HOW TO: pick the right carburetor. SIZE MATTERS!!! Don't overestimate, do the math. 

I have to say you never know till you try, I ran a 1475 cfm Holley gen 3 carb on a 454, it ran great. So I believe anyone can say anything. Test and tune

Currently have a Ford 460 running a 750 carb with vac secondaries. Runs great

Having owned a 4valve ford, it ran a very high VE of 93%, hemispherical chamber. Measured at under 600cfm, so the math is accurate!

For the first time in my life, mines too big. Lol Thanks for the education Mike! RAWK!!

Thank you for making me understand how to calculate for the correct carb to use. I have been trying to figure out what to pick the correct carb for my bike. Now I have a clear picture. Thank u so much

A good ve comes from a good cam bigger than stock with at least 11to 1 compression . Great carb , intake manifold ,, CNC cyl head ,,, headers that fit the ports ,,, 100% ve will occur some times

One thing I might have missed, but single plain intake vs. Dual plane intakes makes a big difference on CFM.

Calculate air flow for cubic inch and max RPM and volumetric efficiency V/E Holley recommends for naturally aspirated 100% ve for most performance . Race only 110% ve . If the carb is to big it will not meter the fuel as well due to lack of vacuum signal . Once the engine is on the dyno the vacuum measured at full throttle should be .5 in hg to 1.5 inhg .

Smart guy+ dry erase board = me feeling dumb. Good video I haven't felt math anxiety in years, all jokes aside thanks for giving your time youtube university salutes you

Also a single plane intake won't tolerate near as big as carb as a duel plane specially on the street.

The worst situation is an oversize carb with being a double pumper with low stall and stock diff gears

Totally agree that for street use smaller carb always runs smoother..But...Dyno numbers and dragstrip mph show specially tuned 950cfm will make slightly more power everywhere above cruising speeds on your typical 450-500hp 383cui.. not talking drivability

Good job explaining both the math and physics involved… you could do some stand-up comedy shows also… keep up the good work…

I always sized carbs the way you describe, but one time I was in a bind. I replaced the Holley 650 vacuum secondary carb on a tight but mild 440 with a Holley 800 spread bore double pumper on what was supposed to be a temporary basis. The car ran better on the street and the strip, so I never went back. I don't have an explanation.

People need to understand something basic about a CFM rating for a carburetor. It is a capacity and not a supply number. In other words the carburetor is not a pump but rather a valve. As the formula is showing an engine that can pull/call for "x" number of CFM. Basically the carb isn't going to "shove" or over-feed CFM but rather the engine is going to "draw" "x' amount of CFM based on cylinders, heads, pistons, and rpms. We could throw the manifold in there but for simplicity sake of the discussion you see where this is coming from. The carb is sized by the manufacturer to have certain dimensions of open space and size of the butterflies to coincide with a certain sizing of fuel metering in order to get an atomization figure that is efficient across a CFM flow range. You can plunk a 600 CFM carb on something and maybe it will be on a motor that will only ever try to draw 500 CFM max. You could conceivably re-jet/meter that carb for smaller fuel flow and be fine. My point is that there is a great misunderstanding about how a carb functions and what it is actually doing. When people say they have a 780 dual feed on a stock 5.0 ford and it seems to run fine it may very well be the case but not because the engine is calling for 780 CFM when it's cranking 6000 rpms but rather because the air/fuel ratio, atomization and quantity is OK for the other factors of engine performance and is not loading up the cylinders. There is after all an efficiency of combustion to take into account long before we worry about 600 or 850 for CFM figures on a carb. Most people do overcarb from a capacity standpoint which is quite different from a supply standpoint. Thought of another way if you put air flow monitoring between the carb and manifold on a 5.0 Ford and the carb was an 850 you might be astonished at the fact it might only be measuring a flow of 400 or 500 at 6500 rpms because that's all that it is trying to suck into the engine and the carb is not limiting it certainly but it has no ability to try and "over feed" CFM. As I said as long as the fuel/air is OK you not be overloading the engine on a CFM basis. Biggest mistake people make is not understanding what is actually happening with an engine. I don't care if a carb is rated for 2000 CFM if the running engine is only calling for 500 then that's it in a NA engine. The same for heads. They may have a flow figure that is way higher than what the cylinder as configured can, or needs to, pull through them. You see guys with huge flowing heads running around on the street and they don't understand that the heads they paid huge money for aren't even approaching anywhere near the flow limit even when they're at wide open throttle. You start in the cylinder and it all follows from there.

