What pressure drop is that "500 cfm" at? How hard are you ****ing on it? DO THE MATH. UNDERSTAND what the numbers you are talking about ACTUALLY MEAN.
First, I'll concede you know a great deal more than I do about this topic. I cannot compete on an equal footing. That said, my reasoning is this: The question posed is what carburetor cfm rating is adequate, or in excess, for a 302 cu in engine. Logic says to me that a normally aspirated 302 cu in engine could not exceed inhaling it's own displacement, at best, and won't under normal operating conditions. However, for examples sake, let's say it can take in 302 cu in in a complete firing of all cylinders (2 revolutions). In the example, I am using the disputed 500 cfm carburetor and an intake tract efficiency of 85%. So, I contend under the given parameters, the engine would have to turn 4863 rpm to use the 434,400 cubic inches of potential carb air flow. My point being to the "500 cfm is not big enough" crowd is that unless you expect to frequently operate at near 5000 rpm, 500 cfm will serve that engine adequately for normal street use and occasional 'spurts' of performance demand. Could one use a bigger carb to good advantage? Sure, but as you stated, unless the COMBINATION of other vehicle factors support the larger cfm, it is wasted and likely a detriment to everyday driving. Obviously, few people understand how to make the corrections/adjustments as well as you have figured out, and I mean that as a compliment, so they will likely suffer from the "bigger is better' approach. Ray
Ok, LETS do some math, I like math. I have been reading this **** on here for years now, and laughing up my sleeve, lets get into just why its so funny. The thing about math is, its no use if you don't understand what it is you are actually calculating. 4 bbl carbs are rated at 1.5" hg on a test device called a flow bench. I happen to have actually built one, and have been using it for a few decades now. Lets take a look at what happens to that cfm number when you change the pressure drop. I know you guys don't know the first thing about this, and I don't want you to think I am making ther formula up so heres a link to it on the internet. http://www.flowspeed.com/cfm-numbers.htm Ok, so lets take that 500cfm@1.5" hg, and see what happens when we convert it to a pressure drop of .8" hg, then we will go in the other direction, and do it at 2" hg. so we divide .8 by 1.5 and take the square root, and multiply the cfm number by that factor, and we get a number for how much cfm that carb will flow it you pull on it at .8 hg. Get out your calculators and follow along, if you can. so .8 divided by 1.5 = .5333333---the square root is .7302967. Multiply 500 cfm by the correction factor, and you get 365 cfm. So if your engine is pulling .8" hg at peak power WOT the MATH says that "500 cfm" carb is now WAY TOO SMALL. lets do it the other way. 3.5 divided by 1.5 is 2.333333--- square root is 1.5275252 x 500 cfm=763cfm ooooh wait, NOW, if your engine pulls 3.5" hg at WOT, NOW ITS TOO BIG! So what does all this mean? It MEANS that a test number, on a flow bench, taken at an arbitrary pressure drop, has NOTHING, ZERO, ZIP NADA to do with how much cfm your carb is actually going to flow ON A RUNNING ENGINE. NOTHING. Lets post some other sources here, so you don't have to take my word for what a complete crock of ******** this precious carb formula actually is. Lets start with David Vizard. Jay Brown... Quoted from Pro Systems Carburators website... "I'm sure you remember this old formula: CID x RPM x V.E. / 3456 = CFM Well that formula is still being quoted by magazines and companies etc...but times have changed and carburetors are operating on almost immeasurable amounts of vacuum. 10 years ago a carburetor would require 10 inches of water to pull signal and shear fuel. Now they can can pull and shear fuel at only 3. Remember 20.4 inches of water (wet) is the cfm rating guide with reputable designers so we aren't looking to match cfm requirements with cfm ratings. 20.4 = 1.5 hg. You can see that going from 10 inches of water as a requirement at launch to only 3 inches as a requirement really allows a serious increase in cfm size. This removal of restriction really pays off in cylinder head flow numbers and hp of course. Imagine altering this upstream restrictor when flowing your heads. Because, most of you have specific application designs, a custom shop/unit is typically the plan. In the future, use this calculation as a general rule on a modified carburetor: CID x RPM x V.E. / 2820 = CFM 350 x 6600 x .9 / 2820 = 737 CFM Now you'll be a little closer. A .9 Volumetric Efficiency (V.E.) number represents a pretty good combination and a 1.1 V.E. number represents an all out ***ault on the engine blocks stress handling capabilities. " I have known why the carb formula is a crock of **** since shortly after I started using my home-built flow bench in 1978, and I haven't bothered to say anything for 5+ years on the HAMB, that's how much of a show-boater I am. The simple fact is, you guys are being fed a complete crock of ********, on a daily basis, by guys who don't know what "cfm" actually means, have never been anywhere NEAR a flowbench IN THIER LIVES, and are regurgitating some ******** that was printed in a magazine 35 some odd years ago, that they A) don't understand and B) haven't even given the most casual thought. And the guy who originally wrote it didn't understand it either. As for me, I am just getting tired of wading through the waist deep ******** on here every day.
