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Why is it every time I read an article or watch a utube video about selecting the correct carb size, it is usually accompanied by a race engine/ performance "expert" of some kind making feel like an idiot. I feel like I am being scolded by my dad who just caught me putting a carb that is too large on my engine. Really? If I hear one more time that an engine is just an air pump and my engine is going to be a "Turd" if the carb is too big, I am going to puke. There is a heck of a lot more going on in an internal combustion engine than an air pump last time I checked. Yes, it is a cute analogy but really not that accurate. The utube video I watched last night was hilarious. The guy lecturing me about carb size clearly did not understand that air flow velocity and flow volume are actually related. Yes... we all understand you need velocity across the boosters for better signal in order to pull fuel into the engine.... A guy on utube from ****** Engines was giving and example using a vacuum cleaner. He put a larger diameter extension using PVC pipe on the end of the hose and actually said that now it can draw more CFM. Wrong... CFM does not change, the velocity changes. The guy did make some excellent points regarding how gearing and converter stall all need to be figured into the carb selection. I do get tired of the equation Cubic inches x peak RPM / 3456 = cfm X .85 (VE) being thrown in my face.... I kinda feel a bit bullied... This would mean a 350 running at 6000 rpm would only need about a 500-600 cfm carb. My 6800 rpm 434 only needs a 750 cfm carb. The issue I have is I have seen a lot of Chevy engines over the years and I don't think I have ever seen a 350 small block street performance engine either in magazines, for sale, at car shows with anything less than a 750. 434's usually have 950's or larger! When shopping around for carbs for my 434, I called Holley, QFT, Barry Grant, Pro-Systems and others and the smallest size that was recommended for my street/strip 434 was 950. That is a long way from 750 this guy on utube told me was all I needed. My issue here is why the disconnect? The equation is based on the volume of air an engine can move at RPM... I get that and if air was an incompressible fluid, that is likely all the air it could move....but it is not incompressible. There is lot more to it than that.... there has to be or all these engines I see would all have much smaller carbs. The bigger is never better folks need to stop feeling like it is their mission to convince us all of that and at least when they try, they need to get their facts straight. My build fly's in the face of the bigger is never better crowd. I got plenty of lectures on the Corvette Forum and guess what, It has the same street manners of any other mild performance engines I have had over the years but lot more top end performance. The main difference is now I seem to be breaking things all the time... Rant Off....
 

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More for Less Racer
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Welcome to the club!
I'm with you though, for hot street and race stuff I take the VE reduction part of that equation everyone tosses around right off the table.... and only use on an application that may actually achieve VE's higher than 100%.
I run a 950cfm Vacuum on my 550hp 406's.
406" x 7500 rpm / 3456 = 882cfm.....the 950 covers that pretty well.

it's not just carburetor specific....you have the same guys in the torque converter stall discussion all the time....."too much stall speed and it will slip at cruise" bullhockey.
 

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Back in the 60’s I asked an old-time tuner about carb sizing, he simply responded vacuum. I shook my head and asked him to explain, he said hook your vacuum gauge up to manifold vacuum and see what it reads at WOT. He continues, ideally you want 0 (zero) manifold vacuum at WOT, but 1hg or less is good especially on the street, more than 1hg at WOT and your carb is too small causing a restriction.
 

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Discussion Starter · #6 ·
Back in the 60’s I asked an old-time tuner about carb sizing, he simply responded vacuum. I shook my head and asked him to explain, he said hook your vacuum gauge up to manifold vacuum and see what it reads at WOT. He continues, ideally you want 0 (zero) manifold vacuum at WOT, but 1hg or less is good especially on the street, more than 1hg at WOT and your carb is too small causing a restriction.
That’s is correct. I hooked a MAP sensor up to my LM1 data logger and checked WOT on a 830 cfm carb with annular boosters and a 940 carb with drop leg. The 830 had 2-3 “hg at WOT and the 950 was close to zero. The 830 did have a slightly better throttle response and only a few less HP at the top end.
 

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The problem is on the dyno a well built 350 picks up power and torque on its way to 6000 RPM using a 750 versus a 650. One might conclude that there are other things going on that the classic equation of half displacement times RPM yields the needed CFM.

Thats not to say that bigger CFM nets better driveability on the street but it does show gains on a dyno programmed to the SAE standard load that maintains an acceleration rate of 300 RPM per second.

