is just as good as a smaller carb and they do everything else better.

Bean...
Your comment that "Holley carbs flow less than their ratings" means what? Usually air flow (CFM) is at a specific pressure drop thru the carb, at a standard temperature and air density and corrected for barometric pressure. I believe Holley's are rated at 3.5" water column drop (I'm sure someone knows the actual number). What happens IF the CFM's are exceeded due to engine demand? The presdure drop thru the carb increases, but it still flows more air and fuel than the published rating. Perhaps I'm missing something....but the engine air flow requirements are a function of displacement, volumetric efficiency and pumping losses, cam, etc.
I'd like to know if there is a rating factor that adjusts the published rating to the actual installed flow....say a 750 CFM rating actually flows 680 CFM @ some stated set of conditions (for example). Perhaps the published CFM is an advertising ploy....the bigger the CFM number the more carbs get sold....influenced by the owner's "mine is bigger than yours factor"..... just my opinion of course.
BOB RENTON
 
Consider that Ma Mopar took the factory stock 4bbl motor, and put a 950 cfm six pack on it. It performed fine in every day driving, but kicked the 4 bbl motor's butt. And of course the factory Carter 4bbl was rated at 750 cfm.

The best running 4bbl on my 440 (smaller cam and heads than yours, and factory exhaust manifolds) was a Holley 800 dp.

IMHO, the factory 6 BBLs seldom "performed fine" in every day driving. They required tuning to make them really perform well. That's why there were so many available used for very reasonable prices by the mid '70's. I bought about four. But with good tuning, they did work really great.
 
The thing with calculators is it is science. A mathematical equation. It doesn’t take into account the restrictions of the intake manifold or exhaust, much less the build of the engine. The 649 cfm the calculator came up with is one thing but reality will show a larger carb will make more power and perhaps, drive better. Why?

The carb does two things besides give headaches. (LOL)
It mixes air and fuel of course but it is also a variable restriction based on drivers needs for the conditions driven and needed. Under a W.O.T. Setting, it is still a regulatory device. The less restriction through it the more power the engine makes. This is why you see this happening at the track, back streets and on TV shows like engine masters.

Even though the TV show reached a point of diminishing returns, what it is showing is that the less and less restriction placed on top allows easier breathing of the engine and thus more power.

Most people have a hard time and freak out when others say use a larger carb and fear a drop in mileage. Which is t the carbs fault since that aspect is co trolled by the state of tune, or lack of and the amount of pedal pressure given from the driver.

An engine will need a certain amount of air and fuel to accelerate the vehicle at a rate of... 3 feet per second or 60 feet in two seconds etc.... This requires the carb, no matter the size, to be opened a certain amount. Once at a cruising speed, the throttle required to keep a steady speed on the interstate will vary per the size of the carb (or front two barrels on a 4bbl. carb) of a certain amount of area. Which will roughly be equal to each other at the butterflies opening no matter the carb size.

The small carb has an edge on throttle response because of the smaller barrels which will constantly have a greater velocity giving it a crisper response but less acceleration at a certain restriction point (rpm’s) and less top end.

On a street driver, smaller is better for the crisp throttle response, high velocity to mix the air and fuel very well and not really miss the top end power differences which are normally small.

On a street strip or track car, it’s a world of a difference.
IMO, your going to be fine with your final choice you made.

One of the things that took me back and made me say “Hummmm” many years ago was a friends (IIRC) ‘67 Imperial with a 440 that had a choke less 600 AFB on top. Interesting food for thought.
 