The point I am really trying to make Ray, and I don't mean to single you out, or knock the stuffing out of you, is that the "carb formula" is just plain wrong. The point isn't just that the numbers it generates are incorrect, its that the whole premise its based on is false. It needs to be put to sleep, just like after Columbus sailed to the new world, we realized the earth wasn't flat. Its based on the idea that a flow number generated on a flow bench at a given pressure drop is an absolute number, as I have CLEARLY demonstrated, its NOT. Engines don't pull a steady 1.5" hg at WOT. It just doesn't work that way. The science is flawed. The formula is meaningless, and has no application in the real world. If guys want to use it, well, knock yourselfs out, hell I wont try to stop you. But if you are deluding yourself into thinking that you are using a "scientific" method to select your carb, or that you are "smarter" than those poor hapless fools that are "overcarburated", well the formula is NOT science, the formula is flawed, and odds are, the guys running bigger carbs understand the formula better than you, and know WHY its flawed.
I think that your logic is actually a good starting place, but one thing that may have an effect on it is how many times per minute does it makes its displacement. For example lets say that the motor is turning 4K then that would be 4k times the total displacement per minute. So 302x4,000 would be 1,208,000 inches per minute divide that by 12 and you come up with 100,666.6667 cubic feet per minute. That is probably a little ways from perfect in the real world, but for the sake of the direction we are taking it will do as far our discussion. One thing to remember is that when a carb is flowed they try to do it under ideal cir***stances and not all carbs are flowed equally. taking a 700 CFM rating for example, that is capable of flowing 700 CFM. now take it a step farther, some carbs are flowed wet and others are flowed dry. A wet flowed carb flows less or at least will show lower numbers. Al of this can get really hairy really fast and for our purposes most of the time we are going to use what we know to work rather than to crunch the numbers, most of us are not tuning to the gnats *** we are just trying to make our old heap motate.
I am somewhat surprised that 2BBL carbs use a different rating system than 4BBL carbs has not entered the discussion. The 2BBL CFM rating is based on a 3.0 inches of mercury pressure drop. The 4BBL CFM rating is based on a 1.5 inches of mercury pressure drop. A WOT 500CFM 2BBLcarb flows more air (@ 3.0"hg) than a WOT 500CFM 4BBL( if we measure at the same pressure drop as the 2BBL of 3.0"hg). 500 vs 354 It is unfortunate that carbs are rated using the two different pressure drops. It is also unfortunate that 4BBL carbs do not specify CFM ratings specifically for the primaries and for the secondaries. It should be obvious by now that small primaries give nice low RPM throttle response, good fuel atomization along with good fuel economy at normal driving speeds. The beauty of the secondaries is that they come in to play when engine RPM is high enough to provide good atomization in the secondaries using a "progressive" system, either vacuum or mechanically operated. Yes, 500 CFM in a 4BBL (in whatever flavor suits you) is what works best on a mild street 302. It also minimizes the number of times you experience the sparkling blue-and-reds behind you! Provided you educate your right foot. reference: Holley Carburetors & Manifolds (H.P.Books)
I'll tell you a dirty little secret. For a couple decades, there have been several carbs in the Holley and Edelbrock catalog that have "cfm" numbers attached to them that actually have NO CONNECTION WHATSOEVER to what they flowed on a flowbench, the numbers were chosen for sales reason, and to slot them into the spot in the catalog that the sellers WANT them slotted into, and this is no big secret, its common knowledge a**** hard-core carb guys. So they don't spell out for you, but the manufacturers know the carb formula is a joke too. But they are in business of selling, and it isn't going to do them any good if they come out and say "oh yes, that carb formula we have been telling you to use for three decades? well, uh, its ********".