I have Asbergers. This video makes more sense to me regarding carburetor sizing than anything else i have seen on the web. I bought a 390gt mustang that has a 750cfm carb. The engine runs well but i am thinking the engine will run better with a 600cfm that i have ordered this morning as the secondary vacuum is not working. Everything has been checked and i have also fitted the lightest spring. I guess the engine is unable to open the secondary vacuum due to the carb been too big.

I had no idea Penn from Penn & Teller was so into cars. All joking aside. The information spot on. I'm personally in the process of building a SBF. Working with a recently retired dragster guy. Who's helping me assemble the stroker motor. Waiting to see what he says about how high we plan on spinning the motor. He's definitely more conservative on carb size. So even with 427ci. I'll probably still end up with a 750dp. It's a 67 fastback with a 5spd and 389 gears. So it's gonna be a fun street car.

thanks for this info, when i was younger i had a 302 with a stock 2 barrel so i though he lets get a bigger 2 barrel carb and get more power, did not make a difference that was very noticeable

You're wrong buddy... I put a 1050 Holley double pumper on a 351 Cleveland with 2v heads and shaved 2s off my quarter-mile time that's all I did you want to know why it got faster the holes in the base plates are bigger than a 650 or 750 more Air..

Cam grinders give you all the info for that specialy cam you want. Like how much carb, intake, ported heads, stall..gears.

Divide by 3456 because there is one intake stroke per 2 revolutions of the 4 stroke engine.

Awesome video. Great detail. I am new to carbs and have been looking to my buddies for help and they all throw out random numbers. Lol. I am a little stumped on my VE still though. What carb would you use in my setup? 68 mustang driven on the street and cruising. 331 stroker, AFR 185 aluminum heads, performer intake, roughly 6500 max, headers and auto Trans C4 with 1500 stall.

Good info, thank you!!!

Thank you for the great explanation of this I have seen some confusing videos on this and your video makes sense bye the math I have a 650 on my engine and it calls for a 653 so I hit it perfect I’d rather be a little bit smaller than over carbonation so I won’t wash out the rings and everything else

Thank you for putting this exce video on this subject I know of guys that have made this video mistake. Your videos definitely rock 🤟🏻

Great Video! This will help not drown out all the potiential Armadillos your motor can make! I might need to run the numbers on a few of my vehicles hahaha Great Job Mike!

Great info. I just subscribed because of this info.

All depends which carburetor , you can go much bigger with a spread bore and still streetable .

Hi. Thanks for putting up this video and other content. I just found you today and I'm subscribing! I have a 390 FE Ford big block engine that we're stroking out.. 428 crank and alot bigger cam. Its our first engine to build. Do I kinda guess the cubic inches of what it could be... Or just go with the 390 original size..? I have a brand new 750 Holly double pumper that I have access to.. Would that be fine since we're stroking it out?

Very good presentation thank you!