Bean...
Your comment that "Holley carbs flow less than their ratings" means what? Usually air flow (CFM) is at a specific pressure drop thru the carb, at a standard temperature and air density and corrected for barometric pressure. I believe Holley's are rated at 3.5" water column drop (I'm sure someone knows the actual number). What happens IF the CFM's are exceeded due to engine demand? The presdure drop thru the carb increases, but it still flows more air and fuel than the published rating. Perhaps I'm missing something....but the engine air flow requirements are a function of displacement, volumetric efficiency and pumping losses, cam, etc.
I'd like to know if there is a rating factor that adjusts the published rating to the actual installed flow....say a 750 CFM rating actually flows 680 CFM @ some stated set of conditions (for example). Perhaps the published CFM is an advertising ploy....the bigger the CFM number the more carbs get sold....influenced by the owner's "mine is bigger than yours factor"..... just my opinion of course.
BOB RENTON
No, you're absolutely correct that the actual flow depends on the demand of the engine, so maybe you're on to something with the last part of your statement that the rating on the label is a little arbitrary? But, that's what they give us.
From my understanding Holley's cfm rating is based on a dry flow as opposed to some carbs that are rated 'wet'. So a wet-flow rated 850 carb may have the same size venturi and bores as a 950- or 1000 cfm dry-flow rated carb to acheive the number the manufacturer is after because, and I know you know this, adding liquid (fuel) changes the flow characteristics--somewhere there's an actual percentage number of the difference between the two. The engine isn't consuming dry air so that's what I meant by 'they flow less' than their rating when on an engine consuming a fuel mixture. I've always taken the company rating to be the max flow the carb is capable of, but to your point, how 'real world' any of it is, would have to be tested on a running engine, and in that case I doubt the full rating would ever be seen..unless the engine demanded it.
For myself, I mainly consider venturi and bore size, and compare that against past experiences.
 
I see the old wives tale that a "bigger carb will run better" is still alive and well... I saw a Holley dominator on a 4 cylinder chevy at the drags one day..
 
No, you're absolutely correct that the actual flow depends on the demand of the engine, so maybe you're on to something with the last part of your statement that the rating on the label is a little arbitrary? But, that's what they give us.
From my understanding Holley's cfm rating is based on a dry flow as opposed to some carbs that are rated 'wet'. So a wet-flow rated 850 carb may have the same size venturi and bores as a 950- or 1000 cfm dry-flow rated carb to acheive the number the manufacturer is after because, and I know you know this, adding liquid (fuel) changes the flow characteristics--somewhere there's an actual percentage number of the difference between the two. The engine isn't consuming dry air so that's what I meant by 'they flow less' than their rating when on an engine consuming a fuel mixture. I've always taken the company rating to be the max flow the carb is capable of, but to your point, how 'real world' any of it is, would have to be tested on a running engine, and in that case I doubt the full rating would ever be seen..unless the engine demanded it.
For myself, I mainly consider venturi and bore size, and compare that against past experiences.

Perhaps the more explicit terminology between wet and dry flow measurement will relate to the flowing density. Air at standard temp and pressure cconditions combined with the fuel factor, either as an evaporated liquid or gas, will change the apparent distributed density. Yes, velocity is a critical factor as large bore carbs tend to have lower flow velocities, especially after immediately exiting the carb, due in large part to the expansion factor (pressure loss), aggravating or decreasing mixture velocity characteristics, resulting in poor throttle response. Multi level manifolds (the old 180° firing order type alternating between carb bores and levels) may also result in turbulent flow causing flow losses. IMO, this is why the old Edelbrock open plenum X design (Torker I & II) worked well for general all around operation. Just thinking out loud.....
BOB RENTON
 
The thing with calculators is it is science. A mathematical equation. It doesn’t take into account the restrictions of the intake manifold or exhaust, much less the build of the engine. The 649 cfm the calculator came up with is one thing but reality will show a larger carb will make more power and perhaps, drive better. Why?

The carb does two things besides give headaches. (LOL)
It mixes air and fuel of course but it is also a variable restriction based on drivers needs for the conditions driven and needed. Under a W.O.T. Setting, it is still a regulatory device. The less restriction through it the more power the engine makes. This is why you see this happening at the track, back streets and on TV shows like engine masters.

Even though the TV show reached a point of diminishing returns, what it is showing is that the less and less restriction placed on top allows easier breathing of the engine and thus more power.

Most people have a hard time and freak out when others say use a larger carb and fear a drop in mileage. Which is t the carbs fault since that aspect is co trolled by the state of tune, or lack of and the amount of pedal pressure given from the driver.

An engine will need a certain amount of air and fuel to accelerate the vehicle at a rate of... 3 feet per second or 60 feet in two seconds etc.... This requires the carb, no matter the size, to be opened a certain amount. Once at a cruising speed, the throttle required to keep a steady speed on the interstate will vary per the size of the carb (or front two barrels on a 4bbl. carb) of a certain amount of area. Which will roughly be equal to each other at the butterflies opening no matter the carb size.