Benno, a conventional four cycle engine has to make two complete revolutions to complete the cycle. That is true regardless of the number of cylinders. So....the 302 V8 under discussion requires two revolutions to have all eight cylinders fill their capacity on the intake stroke. So an engine's rpm has to be divided by two when calculating the cubic inches or cfm it is 'inhaling' at any given rpm. Or conversely, starting with the volume of air, when determining the rpm required to consume that volume. edit: just read Falcongeorge's reply......apparently none of this is worth the time it has taken to type it.....or read. it isn't the first time I have been wrong and most ***uredly, not the last either. Oh well, I hear that new fangled...'fuel injection'... is the coming thing......think I'll start absorbing misinformation about that..... Ray
Ok then 50,333.33335 cubic feet per minute, that is still several thousand cubic feet per minute more than 700 isn't it?
Falcon: I failed to look back while I typed my response and I agree with your contribution. David Vizard is a**** the best flow testers and engine researchers ever to publish his findings. Thank you to the original poster for asking advice and for the stimulating discussion.
This sounds reasonable but actually, booster venturi design plays a much bigger role than overall venturi size. I say this on here all the time, but it falls on deaf ears. David Vizard again... In the type of applications you guys are usually discussing, stock or near stock motors pulling highway gears, I would take the Summit carbs HANDS DOWN over Edelbrocks re-hashed afb. The Summit and old Holley 4010 series carbs its based on uses a particularly good annular booster. When Holley first brought the 4010 to market, it didn't have replaceable pump shooters, and the original size was too small, which resulted in a lean bog when you mashed the throttle. It was a simple matter to take a pin vise and a number drill set and re-drill it to a larger size, and when you did, you had one HELL of a street carb, but unfortunately that out of the box lean bog pretty well turned most guys against it, and the carb didn't sell well. The revised Summit version has separate shooters, and they are calibrated better out of the box. VERY under-rated carb, and I aint the only one who loves them David Vizard does too.
If you already have cubic feet in one column and cubic inches in the other column and want to convert one to the other all you have to do is divide or multiply by 12 depending on which direction you want to go. Both dims are already cubed. it also works with squared dims.