Your video is fantastic,i don't work anymore on cars because of my health but i enjoy very much all the lectures .I want to know in the formula how do you know how high the rpm will be before you do a dyno test so you can achieve the formula right ? thank you

This is great there are so many opinions out there especially on the aftermarket parts reviews jegs / Summit /Forums and alot of them are wrong. I Iisten to too many and it cost me in the long run I have read put 750 nothing less i would rather be over carb than under carb . Intake manifold as well i have a Ford 390 truck engime 9:1 compression truck heads puller motor basically i wanted to go from a 2bbl to 4bbl i bought the cast iron boat anchor intake then read all.the jegs and summit and forum guys saying ditch that boat anchor ⚓ get a Edelbrock but dint get the perfomer thats just wieght savings yiu will never hit 5,000 Rpm with a regular performer you want the RPM performomer . Ok so i listen to them that thing was a beast the runners were huge compared to stock anyone work on a 390 Ford ? The pushrods go down through the intake first drive i was not impressed i had actual better performance with the stock 2bbl and 350 cfm Holly . The low runners gave me tons of tourqe and mid range power matched with the bigger base plate 350 2bbl if flowed better at take off and cruise . The high runners of the RPM needs heads that flow well to match and i woukd nevef hit anything over 4,800 with that stock crank the Gt 390.was rated at 4,800 RPM sure it runs good but in this case i think it woukd run way better with the regular eddy or even the cast iron 4bbl all because the internet new better see the price of thst thing lately ? A eddy performer RPM is pushing 700 bucks thanks guys 😮 whew had to vent ....

We did this at the engine shop came out to be a 650 cfm ran it then a 750cfm and made more hp with the 750,put a 800 and did even better but lose a little at the bottom end.

I have a Pontiac 428, had to bore .060 over because of cylinder scaring. My professional, lifetime experience engine builder said my 650 was too small. Ihe suggested a Holley 750 double pumper. It’s a street rod not race car.

I always find this issue interesting due to the fact their are so many variables involved. I run a 94 Sonoma on the street with a 10.7 to 1 1st. Gen 399 stroker with 17* heads with 261cc int.runners that I believe most would consider way overkill for the street. My combo features a 4l80E w/3000/3200 stall with balloon plate and 9" with 3.55 gears. On the dyno w/89 pump gas the engine did 266 hp and 400 ft lbs@3500.....440 hp and 513 ft lbs.@4500......608 hp. and 560 ft lbs@5700 max tq. ..........641 hp. max and 526 ft lbs.@6400. The reason I point these #'s out is the fact my engine does not studder or shudder or cough or eat plugs it just runs as smooth as could b with a QFT 950 Black Diamond on the street and I honestly don't believe there's a chance in hell a 739 cfm which is the size carb recommended for my cubic inches could support any of those #'s except at the lowest power levels. Cam/ 260/266@50....679/669...110 split solid roller. Heads flow 404 cfm. What is your opinion?

This is a great great video Shame I'm from and in a metric country😂 But awesome video, gonna have some conversion to do in a bit. Best part is I'm only building a 85cc 12-14K rpm engine right now😂 Aimed at 4500-9000 in use regularly but I was just thinking about how little suction the engine could create in a carb with a venturi bigger than the manifold. Hope I can translate this well to my case got a 20mm stock at 50cc now, percentage wise from 50cc to 85cc 26mm would roughly be the same ratio but with only a 19,5mm manifold that seemed like a bad idea. When tuning two strokes I used to get crazy performance on small racing bikes with 14mm on 62,5cc though. 13,5 honed intake manifold that was, (12 stock I believe with a 9/12 at 50cc) Anyway, great video to help me on the way. And gonna be a shame when this project is done, gonna need me something new to build.😢😂 But for now a whole lot of calculating, flowing and tuning to do.

This is a great lesson.

Well explained

The carb flow is rated at a certain pressure drop across the carb. This basic volumetric calculation doesn’t account for this. There is a video on RU-vid that covers a more real world method for determining carb size. That’s why several commenters have had success with bigger carbs than this basic theory recommends..... This can be a basic starting point, but pressure drop across the carb is a dynamic situation and constantly varies. I haven’t spent dyno time, but been tuning carbs since I was 16 years old and have had fluid dynamics class in engineering school. I had DaVinci change the IFR’s and install downleg boosters in a 3310 VS for a street driven BBC in my ‘72 C10 and it flowed 830 cfm. He ran it on a sbc test engine and got it tuned in. I get it home and put it on my big block and it has a massive hesitation when you stomp on it. Seemed like a lean stumble so I pulled the VS spring to find it was one spring from being the weakest spring in the spring kit. I put in a spring one away from the strongest springs and now VS kick in slower. When you hit it now no hesitation and the old truck jumps sideways .... DaVinci’s test engine isn’t under load and VS’s have to be tuned under load. Even the experts get it wrong at times....