The small carb has an edge on throttle response because of the smaller barrels which will constantly have a greater velocity giving it a crisper response but less acceleration at a certain restriction point (rpm’s) and less top end.

On a street driver, smaller is better for the crisp throttle response, high velocity to mix the air and fuel very well and not really miss the top end power differences which are normally small.

On a street strip or track car, it’s a world of a difference.
IMO, your going to be fine with your final choice you made.

One of the things that took me back and made me say “Hummmm” many years ago was a friends (IIRC) ‘67 Imperial with a 440 that had a choke less 600 AFB on top. Interesting food for thought.
I think this is spot on. Typically one wants to use the smallest carb that can provide the needed CFM at WOT. They typically are quicker responding. Tend to start faster. Bigger carbs also seem to be less forgiving? When driving conditions change? Most all of us can get a carb to tune nice in ideal situations. But now see how the engine responds when temperature and pressure drops. And humidity goes up? A smaller tighter carb does seem to handle better. JMO.
 
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Carb calculators say for a 440 at 100% at 6000 = 760 cfm .
 
so I put togther a mild 440. Stock bore, edlebrock rpm heads and manifold, lunati cam.
According to the math my carb should be 649CFM. Now that seems a little small to me but thats what the numbers say. I have an old 1412 edlebrock, 800CFM, I could install that but I think I won't be happy because the lower carb signal with that size carb will be constant idle issue and tuning problems.
So the next choices are 670 street avenger or a 750 avs or just go with the 650 avs. Thoughts on this?
I'm sure this has been covered ad nauseum, just seeing what you guys did
Same thing with home AC. The book calls for around 3 ton AC unit but a 3 ton ran all afternoon in my home. 5 ton does wonders and my electric bill is even lower by a bunch. I throw on what I think will do what I want it to do and keep on testing bigger and bigger until it's not going any faster lol. Imo, the calculators are more about efficiency than maximum performance......and most will have their own definition on what maximum performance is. Hey, if you want some MPG too, go with the big TQ.
 
I once had the opportunity to replace my 850 DP on built up 451 with a dominator 1050. Car ran same E.T. I picked up slightly on the big end. Lost a little on the 60'. Just as we predicted. But the big Dom seemed to use more fuel for nothing? And that's a 4150 vs 4500? We wanted to see if the advertised 4500 style was faster responding? Not what we saw? Of course this is just one instance. Certainly not scientific. And a high HP setup could very well beg to differ?
 
Same thing with home AC. The book calls for around 3 ton AC unit but a 3 ton ran all afternoon in my home. 5 ton does wonders and my electric bill is even lower by a bunch. I throw on what I think will do what I want it to do and keep on testing bigger and bigger until it's not going any faster lol. Imo, the calculators are more about efficiency than maximum performance......and most will have their own definition on what maximum performance is. Hey, if you want some MPG too, go with the big TQ.
Could not resist your analogy re home A/C systems. The secret is to a good performing, efficient system is to add ~25% more sensible heat load adjusted to home's orientation and construction, R value of the insulation and windows. Additionally, up size the evaporator coil to the next size larger. This increases the surface area ~ 30% larger for better humidity control and less air flow pressure loss, because more surface area alliws for more air flow, CFMs, thru the ductwork. A thermostatic expansion valve (TXV) to the evaporator (on the refrigerant inlet line) optimizes the outdoor condenser unit for maximum cooling for the kWh consumed.
The moral of the story is: you have to provide accurate sizing information in conjunction with the chosen equipment's capabilities AND what you, the purchaser, wants. It sounds as if the same criteria may apply to carburetor-manigold selections...... You must, or should do your own do dilligance, not just accept what being offered. When I was buying major capital eauipment for industry, my philosophy with the vendor was: "I don't give a s*** what you are selling, this is what I'm buying, subject to my T&C".....almost always results in the best equipment at the best price. Just talking out loud.....
BOB RENTON
 
