The Myths And Realities Of Carburetor Sizing- History Proves The Formulas Don't Work

so let's talk about carburetor sizing we did a video a few days ago about our tunnel ram with Paris 600 time and reading through the comments it made me realize just how much misconception there is about carburetors size their function their place the house and the whys so I figured let's do a video on this and if if you're looking for an exact formula that's going to tell you give you a mathematical equation on how to pick the right size carburetor for your car you're not going to get it here click away now because that's not what this is about the thing with those formulas is that they work on paper you know it's it's a it's a okay guideline it's a decent guideline on paper but like a lot of things what works on paper does not necessarily work in the real world so at the risk of being long-winded with all of this I have to start this with a story okay back in the 70s it was teenager 16 17 years old and my friend Danny had a 340 four-speed Duster and we were at one night on Highland Boulevard and we were going stoplight to stoplight there were a lot of cars out there that night and uh it was that okay running car it was just it was a stock 340 Duster with headers and and like glass packs that was about it other than that it was stock and it ran okay it ran typical for what you would expect and all night we were just lining up against all the cars and going a couple of years off the stop lights and having fun so at the same time there was this fella in the area we used to call him dad right so he this this guy's really conservative looking he had like abuse with these like button-down shirts and he had like very neat hair and he's still really thick black rim glasses and he drove a Pinto station wagon and like it was a wood grain Pinto station was a brown Pinto State wood grain wagon and Brett Elite and Dad pulls up next to us and he pops the clutch a couple of times like you know let's go so it's like it's a joke right we're going to raise dad well the light turns green and dad walks us with this Pinto now it didn't have it wasn't like he had dropped a 302 or anything in it it was still a four cylinder pinto and the motor was essentially stock bottom end of it was all stock it was 122 cubic inch Pinto motor which were great engines it had a cam it had a header it had a four barrel intake and it had the biggest Holley carb you've ever seen I don't know what it was this is a long time ago it probably is 650 or so double pumper maybe a vacuum secondary I don't know but it was it was way bigger than it should have been and at the time I'm learning about the formulas and how to choose the right size carburetor and so on and so forth but all of the examples I'm seeing in real life contradict these formulas so dad's car was one of the one of the examples he took me for a ride nothing later on in it marched it had power on top of power but it shouldn't have run by the formulas this thing shouldn't have even started and I saw him looking around I'm saying okay it's time Pro stockers were all they were used they were running off of cubic inch to wait at the time so a lot of them were using very small engines 320 330 340 cubic inches and they were topped with a pair of dominators and they were the fastest gasoline carbureted cars in the world how does this work and how is it 330 inch engine with two dominators on top of it makes so much power when the formula says no the biggest thing that should have is a 550 CFM carburetor that's about right as far as the formulas are concerned and then you look at factory cards so look at factory examples so you had the uh let's say the 446 pack which is the same as the the 427 435 vet they use the same carburetor the same tri-power or n6 pack setup which was three Holley two barrels at 350 CFM Center and two 500 Outboards on paper it's 1250 CFM in reality it actually flows 975 CFM because two barrels and four barrels are rated differently but the point being it's still 975 CFM on a 440 or 427 cubic inch engine and according to the formulas well this thing shouldn't run at all but nobody ever argued about how much power 446 pack or 427 that made and then you look at the 346 pack now nobody ever accused the 346 pack of being over carbureted the 346 pack used the same exact carburetors as the 427 435 on the 446 pack 975 CFM on 340 cubic inches from the factory nobody ever said that's over carbureted if you looked at the formula if you went by the formulas we'll just tear this stuff up and throw it away because it means nothing so it was my experience early on that if you want maximum power you go with much more carburetor than you're supposed to have so then you have to look at you say well how do these formulas involved and how did the automakers choose the size carburetor that went on their packages and for that you have to kind of forget about the way we think of things so we're hot rodders when muscle car is we're hot rodders we're looking for performance the manufacturers they wanted performance obviously building a muscle car it has to perform but maximum performance maximum output was not the goal the criteria they had was that this car this muscle car whatever it happened to be had to function in everyday real-world traffic scenarios so in other words on a day that has the worst possible