Fuel system engineer? Why wouldn’t you consider booster design and the considerable changes made to modern carburetors? You’re calculations are for 1960’s carburetor technology. By the way, real race engines are well above 100% VE.

The formula doesnt stay specific to everything. Its a good starting point. If your using separate individual runners, it doesnt work at all. And most carbs dont flow thier advertised numbers.

Most vacuum secondarys with stock springs on street smallblocks won't open all the way=smaller CFM.

This formula will give you the smallest option carburetor. A step or two above the size carburetor this formula gives you is the one to go with.

Thanks!

I have built 360 put 625 Carter it would not run well put 750 holly vacuum secondaries ran great surprised us

I went and bought a 600cfm vacuum secondary for my 351w. I removed my 750cfm with vacuum secondary. I have a AFR wide band so I can see exactly which direction my tuning is going. After spending 4 hours and 10 different combinations I put the 750 back on and jetted the front and back the same. The 650 out of the box ran so lean it wouldn't idle and to get it to run I had to use jets bigger than what was in the 750.Then it ran richer than the 750 at idle but lean at cruising speed. After some research I found that gas with 10% ethanol is 14.1 roughly for perfect burn and some motors just need more fuel like 13.0 to1.to run good. At idle with 14 to 1 my motor pops and is starved for fuel. I sent the 600cfm back for a refund and deal with the slightly rich 750 with maybe a next size hotter plug and see what that does.I have no gas in my oil so I will probably just leave it alone. The 750 has great throttle response.

One things for sure, our cylinder heads and camshaft technology have come a long way from what it used tho be. But I agree with you that most people over carb their rides. How many classic rides have you seen at a car show that the fumes from being over rich chokes you out?

Once ive done the math …where would the Cfm be on the carburetor be? Cause all the carbs that I’ve checheck On eBay they don’t come with cfm on the carbs .

I have a 350 sbc with a 671 blower and 7k stall what should I run?

How do you explain how my 460 ford went from 11.50 to 11.20 in the 1/4 mile just by switching from an 850DP to a Dominator 1050

Isn’t it weird that Chevy had two four barrel carbs on a 302 ? I don’t think they used this formula.

very good video my brother

Im still confused, I had a 71 Cutlass bone stock and i bought it from the original owner. 350/350 turbo transmission. It came from the factor a single pumper 750 carburetor. Would that formula go out the window. I know that by the build sheet and I had to get the carburetor rebuilt and the old man told me that when he worked on it.

If by chance you still read the comments, ... Picked up a stock '70 LA 318 Dodge W200 Power Wagon, truck gears, Carter BBD 2 barrel.. rarely over 3500 rpm. .. Shop manager, and my mechanic suggested a low rise Edelbrock/w 4 barrel. .. Edelbrock customer service recommended an Edelbrock #1405, 600 cfm. .. Questioned him about the size, .. he validated it. ... the formula points to a 350 cfm. ... Will the 600 be more trouble than it's worth?

How do you take something so simple, and make it so complicated?

Well, I know this math and used this math. But it seem when you use a Dyno and experiment you find that on a 355 or 383 stroker pushing around 450hp and up at around 6500 rpm’s the 750 Holley really shines. The 650 works great but you get more out of the 750 but as soon as you go to the 850 you loose performance. “Watch Engine Masters” Back in the mid 80’s I had a stock 350 with a 2 barrel and a automatic transmission. 72 Chevelle I rebuilt it . All I did was add a performer intake and cam for low end shaved the iron heads 20 thousands. 5500rpm was max and that was pushing it. I installed a 600 Holley double pumper mechanical secondaries. Was thinking I should’ve used a 650 vacuum secondaries. No race engine but I sure had fun with it. So, in my opinion……. I think that calculation works great for stock or mostly stock engine and not so much when you are push 450hp and up. I use that calculation as a good starting point.