I was thinking about this walking my kid home from school, I was thinking guys might be missing the full gravity of the information posted here RE: booster design. I think this should be emphasized. Note that the annular booster shows an increase in metering signal over the nozzle bar booster (standard in your much loved Holley 600 vacuum) of roughly 1/3. So what does this mean? When we talk about the negative characteristics exhibited by an engine being "overcarburated", what we are really talking about is that the main venturi is so big, that the airflow through the venturi is so slow that it wont "pull" fuel out of the mains. So what David Vizards test above is saying is that at the same flow bench pressure drop, and the same size main venturi, the annular booster is "pulling" on the fuel in the main circuit 33% harder than the straight-leg booster that comes stock in your 600 vacuum. In effect, what this means is that the annular booster 750 double pumper on my "overcarburated" 355 is actually pulling harder on the fuel in the main well at the same rate of airflow than your little "formula correct" 600 vacuum ( 750 has a 20% larger venturi, with a booster venturi that pulls a 33% higher signal). In other words, its pulling a stronger metering signal than your 600. David Vizard just said so, and I have run the same tests on my own flow bench. If anyone is wondering about test procedure or how this is done, its simple, you mount the main body on your flow bench without float bowls or metering blocks, block the other three venturis, set the pressure drop, and stick the hose from the manometer in the main p***age leading to the booster venturi, the manometer reading will then indicate the pressure drop across the booster. This is just one more example of how meaningless the numbers generated by the carb formula are, and why they just do not apply to whats actually happening on the engine, and why I say booster design is more important than overall venturi size, which is what guys are talking about when they talk about being "overcarburated". The next factor is reducing restriction in the carb without increasing main venturi diameter. Anything you can do to increase airflow through the carb without increasing the main venturi diameter will increase metering signal. In other words, if I take my 750 double pumper and reduce internal restriction by slabbing the throttle shafts, it will go from around 800 cfm (750s are rated slightly under what they actually flow, see my earlier comment re: cfm ratings and marketing) to around 840 cfm, so according to the "formula" I just made my 355 "more overcarburated". In fact, the increased airflow through the same size venturi means that I have actually INCREASED the metering signal, and made the carb more responsive.
And yes, I still realize that after ALL this, I will get up tomorrow morning and there will be at least THREE more threads saying"Oh man, that carbs to big, the formula says..." And I will go back to just rolling my eyes, shrugging and moving on...
For what it's worth, I put a Summit 600 cfm carb on my mild 327 last fall and it runs great in all types of driving. The only thing I adjusted was the curb idle speed. I didn't have a chance to get over 300 mi. on it before the snow came, but I'm very happy with it so far. Gary
Edelbrock would be my choice, put it on adjust it and forget it It will work fine forever. Hollies are a tuning nightmare, you always have to keep adjust the stupid things, don't let if backfire, it will blow out the power valve.
All Holleys have had power valve protection for what, three decades now? before that, I used to do it myself. You drill partway down into the air p***age in the baseplate, drop a spring in on the lip that's left behind, and drop a checkball on top of the spring. If your holley is less than about 25 years old, you don't even need to do this. EDIT: just looked it up, it was changed in 1992. So 24 years ago.
If you choose Edelbrock, make sure you do not have over 5 pounds of fuel pressure. They will flood and it may not be all the time.
Just stumbled across this, this is a good clear photo of a guy using a flow bench to check booster signal as described above. The yellow arrows indicate the main p***ages that lead into the boosters, these are fed by the main jets via the main wells.
Carbs appear to be a hot topic.... A topic that's very interesting Can someone explain to me what the differences are ,which is better for performance and street use, in laymans terms ./ spread bore and square bore in reference to carbs and intakes.
Even new Holleys will blow out power vaves It is just one of the hazards of running a Holley card. Overall the Edelbrock ( just a copy of the old Carter AFB) will be the easier carb to run and maintian
Factory box stock from later years 5.0 with a carb . Annular Discharge boosters . Electric choke . Hey Ford has a lot more money to design a fuel system than we do , try one you'll love
Sure would be fun to have the stuff to play with for this kind of testing. My question is what do they use for wet testing these boosters?
I have no experience with wet flow benches, beyond the knowledge that getting my hands on one is well beyond my means.
I did briefly. Last paragraph of post 43. I actually really like Q-jets as well, The primary booster is an excellent design, and you can dial in the fuel metering with more precision than a holley, but I get that a guy may not want one on a sbf. Death Row Dave also made an excellent suggestion, and I forgot all about that carb. There is ZERO reason to use a "450" or "390" cfm carb on a sbf, unless its a matter of bragging to your friends about it, that seems to be real popular these days. But theres no functional reason, either the Summit I mentioned or the factory ford annular holley Dave mentions will outperform it in every single area. The 390 has one of the lowest gain boosters you will find. It was actually originally designed for circle track cl***es that were restricted by the rules to a small venturi. later Holley took the same main body and started making a carb to sell to the ford 3.8 v-6 crowd, when modifying those motors developed a very short-lived following in the late '70s.
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