atmospheric conditions and the car loaded down with luggage and accelerating from a stop up a steep hill the driver has to be able to just punch this thing and have no bog no hesitation no nothing so now if you're talking about a mechanical secondary carburetor like in the muscle car or the vast majority were mechanical secondary carburetors like these Carter afvs and edelbro you know Edelbrock today but Carter AFB back then so what that means now is that here's here's closed throttle right at 2000 RPM let's let's round it off let's say it's a 2000 RPM stall so off idle to a 2000 RPM stall going up a hill loaded down in bad weather the car had to take this and not bog or hesitate or or stumble you did the driver didn't have to modulate back the driver didn't have to say Well it it'll oh my engine will only take 185 CFM at 1800 RPM so therefore I only have to go like this no this caused these packages were engineered for non-thinking non-responsive drivers who don't know any better they just like bought a muscle car and I'm going to drive it so like that and this thing has to get up and go the manufacturers on their muscle car offerings used the largest carburetors they could given that scenario and then if you got lucky it made a lot of power on the top the bottom line is if you take that if you take that same 625 CFM 383 let's say the 335 for three to three that has a 625 CFM carburetor if you go to an 850 900 CFA 850 CFM carburetor or you put a six-pack on it 975 CFM it will make a lot more than that 375 horsepower but you're doing it at the sacrifice of what the average person with a loaded down car in you know trying to accelerate you know merge into traffic or whatever they flat foot it and it has to be able to perform so that's how the manufacturers for the most part chose their carburetion and then of course here you get into the factory packages so you had Factory race car packages you know like here's a perfect example um Factory super stock is what not um Mr Norm in 1965 got a load of a990 cars the the factory super stock crossframe 426 Hemi cars and the transporter pulled up and there was like six or eight of these things on the transporter and not a single one of them would start so all of the cars had to be rolled off the transporter rolled into the shop and they had to pull little plugs out and clean the plugs and there was it was like a major Fiasco getting these cars you know to where they were operable that's why they were Factory race cars that's where they were super stock package cars because they weren't you weren't able to operate them in regular traffic and that holds true for pretty much every Factory race or factory race package and then you look at other examples like for instance um dealer install so if the factory had a package that they wanted to offer for performance but it didn't meet that criteria of being drivable in everyday traffic by just any old person they would make it a dealer option they put the parts in the trunk when you ordered it they put some parts in the trunk and then the dealer would install it so this way it removed them from the liability of a car that didn't function in traffic so an example of this is the hyper pack you go back to this this Lane six the hyper pack the 170 and the 225 cubic inch both of those had a 485 CFM 4 barrel carburetor those things are miserably sluggish off the the get-go but once they get up in the three thousand four thousand five thousand RPM range they Rock it was a dealer installed package way over carbureted if you take the formula 170 or 225 cubic inch engine by those formulas of volumetric efficiency cubic inch RPM tells you the CFM it's supposed to be well just take that formula and throw it away it's meaningless there is the power other examples of that were um the Trans Am the the Chevys the the 302. cross ramp they you you ordered the car it came with a single four barrel carburetor but if you ordered it you could get the Crosman with a pair of 780 CFM holleys on a cross ramp and the dealer would have to install this was 302 cubic two a pair of 780 CFM Hollies on 302 cubic inches the formulas say no way that's not going to work those things were ran hard really hard um actually AMC had had a setup like that also available on the AMX where it was dealer installed they ordered this stuff and they come in the trunk and the dealer would install it it was a course frame with the same the same Holley carburetor to set up I think I think it was available with the uh with the AMX and with the um the Rambler scrambler so this is what I'm saying the the formulas don't work for maximum performance and if you go by the formulas you're going to be disappointed you're going to be disappointed in overall so now let's talk about now in a multiple Corporation this setup um I'm I'm leaving a lot of stuff out of this video because there are so many different combinations and packages and so on and so forth that happened over the years and each have their own specific like idiosyncrasies and qualities that make them very unique but I have to kind of gloss through those to to cover the topic the bigger Topic in general if there's enough interest we'll go back and talk about some of the specific packages but now you're talking about a multiple carburation setup and we'll go back and let's use our 383 with the tunnel ram and we're using a pair of