Mike, which carburetor should I use for 1969 Lincoln mark 3...460.engine

A 5 liter spinning 5000, with 80% VE only needs about 385CFM, so even a 600 CFM is overkill. A 454 would be still be less than 600Cfm at 5000 with 80% VE

Nice intro song Mike 👍🏼

My 800cfm edelbrock carburetor is likely to big for my 383 stroker. 3800 Stahl 3 speed.Having idle issues & fuel pooling in intake. I used your calculations & came up with a 650 cfm carburetor. What do you think?

What would u consider a 351w. Stock bottom end throw down 370 rwhp and 460 ftlbs threw a c4 trans ve%

Makes sense but why does gm recommend a 750 on their 350 zz crate motors?

Two words, Thermoquad. No math needed. 😉😜 Thanks for sharing Mike, excellent explanation for all us cavemen.

I seen a video on RU-vid when a guy took a sound and stock 350 and put it on a Dino. He then tested different cards from 500 cfm two barrel to 1100 cfm four barrels and got the most power with the 1100 ,was this a scam.

Awesome T-shirt!

These formulas make sense on paper, but then you think about real world applications like the 302 z/28 had 2 780cfm carburetors and performed extremely well. The 340 six pack had the same carburetors as the 440 six pack and both performed extremely well. Maybe someone can shed some light on this.

On a race car I double cid 355 I'd run 700 cfm street car I run the 3456 method

This didn’t work for my ford 545 Holman Moody heads and 10’000 rpm’s .

I keep doing the math and it keeps telling me I need a carb that is 470 cfm. That seems way to small for a 350 small block Chevy. It's not a built engine, it's a stock 1987 suburban tbi engine that I'm swapping out to a carburetor. The only performance modification's will be an aftermarket dual plane intake manifold and possibly headers. I'm looking at the Holley 90470 but part of me thinks I should purchase the Holley 90670. Any advice would be greatly appreciated. This engine won't see much time over 5500 rpm and it's going in a 77 GMC Sierra K15 4x4.

Please help i have a 347 stroker with 830 cfm is that to much ??

Hi I have a 383 stroker engine but it is unknown. I have no specs and don't want to tear it apart. What carburetor would you run I have a 2800 stall converter in my 700r4.

Great content Thank You . 🇺🇲🇵🇷🦊👍

Wouldn't carburetor size change the volumetric efficiency too?🤔

This is wrong, and you will buy a carb that's too small. The way manufacturers flow and rate carb cfm is at higher vacuum than you will see on an engine. The carb can be much bigger than the minimum required without hurting drivability .. watch dyno videos from motor trend or Richard holdener and you will see this proven.

Now how do those number get affected with towing or hualing etc?

What is the difference between a stick and a manual?

Does the same equation apply to a four stroke motorcycle engine? How do you determine your volumetric efficiency if you don't know what it is?

Pontiac 455 should i get 650cfm carb ?

So I'm very confused about the formula behind determining the size. This say you have a 351windsor. 351 cubic inches and say it goes to 6,000 rpm which I think is at least 500 to 1000 more than a stock engine will take.Now we take the value at 85% which again is at a very high end for a stock engine.Now after ÷ this by 3456 we come up with 518 cfm. So my stock 1987 F150 came with a holly 600cfm carburetor. When I use 351 cubic inches and 5500 rpm 80% value this goes down to 447cfm. With that being said, why would Ford use such a big carburetor that would cost more ? I think the Ford engineering department had figured out exactly what the engine required. That's why I think this mathematical formula is misleading at best. I'm not trying to start some war with you, I watch your channel and really enjoy your video's. On another subject I'm going back to the drag strip tonight with new micky Thompson 295 ss drag radials to try and get into the 14 second bracket. Stay cool and keep making videos, really enjoy your shows.

