How to Choose the Right Size Carburetor

today we're going to talk about how to choose the right size carburetor for your engine the popular way of doing it and some problems with using that math formula so let's get started so what is cfm rating out of a carburetor all that does is it really measures how much air will flow through the carburetor by the size of the venturi that's in it now some carburetors are smaller than others and some are much bigger now we're not really concerned how they're configured whether they're a vacuum secondaries double pumper none of that really matters at this point in time we're just thinking about how much air is going to flow through there and that's the equation that we need to figure out is to figure out what size carburetor is going to go on the engine that you're working on now the bigger the carburetor doesn't mean the bigger the power the more performance you can overdo this you can put too much carburetor on there and you can put too small of a carburetor on there and that's why the the guesswork of putting the right size carburetor on the engine comes down to a little bit of math and a little bit of understanding of what the car is being used for and what are all the other pieces and components that are on the engine when you choose the wrong size carburetor you can still probably tune it and have it operate okay depending on how big you go but you can't tune away cfm you cannot change that all you can change is how much fuel goes through there and even that is very difficult sometimes carburetors are based on how much vacuum is in the manifold and how much that pulls air through the system and how much fuel that it pulls with it through the venturi so it is very very difficult if you get it wrong and you go too big because you can't like i said you can't tune out the cfm you have to get this right the first time you can be a little bit off but it's better to be as close to what you need to be as possible now before we get into the carburetor let's talk about really quick fuel injection and specifically throttle body fuel injection because this one throws people off quite a bit you may have a 302 cubic inch small block ford that you figure will use a 600 650 cfm carburetor but this throttle body efi may work on there very well and it will typically flow 900 to a thousand cfm throttle body efi is not rated by cfm it is rated by how much injector is in it the larger the injector the more injectors there are the larger they are the more horsepower it will support that's why throttle body efi is rated by horsepower not by cfm the cfm really doesn't mean anything in this this case because the injectors will feed the engine how much fuel it's going to need and the cfm of the throttle body doesn't matter at that point it's just there to flow the amount of air that needs the the engine and the computer will control the timing it'll control the how much fuel it gets so just want to talk about that really quickly because for whatever reason that confuses people i think is that's a lot of cfm on that engine and it works from 200 horsepower engines typically all the way up to 600 625 horsepower depending on the size of the injector so just wanted to hit that really quick now one part of this equation when you're trying to figure out what size carburetor you need is to go back to the basics and we've talked about this fuel chart quite a bit in the past but it becomes a very very important piece to the puzzle here whether you're using a carburetor a throttle body efi it doesn't matter all of these things are critical to making sure that the cfm that you chose or you're going to choose is supported by the fuel system that you have and if it's not you need to get it right if it's carburetor you need to run a fuel pressure regulator if it's a throttle body efi obviously you're going to run a pressure regulator in both instances i believe you have to have it it's another adjustment point on the carburetor side that you don't normally have but again carburetors are based off of that you know the amount of air that will flow but it's the signal that the venturi are seeing that is the key to this and the more vacuum engine vacuum that the engine has the more demand it has for air the more that's trying to pull into the cylinder then the more fuel it's going to require and the more cfm it's going to require and the signal that vacuum that goes down through the the venturi that pulls the fuel through the system is a big part of this now i don't know and that's something that we're going to have to look at down the road to see what the volume of flow is between a down leg style and an annular style but this is an annular venturi out of an edelbrock carburetor and the more air that flows through here the more fuel that it will pull through that venturi now again i don't know what the difference is between how much a a down lego flow versus a annular discharge will flow but the style of this carburetor isn't going to make that much of a difference in the size carburetor you choose but i think it's going to make a big difference in the performance of the engine and how it responds if it's a high-end race engine you just need a ton of fuel and as much as possible at the high rpm and i think the down leg probably does a little bit better job at that annular is probably better on the street engine so depending on where your rpm range is and the operating rpm range is you may end up choosing an annular discharge nozzle in it so it's a very critical piece to deciding all of this in picking a carburetor but again it doesn't really affect the cfm size but it's something you need to think about as you're choosing a carburetor now let's talk about the two common math problems that there are on how to figure cfm so cfm equals cubic inch displacement times rpm now that is a very basic way of looking at it once you get that number divide the entire thing by 3456 and that will give you the ballpark estimate of what cfm you're