Bogie
 

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There is a lot more to it then just the carburetor size. There is also a lot more to it then just using a certain size. I get a lot of heat from several over the years because on all of my builds I use a smaller 600 to 650 size carburetor on every 350 build I have ever done for the most part and have had excellent results. It all depends what the person is looking for out of there build and also how much power does the build make in power and how much does the heads flow and what cam size is being used and how and what is the person going to drive the thing and what do they want out of it.

There are many variables on carburetor sizing and there effect on different style builds which is a lot more to it then formulas. Take a stock 350 making around 250 horsepower and maybe 325 ftlbs of torque and it tops out at 4000 rpm with a very mild cam like a 196/[email protected] duration and smog heads.That car or truck could use a small holley 500 cfm two barrel and run fine or you could step up to a 600 cfm vacuum secondary and the car would run still just fine with a four barrel vs a two barrel with out the carb being to big for said application. Put a 750 vacuum secondary on it and that makes it a carb that is too big for that motor and will never use all the cfm it can flow as the motor itself won't be able to have enough velocity and air going through to make it open up the secondary side all the way.

Now many will say well GM used a Quadrajet that was 750 cfm rated on many stock engines. But the thing is they primary side was super small to give more better signal for throttle response and the secondary side would only open up to what the engine could use similar to a holley vacuum secondary. You can't compare the two together because even though they both can flow the same amount of cfm, the Qjet will still operate much better on a bone stock motor vs a Holley style carb of any cfm up to a certain point.

On certain engines depending on the build a slightly smaller carb will give better signal on the bottom and mid range, but once you get in the higher mid range and high rpm then the smaller carb will start to fall off and either choke or drop off some and the bigger carb will out do it every time. Just depends on how its going to be used and what the person wants and likes. Bigger does not always hurt it like some say it will like its too big. It depends on what carb they are talking about. An edelbrock AVS2 and a Qjet is different on how its going to do vs a Holley style carburetor of the same size range cfm rating and how it will perform to a certain degree.

Bottom line its all in the build and what the persons preference is and what do they want out of it and what is the best option for what they are wanting from there engine that will make them happy and satisfied for how it performs. For all out racing then you most definitely will want the biggest you can use on your build as that is what you look for the most power you can get and it all counts.

I have currently a Dart SHP 377 small block (4.155 bore x 3.48 stroke) with a set of Dart pro 1 platinum 200cc cylinder heads with close to 10:1 compression and a previous 276/280 228/232 and 110 lsa. Lift was over 535 or more but forget the exact specs. The heads have all been ported out and bowl blended and ran an Edelbrock victor jr intake with that build and a 3000 plus stall convertor and a set of 3.42 rear gears and a turbo 350 in a chevy s10.

That build can use all of a 750 cfm and then some and I ran a Holley 750 HP and also a 650 HP style carburetor and I had a lot of top end with it past 4000 plus rpm. The HP Style is nice but I also needed a choke for the colder months here in Ohio, so I also still had a 650 Holley double pumper and also a Holley 600 vacuum secondary carb from previous builds. On the bottom end and to lower mid range the 600 and 650 carbs gave me more crisper throttle response vs the 750 and would give for a more snappy throttle and better cruising at lower speeds and slightly better mpg but not by much. Only about 2 mpg difference.

I ended up wanting to tune my engine down and make it more for street cruising as I don't race mine and never take it to the track and hardly am ever over 4000 rpm ever unless I get on it but I like to respect the laws and don't do street racing like some younger folks do. I just like the part of having something nice and don't mind if I loose a little off the top to get a little bit more on the bottom. I put in a smaller cam a few notches down and also put in an edelbrock rpm performer air gap and stuck with a 650 ish range carb of whatever one I choose to use since I have several ones.

I like the nicer throttle response and better signal on the bottom end and lower mid range which is where I am mostly at vs the top end. The intake swap and the cam swap added more for what I was looking for and liked better and should have planned it out that way the first time. Over the years I like the bigger and higher reviving engines and higher end rpm rumpy rumpy cams but as time went on for me at least it got old and no longer like the more radical builds.

I am also de tuning my engine for the last time and am done with the bigger rpm builds and rump cams. The 750 carbs were good all the way around don't get me wrong and after 4800 rpm you could tell a bit of a difference on the top end especially once you got up to 6000 rpm as the engine still seemed to want to go as with the 650 carbs it would just be out of air and would just flatten out. Off idle the smaller carbs gave more stronger signal for my build to what I was used to and more to my liking so I kept that over the making more on top end while giving up some signal on the bottom end.