vacuum secondary 600 holley's on it so you say oh my God that's 1200 CFM and it drowned the motor and it's just no way that thing's going to run which was the comments over and over again now there's no way it's way too much carburetor to put a single four Barrel in it a pair of vacuum secondary 600 holley's means that when you open the throttle you've only run it on the front two barrels of the carburetor the back two barrels don't open until the vacuum signal until it's not a strong enough pull through the primary side to open the secondaries so in other words when you flat punch this thing with a pair with two 600 holleys on it what you're doing is exactly this with oh that's stuck oh my God my secondaries are stuck right here what did I do I broke it hmm actually this makes me think of something else entirely pretend that never happens it's the same as if I had a 383 and had a single 600 CFM mechanical secondary Carter or Edelbrock carburetor on it now nobody would ever look at a 383 and say oh 600 CFM that's way too much but a pair of 300 CFM vacuum secondary carburetors gives you exactly the same reaction when you hit the gas so you have to you have to think you gotta you gotta you gotta open your mind a little bit foreign I do want to talk about the diminishing returns of throttle opening because and this doesn't really fit into formulas or sizing or anything but this is something that like you can pretty much I wonder why these aren't opening it's what the hell is going on with this carburetor um there we go it's got the choke lock off on it so if you notice when you're accelerating in your car throttle opening has a diminishing a point of diminishing returns so if you from it from a standstill from stock you go to let's say a quarter throttle the car jumps right it jumps hard but it doesn't jump far right so you'll get an initial hit but then there isn't quite enough for the RPM so then you go to half funnel right it's the same thing the car will hit harder and that pull will last a little bit further and then it'll flatten out well then you flat punch it and it hits pretty much the same as it did the first two but now it'll run all the way through it'll accelerate all the way through you notice that there's no bog difference you don't have to if you've got let's say a an average size carburetor a 600 CFM carburetor on your 350 cubic inch engine you don't have to modulate the throttle you just flat punch it and go now if that's that carbon at 600 CFM carburetor was too big and let's let's talk about now a 700 or 800 CFM carburetor if it was too big you would have to when you flat punched it you would actually it would bog and then you would have to bring the throttle back and that would be a good sign that you've gone too big that that would be a good sign you've gone too far on your carburetor you've you have to modulate the throttle back in order for the acceleration to kick that means that there isn't enough pull through the boosters to get the fuel to come through and atomize correctly so the motor won't run properly but if you've got a carburetor if you've got a setup that you can just flat punch and hold it to the floor and not have to back off to have an accelerate efficiently you've actually your your carburetor is the right size or too small you'll know you've gone too far when you put a carburetor on there and it just you have to bring the throttle back in order for it to keep pulling and that goes back to the manufacturers when they chose the carburetors to put on a specific package again they weren't they weren't concerned with Peak power like we are they were concerned with drivability for the average like no nothing who's just going to just type on a muscle car I'm going to drive it in traffic I'm going to punch it all the time right and this thing has to they're not going to modulate you know what I'm saying they're not there to modulate they're just at a flat punch and the car go so how do we pick the right size carbon if we're not going by formulas how do we pick the right size carburetor that's a tough one and even the formulas vary for you know whether whether you're doing doing a street car you know regular everyday driver car in which case size isn't the issue velocity is the issue and that's where spreadboard carburetors are so so they shine spreadboard carburetors have those tiny primaries and huge secondaries so for your daily driver you're into you're into the primary side you've got super high velocity through the boosters good atomization good throttle response good economy you can't beat it you're not so much worried about the overall size and there's another thing we'll get into too so the way the manufacturer is sized carburetors if you go back to the uh the later 70s the early 80s General Motors was putting four valve carburetors on things that really never should have had four well carburetors like for instance the 301 Pontiac came with a four barrel carburetor a lot of the v6s a lot of v6s turbo charger not Turbo Charge came with four belt came with the Rochester Quadrajet and if you look at those cars those carburetors and those aren't oh there are many other examples but if you look at those Rochester's that were the quadrajets that were fitted to those otherwise tiny low compression low flow Motors they all had limited secondaries so the secondaries were only able to