gonna need there's another way of doing this as well that factors in one other small factor and that is same thing cfm equals cubic inch displacement times rpm times v e now v e is volumetric efficiency and v e is a guesstimate of how efficient the engine is in its configuration typically a street car you can look at uh ve in some terms i think most people agree that you know 85 percent of the the efficiency of the engine operates and that's the common number on a race application you can look at you know being closer to 1.0 it's a lot tighter the clearances sometimes are a lot tighter sometimes on a race application because you're operating at higher rpm you can calculate that higher but it's a guesstimate the ve number is a guess and most of the time you're going to operate on that 0.85 and that's what i'll show you the math problems here is the ve calculated on that 0.85 now we're going to take a look at three different examples here so cfm equals and then we're going to figure out plug in some numbers here into the equation so in this first example we're going to use a 440 cubic inch engine and we're going to assume that the operating rpm on this engine is going to be 6000 rpm multiply those two numbers divide by the 3456 and that is going to give us our ballpark cfm so when you do that it gives us 763.88 now that is your ballpark cfm so we'll round that number down to the closest cfm carburetor and we can assume that that engine will take a 750 cfm carburetor now we're going to take a look at this in the other way with the ve equation and see how this changes so again cfm equals 440 times 6000 rpm divided by 34.56 but in that we'll add in the 0.85 on the ve side as well so let's do that math 440 times 6000 times 0.85 divided by 3456 and we are going to get a much different number in this scenario it comes out to 649 30. now that would denote a 650 cfm carburetor much different response in this equation the higher the cubic inches and the rpm range at 6000 and above you're going to get some very very funky results using both of these equations in this instance it is the worst example i'm going to give you in those two 750 and 650 are way apart let's take a look at the second example now in this example we'll cfm equals and let's say we're working on a 351 cubic inch windsor engine and we're going to assume that that thing is going to wrap out to 6500 divide that by 34.56 once you do the math on that you get 660.15 is the math on that one so that one is about a 650 cfm carburetor for the 351 at that rpm now let's work that ve cfm equals 351 times 6500 times 0.85 on the ve divided by 3456 when you do the math there you get 561.13 in that instance i would round up 600 cfm carburetor in this example smaller cubic inches a little bit higher rpm you're going to get a closer result with this now this works out pretty universally any which way you do this 348 cubic inch engine uh 360 degree or excuse me 360 cfm engine you're all going to be a little close but you'll see how far off we were in the in the 440 example smaller cubic inches it gets a little bit closer now let's take a look at the third example okay in our third example same thing cfm equals and in this instance we're going to use a 366 cubic inch dump truck engine and we're going to make the assumption on the high side that that operates at 5000 rpm at its highest rpm range it's probably too high but we're going to go ahead and figure it that anyway so when you do the math on that 366 5000 divided by 3456 we'll give you 529 51. now in that example i would round that down so that is going to give us a 500 cfm carburetor on that engine now on the v side of that cfm equals 366 times 5000 times 0.85 on the ve side and the math works out too for 50.08 in that example as well just given what carburetors are out there round that up and you get the same answer 500 cfm in this example because we're under that 6000 rpm range and work our cubic inches are still up there in that 350 360 cubic inch range we get a much closer answer now let's talk about all three of those now when you look at this you get some wildly varying answers to this equation depending on the the cubic inch displacement and the rpm of the engine between the ve table or the ve equation and the non-va equation so let's take a look at some other factors here because it'll help us determine which one of these is the better one to use now while this mathematical equation is really cool and all it does not answer every question that you need to ask yourself when you're doing it and the first one when you're trying to pick the cfm and a carburetor is how are you going to use the ant car how are you going to drive it what what do you what are you using it for how will you drive are you going to drag race it are you getting a street driver is it an auto crosser all three of those have different really engine operating rpms that you need to consider the other side is what cam profile are you going to use is it boosted what type of fuel are you going to use all of those are extremely important questions to ask that are not in that equation that equation is just very very generic it doesn't really have enough information in there to properly pick the cfm size of a carburetor because when you start drilling down and getting into the real nitty-gritty of what you're doing with the car or the engine all of these things are going to make a huge difference in how you pick the correct size cfm for the application that you're working on and when you don't account for boosted or nitrous when you don't account for e10 e85 c16 race gas all of those things become a huge huge factor you know how you're going to drive it a race car operates at that higher rpm side most of its life a street car it's off idle to maybe forty five hundred typically you know you may hit that five six thousand sixty five hundred occasionally but it is really super critical to ask all of these questions and none of these formulas no matter whether it's