That is how I felt it performed and my opinion so some will vary on how they feel on how there build is doing and how it works for them as all hotrodders are difference on what there opinion on how things run. That is why there is many options out there. Oh and the old myth that a double pumper is crappy on fuel mileage vs a vacuum secondary Holley is just an old wives tale if you don't keep going wide open throttle over and over again but for regular street driving and not getting into the secondary side that much I have gotten identical fuel mileage from both carb styles. It is having the knowledge to do advance tuning such as the idle circuit with the idle feed restrictors in the metering block and also the power valve channel restrictor and some other things that many tuners fail to learn and just do jet changes and adjustments on the idle mixture screws and not much else.

Learning to understand how the different designs works and how to tune each area at at time can bring in much more from the carb itself vs the regular basic tuner that mostly does not know how to go about with the advance tuning many carburetors need out of the box.

Other factors in carburetor application is the booster style that the carb is using itself on the Holley style carbs. Straight leg booster vs down leg boosters will have a different effect on things with also annular boosters being thrown in the equation. The old formula does not always work as like you stated there is a lot more to an engine then just sucking in air and a carburetor does a lot more then what the average tuner realizes as they don't actually read up on how a carburetor functions and how it works in relation to the engine its on etc. It does more then just suck in air and squirt some fuel.

The different styles of carburetors from a Qjet and an Edelbrock AVS 2 and a Holley all have there pros and cons with there designs across the board depending on there use. Same thing with a vacuum secondary vs a double pumper carb. Many things can change how they work and perform vs way more then the cfm rating itself. The transmission being used along with stall rating if using an automatic and the rear gears and cam size and engine rpm cam range along with the weight of the vehicle and how it is setup can effect things across the board.

Bigger is not always better but sometimes bigger is better depending on what you want out of your build and how you plan to use it. Since I am just all street and mostly just cruise and am under 4000 rpm 99 percent of the time I like to be on the slightly smaller conservative side of things, but not to the point I choke it to death.
 

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Not one person mentioned that a dual plane intake, especially if the divider in intact needs a bigger carb than open plenum. Ive used 750s on 283s, 850s on 350s and 950 cfm on larger sbc s. I recommend 600/650 double pumpers on hot 283s, a 750 on a hot 327 and after that depends on person driving.
One day at the track with my 350 powered car I swapped out the 750 for an 850 and picked up 3 mph top end. Not as crisp for daily driving. Like conservative Eric says he likes smaller carbs for the crispness and everyday driving. I personally look for those extra 5&10 hp options when choosing my parts. Thats why I wont use a hydraulic flat tappet cam,,,
 

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I guess to further things I should mention that on the intake part being the dual plane intake, I have not had much different results between a dual plane with the divider like the air gap or the regular performer rpm intake from Edelbrock. I can say the air gap intake will deliver a slight bit more horsepower on the top end vs the closed gap design but I would bet it will vary depending on the build and camshaft size and flow etc. I also will never use a flat tappet cam ever again and just stick with hydraulic roller cams for my builds and now just stick with mild cams in the lower 220's @ 50 range with wider lobe separation angles for more vacuum and less overlap.

I also should have mentioned that there is also the option of using a carburetor spacer of an open hole vs a 4 hole or a spacer that is a combination of both and is a 4 hole tapered design to an open spacer. Those 3 different designs will alone effect the carburetor in different ways as well. I myself have used an open spacer on a dual plane intake manifold before and it took away a lot of bottom signal and made more on the top end which is what its used for. I have then used a straight 4 hole spacer and it helps a lot at least on my builds and makes the carb wake up even more on the bottom end and improve throttle response and makes for a better throttle response from the carb alone.

I have used the 4 hole and the 4 hole tapered spacer on the former Edelbrock victor jr. intake which both of them helped for me to get more signal on the bottom end and more response from my carburetor I had used regardless on what size it was. The 4 hole gives a little more response vs the tapered design but the tapered design was not far behind and also gave more on the top end of things vs the regular 4 hole and worked quite well across the board.

The open spacer would have just taken away more from me as that is the purpose for the open spacer is to make more air for the top end and more power. The thing is with spacers they can be used as a tuning aid and sometimes they will give negative results and sometimes positive results depending on the build and what the application is being used for and the engine combo. Heads up to Moosecountry for mentioning that one.
 