Thats a 10% difference in CFM, when you went from the 85% to 95%...thats more than not much.

Holley tech told me 650 mechanical secondaries for a 355 vortec, (EQ ch350 heads), RPM to 5,500, mud bog jeep motor. According to your math I’m way over carbed. Any opinion on this? Thanks jack.

So with a smaller carberator would the vehicle get better gas mileage

Maths wrong…. Cubic Inches x RPM / 3456 = CFM (cubic feet per minute) / Volumetric Efficiency First calculate the CFM. CI x Engine RPM/ 3456. That will give you the CFM. Then divide by the volumetric efficiency, that will give you the carb size you should shoot for.

So 427 with a 780cfm red line 6000 ? 550hp ???? I 🤔 do I need a 850cfm ? Hell I don't know...

Isn't VE on a stock 60's muscle car closer to 65%? (it would have to be in view of the low power outputs compared to engine size) About 20 years ago, I remember asking VE on Pontiac 400 motor and 'Pontiac Owners Group' were 'convinced' VE was 100% 🙄 At least I now know why 3456, doesn't it matter how many cylinders are drawing through carb though? Street motors running 8,000 rpm are out there, but (big BUT) most of the time they are only turning 3¬4,000 or less Ram air is pretty negligible below 400mph compared to what the engine is already drawing in (for anyone else's benefit as you already know that) BTW, I almost exclusively work on motorcycles, Suzuki were getting 130% VE at 150mph but that was a modern design optimised for airflow

Why does the math basically call for a Holly 600 on a mild 350, yet the factory Q-Jet from Chevrolet is a 750?

Soooo…. No Dominator and 2” headers on my 305? 😂

This is incredibly inaccurate to what CFM my engine ran quickest with. 15.5:1 alcohol 310”sbf shifting at 8800 with a liberty style five speed. Based on your calculation it would run best with a 750. The 750 4500 went drastically slower than the 1050 4500 which went marginally slower than the 1140 4500 on the same track with the same AFR as well as nitrous jetting and bottle pressure.

Thanks...I'll just stick to my 650

You’re wrong. Bigger is better After installing a 750 double pumper on my lawnmower I am able to shave off .2 sexy off the time it takes me to cut my lawn .

i have seen a STOCK 305 make its best hp and torque with a 950 cfm on the dyno . Every drag racer out there seen much improved et time slips with a 750 over a 600 at every drag strip around . Just because it is math does not make it right and your made up number of 3456 is wrong ! Ps every body and their bother has a dyno now so !

😎😎😎😎😎😎😎😎😎😎😎😎😎😎

When it comes to carbs...yes it does you can easily over carb..and your motor will burn more fuel foul more plugs and make no power...fact...

The math says. a. FE. 390. At 6k. .85. ÷ 3456. Wants. 575. Cfm. ? Do why does a FE. 390. Make more power with a 750 dp. 25 ft tq. Over a 650. Carb. On the Dyno. The Dyno don't lie. Something is not right with the formula

That formula only works for mild street where you have a broad range your engine may run in. Throw it out the window for anything else! At 100% VE my 355 would churn out a 715 cfm at 7000rpm. I was pulling so much vacuum as the tach rose it was ridiculous. 850 cfm and power was better. Granted I had to tune it for that, but one needs to tune regardless. Lack of tuning skills won't allow you to run an optimal cfm carb. I'm not talking just changing jets and or idle mixture screws. Your myth has been around forever and is completely wrong. It's been proven wrong way back. "the truth you know might not be the real truth"

302cid × 6k rpm × 0.8 VE ÷ 3456 = 419.444444 Hmmmmm 302 × 8k × 0.85 ÷ 3456 = 594 Hmmmm 302 × 8k × 0.95 ÷ 3456 = 664.12 1969 Z/28 DZ302 Cross Ram: Factory equipped with twin 585 cfm Holley carbs for 1170 total cfm. Your math isn't even good enough to be considered horseshit

Get a haircut and buy the biggest carb and cam you can afford