Could not resist your analogy re home A/C systems. The secret is to a good performing, efficient system is to add ~25% more sensible heat load adjusted to home's orientation and construction, R value of the insulation and windows. Additionally, up size the evaporator coil to the next size larger. This increases the surface area ~ 30% larger for better humidity control and less air flow pressure loss, because more surface area alliws for more air flow, CFMs, thru the ductwork. A thermostatic expansion valve (TXV) to the evaporator (on the refrigerant inlet line) optimizes the outdoor condenser unit for maximum cooling for the kWh consumed.
The moral of the story is: you have to provide accurate sizing information in conjunction with the chosen equipment's capabilities AND what you, the purchaser, wants. It sounds as if the same criteria may apply to carburetor-manigold selections...... You must, or should do your own do dilligance, not just accept what being offered. When I was buying major capital eauipment for industry, my philosophy with the vendor was: "I don't give a s*** what you are selling, this is what I'm buying, subject to my T&C".....almost always results in the best equipment at the best price. Just talking out loud.....
BOB RENTON
The funny thing was the last AC tech here lived down the street and I knew his rep but he kept trying to tell me I didn't need a 5 ton unit......then he started talking about his house having one and bla bla bla (I hate using the bla bla bla crap). That's when I asked him what his square footage was and if he had a cathedral ceiling. His house was 200 sq ft smaller and no the cathedral ceiling. I just gave him a dumb look and he said "I'll go get the unit" lol
 
The funny thing was the last AC tech here lived down the street and I knew his rep but he kept trying to tell me I didn't need a 5 ton unit......then he started talking about his house having one and bla bla bla (I hate using the bla bla bla crap). That's when I asked him what his square footage was and if he had a cathedral ceiling. His house was 200 sq ft smaller and no the cathedral ceiling. I just gave him a dumb look and he said "I'll go get the unit" lol

When i built my last house, ~ 3000 sqft on 2 levels with open foyer, 4 br, 3 baths 2 powder rooms, full finished basement, etc. , i asked the HVAC contractor for a copy of the heat balance calcs (heat gain in summer and heat loss in winter + air duct velocity calcs)...he said for a house of this size we always use 3 tons (1 ton = 12,000 Btu/hr cooling). I wanted his calcs...he said we don't do calcs anymore. I asked him what is the best delta T his proposed system could do (coldest inside temp vs hottest outside temp 95°F @ 75% humidity)....he said 12 degrees....for 95 degrees outside -12 degrees difference the best his system could do was 83 deg F inside. My selected system could produce 65 degrees inside with 95 degrees outside and cycle on off every half hour. Worked great at a lower operating cost and the same installed equipment price.....who knows best??? Results speak for themselves.
BOB RENTON
 
I see the old wives tale that a "bigger carb will run better" is still alive and well... I saw a Holley dominator on a 4 cylinder chevy at the drags one day..
Better or faster since you state “Better” in your quote but then make a joke about a Dominator on a Chevy 4 banger. This is two completely different scenario’s. But then again, there can be truth in both (OK, not so much as a dominator on a 4 banger I think) which would be build dependent.

I think this is spot on. Typically one wants to use the smallest carb that can provide the needed CFM at WOT. They typically are quicker responding. Tend to start faster. Bigger carbs also seem to be less forgiving? When driving conditions change? Most all of us can get a carb to tune nice in ideal situations. But now see how the engine responds when temperature and pressure drops. And humidity goes up? A smaller tighter carb does seem to handle better. JMO.

Thanks. IMO, a bigger carb, well, actually, too big of a carb can be a bitch to cover all the bases the way you want it to. AKA Flawlessly
 
I use an Edelbrock 1407 750 cfm on both my 440's. They are built to factory Magnum spec's except for '452' heads for lower compression. I increase secondary jets up one size and run an electric choke. Both run well. Both are in cars with stock dual exhaust systems, no headers.
 
Better or faster since you state “Better” in your quote but then make a joke about a Dominator on a Chevy 4 banger. This is two completely different scenario’s. But then again, there can be truth in both (OK, not so much as a dominator on a 4 banger I think) which would be build dependent

The first part is always quoted as " If my car runs this well then a bigger carb will run even BETTER"

Dominator was on a car at the DRAGS....

My ford 302 block ( I know) is at the machine shop.. Its a 98 explorer engine ( good lower end and best iron heads) and all I am going to do to it is a Flow tech cam.... The calculator for it states 472 CFM.... I am going to try one of Summits 500 cfm 4 barrel carbs.... Curious myself on how it will run....
 
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