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Discussion Starter · #11 · (Edited)
There is a lot more to it then just the carburetor size. There is also a lot more to it then just using a certain size. I get a lot of heat from several over the years because on all of my builds I use a smaller 600 to 650 size carburetor on every 350 build I have ever done for the most part and have had excellent results. It all depends what the person is looking for out of there build and also how much power does the build make in power and how much does the heads flow and what cam size is being used and how and what is the person going to drive the thing and what do they want out of it.

There are many variables on carburetor sizing and there effect on different style builds which is a lot more to it then formulas. Take a stock 350 making around 250 horsepower and maybe 325 ftlbs of torque and it tops out at 4000 rpm with a very mild cam like a 196/[email protected] duration and smog heads.That car or truck could use a small holley 500 cfm two barrel and run fine or you could step up to a 600 cfm vacuum secondary and the car would run still just fine with a four barrel vs a two barrel with out the carb being to big for said application. Put a 750 vacuum secondary on it and that makes it a carb that is too big for that motor and will never use all the cfm it can flow as the motor itself won't be able to have enough velocity and air going through to make it open up the secondary side all the way.

Now many will say well GM used a Quadrajet that was 750 cfm rated on many stock engines. But the thing is they primary side was super small to give more better signal for throttle response and the secondary side would only open up to what the engine could use similar to a holley vacuum secondary. You can't compare the two together because even though they both can flow the same amount of cfm, the Qjet will still operate much better on a bone stock motor vs a Holley style carb of any cfm up to a certain point.

On certain engines depending on the build a slightly smaller carb will give better signal on the bottom and mid range, but once you get in the higher mid range and high rpm then the smaller carb will start to fall off and either choke or drop off some and the bigger carb will out do it every time. Just depends on how its going to be used and what the person wants and likes. Bigger does not always hurt it like some say it will like its too big. It depends on what carb they are talking about. An edelbrock AVS2 and a Qjet is different on how its going to do vs a Holley style carburetor of the same size range cfm rating and how it will perform to a certain degree.

Bottom line its all in the build and what the persons preference is and what do they want out of it and what is the best option for what they are wanting from there engine that will make them happy and satisfied for how it performs. For all out racing then you most definitely will want the biggest you can use on your build as that is what you look for the most power you can get and it all counts.

I have currently a Dart SHP 377 small block (4.155 bore x 3.48 stroke) with a set of Dart pro 1 platinum 200cc cylinder heads with close to 10:1 compression and a previous 276/280 228/232 and 110 lsa. Lift was over 535 or more but forget the exact specs. The heads have all been ported out and bowl blended and ran an Edelbrock victor jr intake with that build and a 3000 plus stall convertor and a set of 3.42 rear gears and a turbo 350 in a chevy s10.

That build can use all of a 750 cfm and then some and I ran a Holley 750 HP and also a 650 HP style carburetor and I had a lot of top end with it past 4000 plus rpm. The HP Style is nice but I also needed a choke for the colder months here in Ohio, so I also still had a 650 Holley double pumper and also a Holley 600 vacuum secondary carb from previous builds. On the bottom end and to lower mid range the 600 and 650 carbs gave me more crisper throttle response vs the 750 and would give for a more snappy throttle and better cruising at lower speeds and slightly better mpg but not by much. Only about 2 mpg difference.

I ended up wanting to tune my engine down and make it more for street cruising as I don't race mine and never take it to the track and hardly am ever over 4000 rpm ever unless I get on it but I like to respect the laws and don't do street racing like some younger folks do. I just like the part of having something nice and don't mind if I loose a little off the top to get a little bit more on the bottom. I put in a smaller cam a few notches down and also put in an edelbrock rpm performer air gap and stuck with a 650 ish range carb of whatever one I choose to use since I have several ones.

I like the nicer throttle response and better signal on the bottom end and lower mid range which is where I am mostly at vs the top end. The intake swap and the cam swap added more for what I was looking for and liked better and should have planned it out that way the first time. Over the years I like the bigger and higher reviving engines and higher end rpm rumpy rumpy cams but as time went on for me at least it got old and no longer like the more radical builds.

I am also de tuning my engine for the last time and am done with the bigger rpm builds and rump cams. The 750 carbs were good all the way around don't get me wrong and after 4800 rpm you could tell a bit of a difference on the top end especially once you got up to 6000 rpm as the engine still seemed to want to go as with the 650 carbs it would just be out of air and would just flatten out. Off idle the smaller carbs gave more stronger signal for my build to what I was used to and more to my liking so I kept that over the making more on top end while giving up some signal on the bottom end.