the standard style of doing this or the enhanced one with the ve to find cfm they don't answer those basic questions and we get some varying responses as it is with these two equations how do you determine which one is better and i will tell you that i i understand the philosophy of using a ve uh volumetric efficiency to to get a better equation but honestly with today's fuel and where we're at today i almost always exclusively if i'm going to use the math problem we'll always use the first one and that's cubic inch displacement times rpm divide by 34 56 i don't even take ve into consideration i'm not saying it's not worthwhile i'm just saying excuse the results here and it's going to throw you off especially when you look back at that 440 where that first equation gave us that 750 cfm and the ve table or the v equation gave us a much 100 cfm lower you screw up and put a smaller carburetor on that 440 depending on what you're using it for it's gonna be much more difficult to tune to drive to race to autocross to road race whatever it is so that's why have understanding all of these other pieces of the puzzle are extremely important and that's why i don't use the ve side i know it's popular i know you'll see it all over the place on the internet it makes really really good sense but when you do the math it really really doesn't work out so let's talk about the other side of that and how to factor in those questions into picking your cfm i made mention earlier this is kind of a moving target it's very difficult to get the right answer here in these equations and this is where it becomes even more difficult now let's take our 440 and we'll just assume that it's a fairly you know mild 270 280 you know two type of grind somewhere in there but let's say we're using e85 fuel if that's the case you need more fuel in that carburetor because e85 requires more fuel i'm going to bump that up to 800 cfm at a minimum now depending on what you're using it for if it's that 282 grind and you're using it more of a of a mostly street but quite a bit of time on the racetrack i may even bump that up to 850. it just all depends that e85 is a big factor in there and if you don't think of these things again you could end up getting a much smaller carburetor than what you're actually going to need so again i understand it's a bit of a moving target and this at this point it's guess work it is more you know understanding and knowledge and this is where you may have to ask a few people to get some consensus on what you're thinking and if you can explain this out and somebody says yeah i think you're on the right path however you didn't consider in that it's a very heavy car or you're a manual or whatever the case may be so it's always good to kind of think through these the longer you do this the better you'll be picking it out but it's it's the difficult side of picking a carburetor and getting it wrong um you know you can get it right i guess hopefully more times than not but when you get it wrong you know it's it becomes costly it's becomes hard to tune that's where people go well that carburetor is a piece of junk it didn't work and then you go down that path so it's really a very very difficult you know equation here to try to figure all this out you know it will go back to the dump truck you know that may really not operate at that high rpm it may only operate at five or four thousand rpm is its highest and that is going to drop that cfm rating down quite a bit and maybe it doesn't need a four barrel maybe you need a two barrel on there but again if you just looked at that and said okay 366 cubic inches time 5000 rpm a lot of people just assume oh it's a street car no it's not it's a dump truck it doesn't rev it doesn't rpm it needs a lot of low end torque and grunt and a two barrel is really good at delivering that so all of these things you have to take these other questions into consideration the math problem is okay but it is not the solution it is not the final piece to the puzzle you're gonna have to think through the rest of them and decide what where you need to go do you need to move up or you need to move down the cfm that's the difficult side of it i may have left too many questions unanswered in this one and for that i do apologize but picking a carburetor size by the right cfm is a little more difficult than just a simple math formula i wish it was that easy but it's really not so you just have to take some of those things to consideration ask yourself all those questions figure out what you're going to use the engine for and if you do then you're going to get this pretty close and certainly if you have any questions on this at all don't hesitate leave them down below i'd be more than happy to try to figure it out with you and see what you're gonna you know what you're working on and and you know what size carburetor you're gonna need to get it done right so drop me a question down below if you got it and we'll figure it out but leave me all the details you know what the cubic inch engine uh that you're working on how much rpm and uh you know all the little details because that's the questions that i'm going to ask you so anyway if you have any questions please leave them down below thanks for watching the video give me a thumbs up you thought it was cool he got something out of it and of course if you haven't subscribed i always appreciate that as well we'll catch you guys on the next video we'll see you

Channel: Muscle Car Solutions

Views: 111,198

Keywords: how to choose a carburetor, edelbrock, holley, carburetor, carburetor vs fuel injection, edelbrock carburetor, holley carburetor, carb, carburetor size, carb tuning, what size CFM, what is CFM, what size carburetor do I need, how to choose the right carburetor, VE, volumetric efficiency, big block, small block, racing carb, street carb, carb size, measuring a carburetor, what size carburetor do i need

Id: Hy4s9B43YM0

Length: 20min 0sec (1200 seconds)

Published: Sat Jan 02 2021