open an eighth of the way a quarter of the way a third of the way just it just depending depending on the combination how far they would allow the secondaries to open they were staked so that they couldn't go past that point because if you went past that point you would have that situation where if you're in traffic and you've got to accelerate in bad conditions or whatever you'd have to modulate the throttle they have to pull the throttle back in order for the car to respond and they did that at the sacrifice of top end power by by putting an unstaked Quadrajet on one of those smaller engines you'd probably probably pick up a third more horsepower or you know Peak power but the thing wouldn't function in traffic the way the engineers needed to function in traffic so again how do you pick the right size carburetor all right so rule of thumb rule of thumb here's your rules of thumb if it's a stock basically a stock car meaning stock cam exhaust manifolds regular quiet kind of exhaust and it's going to be used mostly for cruising for for just you know occasional high performance use but you're really more concerned with drivability and and function and fuel miles and everything else go with what the engineers put on there you're not going to out think the engineers if the rest of your package is what the engineers had designed then go with the size carburetor that the engineer is designed into it now if you you go in the next step and you talk about moderate performance so we're talking about you've got free flow a free flow exhaust you've got headers you've got maybe a cam you've got decent heads you got an intake manifold at that point double your cubic inches to arrive at your cfms the formula I've always used I've employed it on I mean I can't tell you how many cars over the last 45 years double the cubic inches and that will give you the approximate good Street and strip performance combination so uh three and rounded up to the next largest size so a 350 Chevy with all of the typical bolt-on performance things will work best or like a 750 Holley or thereabouts in that range mechanical or vacuum secondary it doesn't really make any difference and I do we're going to do a whole video on the difference the the main functional differences between vacuum and second vacuum and mechanical secondaries beyond what the obvious vacuum opens when linkage opens the other um but not now because this has already gone way too long I see people drop it off like flies um double the cubic inches simple easy formula and it works almost every time and if you look at the maximum performance packages like the 446 Street packages not super stock or a factory race packages but regular Productions three packages it actually comes pretty close 440 cubic inches double that that's 880 cubic inches you've got the tri-power set up there six pack set up with 950 CFM 975 CFM it comes to about right double the cubic inches and round up to the next largest carburetor size and you will be happy car will function well in all situations now if you go on past that and you've got a race cam you've got race heads you've got a tuned exhaust not just a free-flowing exhaust but a tuned exhaust you've got all of the elements that make for a race car well then there is no real formula other than go with the biggest carburetor you can it's a trial and error and it can get expensive trial and error you know because like oh God I'll put an 850 on here well let's try a 950 well let's try a dominator let's try two dominant right you know you can keep going with it but then again if you're playing the game at that level you have to expect to spend the money and spend the time and the bottom line is go with the biggest carburetor that'll net you the numbers that you're expecting out of your car you know what it should run take your car the weight the the power put your put it through one of the power speed calculators you know what it should run based on those based on the incrementals that you find on your time slip go with the biggest carburetor you can to get as close to what the car should run via those calculators and that's the right size for you I know it's broad but the bottom line is this if you go by the formulas for anything other than a completely stock daily driver car you will be disappointed and like I said don't take my word for this this isn't my opinion go back and actually look at history look at the carburetors that the manufacturers put on their maximum performance Street packages and you'll see those formulas have no bearing in the real world I hope you got something out of that we'll do we're going to do a lot more on this in the future because this is a topic that I'm really I'm really into uh but that's it I'll see you tomorrow

Channel: Uncle Tony's Garage

Views: 190,352

Keywords: carburetor, first start in, will it run, restoration, dart gts, dodge dart, challenger, pick your carb, engine rebuild, restored, chevelle, first start, size matters, engine repair, abandoned cars, jay leno garage, rescue, charger, 69 charger, muscle car, carb sizing, carb size, how to make horsepower, engine combination, cubic inches, volumetric efficiency, holley, edelbrock, utg, uncle tonys garage, which carb to pick, how to pick a carb, match carb to engine, how to, choose

Id: HdF7IAWFT8w

Length: 22min 43sec (1363 seconds)

Published: Sat Oct 08 2022