That is how I felt it performed and my opinion so some will vary on how they feel on how there build is doing and how it works for them as all hotrodders are difference on what there opinion on how things run. That is why there is many options out there. Oh and the old myth that a double pumper is crappy on fuel mileage vs a vacuum secondary Holley is just an old wives tale if you don't keep going wide open throttle over and over again but for regular street driving and not getting into the secondary side that much I have gotten identical fuel mileage from both carb styles. It is having the knowledge to do advance tuning such as the idle circuit with the idle feed restrictors in the metering block and also the power valve channel restrictor and some other things that many tuners fail to learn and just do jet changes and adjustments on the idle mixture screws and not much else.

Learning to understand how the different designs works and how to tune each area at at time can bring in much more from the carb itself vs the regular basic tuner that mostly does not know how to go about with the advance tuning many carburetors need out of the box.

Other factors in carburetor application is the booster style that the carb is using itself on the Holley style carbs. Straight leg booster vs down leg boosters will have a different effect on things with also annular boosters being thrown in the equation. The old formula does not always work as like you stated there is a lot more to an engine then just sucking in air and a carburetor does a lot more then what the average tuner realizes as they don't actually read up on how a carburetor functions and how it works in relation to the engine its on etc. It does more then just suck in air and squirt some fuel.

The different styles of carburetors from a Qjet and an Edelbrock AVS 2 and a Holley all have there pros and cons with there designs across the board depending on there use. Same thing with a vacuum secondary vs a double pumper carb. Many things can change how they work and perform vs way more then the cfm rating itself. The transmission being used along with stall rating if using an automatic and the rear gears and cam size and engine rpm cam range along with the weight of the vehicle and how it is setup can effect things across the board.

Bigger is not always better but sometimes bigger is better depending on what you want out of your build and how you plan to use it. Since I am just all street and mostly just cruise and am under 4000 rpm 99 percent of the time I like to be on the slightly smaller conservative side of things, but not to the point I choke it to death.
I have had a 350, 427 and now a 434. 950 seems to e a good size for the 434. I just think the people who lecture the unwashed regarding size are missing the point. Maybe they have just been in the business too long and think we are all just idiots..... I don't know. Heck, if my throttle response was any snappier, I would just be spinning my wheels. I say throw the calculation out the window and start big (within reason) and work your way down until you are happy or buy 2 carbs, one the street and the other for the strip but please lets stop with the air pump analogy... it is so cliché, incorrect and misleading. It is lot more complicated than that. I have been tuning carbs for 25 years and still learning. There are so many variables.... I watch Engine Masters and find that often times what they think will happen does not. What does that tell you? The only way to know for sure is to test it on your combination. Actual strip time would be best but not always available. I rely on dyno testing. Great for apples to apples comparison stuff.
BTW, I am a big fan of the Edlebrock dual plane air gap. I am currently running an AFR Titan single plane. I wanted to get the dual plane spider for it for street use but they stop making them it appears.
 

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most people don't want Max power in their street car, even if they think they do.
Race engines can get by with minimal pressure drop through the venturi, but there has to be a delta-p for any fuel to flow.
Fuel injection needs no delta-p and should/could make a little more power in a NA engine.
 

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well alot said i run a 750 xp holley with world aluminum220 heads on a 400 cid with matching world single plane with the dog bone in the floor and that makes good bottum end torque and good street manners.it has a flat tappet 580 lift [email protected] duration.it dyno numbers were 542 @6250 rpm.i have a 850 on hand but have not tryed it yet.nor have i made it out to the track yet but its pretty bad ass in my monte carlo.
 

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Imma throw this out there.
The type of fuel also is included in the considerations.
Example. Methonol and gas. A methanol carb will typically be bigger in CFM than a gas carb on the same the engine because it displaces the air.
 

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If you have access to a couple of sizes of carb, try each and use the one that is most fun to drive. Realize that most carbs need a degree of tuning, even the "right" size can be a dog with the wrong jetting / power valve.
 

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I think the most important thing on a street car is primary venturi size. A 600 CFM carb on a 327 or 350 will make the engine more responsive on the street than a 750. Been there, done that. Now if that 750 was a Q-jet it would be an entirely different story. In fact that Q-jet would probably work just fine on a 283. It probably has smaller primaries than than a Rochester 2bbl.

As far as WOT goes, maybe computing CFM size based on cubic inches and VE may be a bit conservative. But it's kinda strange that even Holley sticks to that formula, especially since they make more money selling bigger carbs.
 
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